Inner World of the Cell

Structure in a Cell (Updated 2022)

The structure of an eukaryote cell is shown in Figure 04, in which the cell organelles are often likened to the various utilities within a city. There is library for the nucleus, power plants for the mitochondria, garbage disposal for lysosomes, post office for Golgi complex, ... The drawing is a combination from observations of the individual components. Table 01 gives a brief description and its function for each of the organelles.

By 2022, technique has been advanced to Cryo-ET, which allows direct imaging of specimen as shown in Figure 05.

T stands for "Tomography", which takes images from different sections of the sample and then combines them to make 3-D structure by computer algorithms similar to those for MRI imaging.
E is for "Electron" microscope, which uses high speed electrons as a source of illumination. As its wavelength can be up to 100,000 times shorter than visible light, it can achieve magnifications up to 10,000,000× (comparing to < 2000x for optical microscope). However, it requires high vacuum to operate, the viewing sample has to be specially prepared (so that it would not explode).

		Cryo refers to plunge the samples (typically small cells, e.g., bacteria, or viruses) to very low temperature (< -150 °C), so that water molecules do not have time to rearrange into a crystalline lattice. The resulting water state is called "vitreous ice" (watery ice). It preserves native cellular structures, such as lipid membranes, that would normally be destroyed by freezing. Such frozen samples are subsequently stored and imaged at liquid-nitrogen temperatures so that the water never warms enough to crystallize.
Figure 04 Cell Organelles	Figure 05 Cryo-ET Imagings	Figure 05 shows 2 examples taken with cryo-ET. Part (a) is for the primitive cyano-bacteria (a.k.a. blue-green algae), while (b) is from a human cell. The legend within the images lists the function of various organelles. See "The secret lives of cells — as never seen before".

STRUCTURE	DESCRIPTION	FUNCTION	PKC
STRUCTURAL ELEMENTS
Cytosketeton	Network of protein filaments	Structural support; cell movement	No
Flagella(cilia, microvilli)	Cellular extensions	Motility or moving fluids over surfaces	Yes
Centrioles	Hollow microtubules	Moving chromosomes during cell division	No
ENDOMEMBRANE SYSTEM
Plasma membrane	Lipid bilayer in which proteins are embedded	Regulates what passes into and out of cell; cell-to-cell communication	Yes
Endoplasmic reticulum	Network of internal membranes; forms compartments and vesicles	Rough type processes proteins for secretion and synthesizes phospholipids; smooth type synthesize fats and steroids	No
Nucleus	Structure bounded by double membrane; contains chromosomes	Control center of cell; directs protein synthesis and cell reproduction	No
Golgi complex	Stacks of flattened vesicles	Modifies and packages proteins for export from cell; forms secretory vesicles	No
Lysosomes	Vesicles derived from Golgi complex that contain hydrolytic digestive enzymes	Digest worn-out mitochondria and cell debris; play role in cell death	No
Autophagy	Vesicles to collect debris within the cell	Malfunction causes accumulation of cell damage leading to diseases and aging (see Malfunction of Autophagy)	No
ENERGY-PRODUCTING ORGANELLES
Mitochondria	Bacteria-like elements with inner membrane	Battery of the cell by ATP synthesis; site of oxidative metabolism	No
ORGANELLES OF GENE EXPRESSION
Chromosomes (during cell division) / Chromatins	Long threads of DNA that form a complex with protein	Contain hereditary information	Yes
Nucleolus	Site of rRNA synthesis	Assembles ribosomes	No
Ribosomes	Small, complex assemblies of protein, often bound to ER	Site of protein synthesis	Yes

Table 01 Cell Structure

PKC - Absence or presence in prokaryotic cells.

Table 01 gives a brief description and its function for each of the organelles. However, this is not a complete picture. It has glossed over the role of molecular tracks and the molecular motors running on these tracks. Without these molecular tracks (Figure 06), the cell would be a shapeless blob and all the material inside would be wandering aimlessly. As discussed below, there are two kinds of visible tracks - the microtubule and the actin, each one is accompanied with a particular motor. The actin also provide a scaffold to support the cell membrane, and the microtubule is critical (providing the spindle) in cell division.