The Animal Cell
Two classes of cells:
(1) Sex cells (called also germ cells or
reproductive cells).
(a) Sperm of a male.
(b) Ovum (oocyte) of a female.
(2) Somatic cells include all the other cells of the
body.
The cell membrane (plasma
membrane or plasma lemma)
• The cell membrane forms the outer boundary of
the cell.
• Functions:
• Physical Isolation: It acts as a physical barrier that
separates inside the cell from the surrounding
extra-cellular fluid.
• Regulation of exchange with the environment
• Structural support: between cells or between
cells and extra-cellular materials.
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Membrane structure
The membrane thickness ranges from 6 to 10nm.
The cell membrane contains :
Lipids: In the form of phospholipids &
cholesterol.
• Proteins.
• Carbohydrates which are in the form of
glycolipids and glycoproteins. .
• Membrane lipids
• The membrane lipids are phospholipids and
cholesterol.
• The phospholipid molecules are arranged in
two layers. Each molecule consists of a
hydrophilic head (towards outside or inside
the cell) and hydrophobic tail (towards the
center of the membrane).
• Membrane proteins:
• Proteins account for 55% of the weight of the cell
membrane
• Two types of proteins are present:
1) Integral proteins: which are part of the membrane
structure and most of them span the membrane width
and therefore called Trans membrane proteins.
(2) Peripheral proteins: which are bounded to the outer
or inner surface of the membrane.
Membrane protein functions:
A.
B.
C.
D.
E.
Anchoring proteins which may attach the membrane to other structures and
stabilize its position.
Recognition proteins (identifiers) which help the body cell to recognize each
other as self and other foreign cells as non-self specially in the immune system.
Enzymes, which catalyze reactions in the membrane.
Receptor proteins, which bind specifically to molecules called ligands. Carrier
proteins bind solutes and transport them across the cell membrane.
Channels, some integral proteins act as channels that form a passage way that
permits the movement of water and small solutes across the membrane.
Channels are of two types:
(1) Leak channels, which permits water and ions movement at all times.
(2) Gated channels, which can open or close to regulate ion passage
according to the cell demands.
• Functions of glycoproteins and glycolipids
(glycocalyx):
1. The glycoproteins and glycolipids form a viscous
layer that lubricates and protects the cell
membranes.
2. The blood group (A, B, AB & O) of any individual
is determined by the presence or absence of
certain glycoproteins on the membrane of red
blood cells.
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Membrane Permeability and
Transport of Substances across
Cell membrane.
• The cell membrane is selectively permeable i.e. it permits
the free passage of some materials and restricts the
passage of others.
• Passage across cell membrane is either passive or active.
• Passive transport does not need energy while active
transport needs energy in the form of ATP.
• According to the nature of the mechanism, the transport
processes are classified into three types :
• (1) Diffusion.
• (2) Carrier mediated transport.
• (3) Vesicular transport.
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Diffusion
• It is the net movement of material from an
area of relatively high concentration to
another area of relatively low concentration.
The difference in concentration is called
concentration gradient. Diffusion takes place
"down concentration gradient" or "downhill".
•
• Factors that influence the rate of diffusion
include:
1. Size of the gradient: the larger the
concentration gradient, the faster is the rate of
diffusion.
2. Molecular size: the smaller the size of the
molecule, the faster is the rate of diffusion
3. Temperature: the higher the temperature, the
faster is the diffusion.
• If a substance has a concentration gradient across the
membrane , its diffusion will depend on two major
factors:
• (1) Lipid solubility: alcohol, fatty acids and steroids can
enter cells easily because they can diffuse through the
lipid layer.
• (2) Size: to diffuse through the cell membrane, water
soluble substances must be small enough to pass
through a membrane channel. Ions such as sodium
potassium, calcium and hydrogen can diffuse through
membrane channels.
• Osmosis is a term used to describe the diffusion of water.
• The term diffusion is used only for solutes.
• Remember the following three characteristics of osmosis:
1. Osmosis is the diffusion of water molecules (the solvent)
across the membrane.
2. Osmosis occurs across a selectively permeable membrane
that is freely permeable to water but not for solutes.
