Origin of Life -

Contents

Origin of Life

more about the concepts of self-assembly, self-organization and the phenomena of emergence (from the prebiotic soup, Figure 01). The explanations are all based on the science of physics and chemistry although ultimately life could be better described at the level of biology. Giving enough time, life is inevitable provided the condition is right. For example, the hydrogen bond so important for maintaining the structure of life will be disrupted by thermal agitation at few tens

Figure 01 Origin of Life [view large image]

of degrees above room temperature; and the structure becomes frozen if temperature is low enough for the van der Waals force to take over.

Self-assembly

• The prerequisite for self-assembly of two molecules demands that the binding partners are equipped with compatible binding sites in a complementary spatial arrangement — analogous to a lock with a matching key, i.e., it is similar to the binding between enzyme and substrate or ligand and receptor (see "Catalyses", and a theory of "Induced Fit". BTW, any broken thing has the lock/key conformation as well ). Non-covalent interactions such as hydrogen bonding, and van der Waals forces then serve as the glue that holds the 2 molecules together. Examples include the interactions between ligands and receptors, the docking of antibodies on antigen molecules, the binding of DNA/polymerases, and the instant self-assembly of the ribosome components in presence of mRNA.

Order - The self-assembled structure have a higher order (lower entropy) than the isolated components. According to the second law of thermodynamics its surrounding would become more disorder (higher entropy). Interaction - It usually involves only the weaker hydrogen bond, which makes the structure more flexible enabling the system to withstand minor perturbation (the ability of reversibility). Building blocks - They are not only atoms and molecules, but span a wide range of nano- and mesoscopic structures, with different chemical compositions, shapes and functionalities. Examples include crystals, colloids, lipid bi-layers. The folding of poly-peptide chains into proteins and nucleic acids into their functional forms are examples of self-assembled biological structures.

Figure 02 Self-assembly [view large image]

Figure 02 shows some examples of self-assembly from peptide components to nano-structures. The alphabet in there represent the individual amino acid.

Self-organization

more elementary, which can be traced back to the electromagnetic force; the interaction in self-organization involves the kind of action that affects many facets. The size of the resulting structure from self-assembly is mostly mesoscopic, but it can be considerably larger for self-organization. Figure 03 illustrates the continuum of open thermodynamic systems from ordered, near-to-chaotic, to far-from-equilibrium states. As energy consumption increases, systems move further from equilibrium and pass through a phase transition between order and chaos. Complex systems such as proto-ribosome exist in this phase transition.

Figure 03 Self-organization [view large image]

The diagram also shows the assembly and self-organizational activity of the proto-ribosome under the sun (for energy infusion). It is supposed to be the ancestor of the modern ribosome making protein from genetic codes.

Emergent Phenomena

Self-organization is different from emergent phenomena. While both are dynamic processes arising over time, emergence emphasizes the presence of novel coherent macro-level emergent property, behaviour, structure, ... as a result from the interactions between micro-level parts (rectangles in Figure 04b). Self-organisation emphasises the spontaneous increase in order (circle in Figure 04a). When both the self-organizing and emergent processes

Figure 04 Three Auto-systems

are involved in creating the system (as in Figure 04c), it is often mistaken that they are the same process. It is this kind of process that is most important for the origin of life.

	macroscopic correspondence offer an example of emergent phenomena without self-organization - it is a change of perspective from individual molecules to thermodynamics without any change in the entropy of the system. The example of slime moulds life cycle is for
Figure 05 Examples of Auto-systems [view large image]	the case of simultaneous occurrence of self-organization and emergent phenomena (Figure 05).

	The self-organization and emergent phenomena occur simultaneously when the slime moulds (see more in "Slime Moulds Life Cycle") run out of food. As shown in Figure 06, The parts are the individual amoebae, the interaction is the adhesive molecules, the energy is supplied by their favorite food - the bacteria and yeast, and the emergent phenomena is the moving slug and fruiting body.
Figure 06 Slime Mould [view large image]

In summary, the minimum requirements for emergent biologic phenomena out of self-organization include :


1. There are interactions between components parts to build the structure.
2. An open thermodynamic system with many parts to allow the infusion of energy and removal of entropy.
3. A positive feedback loop causing nonlinear dynamic behavior (for amplifying small fluctuations).

It follows that the resulting characteristics of the biological emergent phenomena are :


1. Novel properties not observed previously in those component parts - the total is more than the sum (via interaction).
2. The resulting entity can reproduce, is coherent and could maintain itself as a whole over period of time (by energy infusion).
3. It can evolve to something else. This is the Darwinian selection, which determines the outcome of the evolution according to changing environment (in positive or negative feedback loop).

2018 Update on the Theory of "Prebiotic World"

Evolution of Ribosome : starting from recent observations to some evidences from the ancient world.

Most theories on the origin of life adopt a "bottom-up" approach from simple to complex and from unknown to known. Another method is to trace the evolutionary path from a known point at present. The path is narrowing down by observing a conversed organelle called ribosome inside all life including bacteria and human. Although difference exists on the ribosome across different species, there is a core structure inside that is present in all of them. The following is an attempt to reach the origin by such scenario from "The Modern Day Ribosome" (structure and function) to "History of Ribosome" (looking backward to LUCA).

Figure 07 RNA World [view large image]

Figure 07 portrays one of the bottom-up theories - the hypothetical "RNA World" in a evolutionary sequence from some nucleotides ~ 4.5 Gyr ago to LUCA (Last Universal Common Ancestor) ~ one billion years later. See relationship to the "RNA-Protein World" in Step C.

• The Modern Day Ribosome (structure and function) - The main function of the ribosome is to make protein (folded 3D version of the peptide chain, Figure 08a) according to the genetic codes (each codon = 3 nucleotides) on the mRNA. There is an intermediary in the form of tRNA, which associates every anti-codon with an amino acid (see Figure 08). The ribosome is composed of a small (SSU) and a large (LSU) subunits each one is a mixture of rRNA and proteins (Figure 09). In the absence of mRNA, all these components have its own existence separating from each others (Figure 10). Mammalian cell contains about 10 million of the ribosomes [either free or bound to the rough endoplasmic reticulum (ER) for exporting the protein outside the cell] and about equal number of tRNA. As a very good example of "Self-Assembly" in action, each one of these parts assembles into a working

  	ribosome at the presence of mRNA. The generation of peptide chain occurs at the active site, where the rRNA in the LSU acts like an enzyme (ribozyme, aka peptidyl transferase) to add another amino acid from the tRNA at A-site to the growing peptide chain at P-site (Figure 10). The SSU is the decoding site. [peptide chain, 08a]. In chemical formula :
  Figure 08 Genetic Code / Amino Acids / Peptide Chain [view large image]	(peptide chain)n + ribosome + mRNA + (tRNA + amino acid) + ATP + H2O      (peptide chain)n+1 + ribosome + mRNA + tRNA + AMP + PPi + 0.64 ev

  See Footnote on ATP

  			The aggregate also recruits proteins called elongation factors (EF) to transfer the tRNA into the ribosome at A-site and to push the mRNA to the P-site for adding another amino acid to the growing peptide chain. The action is self explanatory by Figures 10, 11; while the video in Figure 11
  Figure 09 Ribosome Structure [view large image]	Figure 10 Ribosome and tRNA in Action	Figure 11 mRNA Translation [animation]	further articulates the processing sequence which moves the whole thing to an ER at the end.

  Termination occurs when the ribosome reaches a stop codon (UAA, UAG, and UGA). Since there are no tRNA molecules that can recognize these codons, the ribosome senses that translation is complete. The new protein is then released, and the translation complex comes apart (see "RNA Translation").
  
  Polymerase is an enzyme for synthesizing long chains of polymers, or nucleic acids of RNA (from genes in the DNA). In particular, the RNA polymerase I (Pol I) synthesizes the rRNA in LSU and SSU, while Pol III makes tRNA, and the very short 5S rRNA in the LSU. Pol II produces a precursor of mRNA, which (depending on species) would synthesize 50 - 80 individual proteins that provide support for ribosome assembly and translation from mRNA to peptide chain (Figure 12).
  
  Figure 13 shows the evolution of common core of the E. Coli bacterial SSU rRNA in the form of helices (single RNA strand folds

  		back to form double helix) labeled by numerals from 7 to 10 (see Figure 13,A) with a total length of 92 nt (nucleotide). The difference to this basic form in other species occurs mostly from the extension of helix 9 (shown in different color in different species). Figure 13,B is the secondary structure, 13,C the 3D super-imposition for H. sapiens, 13,D shows the region of insertion (fingerprint). According to this sequence of variations, the common core can be considered as the basic structure of the SSU rRNA soon after the emergence of life. There is similarly a basic core for the LSU rRNA.
  Figure 12 Ribosome Assembly [view large image]	Figure 13 Ribosome SSU Evolution	see 3-D Pol III and a video on "cryo-EM" (for making such image).



• History of Ribosome (before LUCA) - Following the investigation of rRNA structures across the tree of life, a model based on observations categorizes the "Pre-LUCA History" into 6 phases as summarized below from ancient to modern (Figure14):
  
  

  Figure 14 Ribosome Evolution     See original article : "History of the Ribosome and the Origin of Translation"

  • Phase 1 - The LSU and SSU start to evolve uncorrelated from ancestral RNA. Primordial stem-loops (precursor of tRNA) also appeared at about the same time. These defect-laden chemicals are stabilized by interactions with metal cations [metal (metal ⁺ + 1 free e^-)] and the folding protects them against chemical degradation.
    Even in this modern day, all these subunits maintain their separated existence until a mRNA comes along. Then through the process of self-assembly they form the ribosome (as one unit) to translate the mRNA codons to protein returning to their separated existence once the task is completed (see Figure 10).
  
