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 sugars
2. lipids / fats
3. amino acids
4. 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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