3. In osmosis water will flow across a membrane toward the
solution that has the higher concentration of solutes ( low
concentration of water)
Carrier mediated transport
• In this type, specific ions or organic
substrates bind to a carrier protein which
facilitates their movement across the cell
membrane.
• The two major examples are :
• (1) Facilitated diffusion; no energy is needed
as the substance diffuses from high to low
concentrations but with the help of a specific
carrier.
• (2) Active transport:
This type needs energy in the form of ATP as the
substance is transported from low to high
concentration, i.e. against concentration gradient.
The common example of his type is the sodium potassium exchange pump in which sodium ions are
transported from inside to outside the cell in
exchange with potassium ions which are
transported from outside to inside the cell across
the membrane.
Vesicular transport
• It is also called bulk transport.
The materials move into and out of the cell by
means of vesicles.
• The two major types are :
• (1) Endocytosis: it is the packaging of extracellular materials in a vesicle surrounded by a
part of the cell membrane and its engulfment
into the cytoplasm.
The two types of endocytosis are :
(a) Pinocytosis; is the formation of small
vesicles filled with extra- cellular fluid. It is also
called cell drinking.
(b) Phagocytosis; in which the engulfed
material is solid and the formed vesicle is called
phagosome. It is also called cell eating.
• (2)Exocytosis: It is the reverse of endocytosis.
• A vesicle is created inside the cell surrounding a material to be
discharged outside the cell such as secretion. This vesicle
fuses with the cell membrane and discharges its contents to
outside.
THE CYTOPLASM
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The cytoplasm consists of:
Cytosol.
Organelles.
The Cytosol
The cytosol is the part of the cytoplasm between the organelles.
The cytosol differs from the extra-cellular fluid in the following:
(1).It contains higher concentration of potassium and lower
concentration of sodium ions.
• (2) It contains higher concentrations of proteins.
• The cytosol may also contain insoluble nonliving materials called
inclusions.
The Organelles
Membranous Organelles
Surrounded by lipid
membranes separating their
contents from the cytosol
include:
(1)Mitochondria.
(2)Endoplasmic reticulum.
(3) Golgi Apparatus.
(4)Lysosomes.
Non-membranous
organelles
In direct contact with the
cytosol.
Include:
(1)Cytoskeleton.
(2)Centrosome
(3)Ribosomes.
(1). Mitochondria
(singular-mitochondrion)
(mitos= thread & Chondrion= cartilage)
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Shape: Long slender - like threads or short thick like sausages.
Number: Depends on the activity and energy production by the cell.
. NB. Red blood cells have no mitochondria.
Structure: Each mitochondrion is surrounded by two membranes.
The outer membrane is smooth while the inner is provided with folds.
These folds increase the surface area of the inner membrane.
The folds are the sites of the respiratory oxidative enzymes responsible for energy
production.
• The greater the energy demands of the cell , the larger is the number of folds in
the mitochondria.
• Functions: mitochondria are the power house of the cell
1. Mitochondria are the sites of the aerobic phase of oxidation of glucose as it
contains the enzymes of the Kreb's cycle.
2. Inside mitochondria the liberated energy is stored in a high energy compound
called adenosine triphosphate (ATP.)
3. Mitochondria are the major sites for fatty acid oxidation.
• (2) The Endoplasmic Reticulum (E.R.)
• Structure:
• E.R is formed of hollow tubes and sheets
surrounded by membranes and connected
together forming a network.
• Two types of E.R. are present:
– Rough E.R.(R.E.R.) provided with ribosomes on the
outer surface.
– Smooth E.R.(S.E.R.) without ribosomes on the outer
surface.
• Function of R.E.R.:
1.
2.
3.
Protein syntheses (as ribosomes are the sites of protein
syntheses).
Most of proteins leave the E.R. surrounded by membranes and
separated as micro-vesicles.
These vesicles fuse with the membrane of Golgi apparatus and
deliver the products of E.R. into it.
1.
• Functions of S.E.R.:
1.
2.
3.
Syntheses of lipid secretions.