  
  • Phase 2 - The evolution and function of the LSU and SSU are uncorrelated. The LSU is a crude ribozyme, catalyzing nonspecific, non-coding condensation of amino acids. The exit tunnel (just a pore at this stage) retains and stabilizes reaction intermediates, facilitating the production of oligomers. Charged CCA tails (see top-right insert in Figure 10) in tRNA, which is in the form of minihelices, deliver activated substrates to the PTC. SSU function may involve association with single-stranded RNA. The RNAs grow by expansions.
  • LSU - The PTC is stabilized, exit pore formed. Expansions develop for the A-site and P-site.
  • SSU - The initial expansions produce a quasi-stable fold.
  • tRNA - Pre-tRNA obtains a CCA tail. It also acquires the amino acid acceptor stem and then the T-stem and T-loop to form a minihelix (a special recognition site for the ribosome to form a tRNA-ribosome complex) .
  
  
  • Phase 3 - Correlations in the evolution of the LSU, SSU, tRNA, and mRNA are initiated. Catalytic efficiency and product length are increased.
    • LSU -The PTC (Peptidyl-Transferase Center) is encased, and the exit pore is extended into a short tunnel by expansion. An embryonic subunit interface is formed.
    • SSU - The evolution is uncorrelated, but it docks into the bridge between the subunits correctly.
    • tRNA and mRNA - The LSU, tRNAs, proto-mRNA, and the SSU form a noncovalent quaternary complex. Proto-mRNA is a random population of single-stranded oligomers.
  
  
  • Phase 4 - The ribosome is a noncoding, diffusive ribozyme. The LSU, the SSU, proto-mRNA, and tRNA can be coupled and evolves together. The subunit interface is well developed, with numerous RNA-RNA interactions. In the LSU, the tunnel is further elongated and rigidified, facilitating the production of longer condensation products.
  • LSU - It gains mass and assumes a hemispheric shape in correlated evolution with the SSU. The exit tunnel is extended.
  • SSU - Newly acquired rRNA creates well-defined binding pockets for tRNAs. Primitive binding sites for elongation factors (EF) G and Tu are formed by expansion.
  • tRNA and mRNA - tRNAs are optimized to form quasi-stable triplets matched to the proto-mRNA, which remains a single-stranded oligomers.
  
  
  • Phase 5 - Ribosomal evolution from this phase forward (to 6, 7, 8) is a tight collaboration between RNA and protein. The ribosome achieves primitive decoding ability.
  • LSU - It acquires the ratcheting system, which moves the mRNA along. The binding sites for elongation factors (EF) G and Tu are established. The 5S rRNA now associates with and reinforces the central protuberance (CP, see Figure 09). The peptide chain exit tunnel is extended. The LSU joints the SSU from each side by bridges.
  • SSU - The P-site and A-site is stabilized. There is expansion for decoding, translocation, and proofreading. Intrinsic flexibility incorporated into the SSU allows the domains (distinct structures) to move relative to each other, providing a mechanical basis for translocation.
  • tRNA and mRNA - Anti-codons in tRNA form specific interactions with mRNA codons. mRNA is polymeric with a defined sequence. The genetic code begins to optimize and expand.
  
  
  • Phase 6 - The rRNA common core is finalized, and its genetic code is optimized. The ribosomal surface is an integrated patchwork of rRNA and mature ribosomal proteins. Many expansions acquired in this phase serve as binding sites for ribosomal proteins. The interaction between the two subunits is further elaborated by bridges involving both rRNA expansions and ribosomal proteins. Other expansions form the translocation stalk from P-site to E-site. The exit tunnel for the growing peptide chain is still under development. LUCA emerges at the end of this phase.
  
  
  • Phase 7 (not shown in Figure 14) - The rRNA develops eukaryotic-specific expansions which are stabilized by eukaryotic-specific proteins.
  
  
  • Phase 8 (not shown in Figure 14) - This phase marks expansions in metazoan (multicelluar organism) ribosome. The additions serve as docking sites for chaperones (escort), and regulatory proteins.
  
  

Recently in 2022, there is a report about "Ribosome Self-replication Outside a Living Cell". It could have a huge implication on the scenario of "RNA/Protein World" for the Origin of Life Theory. Here's an attempt to illustrate its significance through the evolutionary history of the ribosome from modern to ancient.



Ribosome facilitates the translation from genetic codes carrying by mRNA to poly-peptide with each amino acid delivered by one tRNA (Figure 14b,c). Figure 14b also shows details of the 2 ribosomeal components (the Large and Small subunits - LSU, SSU) in the modern day version together with the mRNA and tRNA. The subunits break down to even small parts of 3 or 4 rRNA strands (Figure 14c) and up to 50 units of proteins (Figure 14d) depending on the kind of cell with size about 10-6 cm.

Figure 14b Modern Ribosome See "Synthesis of Ribosomal Subunits"

The Svedberg unit (such as 50S) offers a measure depending on the particle's size, mass, density, and shape indirectly based on its sedimentation rate under acceleration.




	It is a remarkable fact that the components of the apparatus for making proteins maintain their separate existence in the absence of mRNA for translation. Each one of them seems to evolve independently in view of :
Figure 14c rRNA Types

The types and numbers of ribosomal RNA (rRNA) in LSU and SSU as well as the scaffolding proteins are different between prokaryotes and eukaryotes (see Figures 14c and 14d). Examples of rRNA accretions across species are shown in Figure 13. The evolutionary process would have to occure continuously starting about 2 Gyr ago (see Figure 19e). The virus do not have ribosome, they have to invade the host to replicate (see "more details on virus"). In the 2015 research paper on "History of the ribosome and the origin of translation", a model has been constructed to trace the evolution of the SSU, LSU, tRNA, and mRNA (see Figure 14). It is based on knowledge about

Figure 14d rRNA Subunits

extant ribosome, and in consistent with further researches on primitive ribosome as shown below.

• Experiment in 2022 shows that poly-peptide can be assembled by naked strands of RNA without the complicated machinery of ribosome (see details in "2022-05-19 Update").



• Finally, the latest (as of August, 2022) research on ribosome (see the preprint article "Reconstitution of ribosome self-replication outside a living cell") shows that ribosomes would emerge in a sort of primordial soup giving enough time (7 years in the lab, ~ 4.5 Gyr in ancient environment). Here's the summary and comment.
  
  
  • The research coactivates (adding coactivator proteins to increase RNA transcription rate) the transcription of an rRNA operon (a cluster of genes that are transcribed together to give a single messenger RNA molecule), as well as the transcription and translation of 54 r-protein genes encoding r-proteins, and the coordinated ribosomal assembly in a cytoplasm-mimicking reaction solution. It is found that such mix is highly efficient in vitro reconstitution of ribosome biogenesis. They also succeeded in engineering rRNA and r-proteins by only adding mutant ribosomal genes in the reaction, enabling high-throughput and unconstrained creation of artificial ribosomes with altered or enhanced functionality.

  Other report (see "Self-replicating protein factories are a step towards artificial life") additionally indicates that by putting DNA coding for ribosome components along with ribosomes derived from E. coli cells into a tube, and adding a tweaked version of the cytoplasm also extracted from E. coli cells, the researchers (Guided by an artificial intelligence) established the precise molecular cocktail that would kick-start ribosome self-replication. The winning formula enabled ribosomes to produce new ones within 3 hours (but it takes 7 years' work to optimise the reaction mixture).

  Figure 14e Uncorrelated Evolution

  Such artificial environment is a much enhanced version of the primordial mixture (Figure 14e) that existed ~ 4.5 Gyr ago. It could be in a small tidal pool or a little pond. The uncorrelated evolution would take a long long time statistically to assemble something like a ribosome. The process through self-assembly may help; anyway, the emergence of life is not so common.

  [End of 2022-10-05 Update]

  Evolution of aaRS : the "Protein/RNA World"

  
  

  		The model above is essentially the theory of RNA world looking backward. It postulates the RNA to perform both the replicative and catalytic functions. While catalyst can be made from RNA folding, RNA self-replication is difficult to create in laboratory. One suggestion is by ribozyme catalyses as shown in Figure 07,
  Figure 15 RNA Replication by Heating and Cooling	Figure 16 Protein/RNA World [view large image]	Another theory considers heat separation of the RNA minihelix and piecemeal assembly as shown in Figure 15.

The alternate is the "Protein/RNA World" (Figure 16), which is devised on the following considerations :


• Failure to self-replicate RNA without enzyme in laboratory (see "Before RNA World", and 2022 update on "Self-Replication").
• The catalytic properties of ribozymes are not sufficient to generate RNA replication as observed so far.
• Contemporary proteins are assembled by a ribozyme (~ribosome) and contemporary nucleic acids are made by a protein polymerase.
• None of the biological reactions (called metabolism in such context) would take place if the participating substances are merely mixed together. It requires catalysts (called enzyme for making complex organic molecules) to speed up the reaction (see Figure 30) and other kinds of proteins to self-assemble everything into a functional unit.
• Amino acids would have been as abundant as the nucleic acid bases in the prebiotic world (see "prebiotic chemistry"). They could as easily (or as occasionally) be combined into peptides (proteins) to serve as catalysts.
• The requisite specificity is low in the initial stages of the Protein/RNA world, but unacceptably high in the RNA world. Low specificity processes occur with greater frequency and hence are more likely to have occurred first.
• Experiment has demonstrated that the two distinct aminoacyl-tRNA synthetase (aaRS, see Figure 16) to link up the tRNA with amino acid are, very probably related to one another. They arose as products of sense/antisense translation by processing from opposite ends of a single gene. These two classes of aaRS still retain some small, highly conserved fragments of the urzymes (ancestral enzymes), which contain two domains for catalytic and anticodon-binding functions respectively.

