Cells

The basic unit of life is the CELL. This is the smallest entity that is alive, that is capable of growth, division, and propagation. Three things are required by a cell to live, grow and multiply. A cell requires:

1. PRECURSORS

2. ENERGY

3. INFORMATION

Precursors (a substance from which another is formed) are small biomolecules. They are used to build the large biopolymers that make up a cell and they are oxidized for energy.

Cells require energy to power cellular processes. Energy is used for such diverse things as forming new chemical bonds, muscular contraction, and thought. Energy is obtained by oxidizing (combusting or burning) certain biomolecules to CO2 and H2O.

Information is required to direct and control the cellular processes. A cell is a highly organized thing. Information is required to establish and maintain this organization.

Cellular Components

All cells are composed of four fundamental types of biomolecules, four types of precursors. These four types of biomolecules are:

1. carbohydrates / monosaccharides / simple sugars2. lipids / fats3. amino acids4. nucleotides

Carbohydrates and lipids are the major source of energy for the cell. They are oxidized to CO2 & H2O and the energy released is trapped in other energy rich molecules that are used to power cellular processes. Amino acids can be used for energy when they are present in excess and/or when lipids and carbohydrates are in short supply.

The large cellular biopolymers are made up from smaller precursors. Disaccharides and polysaccharides are composed of the monosaccharides (simple sugars). Proteins and other nitrogen containing molecules are synthesized from the amino acids. Lipids are composed of fatty acids, glycerol, sphingosine, and other precursor molecules.

Nucleic acids are polymers of nucleotides. Cellular information is stored in and carried by nucleic acids. This information is used to direct the synthesis of proteins. Cellular information is expressed as a specific set of proteins present in the cell.

Types of Cells

There are three types of organisms - BACTERIA, ARCHAEA, and EUKARYOTIC. BACTERIA and ARCHAEA are single cell organisms. EUKARYOTES are the higher organisms, the multicellular organisms. The different organisms are composed of different cell types.

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BACTERIAL CELLS

Bacterial cells are structurally much simpler than eukaryotic cells. Starting from the outside there is the:

1. The FLAGELLA or CILIA. The flagella or cilia are protein tubules used by the cell for movement. Not all bacteria have flagella or cilia.

2. The CAPSULE. The capsule is composed primarily of complex carbohydrates and a small amount of protein. The capsule is very hydrophilic (water loving), it absorbs and contains a large amount of water from the external media. Not all bacteria have a capsule. The capsule inhibits phagocytosis by white blood cells and makes the bacteria more infectious. In general, the larger the capsule the more virulent (infectious) the bacteria.

3. The CELL WALL. The cell wall is a rigid polymer composed of carbohydrate and protein. The cell wall gives structural stability to the cell. It prevents the swelling and bursting of the cell if it comes in contact with a hypotonic medium.

4. The CELL MEMBRANE or PLASMA MEMBRANE. The cell membrane or plasma membrane is composed of lipids and proteins. It surrounds / delimits the cell. It keeps the inside in and the outside out.

5. The PERIPLASMIC SPACE. Between the cell wall and cell membrane is the periplasmic space. Waste products from the cell are transported into this space and they then diffuse into the surrounding medium. Aerobic and anaerobic bacteria that perform respiration use this space to store/partition metabolic intermediates.

6. The CYTOPLASM or CYTOSOL. Inside the cell is the cytoplasm or cytosol. Since most of the cell is cytoplasm, most of the biochemistry of bacteria occurs in this space. The cytoplasm of a prokaryotic cell is a complex solution/colloid of lipids, carbohydrates, proteins, nucleotides, and inorganic ions. It contains all of the components / compounds necessary to sustain life.

7. RIBOSOMES. The cytoplasm contains large complexes of protein and nucleic acid, the ribosomes. These complexes play a direct role in protein synthesis.

8. The NUCLEOID REGION. Immediately before cell division, a nucleoid region can be defined. This region of the cell is enriched in nucleic acids.

