Unicellular Organisms

Cells

		There are two types of cells: prokaryotic and eukaryotic. Prokaryotic cells have no nucleus and form unicellular organisms such as bacteria. The cells in protista, fungi, plants and animals are eukaryotic cells, which have a nucleus. In a eukaryotic cell, the plasma membrane is a lipid bilayer that separates the materials inside the cell from the environment surrounding it. The outer surface of the membrane contains structures that allow cells to communicate with each other.
Figure 11-30a Eukaryotic Cell [view large image]	Figure 11-30b Cell Nucleus [view large image]

The nucleus contains the genes that control DNA replication and protein synthesis of the cell (Figure 11-30b). The cytoplasm consists of all the materials between the nucleus and the plasma membrane. The cytosol, which is the fluid part of the cytoplasm, is an aqueous solution of electrolytes and enzymes that catalyze many the cell's chemical reactions.

Within the cells are specialized structure called organelles that carry out specific functions in the cell. The cell structure is shown in Figure 11-30a, the functions of the cell are shown in Table 11-02 below.

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 11-02 Cell Organization

PKC - Absence or presence in prokaryotic cells.
See more about the internal detail of the cell in : "Inner World of the Cell".

The nucleus is of primary importance in the cell because it is the control center that oversees the metabolic functioning of the cell and

ultimately determines the cell's characteristics. Within the nucleus, there are masses of threads called chromatin, which is indistinct in the non-dividing cell. The long string of DNA winds around a spool made with proteins called histone, which also plays a role in gene regulation. The chromatins condense to chromosomes at the time of cell division. Figure 11-31 shows the packed chromatin unwinding to a DNA strand. It also shows the appearance of chromosome in mitosis stage. The nucleolus is the specialized part of chromatin in which the ribosomal RNA (rRNA), is produced (Figure 11-30b).

The telomeres lie at the tips of the chromosome. They have hundreds to thousands of repeats of a specific 6-nucleotide DNA sequence. The telomeres lose 50 to 200 of these nucleotides at each mitosis; gradually shortening the chromosome. After about 50 divisions, a critical amount of telomere DNA is lost, which somehow signals the cell to stop mitosis. The cell may remain alive for a while but is unable to divide further. This is the cellular clock, which pre-determines the life span of the cell.

Figure 11-31 Chromosome

See "DNA orgainization".