Here's a summary of the evolution of the Protein/RNA World (forward in time) with the development of the aminoacyl-tRNA synthetase (aaRS) playing the central role as illustrated in Figure 16 (see more in "Peptide/RNA Partnership") :


1. Biology emerged from a reciprocal partnership in which small ancestral oligopeptides and oligonucleotides initially both contributed rudimentary information coding and catalytic rate accelerations.
2. The superior information-bearing qualities of RNA and the superior catalytic potential of proteins emerged from such complexes only with the gradual invention of the genetic code.
3. The two aminoacyl-tRNA synthetase (aaRS) ATP binding sites (including a 46-amino acid sequence - the 46mers, see Figure 16 for location of the site) arose as translation products from opposite strands of a single gene. This is the key intermediates on a path from simple catalytic peptide/RNA complexes to contemporary aminoacyl-tRNA synthetases. This scenario provides a path to increasing complexity that obviates the need for a single polymer such as the RNA to act both catalytically and as an informational molecule.
4. The pattern persisted until the evolution of the modern aminoacyl-tRNA synthetases (aaRS), tRNAs, and ribosomes enabled higher-specificity genetic coding. The LUCA emerged at this stage. The early steep raise of the evolutionary curve in Figure 16 indicates relatively easy steps in synthesizing macromolecules such as RNA and protiens, while the nearly flat portion of the curve later signifies the more difficult task of making them to perform useful functions such as replication of a strand of RNA. That is, synthesis is relatively easy by just adding another unit randomly selected, while replication involves one to one copying of a specific sequence.

role of aaRS is only implied in the formation of the proto-CCA (Figure 10). The RNA World follows the evolution of ribosome to arrive at the ancestral rRNA. While the Protein/RNA World as portrayed in Figure 16 emphasizes the role played by aaRS, which bears evidence of ancestral protein (enzyme) in the very beginning of the evolution of life. These two theories are complementary to each other. Evolution on both ribosome and aaRS should be considered in any future investigation into the "Origin of Life". For example, there would be no translation by the ribosome without the tRNA charged by the aaRS. See "Origin of Translation" for more info on aaRS in action.

Figure 17 aaRS in Action [view large image]

BTW, most cells contain 20 different aaRS enzymes, one for each amino acid. The editing function of the aaRS as indicated in Figure 17 is to ensure high fidelity of tRNA charging.

There are 2 pre-requisites for the transition of non-life to life :

• Reaction between complex molecules (a pre-requisite for life) requires the helping hands of enzymes (a special kind of protein), otherwise, the reaction rate would be too slow to produce any new compound (see "enzyme").
• Even when an useful molecule has been created (by the enzyme), it needs to keep the plan and the facilities for re-creating the synthesis. This is "replication" (to make copies), and "translation" (to reproduce the molecules from a copied plan). Such functions are fulfilled by either DNA or RNA. and the ribosome respectively in modern days.

Here's a summary of the novel experiment and comparison to the modern process (in italic, see also corresponding diagrams in Figure 17b,b). The formation of bonds in the process requires energy, which the researchers provided by supplying various reactants in the solution (in place of the energy supply from the ATP to the aaRS in charging the modern tRNA, see Figure 17b,b).



A RNA molecule is synthesized by joining two pieces of RNA commonly found in living cells with two exotic nucleosides. At the first exotic sites, the synthetic molecule could bind to an amino acid. These RNA segments correspond to the charged/un-charged modern day tRNA but carrying only 1 nucleoside instead of 3 (the codon carrying the genetic code in modern version). The amino acid then moved sideways to bind with the second exotic nucleoside adjacent to it. This step corresponds to the transfer of amino acides from the 1st tRNA to the next one within the ribosome (see Figure 10). The original RNA strands are separated and a fresh one is brought in, carrying its own amino acid. This step corresponds to the advance of mRNA fetching the next tRNA to the ribosome (Figure 10). The second amino acid would form a strong covalent bond with the first amino acid. The process

Figure 17b RNA/Protein World [view large image]

then returns to step 3 and keeps on repeating, growing a short chain of amino acids — a mini-protein called a peptide — that grew while still attached to the RNA - the process is uncontrollable. This one corresponds to the elongation after processing each tRNA in the ribosome (Figure 10) - the process is regulated by mRNA in the ribosome.


Here's an illustration with chemical formulas for the exotic processe :

		In Figure 17c (see "Is This RNA a Key Ingredient in the Origin of Life?") : (a1) shows the location of the proto-ribosomes within the PTC of modern ribosome. (a2) reveals the 3-D tertiary structure of the proto-ribosome pair in blue and green (short line = nucleobase). (a3) displays the evolutionary change of the proto-ribosomes over the eon in secondary structures.
Figure 17c PTC	Figure 17d Evolution of Ribosome	Figure 17d shows the evolution of ribosome in quaternary structures.

As shown in Figure 17c, the appearance of the ribosome and its components depends on how we look at it, as different techniques and methods can provide different perspectives (see Figure 17g).



	Primary structure: This refers to the linear sequence of nucleotides in the mRNA molecule, which is determined by the genetic code (see Figure 17e). The primary structure of mRNA varies depending on the species, but it is highly conserved within each species. This structure can be determined by genetic sequencing techniques. [structure of peptide, side chains (residues) in colors].
Figure 17e Genetic Code and amino acids [view large image]	In the translational process, the amino acid is joined by its carboxyl group to the 3' OH of the tRNA (see Figure 17f), while the amino group at the other end (of the incoming amino acid) is added to the growing peptide chain with the release of a water molecule (H2O).



• Secondary structure: This refers to the base-pairing interactions between nucleotides within the primary structure. The secondary

  structure of rRNA consists of stem-loop (ladder-bubble) configuration looking somewhat like hairpins, formed by base pairing between complementary nucleotides. These structures are stabilized by hydrogen bonds between the base pairs, from which the shape is predicted. The pairing is absent inside the loop/bubble leaving it exposed to interact with external molecules such as the codon in modern tRNA (see Figures 17f, and 17g). Tertiary structure: This refers to the three-dimensional folding of the rRNA molecule, which is determined by interactions between distant regions of the molecule. The tertiary structure of rRNA is essential for ribosome function, as it allows the ribosome to interact with other molecules involved in protein synthesis. It is shown with the backbone represented by wavy curve, and the nucleobase (if shown) is indicated by short line or polygon.

  Figure 17f Modern tRNA

  The structure is revealed by various experimental techniques such as X-ray crystallography or cryo-electron microscopy.

  
  

  Quaternary structure: This refers to the arrangement of multiple rRNA molecules and protein components within the ribosome. The ribosome consists of two subunits, each of which contains rRNA molecules and protein components. The quaternary structure of rRNA is essential for the function of the ribosome, as it allows the ribosome to interact with messenger RNA (mRNA) and transfer RNA (tRNA) during protein synthesis. Its structure is often studied in the context of ribosomal assembly.

  Figure 17g RNA Types

In digram (a) :


• A shows the location of the PTC within the LSU of the extant ribosome. The symmetrical pair is marked in blue for the one in A-site and green in P-site. These sites are related to the A and P tRNAs during the process of amino acid polymerization (adding amibo acid to the peptide chain).
• B is a close-up of the symmetrical pair (now identified as proto-ribosome to indicate its ancient origin). The view is along the symmetrical axis with the center of the PTC marked by an orange ellipse.
• C depicts the secondary structure surrounding the PTC. Nucleotides in the A and P sites are marked in blue and green as usual. The connecting lines indicate hydrogen bonds between the nucleotides.
• D presents the overlay of the symmetrical pairs from various organisms (note that all of them are identical). The central red dot marks the position of the symmetry axis, which is perpendicular to the plane.
• E shows the CCA-3' end of the tRNAs superimposed on the symmetrical region of a bacterial ribosome. The amino acids and peopides (proteins) are drawn as some sort of lumpy substance. The view is perpendicular to the symmetrical axis (in red line). The circular red arrow describes the process of polymerization, during which the tRNA in A-site turns 180^o to the P-site and adds the incoming amino acid to the base of the growing peptide chain; while the previous tRNA at the p-site exits from the system.


In Digram (b) :

There is a close-up view at the "heart" of the PTC pair associated with P-site and A-site tRNAs. The effects of PTC-tRNA pairings are summarized below (P-site rRNA backbone and residues are colored green, and blue for the A-site counter part. P-site tRNA is colored orange, while A-site tRNA is colored yellow) :

• The CCA end of P-site tRNA is mainly coordinated by the base pairs of G2252:C74 and G2251:C75 (with 4-letter for the symmetrical pair and 2-letter for the tRNA).
• There is an A-minor interaction of A2450 with A76 from the P-site tRNA. An minor interaction between the nucleotides of the same type cannot form a hydrogen bond. It would result in a break in the backbone.
• Both P-site residues A2450 and A2451 are considered important to the trans-peptidation reaction.
• The CCA end of A-site tRNA is mainly stabilized by the base pair G2553:C75, while U2554 establishes a hydrogen bond with C74. A2583 creates an A-minor interaction with A76 from the A-site tRNA.
• The “rotatory motion” mechanism implies that the 3' end of the tRNA passage moves in a helical motion of almost 180° coupled with peptide bond formation.
• The tRNA transitions from the A- to the P-site along a rotation axis and around U2585 and A2602.

