THE CELL
CELL THEORY
The scientific study of cells- their structures, functions, and activities is called
cytology. The cell is the smallest unit of life. In 1590, Francis and Zacharias
Janssen constructed the first compound microscope. The cell as a structural unit was
first observed by Robert Hooke in 1665 in thin slices of cork and other plant tissues. At
about the same time (1674) the Dutch lensmaker Leeuwenhoek reported seeing
"animalcules" or single-celled organisms through his microscope. Marcello Malphighi
used a microscope to examine and describe thin slices of animal tissues from brain,
liver, kidney, spleen, lungs, and tongue. In time, the quality of microscopes improved.
Thus in the early 1800 the concept of the cell theory began to gain currency in scientific
world. Theodor Schwann (a zoologist) wove the various loose ends into a coherent
theory which proposed that all organisms are composed of cells and the product of cells. In
1831, Robert Brown described and named the nucleus of the cell. At the end of the
decade Purkinje proposed using the term protoplasm to describe the living material
within the cell. Thomas Huxley later called protoplasm the "physical basis of life".
Another advancement came about when Rudolf Virchow stated an important principle:
All cells arise from pre-existing cells. That there is no spontaneous creation of cells from
non-living matter was supported by the experiment conducted by Louis Pasteur. This
marked the end of a belief in spontaneous generation theory and the establishment of
the principle of biogenesis.
CELL STRUCTURE
I. CELL SURFACE
1. Cell wall - Plant cells are encased in an outer protective and supporting structure
called the cell wall. It is composed of the polysaccharide cellulose in the form of
threadlike structure called fibril. The first portion of the cell wall laid down by a young
cell is the primary wall. Between the walls of adjacent cells is an intercellular layer
called middle lamella which contains pectin. The middle lamella binds the cells
together. The cells of the more woody portion of plants form an additional layer
called secondary wall. This is located internal to the primary wall and contains lignin
which adds stiffness and rigidity to cell walls.
2. Cell Membrane (plasma membrane or plasmalemma). This is the living boundary
between the external and internal environment of the cell. It is a sturdy envelope that
encloses the cell and behaves as a selective "gatekeeper" that determines what can
and can not enter or leave the cell. The electron microscope together with
biophysical and biochemical studies revealed that the cell membrane consists of
a
phospholipid bilayer
sandwiched between layers
of protein molecules that
may penetrate or extend
through the lipid membrane.
The proteins may act as carrier
molecules to allow passage of
substances or as enzymes with
biological functions such as
converting nutrient molecules
into ATP. These
constantly
moving
and
reorganizing
protein
molecules are
responsible for the selective permeability of the cell membrane. The concept of
fluidity refers to the fact that both lipids and proteins may have considerable
lateral movement within the bilayer.
Normally, these lipids are liquid at body
temperature and the main
consideration is the degree of saturation of the
hydrocarbon chains.
Functions:
1. protects the cell
2. seat of interaction of a cell with its surrounding
3. selective pathway for the transport of substances into and outside the cell
4. helps in cell adhesion and mobility
3. Cilia or flagella - Many cells bear specialized structures of locomotion called cilia or
flagella. These are vibratile extensions of the cell surface.
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II. NUCLEUS
The most conspicuous protoplasmic structure is a spherical or ellipsoid body
called the nucleus. It is the reproductive, metabolic, and dynamic center of the cell. A
few unicellular organisms such as bacteria and blue-green algae do not show a
distinctive separation of nuclear and cytoplasmic constituents. Cells without nuclei are
called procaryotic cells in contrast to those with nuclei called eucaryotic cells.
1. Chromosomes- These are principal nuclear structures which consists of DNA and
proteins. They are conspicuous only during cell division. at other times they appear as
a network known as chromatin network. They bear in linear arrangement the basic
units of heredity called genes
Functions:
1. carry genetic information
2. play an important role in determining the phylogeny and taxonomy of the species
2. Nucleolus (little nucleus) - This is a highly compact and darkly staining
spherical body which consists largely of RNA and proteins. It is now believed that the
region of contact between the nucleolus and associated chromosome, the
nucleolar organizer, is the site of large-scale production of RNA which is stored in
the nucleolus and then sent into the cytoplasm, where it plays a key role in
protein synthesis.