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EUKARYOTIC CELLS

Eukaryotic cells are structurally more complex because they contain SUBCELLULAR ORGANELLES. SUBCELLULAR ORGANELLES are membrane enclosed structures that contain the metabolic machinery necessary to perform a specific cellular task.

1. The CELL MEMBRANE or PLASMA MEMBRANE. The cell is surrounded by the cell membrane or plasma membrane. Like the prokaryotic cell membrane it is composed of lipid and protein. Unlike the bacterial membrane the outer surface of the eukaryotic cell membrane contains a variable amount of carbohydrate. The membrane keeps the inside in and the outside out.

2. The CYTOSKELETON. Directly adjacent to the interior surface of the cell membrane is a complex arrangement of protein fibers, the cytoskeleton. This structure gives the cell shape and mechanical strength. A less dense arrangement of protein fibers criss-cross the entire interior of the cell. These fibers are continuous with the cytoskeleton and function to hold organelles in their proper position within the cell and they serve to guide the movement of organelles within the cell and the movement of secretion vesicles to the cell membrane.

3. The NUCLEUS. The nucleus is a double membrane bounded organelle. It contains the genetic information necessary to sustain and propagate life. The nucleic acids (DNA & RNA) and associated enzymes and proteins are found in the nucleus.

4. The ENDOPLASMIC RETICULUM. Closely associated with the nucleus is a system of membrane bound tubules, the endoplasmic reticulum (ER). The membrane surrounding the ER, like all membranes, is composed of lipid and protein (little if any carbohydrate). The endoplasmic reticulum serves several functions within the cell. It is involved in protein synthesis, protein processing, protein modification, and it is a storage depot for small inorganic ions especially metal ions, e.g., Ca+.

5. The GOLGI APPARATUS. Proteins and other molecules that will be exported from the cell travel in membrane bound vesicles from the endoplasmic reticulum to the Golgi apparatus. The Golgi apparatus prepares proteins for export from the cell. Small membrane surrounded vesicles bud off from the Golgi. These vesicles contain the proteins that are to be secreted.

6. The MITOCHONDRIA. The mitochondria are double membrane surrounded structures. Mitochondria are the energy producers of the cell. They take a particular defined set of cellular biomolecules, oxidize them to CO2 and H2O, and convert the energy released into a form usable by the cell. Mitochondria are unique in that they contain some of their own DNA, some of their own information. The presence of DNA in mitochondria is supportive evidence for the theory that eukaryotic cells arose from two prokaryotic cells that established a symbiotic relationship.

7. The LYSOSOMES. The lysosomes are the cellular scavengers. They digest damaged cellular organelles and / or foreign materials that have entered the cell. The products of the digestion are recycled by the cell.

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8. The LIPOSOMES. The lipisomes are small vesicles full of lipid. The stored lipid can be used as an energy source or used to synthesize membrane lipids.

9. The PERIOXISOMES. The perioxisomes are small membrane enclosed vesicles. They are involved in the modification of cellular lipids and they also play a role in the killing ingested bacteria. Many oxidation / reduction reactions occur in perioxisomes.

10. RIBOSOMES. Associated with the endoplasmic reticulum as well as being free in the cytoplasm are ribosomes. These large complexes of protein and nucleic acid play a direct role in protein synthesis. Cytoplasmic ribosomes are involved in the synthesis of proteins that will remain in the cytoplasm and be utilized by the cell. Proteins that will be incorporated into cellular membranes or secreted by the cell are synthesized by ribosomes attached to the endoplasmic reticulum.

11. The CYTOPLASM or CYTOSOL. The part of the cell not enclosed in subcellular organelles, the remaining part of the cell is the cytoplasm or cytosol. The cytoplasm is a complex solution of lipids, carbohydrates, proteins, nucleotides, and inorganic ions. Many biosynthetic reactions occur in the cytoplasm of the cell.

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