• individual mutations at positions C2063, G2447, A2450, A2451, A2453, and U2585 and the combination of A2451U and A2602G did not eliminate the transpeptidation activity.
• Positions A2451, U2506, U2585, and A2602 were defined as forming part of an “inner shell” of the PTC. Mutations at these positions affected the peptide release significantly but did not equally affect the PTC activity. Three of these residues, namely, A2451, U2506, and U2585, along with G2252, are important for the binding of peptidyl-tRNA to the P-site of the large ribosomal subunit.
• A2572 is in the immediate neighborhood of the aminoacyl-tRNA in the A-site and is thought to be a sensor of conformational changes in the PTC. It was shown to be important for PTC activity.
• The next adjacent base, C2573 (in collaboration with U2492 and C2556), facilitates the placement of the aminoacyl-tRNA at the A-site, tentatively labeled the “accommodation gate”. Mutations at A2572 and C2573 affect the peptide release but not the PTC activity.
• Several highly conserved residues are particularly important for their roles in positioning the tRNA at the PTC center. G2251

  	is essential for positioning the P-site tRNA during peptidyl transfer, and its mutation severely affects the PTC activity. Mutations of G2252, which base pairs with C74 of the P-site tRNA (54), can severely impact peptide release while not affecting the PTC activity itself. G2553 makes base-pairing interactions with C75 of the A-site tRNA, and its mutation reduces the PTC activity.
  Figure 17h Ribosomal, Details and Innards [view large image]	See "The Peptidyl Transferase Center: a Window to the Past" for a lot more information on functional implications (TABLE 1), the position of the nucleotides in secondary structure (FIG 1), ...

  
  
  Based on its highly conserved structure in all living organisms, the symmetrical pair of rRNA monomer units is considered to be the very ancient relic leading to the emergence of life. The lifeless phase of this substance is now referred to as proto-ribosome.

  Initially, the primordial soup contained single nucleotides, short RNA segments, and RNA chains of significant size (~ 50-90 nucleotides), which survived since they acquired a stable conformation. These surviving ancient RNA chains could have been the ancestors of RNA chains now referred to as proto-ribosome which underwent dimerization (formation of a dimer molecule form 2 monomer units) subsequently, and thus constructing a moiety resembling the symmetrical region in the extant ribosome. Figure 17i shows the proto-ribosome formation from a RNA precursor, undergoing simple catalytic peptidyl transferase activity. The substrates (substances on which an enzyme acts) could be one of the amino acids without codon association.

  Figure 17i Proto-Ribosome, Orgin [view large image]

  See "The Proto-Ribosome: an ancient nano-machine for peptide bond formation".

  The origin of the tRNA is also linked with the origin of the PTC. The evolutionary history of ribosomes suggests that tRNA is an ancient component of ribosomes that arose in the early pre-biotic world. The ancestral small RNAs of extant tRNAs, (i.e., the pre-tRNA = minihelix), were aminoacylated (amino acid added to tRNA) based on the amino acid affinity of the CCA stem. It is an ancient molecule that has evolved very little over time (see Figure 14 for its role in ribosome evolution before there is life). During the initial stages of evolution, the proto-ribosome and the CCA end of the tRNA likely coevolved together (Figure 17h,b).
  
  Figure 17j adopts the evolutionary model of "The Origin of Prebiotic Information System in the Peptide/RNA World" with some re-interpretations in view of the recent discovery. The evolutionary sequence illustrates :
  
  
• (A),(B) in Figure 17j shows the combination of 2 folded RNA molecules in the form of hairpin. Its stem is maintained by hydrogen bonds similar to the coupling of nucleotides in DNA. The coupling is absent inside the loop/bubble leaving it exposed to interact with external molecules such as the codon in modern tRNA (see Figure 17e). The paring of RNA molecules in this case is similar to the dimerization shown in Figure 17i, but the path of further development is different. This one eventually becomes the extant tRNA, while the other stays inside the modern ribosome as catalyst in the PTC.
• (C) indicates the formation of the D- and T- loops (for the modern tRNA), while an anticodon (ANT) site formed between the two stems. In this newly configured molecule, the acceptor site at the 3' tail end and the anticodon site are very close together.

(D) attests that such structures combined to form a new entity called pre-tRNA as shown in the middle of the diagram. It is remarkably similar to the figure at the top taken from the Phase 1 proto-ribosome in (a3) of Figure 17c. The figure at the bottom is the minihelix reputed to be the ancestor of modern tRNA. (E) is the modern tRNA together with the mRNA and amino acid. (F) illustrates the structural similarity to the cloverleaf. (G) is a schematic diagram showing the salient features of the tRNA molecule. It emphasizes that each amino acid is paired with certain codons (see Figure 17e). Such rules should have been established in a very long process of trial and error for charging tRNA by aaRS; each amino acid would have the chance of meeting up with all kinds of anti-codon. Eventually, the most stable and frequent configurations become the modern genetic codes

Figure 17j Pre-tRNA, Model [view large image]

(H) is the tertiary form of the modern tRNA. (see Figure 17e, "Quick Guide - Genetic Code", and "Errors in Translational Decoding").

By 2023 this model of proto-ribosome and pre-tRNA finally receives some experimental supports. The lab work started in 2006 when a stripped down version of the symmetrical pair in the PTC is obtained to represent the proto-ribosome. For 17 years researchers failed to verify the dipeptide of amino acids by measuring its size. Eventually, a tiny amount is obtained by measuring its mass (see "Origin of life: proto-ribosome forms peptide bonds and links RNA and protein dominated worlds"). Similar result also come from 2 mini-helices (this 74-nucleotide seems to be too large for an initial structure emerging into being). Dipeptide of amino acids is also reported in 2022, by placing an exotic nucleoside at the end of the RNA chain.

N.B. A dipeptide is a molecule consisting of 2 amino acids linked together by a peptide bond (= covalent/ionic bond). In the present case, it is the adding of amino acid between 2 pre-tRNAs to mimic the peptidyl transfer from A-site tRNA to P-site tRNA.

[End of 2023 Update]

This is just a possible scenario about the origin of life. It is certainly not the final words on the subject.

Requirements for the Origin of Life : Self-assembly, Physics and Chemistry

• Self-assembly of RNA Virus - Most of the time some virus would just lay dormant as an inert crystal, lifeless as a rock and perhaps staying that way for centuries or millenniums. Yet, unlike a rock, it may wake up at any moment, when there is a suitable entry

  point at the surface of some vulnerable cell (see COVID-19). Figure 18 shows the replication process for the RNA virus. After entering the cell by endocytosis, the virus becomes uncoated. The DNA then replicates more of its kind and simultaneously making new coating proteins. These parts assemble to form more viruses, which exit from the host to infect more cells. One of the theories proposes that viruses originated and coevolved along with the most primitive cells that first contained self-replicating abilities. Irrespective of the various hypotheses, the fact that the viral parts re-pack themselves, constitutes the strongest support for the role of self-organization and emergent phenomena in the origin of life. This case involves the self-assembly of macromolecules such as RNA, and proteins (the parts) inside the host. The self-organization occurs within the host at its expense. Finally, a new virus (a coherent whole entity) appears with emergent property which can infect another cell. This is everyday occurrence in modern time. It seems that the virus stops further evolution at a very primitive stage, while the cell continuous on with further development to very complicated organism. The coevolution stopped at a point when the virus adopted its parasitic existence with no need for much improvement. See "coevolution scenario".

  Figure 18 RNA Virus [view large image]

  The followings provides further details on RNA virus replication in 7 steps (Figure 18) :

  
  
  There are numerous kinds of viruses, this summary only considers the single-stranded, positive-sense (its RNA is the same as mRNA) RNA virus (such as the Coronaviridae), which replicates solely in cytoplasm of the host cell.
  
  
  1. Adsorption - Viruses can infect only certain species of hosts and only certain cells within that host. Cells that a virus may use to replicate are called permissive. It is the special kind of membrane receptor that allows certain kind of virus to enter. The permissive cell must also possess the materials required by the virus to replication.
  2. Entry - A channel forms across the cell membrane and the virus enters through invagination from which process it enters into the host encapsulated inside a vesicle. It is thus protected from the host's antibodies.
  3. Uncoating - Host cell enzymes (from lysosomes) strip off the virus protein coat (a mistake committed by the host). This releases or renders accessible the virus RNA.
  4. Replication - Viral RNA polymerase copies plus-sense genomic RNA into complementary minus-sense RNA, the whole thing becomes cDNA (complementary DNA). The cDNA is then integrated into the host DNA for replication using the host's replicating machinery.
  5. Synthesis of viral Proteins - For single-stranded, positive-sense RNA virus, the RNA itself is the mRNA, which uses the host's ribosome and tRNA (also aaRS by implication) to make the viral coating proteins and viral RNA polymerase.
  6. Assembly - When sufficient plus-sense progeny RNA and virion proteins have accumulated, self-assembly begins with the virions (newly formed viruses) enclosed inside a vesicle.
  7. Release - The viruses, now being mature are released by either sudden rupture of the cell, or gradual extrusion (budding) of enveloped viruses through the cell membrane. In the case of bacterial viruses, the release of progeny virions takes place by lysis (destruction) of the infected bacterium. However, in the case of animal viruses, release usually occurs without cell lysis.


Some Comments :


The RNA virus equips only with (a) a single strand of RNA as template for replication as well as for translating its codes to viral proteins, (b) the viral polymerase for inducing replication by the host, (c) the protein coating to provide an enclosure. It relies on the host's DNA polymerase for the actual replication of its RNA, and the host's ribosome, tRNA, aaRS, etc. for the translation of its RNA codes to proteins. The viruses do manage to generate a system capable of self-assembly (Figure 19a). However, they fail to incorporate ATP usage into the system and thus no energy is available to promote the system to non-equilibium state - a prerequisite for self-organization and onto real living organism. There are numerous kinds of viruses beside the RNA virus. It seems they belong to a class of "organisms" failing to complete the transition to life. They lay dormant until a favorable condition to wake up. Relic of such death-end products from the prebiotic era could still be around somewhere that we are not aware of. See "update" for latest information.