Functions:
1. believed to be the site of ribosomal manufacture
2. collects freshly produced mRNA for cellular protein synthesis
3. plays an important role in cell division.
3. Nuclear Membrane - It has
a
double
unit
membrane
structure.
It
communicates with an internal membrane network, the endoplasmic reticulum
by minute pores. These pores may facilitate an exchange of materials between the
nucleus and the cytoplasm. It is similar to the cell membrane in functions.
1. it bounds and protects
the chromatin material in
eukaryotic cells
2. gives definite shape to
the nucleus
3. helps in the exchange of
materials between the
nucleus and cytoplasm
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III. CYTOPLASM
The largest volume of the cell is taken up by the cytoplasm, a transluscent,
colorless, viscous mass where most of the organelles of the cell are situated. The
soluble portion of the cytoplasm, the cytosol has a very high protein concentration
exceeding 20%. Most of these are enzymes required in metabolism.
1. Mitochondria - These are round or filamentous structures with predominantly fatprotein composition. They are found in all cells.
An outer layer covers the
mitochondrial surface, whereas the inner surface
is much folded and forms projections called
cristae that
extend
into the interior fluid or
matrix. In the matrix are a number of enzymes of
the Krebs cycle, salts, water. The cristae is the site
of protein synthesis for respiration. Research
shows that the mitochondria are the principal sites
for cellular respiration. The mitochondria, as the
respiration centers have been aptly called the
"powerhouse of the cell".
Function:
1. production of energy during cell respiration
2. Golgi apparatus (Golgi bodies or dictyosomes) These structure were discovered
by Camilo Golgi. They consist of flattened
sacs which appear as parallel membranes,
cluster of small, tightly packed vesicles
and large clear vacuoles. Functions have
not yet been completely defined but
some investigators believe that they play
an
important
role
in secretion
(cellulose) and provide a temporary
storage area
and repackaging center
for substances like proteins and
lipids.
Lysosomes are believed to be derived from
the golgi bodies.
Functions:
1. Its main function is cell secretion not only of exportable proteins but also
enzymes present in lysosomes and peroxisomes
2. Provide a temporary storage area and repackaging center for substances like
proteins and lipids
3. Lysosomes- These organelles have been observed primarily in animal cells and
protists. They are enclosed by single membrane and contain powerful digestive
enzymes. These enzyme-filled sacs fuse with food vacuoles where intracellular
digestion takes place. They also take part in the phagocytic activity of white blood
cells, the degeneration of the tadpole's tail during metamorphosis and the digestion
of certain bone cells. When the cell dies, the lysosomes rupture causing self4
digestion or autolysis of the cells. Thus these organelles are often referred to as
"suicide bodies".
Functions:
1. digestion of extracellular and intracellular substances
2. take part in phagocytic activity of white blood cells
3. lysis of organelles during cellular differentiation and
metamorphosis (degeneration of the tadpole's tail)
4. when the cell dies, the lysosomes rupture causing self-digestion
or autolysis of the cells. Thus these organelles are often
referred to as suicide bodies
5. involved in cell aging
6. active in remodelling of tissues
4. Centrioles- These are two darkly stained bodies located just outside the
nucleus in most animal cells. They determine the orientation of the plane of cell
division.
Functions:
1. determine the orientation of the plane of cell division
2. concerned with the movement of the chromosomes
during cell division. The
spindle is made up of
microfilaments and microtubules.
5. Endoplasmic reticulum - This is an elaborate system of channels and cavities
present in all nucleated cells. This double-layered system is continuous with the
plasma membrane and nuclear membrane. It has often been described as a type of
cytoskeleton that provides surface for chemical
reactions, pathways
for
the
transportation of cellular molecules and storage for synthesized molecules.
Two types of endoplasmic reticulum have been observed: rough
endoplasmic
reticulum (RER) and smooth endoplasmic reticulum (SER). The rough endoplasmic
reticulum has on its outer small granules called ribosomes. It is abundant in cells
where protein synthesis is active. The smooth endoplasmic reticulum do not have
ribosomes and occur in cells active in lipid synthesis.