Detoxification of some drugs and harmful substances.
In skeletal muscles, S.E.R. is the site of storage and release of
calcium ions during contraction and called sarcoplasmic
reticulum.
• (3) Golgi Apparatus
• Structure:
• It is composed of flattened disc-like sacs and a
number of associated vesicles.
• Number:
• It is larger in secretory cells.
• Functions:
1. Concentration, storage and packaging of
secretory products of the cell.
2. The Golgi apparatus participates in the
formation of lysosomes.
3. The Golgi apparatus is also involved in the
synthesis of membrane components (because
during exocytosis the membranes 0f the
secretory vesicles are added to the cell
membrane).
• (4) Lysosomes
• Lysosomes are vesicular organelles each surrounded by
a single smooth membrane
• Structure:
• Lysosomes are vesicles, filled of hydrolytic (digestive)
enzymes that are capable of degrading (breakdown)
most of the material.
• The membranes of the lysosomes protect the cytosol
and other organelles from the hydrolytic action of the
enzymes.
• Functions:
1-Intra-cellular digestion:
Lysosomes can digest substances inside the cell. This function may take place in two
forms:
Heterophagy: in which the digested substance is engulfed from outside the cell by
endocytosis. This mechanism is important in feeding of some protozoa (amoeba) and
in leukocytes (white blood cells) during killing foreign organisms.
Autophagy: in which the digested substance is a part of the cell (example, an old or
injured organelle).
2-Extra-cellular digestion:
Lysosomal enzymes may be discharged outside the cell by exocytosis and produce
their effect on the surrounding structures.
3-Autolysis:
The lysosomal membranes rupture and release the enzymes inside the cell
leading to the digestion of the cell itself.
The Cytoskeleton
• It is an internal protein framework that gives
the cytoplasm strength and flexibility. It has
four major components:
1. Microfilaments.
2. Intermediate filaments.
3. Thick filaments.
4. Microtubules.
• I. Microfilaments :
– Microfilaments are protein strands less than 6 nm in diameter.
– Most microfilaments are composed of the protein actin.
– Functions of microfilaments are :
• Muscle contraction
• Formation of microvilli
• II. Intermediate filaments:
– The diameter rages from 7 to 11 nm.
– Functions :
Provide strength & stability to cell shape.
III. Thick filaments:
– The diameter may reach 15 nm.
– Composed of myosin protein.
– Function: muscle contraction
• IV. Microtubules:
– Microtubules are hollow tubes formed of a protein
called tubulin.
– The diameter is 25 nm.
– Functions :
1.
2.
3.
4.
Cell strength & rigidity.
Transport of certain materials .
cell division.
Enter in the structure of centrioles, cilia &
flagella.
Microvilli
– Microvilli are small, finger-shaped projections of the cell membrane, that
greatly increase the surface area of the cell, such as cells lining the intestine to
increase absorption.
– Each microvillus consists of a core of microfilaments covered by a projecting
cell membrane.
Centrosome
• Consists of two centrioles at right angle to each other
• Each centriole is a cylindrical structure composed of 27 short
microtubules arranged as nine triple groups at the periphery
without any microtubules in the center. This organization is called
(9 + 0).
– During cell division the 2 centrioles migrates to the opposite poles of
the cell and form the spindle apparatus to which the chromosomes
are attached.
– Centrioles are absent in cells that are not capable to divide. These cells
are :
1.
2.
3.
4.
Mature red blood cells.
Skeletal muscle cells.
Cardiac muscle cells.
Neurons (Typical nerve cells).
Cilia
• Cilia are cylindrical structures that project from the cell
membrane and each cilium is attached to a structure beneath
the cell surface called basal body.
– The organization of the microtubules in the basal body is
similar to that in centrioles, i.e. (9 + 0).
– The organization of the microtubules in each cilium is
known as
(9 + 2), i.e. 9 groups of double microtubules at the
periphery plus two microtubules in the center.
Flagella
– Flagella (singular, flagellum) resemble cilia(9+2)
but they are much longer.
– It helps the cell through a surrounding fluid.
• The only human cells that have flagella are the
sperm cells