Figure 19a Virus Self-assembly [view large image]

There are even more primitive organisms (?) such as viroids, which do not have protein coating to shield the RNA, and the VLPs (Virus-like Particles), which do not contain genetic materials.




A 2020 article on "Virosphere" reports that the number of viral species identified has increased 20 fold (to about 195,000 species) in 5 years as the ability to detect them has improved exponentially. One study estimates the total virus population on Earth to be about 1031. Some 800 million viruses attached to dust particles fall onto every square meter of Earth's surface each day (1023/day on the whole Earth). Some survive only seconds while others can persist for decades depending on environments (more vulnerable to higher temperature, sunlight, ...). Study also reveals that most viruses have a narrow range of habitats, infecting only one or two types of host. Figure 19b shows the number of viral groups in 7 environments. The role of virus on the evolution of complex life is mainly on horizontal gene transfer, which can add useful function to the organism (the host).

Figure 19b Virus Habitats

The number of virus groups in various environments is inscribed on the perimeter of the circle in Figure 19b.

2020 November Update :


• Mimivirus - Giant virus was first discovered in 2003 and then hundreds more, they are actually widespread. They all encode a full transcription apparatus allowing them to replicate DNA into mRNA in the host’s cytoplasm (in a process called transcription). Their genome include multiple genes encoding a number of aminoacyl tRNA synthetases (aaRS), enzymes that charge the tRNA with amino acid for protein production by mRNA (see Figure 10). Giant viruses replicate within large spheroidal virus factories located within the cytoplasm of the infected host cell. The two main hypotheses for their origin are that either they evolved from small viruses, picking up DNA from host organisms, or that they evolved from very complicated organisms into the current form which is not self-sufficient for reproduction, i.e., evolution in reverse.


• Pandroravirus - It is discovered in 2013, the second largest in physical size of any known viral genus. Pandoraviruses were originally mistaken for bacteria, however, they are missing some of the distinct features of bacteria such as the ability to make their own proteins. The dissimilarity of the remaining genes to any cellular genes led researchers to speculate that this virus may represent a previously unknown branch of the tree of life (Just 7% of their genes match those in existing databases). Further research in 2020 shows that it has membrane potential used for ATP production. At least one type has many genes that code enzymes for energy generation. The Pandoraviridae genome appears to be “open” (i.e., unbounded, see "Microevolution Processes at Work in the Giant Pandoraviridae Genomes"). Pandoraviruses infect amoebas, which are single celled eukaryotes. It enters amoebas in marine environments through fusion (via virion apical pore, see Figure 19c,a) with membrane vacuoles, and integrate their DNA into the host cells. The host cell replicates the viral particles and eventually splits open releasing the virus. The process of replication cycle lasts 10–15 hours.



	There are many interpretations on the origin of viruses. A single evolutionary sequence can be constructed by considering the progressive change of their characteristics such as the length of genome, physical size etc. (Figure 19c,c).
Figure 19c Virus Evolution - Origin of Life [view large image]	Following is an attempt to link the various kind of microscpoic organisms together in an evolutionary sequence (aka Co-evolution Theory).

1. Viroids - These are simplest organisms consisting only of a short chain of naked RNA containing 240 - 375 base pairs (bp). There is no capsid to house the genetic material. They should be around even earlier than RNA virus in the "Pre-RNA" era about 4 billion years ago (see Figure 19d).
2. RNA Virus - According to the 2010 JCVI research on synthetic life, minimal length of 531,560 bp in 473 genes is required to produce a viable bacteria. Common virus such as the Influenza comes short of this number. They have to hijack some of the genes in the host cell to reproduce and considered to be lifeless. The First Appearance (FA) of such virus would be about 3.7 Billion Years Ago (BYA) between the RNA world and DNA/protein life (Figure 19d).
3. Mimivirus - The genome of this kind has evolved to DNA base and the number of base pairs has exceeded the minimal (Figure 19c,b). It seems to have enough apparatus to evolve to a living cell except that it lacks the crucial element of ATP production, the absence of which cannot maintain the non-equilibrium state - a signature of life. This stage could be close to the 3.8 BYA time frame for FA of LUCA (Figure 19d).
4. Pandroravirus - This one has the ability to make ATP, but lacks the proteins to complete the process for life. It seems during the one billion years prior to LUCA, there was a lot of attempts to arrive at that stage. Actually, there is a third type of giant virus combining the Pandoravirus morphology with a gene content more similar to that of mimiviruses (see "Thirty-thousand-year-old distant relative of giant icosahedral DNA viruses with a pandoravirus morphology").
5. Bacteria - Finally, the ascension toward life was achieved about 3.8 BYA. The primitive cell is very similar in size to the Pandroravirus but the size of genome is about twice as much (see Figure 19c,c).
6. Eukaryotic cell - Further evolution proceeded slowly for the next 3.3 billion years until about 0.5 BYA when certain bacteria acquired the mitochondria and chloroplast through the endosymbiotic process. Then, the genome size becomes ~ 1000 times larger than the one for bacteria. It took that long to assemble all the pieces. Once it was done, the complexity increases by many thousand folds suddenly and hence labeled as Cambrian Explosive (see Figure 19e). The physical (linear) size also increased by ~ 20 folds accordingly. It seems that size does matter in competition for resource. However due to the limitation imposed by area to volume ratio, an organism cannot bump up its size infinely. The solution is to go multicellular.



Figure 19d Transition to Life [view large image]	Figure 19e Complexity of Life


2021 Update :


• An update of the viral infection process (Figure 19f) indicates that the basic material for viral replication had already been established in the genome of the RNA virus. They were not able to implement the scheme mainly because they have no ATP to execute the processes which are now hijacked from the host. As shown in Figure 19f, the SARS2 generates many proteins via the host's ribosome in step 6, these proteins are then self assembled into a complex to replicate the viral RNA in step 8.

  		This kind of processing is very similar to the functions of the various polymerases (Pol's) and ribosome. Thus, the RNA virus misses only a few steps before coming alive. The crucial missing link is the absence of ATP. There is no way to attain a non-equilibrium state for living entity without the continuous supply of energy (see "Energy Supplier for Life"). Table 01 below compares the viral replication with similar modern processes in living organisms (see a pictorial illustration in Figure 19g).
  Figure 19f Virus Infection	Figure 19g Viral Replication, Pol II, I, III, mRNA Translation

  Process	Template	Output	Composition	Function
  Viral Replication	RNA	RNA	Complex of 6 enzymes	Replicate viral RNA
  RNA Polymerases II	DNA	RNA	Complex of 12 protein subunits including TBP and TRF2 proteins	Transcribe DNA into precursors of mRNA
  RNA Polymerases I	DNA	rRNA	Enzyme of 14 protein subunits	Transcribe DNA into rRNA
  RNA Polymerases III	DNA	tRNA	Complex of many transcription factors	Transcribe DNA into tRNA
  Ribosomal Translaton	mRNA	Protein	65% rRNA and 35% proteins	Translate mRNA to protein

  Table 01 Comparison of Viral Replication with Pol II, I, III, and Ribosomal Translation

  The similarity between the process of viral replication and some modern biological processes as shown in Table 01, suggests a coevolution scenario for the virus and cell. Futhermore, the viral structural genes seem to be an addition to its genome later after the non-structural polyproteins (nsp's) (see "SARS2 genome") indicating the progression to its parasitic existence.

  

  Figure 19h Virus and Cell Coevolution Sequence

  According to the illustration in Figure 19h, a rough sketch is composed for the coevolutionary sequence :
  
  
  1. It started with a pool of olig-nucleotides about 4 billion years ago (4 Gya), see "Transition to Life".
  
  
  2. The olig-nucleotides progressed to poly-nucleotides (aka RNA) in about 200 million years. Such entity can be identified to the viroid still alive (as parasite) today.
  
  
  3. Another 200 million years later, the poly-nucleotides have attained functional capacity to replicate or transcript. This could be from the pp1ab gene in the SARS2 genome. Then at some point in this stage,
     • The cell acquired the capacity to make ATPs at the membrane, which activates the system to an non-equilibrium state turning it alive. Such energy generating apparatus could be very primitive running on diffusion in the beginning. The ATP synthesis would be established only later on.
     • Meanwhile, the virus went the other way to develop the spike proteins on its membrane. The purpose is to hijack the functions within the cell instead of expanding its own genes.
     This stage could be similar to phase 4 in "Ribosome Evolution" when the translocation function (which requires energy to move the molecules) has not yet been developed.
     
     
  4. Since then while the cells have undergone very complicated development, the viruses do not change very much. They only need to adopt to the host's environment such as anti-viral immunity.



• Physics and Chemistry - (a) Energy the prerequisite for life, (b) Solvent to promote chemical reaction, (c) Enzyme a necessary substance for metabolism (chemical reaction for complex molecules).
  
  
  The requirements in detail :
  
  
  • (a) Energy -
    
    
    • Carbon Atom - The ability to form covalent bonds with other carbon atoms in long chains and rings distinguishes carbon from all other elements. This property of carbon, and the fact that carbon nearly always forms four bonds to other atoms, accounts for the large number of known compounds. At least 80 percent of the 5 million chemical compounds registered as of the early 1980s contain carbon. The affinity of carbon for the most diverse elements does not differ very greatly - so that even the most diverse derivatives need not vary very much in energy content. This ability allows the organic world to exist in a special form of thermodynamic stability.

      		The electron configuration of the normal carbon atom has 2 electrons in energy level 2S and 2P respectively. By supplying about 2 ev to a carbon atom, the 4 electrons in the 2S and 2P states are rearranged to the SP3 state (Figure 20). The four electrons in the SP3 state form the tetrahedral arrangement (Figure 21) of orbitals (probability distribution of electrons), which can form stable covalent bonds with other atoms.
      Figure 20 [view large image] SP3 State (Hybridization)	Figure 21 Tetrahedra	It is no accident that photosynthesis supplies 36 ATPs each carrying ~ 0.32 ev to synthesize 1 glucose molecule.