Functions:
1. it has often been
described
as a type of
cytoskeleton that provides mechanical support to the
cytoplasmic matrix
2. provides surfaces for chemical reactions, or as a
transport system (circulatory system) that
provides pathways for the transportation of cellular molecules and chemical
reactions
3. mitochondria, golgi bodies, lysosomes are produced by endoplasmic reticulum
4. storage for synthesized molecules
5. transmits impulses in muscle and nerve cells
6. participates in excitation-contraction
7. involved in Cl- secretion in stomach cells
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6. Ribosomes - These tiny spherical bodies contain relatively large amounts of RNA
and are sites of protein synthesis. It has recently been
discovered
that
protein
synthesis
involves
the
association of a molecule of messenger RNA sent out from
the nucleus with a group of ribosomes called polyribosomes.
Function:
1. site of protein synthesis
7. Plastids - These are organelles found in plants and serve as sites of synthesis of
complex organic compounds and storage. They are classified according to color and
function:
a) Chloroplastids - are the green plastids containing the light-sensitive pigment
chlorophyll. They are the centers of photosynthetic activity.
b) Chromoplastids - these are yellow or red in color and owe their color to the
presence of various carotenoids. They are responsible for the color of ripening fruits
and autumn leaves.
c) Leucoplastids- these are colorless plastids that serve as starch-forming and
storage centers of plants. They are generally found in colorless leaf cells of stems,
roots and other storage organs.
8. Microbodies - These are small cellular components possessing oxidases and
catalases. One type of microbody is the peroxisome. The peroxisomes are
membrane-bound organelles rich in peroxidase. They are abundant in liver and
kidney cells and are concerned with peroxidation and beta oxidation of fatty acids.
IV. CYTOSKELETON. The cytoskeleton is a network of filaments and fibers within the
cytoplasm that helps maintain the shape of the cell, move substances within cells
and anchors various structures in place. The cytoskeleton contains three types of
fibers; the microfilament, the microtubule and the intermediate filaments.
.
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The microfilaments are the thinnest, twisted, double chain of protein molecules.
They run parallel to the cell membrane in bundles. The microtubules are the
thickest, consisting of a chain of proteins wrapped in a spiral to form a tube. They
radiate from an area near the nucleus; they also provide intracellular support in nondividing cell. The intermediate filaments have “in-between size, thread-like protein
molecules that wrap around one another to form “ropes” of protein. In a skin cell,
these form thick, waxy bundles that probably provide structural support.
1. Microfilaments - These represent the active or motile
part
cytoskeleton consisting of actin, myosin, tropomyosin, and troponin.
of
the
Functions:
1. cellular motility
2. cyclosis
3. ameboid movement
2. Microtubules - These are tiny tubules which are either free in the cytoplasm or
forming part of centrioles, cilia and flagella. The main protein component is tubulin.
Functions:
1. provide structural support to the cell
2. determine the polarity and direction of cell movement
3. play a role in the contraction of the spindle,
movement of chromosomes and centrioles, as well as
ciliary and flagellar movement.
V. CELL INCLUSIONS - They are mostly organic in nature and are products of
cell action. They may appear or disappear at different times in the life of the
cell.
a) Vacuoles (food or contractile). These are the most important inclusions of plant
cells. The
major
components of vacuoles are water and some dissolved
substances such as gases, salts, sugars, organic acids, soluble proteins and
certain pigments called anthocyanin, responsible for the red, blue, and purple color
of leaves, fruits and stems. Vacuoles facilitate exchange of gases with the
surrounding medium. Contractile
vacuoles
found
in numerous freshwater
protozoans maintain water balance by pumping out excess water from the cell.
Plants vacuoles contribute to cell turgor, contributing to the rigidity of plant parts.
Food vacuoles enclose ingested food particles. Waste vacuoles serve as organs
of excretion (metabolic trash cans).
b) Other inclusions are starch granules, inorganic deposits in plant cells such as
calcium oxalate crystals, crystals of calcium carbonate, calcium sulfate and silica,
gluten and aleuron grains, and oil globules.
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