    
    
    • Non-equilibrium State - Creation of a tetrahedral configuration is the basic reason for pumping energy into biological system to maintain metabolism and cellular structure. Therefore, the biological system is said to be in a non-equilibrium state. The electrical discharge in Stenley Miller's experiment is an example of infusion energy to move the molecular configuration into the non-equilibrium state. The continual free energy input from the environment will lead to a dissipative structure (Figure 22), which is a necessary condition for life. The process is called dissipative because it continuously dissipate free energy (work) into high-entropy energy namely heat. However, not all dissipative

      structures are living systems, non-life examples include convection, hurricanes, the Solar system, and galaxies, ... Living system can form only when the dissipative structure begins to perform work. As the hybrid orbitals of the tetrahedral configuration do not exist in an isolated atom, but arise while the carbon atom is interacting with others to form a molecule, it will dissolve and return to an equilibrium (lowest energy) state once the input of free energy ceases causing the removal of the associated constituents.

      Figure 22 Dissipative Process [view large image]

      BTW, life is optimized to an near-equilibrium state to make it a highly efficient process in using free energy.

    
    
    • Entropy - Beside the infusion of energy to support life, the organization of complexity requires the removal of entropy

      as shown in Figure 22. Such requirement is fulfilled by re-transmission of lower frequency radiation back to the cold dark space as illustrated in Figure 23. The process re-emits more photons (at lower frequency) back to the space. By Boltzmann's definition of entropy, it has the effect of enlarging the phase space volume and hence returning more entropy to space than receiving from the sun in accordance with the second law of thermodynamics.

      Figure 23 Entropy and Life [view large image]

      Ultimately, it is the stars which generate the carbon atoms (see "Origin of Elements"), and the Sun which supplies energy to all life on Earth.

    
    
    
  • (b) Solvent -
    
    
    • Property - A solution is a mixture in which one substance called the solute (the component that changes state upon dissolving in smaller amount) is uniformly dispersed in another substance called the solvent. Because the solute and the solvent do not chemically react with each other, they can be mixed in varying proportions (see Figure 24). Solutes and solvents may be solids, liquids, or gases (see Figure 25). Gases form solutions easily because their particles are moving so rapid and far apart so that attractions between particles are not important. When solids or liquids form solutions, there must be an attraction between the solute particles and the solvent particles. Otherwise, the particles do not mix and no solution forms. For example, compounds containing nonpolar

      		molecules such as iodine, oil, or grease do not dissolve in water because water is polar. The polarities of a solute and a solvent must be similar in order to form a solution. It is found as early as the 1800s that whenever a substance is dissolved in a liquid the vapor pressure (the pressure at which the rate of evaporation is equal to the rate of condensation) of the solvent is lowered. Such behaviour has the effect of rising the boiling point and lowering the freezing point of the solution. It also produces osmosis, which is the flow of a solvent,
      Figure 24 Solution	Figure 25 Types of Solution [view large image]	usually water, through a semipermeable membrane into a solution of higher solute concentration, i.e., diluting the solution.

    
    
    • Water - In the H₂O molecule, the electric field around the oxygen atom is stronger than that around the hydrogen. The electrons from the hydrogen atoms are drawn close to the oxygen. This leaves the hydrogen atoms positively charged at one end as shown in Figure 26. Hydrogen bond is different from the covalent bond since there is no orbital overlap; it is not an ionic bond since there is no charge transfer from one atom to another. The strength of the hydrogen bond

      		is about 10 times weaker than the covalent and ionic bonds. It is important in fixing properties such as solubilities, melting points, boiling points, and in determining the form and stability of crystalline structures. Molecules such as water carrying hydrogen bonds are called polar molecules. They play a crucial role in biological systems.
      Figure 26 Hydrogen Bond	Figure 27 Water [view large image]

      Water has some special properties crucial to the existence of life courtesy to the hydrogen bond (Figure 27) :
      
      
      1. Heat Capacity - Water has high heat capacity (needs more heat to rise the temperature) and boiling point by virtue of the additional hydrogen bonds that keep the water molecules packed together with extra strength (see Figure 27a, white bar represents covalent bond, grey and blue bars for hydrogen bond). This property moderates the temperature of the environment. Lack of water turns the land into desert. The same hydrogen bonds also pull water up a tree, in a phenomenon called capillary action, and maintain surface tension, which enables small bugs to walk on water.
      2. Ice - When water cools to below 4^oC, it suddenly becomes less dense. It is because the thermal agitation cannot overcome the hydrogen bonds and solid ice begins to form. The angle subtended by the two hydrogen atoms then increase from 105 to 109 degrees resulting a less tightly packed configuration (Figure 27b). That's why ice can float on water and protects the water beneath from frozen allowing fish to live in the polar regions or creatures under the icy surface of Europa (Figure 27f).
      3. Solution - Water is a good solvent able to dissolve many polar substances (Figure 27c). That's why water-based liquids, like blood, are perfect transporters of essential substances such as salt, sugars and hormones.
      4. Protein folding - Some amino acids are hydrophobic (repelled by water); while others are hydrophilic (attracted to water). In a a liquid environment such as the inside of cells, these properties dictate how a protein "assembles", or folds (Figure 27d). DNA too, depends on water. Experiments have shown that the double helix would fall to pieces without water. This is because water molecules help form hydrogen bonds between DNA's phosphate groups.
      5. DNA binding site - Some biochemists think that water molecules may play an active role in guiding enzymes to certain spots on the DNA (Figure 27e). It is found that water molecules spend more time around certain areas of DNA when cell divides. Such tendency seems to suggest that water is needed for the expression of genes - a process that is kick-started by binding the transcription factor to the gene switch.
      6. Origin of Water on Earth - Water molecules are one of the components in interstellar dust. It would be incorporated into the primordial Earth naturally. Water as many other gaseous materials are trapped by the Earth's gravity when it acquired a sufficient amount of mass. It might be in the form of solid when the temperature was too low in the very beginning, and in the form of gas when it was covered with magma produced by planetesimal impacts. Only in a very narrow window of temperature (from 0^oC to 100^oC) it turned into liquid state suitable for life. Later on more water are added via the impacts of comets, which contain a lot of water and the impacts occurred more often in the past.
      7. Search for ET - All known life on Earth depends on water. Typical enzymes, just like DNA, need water to function properly. Nobody know if enzymes can function in another liquid, or there might be fundamentally different life-forms in other parts of the universe - a possibility that would not be ruled out by many scientists. Nevertheless, NASA has chosen the strategy of "Follow the Water" in its programs to search for extra-terrestrial life (to Europa for example, see Figure 27f).
    
    
    • Internal Sea - Some theories on the "Origin of Life" suggest that life arose from the sea. Specific environment varies according to different scenario. It could be in hydrothermal mounds or mud pool or tidal pool. While composition of the ancient sea 4 billion years ago could be different from the present; very recent sample shows that it is not much different from the interstitial fluid bathing all cells in multicellular organisms, i.e., it is salty with lot of Na⁺ and Cl^- ions (Figure 28). The importance of interstitial fluid was observed back to 1887 by Claude Bernard. Its role has been reinforced in modern time by such similarity.

      While unicelluar organisms can acquire nutrients and expel waste directly in aquatic environment, they could not survive on dry surfaces with a humidity of less than 10%. The same situation is applicable to the cells in multicelluar organisms which solve the problem with the "Internal Sea" in the form of Interstitial Fluid (ISF). Figure 28 also shows that the fluid composition inside the cell (Intracellular Fluid, ICF) is quite different from sea water as the constituents are altered by the metabolic process of life.

      Figure 28 Sea Water and Body Fluids, Composition [view large image]

      See "Interstitial Fluid" for more detail.

    
    
    
  • (c) Enzyme - The rates in a chemical reaction is measured by the amount of reactant used up, or the amount of product formed, in a certain period of time. Reactions with low activation energy E_a go faster than reactions with high activation energies. The rate of a reaction can be affected by temperature T, the amounts of reactants in the container (concentration [A]) as shown by the formulas in Figure 29, pressure and cross-section of the reactants have similar effect of concentration by varying the chance of collisions; while for large macromolecules, catalysts is required to match the reaction sites and to lower the activation energy as shown in Figure 30. Reactions almost always go faster at higher temperatures, because the increase in kinetic energy makes the reactants move faster and collide more often. The rate of a reaction also increases when reactants are added (resulting in higher concentration) since there are more collisions between the reactants. Another way

    		to speed up a reaction is to lower the activation energy Ea. This can be done by adding a catalyst, which speeds up the reaction but is not itself changed or used up. Enzymes are known to catalyze more than 5,000 biochemical reaction types. Most are proteins, although a few are RNA molecules (ribozymes). The catalyzing function comes from the unique 3-dimensional structures.
    Figure 29 Reaction Rate for Simple Molecules [view large image]	Figure 30 Enzyme [view large image]	Enzymes are classified according to the chemical reactions they catalyze. See "Enzyme Commission (EC) number".

    
    Two opposed trends on the formation of an enzyme : on one hand it needs to fold into a stable and compact structures, while on the other hand they must also be flexible in order to catalyze chemical reactions. The active site of an enzyme is highly strained because it is designed to develop favorable interactions with the transition state of the catalyzed reactions. This strain diminishes the stability of the global structure of the enzyme and thus a trade-off between stability and function has to be established. It is found in the lab that enzymes exhibit a remarkable evolutionary adaptability. Many enzymes evolved from pre-existing enzymes via gene duplication thus allows the evolution (of enzyme) from a simple version to more complication one via the process of Darwinian selection (see "A Darwinian Process: The Molecular Evolution of Enzymes").

  A Scenario of Prebiotic Evolution : based on the sequence of events and related information cited above, a scenario is constructed below for the evolution of inanimate matter in a prebiotic world toward life (see illustration in Figure 31).

  ( The Beginning,  Enzyme Catalysis,  Uncorrelated Evolution ,  Self-assembly ,  Pre-requisites for Life )
  
  
  1. The Beginning -
     • In the beginning energy provided by the Sun had made the Earth warm enough to have H₂O molecules in liquid phase forming the oceans, rivers, lakes and ponds.
     • The atmospheric composition at that time had N₂, H₂O (water vapor), CO₂, and some CH₄, NH₃.
     • In a rather similar environment, the Stanley Miller's experiment is able to make : carboxylic acids (R-COOH), nucleic acids base, and amino acids. Since the experimental environment is not exactly the same as the atmospheric composition of the early Earth (note the presence of CO₂, and the absence of H₂), it could be that those organic molecules were produced in localized spots where the chemical composition may be different from the global environment. Another possibility is the delivery by meteorites. Regradless of the question of its original source on Earth, astronomical observations have detected lot of those simple organic molecules in space. They had to be around in the environment of early Earth.
     • The amino acids can be synthesized to oligopeptides (short peptide chain folding to form protein) by removing the water molecule between the link. However, it can also became hydrolyzed (breakdown) by adding water to the product (same applied to nuclotides). Thus the more complicated polymers are easier to make in mud pools or tidal pools where the wetting and drying cycles permit easier synthesis and also allow the necessary degrading back to monomers for making new products (see "How the first life on Earth survived its biggest threat — water", and Figure 31).

     Polymerization of oligonucleotides (short polymer of nucleotides in the form of RNA) joins together the nucliotides, similarly to other biological molecules, by condensation reaction that releases a small molecule. Unlike proteins, carbohydrates, and lipids, however, the molecule that is released is not water but pyrophosphate (O3-P-O-P-O3 = ppi). This is essentially the ATP hydrolysis to AMP reaction with the release of a lot more free energy resulting in higher binding energy. That's why polymer of nucliotides, e.g., RNA or DNA does not break down easily.

     Figure 31 Prebiotic World [view large image]

     All such short polymers can be produced at moderate temperature range and sufficient concentration without the assistant of enzyme (Figure 29).

  2. Enzyme Catalysis -
     • As the molecules become larger and the configuration more complicated, reaction between two of them requires more precise contacting point (active site). It needs an enzyme to arrange them in proper orientation for the reaction to proceed.
     • The oligopeptides or oligonucleotides would coil into a three dimensional configuration from the linear chain by hydrogen bonds. The product is somewhat malleable and can accommodate two particular molecules for a reaction to take place. This is called catalyzation without which there would be no macromolecules and no life.
     • Beside the lock-and-key specification by the enzyme, some enzymes also supplies energy in the form of ATP (AMP + energy) to establish the C-C, C-N, C-O, or C-S bonds (see Figure 17 and Figure 31,2).
     • Other classes of catalysis do not use ATP, those enzymes could be the earlier version of the more complicated one. For example, the hydrolases are the enzymes in hydrolysis (breakdown) of many polymers without the involvement of ATP. In fact, enzymes in Enzyme Classes EC-1 to EC-5 do not use ATP, the one class that does belongs to EC-6.
     • The catalysis can either join two molecules together or breakdown a molecule. The appearance of such mechanism on the scene accelerated all kinds of reactions by ten- to a hundred-million-fold permitting lot of trial and error processes to run - often without any consequence, i.e., coming to a death end.
     
     
  3. Uncorrelated Evolution -
     • The system is very slowly evolved to a collection of increasingly complicated molecules, everyone of them acting alone without regard to others. The lack of design is compensated by the sheer number of trials, very small incremental change in atomic scale, and plenty of time in the order of billion years. The tidal pools could serve as enclosures to retain the changes which would be lost in the vast ocean otherwise. Among the multitude proteins in such environment, two enzymers stand out for their role in the evolution toward life. They are the proto-polymerase and proto-synthetase (proto=primitive). There are also specialized forms of RNA such as the mini-helix and proto-mRNA.
     • Mini-helix is single RNA strand folds back to form double helix, which becomes part of the pre-tRNA.
     • Proto-mRNA is the precursor of modern mRNA, which has a AUG start codon at the 5' end and a UGA codon at the 3' end together with the untranslated regions (UTRs) before the AUG and after the UGA.
     • Modern polymerase is an enzyme that synthesizes long chains or polymers of nucleic acids. It is used to assemble DNA or RNA molecules, by copying a DNA or RNA template strand using base-pairing interactions. There are

       	different types of polymerase; for example, the RNA polymerase is composed of a dozen different proteins. Together, they form a machine that surrounds a DNA strands, unwinds them, and builds an RNA strand based on the information held inside the DNA. Once the enzyme gets started, RNA polymerase marches along the DNA copying RNA strands thousands of nucleotides long (Figure 32).
       Figure 32 RNA Polymerase in Action [view large image]	Another example is the DNA replication expressed by the formula : deoxynucleoside triphosphate + DNAn + DNA-pol diphosphate + DNAn+1 + DNA-pol

     • Modern synthetase is another enzyme that catalyzes the linking together of two molecules usually using the energy derived from ATP. The most cited example is the aminoacyl-tRNA synthetase (aaRS) as shown in Figure 17 in the process of translating the genetic codes to peptide chain. The process can be expressed by the formula : Amino Acid + tRNA + ATP + aaRS Charged-tRNA + AMP + PPi + aaRS.
     • The proto-polymerases and proto-synthetases did not perform the functions executed by their modern cousins. In fact, all the macromolecules did not have permanent association to each other - their evolutions are uncorrelated (Figure 31,3). This step is illustrated by Phases 1 and 2 in Figure 14. Expansion of these primitive RNA and peptide was achieved by appending basic unit or short oligomer one at a time to the linear sequence. The 3-D structure is produced by folding.
     • It is the interplay between activation energy and concentration that bumps up complexity of molecules. Lower activation energy for certain reactions would increase the concentration of such end product, while higher concentration of that species would overcome a little bit the activation energy for another reaction to produce a new

       kind, and so on, ... with the enzyme along the way to facilitate the process by lowering the activation energy. (see Figures 29 and 30 respectively for the relationship between reaction rate and concentration and the effect of enzyme in lowering the activation energy). The absence of reproduction to accumulate the same kind is covered by stability, which is in turn related to stronger binding energy (Figure 33). Thus, such chemical species would be preserved. Anyway, only perishable item needs to be replaced or reproduce in order to keep its presence.

       Figure 33 Binding Energy [view large image]

       For example, the binding energy of water H2O is stronger than either H2 or O2 (see Figure 33). It sticks around on Earth for 4 billion years without any mean of reproduction.

       BTW, the hydrogen bond in most macro-molecules is about 10 times weaker than the covalent bond in H₂O (see Figure 26). It is found that the secondary structure of macromolecules can be affected by external conditions, including pH, temperature, and ionic interaction beside the binding energy in an isolated environment. Proper native secondary structure is an important aspect of the development of stable formulations. See a comparison of stability between DNA, RNA, and Protein.
     
     
  4. Self-assembly -

     A space enclosed by a wall is a prerequisite to assemble complex chemicals. It would keep those macromolecules inside but allow other molecules to pass through. Surfactant molecules (such as the lipids) contain both hydrophobic groups (the tails) and hydrophilic groups (the heads). They form double layers spherical vesicle naturally with the hydrophilic heads facing the aqueous solutions on the outside and the hydrophobic tail pointing to the space between the layers (Figure 34). The modern cell membrane is a form of such vesicle.

     Figure 34 Vesicle [view large image]

     The capsid of the virus is a protein shell in various forms as shown in Figure 19a. This special coating is made for penetrating the membrane of the host. It may also contain a lipid envelope. The

     minimum linear size for modern cells is about 10^-5 cm, viruses can be 10 times smaller with less biochemical machinery inside.

     	In a nutshell, self-assembly just follows the law of physics, which tends to drive a system to an equilibrium state with minimum energy. It is as natural as a rock rolling down from hilltop to the bottom of the valley where the gravitational potential Ep is at its minimum (Figure 35). Such state would be easier to achieve in a small volume than globally with a lot of things to be adjusted.
     Figure 35 Minimum Energy

     • A good example of self-assembly is the operation of ribosome in modern cells. It collects all the components such as the LSU, SSU, tRNA, and a host of assorted proteins together in the presence of mRNA (see "Modern Day Ribosome"). After the assembly, the aggregate switches to self-organization to run the translation. It turns into a state of non-equilibrium - the trademark for life.
     • As mentioned earlier about virus assembly, it lacks some important components to replicate itself. The problem can be attributed to mistake in self-assembly, i.e., it fails to include all the necessary apparatuses into the enclosed volume. In the game of trial and error, somehow some self-assemblies (in small tidal pools for example) did incorporate everything correctly for use in the future. Such assembly could become the precursor for a living entity (see Figure 31,4). This step would be represented by Phase 3 in Figure 14. As shown in the same figure, there is a few more steps (Phases 4. 5, 6) before reaching the "Origin of Life" symbolized by LUCA. By then the system will be running in non-equilibrium state and its behavior will be dictated by the rule of self-organization.
     
     
  5. Pre-requisites for Life : There are some components that have to couple and evolve together before life can start to run on its own.

     RNA - For the positive sense RNA, it can also serve as mRNA to encode the genetic code for translation. Initially, it would be just a collection of randomly arranged nucleotides. It has to be optimized to become gene for making protein and other functions (Figure 36). RNA Polymerase - It uses the reverse transcriptase (RT) enzyme to create a double stranded cDNA, which is a copy of the RNA itself matched by a complementary strand to form a DNA double helix (Figure 36). In virus, replication of the viral RNA is make from its cDNA by hijacking the host DNA polymerase. In modern laboratory, the process is used as one of the methods in polymerase chain reaction (PCR), and also in molecular cloning. In the prebiotic era, the cDNA is very likely to be the intermediary in the transformation from RNA to DNA as the genetic codes keeper.

     Figure 36 Pre-requisites for Life [view large image]

     tRNA - It serves the important function in the translation of genetic codes to amino acids. The bending and looping structure seems to indicate its ancient root as "ribonucleo-protein".

     • aminoacyl-tRNA Synthetase (aaRS) - This is the enzyme to charge the tRNA by linking it up with an amino acid (corresponding to the tRNA's anti-codon). The translation would not work without it. It started to evolve from ancient past as described by the theory of "Protein/RNA World".
     • Ribosome - Every living organism has a version of this organelle running all the time. At the presence of mRNA, the ribosome self-assembles many parts into a whole to make protein according to the genetic codes in the mRNA. It's evolution has been described at length in the "Top-down Theory".
     • Membrane - The enclosure needs pores to drain out wastes such as CO₂ and to admit nutrients such as small monomers and ions. It should also have the capacity to expand and to divide when there is too much material inside - a trait for cell division.
     
     When all these components have self-assembled and self-organized, the entity starts to live. The imperfection and imprecision will be resolved according to the rule of "Natural Selection" - that's another story Continuation.
     
     
     This footnote has been incorporated to the topic of "ATP - Energy Supplier of Life (2020 Edition)".
     
     

     	Footnote : ATP Hydrolysis ATP (Adenosine TriPhosphate) is the adenosine nucleotide with 3 phosphate groups. Hydrolyzing each phosphate would release an energy of ~ 0.32 ev, i.e., ATP + H2O ADP + Pi + 0.32 ev, or ATP + H2O AMP + PPi + 0.64 ev. The A, G, C, T (or U) nucleotides in DNA or RNA retain only one phosphate group as the backbone to link up the structure (Figure 08a).
     Figure 08a ATP, ADP, AMP, and RNA Backbone	ADP = Adenosine DiPhosphate AMP = Adenosine MonPhosphate

     		Sun light is captured by chloroplasts to make glucose, which in turn is used by the mitochondria to produce ATP (Figure 08b). The ATP Synthase is ultimately responsible for the chemical process. The chemical formula on the (peptide)n to (peptide)n+1 reaction (and the ATP to ADP, AMP formulas above) actually require a H+ ion on the right-hand-side to make them balanced. It is the concentration gradient of such ions across the cell membrane to complete the conversion from ADP to ATP. This ubiquitous molecule is "used to build complex molecules, contract muscles (including heart beat), generate electricity in nerves, ...". Figure 08c is a cartoon to show the role of ATPs in life.
     Figure 08b Eco-cycle	Figure 08c Food Cycle [view large image]	See the review article "ATP: The Perfect Energy Currency for the Cell" for detail.

     The total quantity (number) of ATP in the human body is about 0.1 mole (~ 6x1022). The energy used daily by an adult requires the hydrolysis of 200 to 300 moles of ATP. This means that each ATP molecule has to be recycled 2000 to 3000 times during the day. ATP cannot be stored and so its synthesis has to closely follow its consumption. ATP is formed as it is needed, primarily by oxidative processes in the mitochondria. ATP is not excessively unstable, but it is designed so that its hydrolysis is slow in the absence of a catalyst. This insures that its stored energy is released only in the presence of the appropriate enzyme (to

     Figure 08d ATP in Action [view large image]

     lower the potential barrier) as shown in Figure 08d. Thus, life is an non-equilibrium process (see Figure 22); it ceases to exist once the replenishment of ATP fails to come through.

Here's further detail on the charging of ATP by various organisms using different methods :


• Autotrophs (do not depend on organic food) - In photosynthesis, when Sun light enters the reaction center, the chlorophyll (P680) in photosystem II captures light energy ~ 2 ev (wavelength ~ 680 nm, red; see Figure 08e,a); while carotenoids absorb in the blue-green region of the solar spectrum then transfer the absorbed energy to chlorophylls, and so expand the useful wavelength range of light (Figure 08e,b).


• The manganese center in there splits the water molecule H₂O in the reaction induced by the Sun light :
  Sun Light + 2H₂O 4H⁺ + 4e^- + O₂ ----- (1) .

When proton ions H+ are squeezed into the thylakoid space by electron transfer from one chlorophyll to another (Figure 08e,b), its concentration goes up causing a proton ion gradient between there and the stroma outside. Such is exactly the situation required to run the ATP synthase (Figure 08e,c). The electrical energy of the ionic gradient is converted to the mechanical energy of the rotor, and finally to the chemical energy stored in the ATP. It is not clear how the rotor manages to transform the ADP to ATP except that ADP goes into the rotor and comes out as ATP. The net effect is for exciting the ADP from ground state to the ATP meta-stable state (see insert in Figure 08e,c). In effect, the rotor just serves as an enzyme to speed the process.

Figure 08e ATP Charging Process [view large image]

And so it turns the system into a state of thermodynamic non-equilibrium.

• The O₂ part is released as waste while the excited electrons move around squeezing the proton ions H⁺ into the thylakoid space. The electron transport chain then passes through a number of electron acceptors, and eventually end up producing NADPH₂, which goes on to photosystem I in the conversion of CO₂ to glucose by ATP (Figure 08f) in the reaction :
  6CO₂ + 6H₂O (reduction by ATP and NADPH₂) C₆H₁₂O₆ + 6O₂.
  As indicated in Figure 20, photosynthesis supplies 36 ATPs each carrying ~ 0.32 ev to synthesize 1 glucose molecule. In addition, there are bonds between the carbon and other atoms making a total of about 30 ev stored in each glucose molecule.
  
  Note that, there was no ATP in the beginning. Photosynthesis has to generate ATP in the initial step for use to synthesis glucose in the next step.



		Figure 08g shows the ubiquity of ATP across all the three domains of life from bacteria (in cytoplasm), to plants (in stroma) and animals (in the matrix of Mitochondria). The mechanism in bacteria is similar to plant's except that for some species, the energy from Sun light is replaced by chemical reaction, the process is called Chemosynthesis.
Figure 08f Photosynthesis [view large image]	Figure 08g ATP Charging Sites [view large image]

• When there is no Sun light to provide the energy, some bacteria manage to extract the minuscule amount from chemical reaction.

  For example, the oxidation of Fe2+ to Fe3+ releases about 0.4 ev in the reaction : 4Fe2+ + 2H2O 4Fe3+ + 4H+ + 4e- + O2 ----- (2), which would dispatch the electrons to perform some works and to create an ionic proton H+ gradient in the periplasm (a space between the outer and inner membranes). It is then used to run the ATP synthase to generate ATPs (see Figure 08h and compare the photosynthesis in Eq.(1)).

  Figure 08h ATP by Chemo

  Fossil evidences have been accumulated over the years that anoxygenic photosynthesis (in absence of oxygen) were used by bacteria 3.4 billion years ago in a sulfuric environment such as the hot spring. It is also known that oxygen-producing form of photosynthesis emerged about one billion years later (see cyanobacteria in Table 02 on "Major Events of Life on Earth"). Such long time interval is attributed to the difficulty of merging Type I Reaction Center (modern photosystem I, see Figure 08f) with Type II RC (photosystem II) as indicated in

  Figure 08i Photosynthesis Evolution [view large image]

  Figure 08i (see details). Finally, the cyanobacteria appeared to possess thylakoid in the cytoplasm as photosynthetic lamellae (see cyanobacteria).

  
  
• Heterotrophs (depend on organic food) - Animals break down glucose to extract the necessary energy.
  
  
  • The first step of carbohydrate catabolism is glycolysis, which occurs in the cytoplasm producing pyruvate, NADH, and 2 ATPs (Figure 08j,b).
  
  

  	The pyruvate molecules generated during glycolysis are transported across the mitochondrial membrane into the inner mitochondrial matrix, where they are metabolized by enzymes in a pathway called the Krebs cycle. The electron transport chain (ETC) uses the NADH and FADH2 produced by the Krebs cycle and a series of cytochrome complexs (acting as electron sources and sinks) to
  Figure 08j ATP Charging by Glucose [view large image]	generate high concentration of H+ ions in the inter membrane space. The gradient enables the ATP-synthase to produce 36 ATPs from each glucose molecule. The ATPs then exit to the cytoplasm for use.

  
  
  • Some bacteria excrete enzymes to dissolve food particles outside the cell to allow them to pass through the membrane via diffusion (through the porin, for example as shown in Figure 08j,a) into the cytoplasm. Carbohydrates (particularly glucose), lipids, and protein are the most commonly oxidized compounds acquired in such process. In bacteria, glycolysis represents one of several pathways by which they can catabolically break glucose. The Krebs cycle and intermediate compounds serve as precursor molecules to build up a H⁺ gradient in the peptidoglycan (a layer in between the outer and inner membranes), from which the ATPs are generated by the ATP synthase (Figure 08j,a). The proton gradient also facilitates the transport of ions and metabolites across the system.
  
  
  • The endosymbiotic hypothesis for the origin of mitochondria (and chloroplasts) suggests that mitochondria are descended from specialized bacteria (probably purple nonsulfur bacteria) that somehow survived endocytosis by another species of prokaryote or some other cell type, and became incorporated into the cytoplasm. They contain their own DNA, which is circular as is true with bacteria; but do not contain anywhere near the amount of DNA needed to code for all mitochondria-specific proteins. A billion or so years of evolution could account for a progressive loss of independence. Anyway, it is just a hypothesis with evidence forever lost in antiquity.