Download Plasma membrane Structure

CELL BIOLOGY
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Cell Biology (Cytology)
is a scientific discipline that studies cells – their
physiological properties, their structure, the organelles
they contain, interactions with their environment, their
life cycle, division and death.
This is done both on a microscopic and molecular level.
Cell biology research encompasses both the great
diversity of single-celled organisms like bacteria and
protozoa, as well as the many specialized cells in
multicellular organisms such as humans.
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Knowing the components of cells and how cells work is
fundamental to all Biological Sciences.
Appreciating the similarities and differences between cell
types is particularly important to the fields of cell and
molecular biology as well as to biomedical fields such
as cancer research and developmental biology.
Research in cell biology is closely related to genetics,
biochemistry, molecular biology, immunology, and
developmental biology.
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Cellular Anatomy
There are several hundred distinct human cell types. There
are between 50 and 75 trillion cells in the human body. [1]
The human body contains two classes of cells:
• Germ cells or reproductive cells
(a) sperm of a male.
(b) ovum (oocyte) of a female.
• Somatic cells include all the other cells of the body.
1- Molecular Biology of the Cell, Fourth Edition, Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, Peter Walter
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Plasma membrane
Plasma membrane
The plasma membrane in plants and
prokaryotes is usually covered by a cell
wall .
Plasma membrane serves to separate
and protect a cell from its surrounding
environment and is made mostly from
a double layer of
lipids (hydrophobic fat-like molecules
and hydrophilic phosphorus
molecules. Hence, the layer is called
a phospholipid bilayer .
Definition
Plasma membrane
Definition
It also be called a fluid mosaic membrane. Embedded within this
membrane is a variety of protein molecules that act as channels
and pumps that move different molecules into and out of the cell.
The membrane is said to be selectively permeable, in that it can
either let a substance (molecule or ion ) pass through freely, pass
through to a limited extent or not pass through at all.
Cell surface membranes also contain receptor proteins that allow
cells to detect external signaling molecules such as hormones .
Plasma membrane
Components
Plasma membrane Function
The general function of the cell membrane include:
• 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: It controls the
entry of ions and nutrients such as glucose , the elimination
of wastes and release of secretions.
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Plasma membrane Function
• Sensitivity:
It contains a variety of receptors that allow the cell to
recognize and respond to specific molecules in the
environment.
• Structural support :
Specialized connections between cell membranes or between
membrane and Extra-Cellular Materials ECM.
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Plasma membrane Structure
Membrane structure
• The membrane thickness ranges from 6 to 10nm .
• The cell membrane contains :
*Lipids: In the form of phospholipids & cholesterol.
*Proteins.
*Carbohydrates in the form of glycolipids.
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Plasma membrane Structure
Membrane lipid :
The cell membrane is called a phospholipid bilayer because the
phospholipids form two distinct layers. In each layer,
the phospholipid molecules lie with their hydrophilic heads at the
membrane surface and the hydrophobia tails on the inside.
In addition, cholesterol molecules are also present and also with
their heads at the membrane surface and their tails on the inside.
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Plasma membrane Structure
Membrane proteins :
Proteins account for 55% of the weight of the cell membrane
Two types of proteins are present:
(1) lntegral 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.
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Plasma membrane Structure
Membrane protein function:
1. Anchoring proteins .which may attach the membrane
to other structures and stabilize its position .
2. Recognition proteins (identifiers) ,the cells of the
immune system recognize the cells of the body as self
on the bases of the presence or absence of recognition
proteins that are characteristic for each individual.
3. Enzymes, which catalyze reactions in the extracellular
fluid or inside the cytoplasm.
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Plasma membrane Structure
4. Receptor proteins, which bind specifically to molecules called
ligands. When a receptor protein binds to its specific ligand , it
triggers changes in the activity of the cell.
5. Carrier proteins, bind solutes and transport them across the cell
membrane. The transport process involves a change in the shape
of the carrier. The carrier returns to its original form when the
solute is released. Carrier proteins may or may not require energy
from ATP.
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Plasma membrane Structure
5. 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.
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• N.B. proteins are embedded in the phospholipid bilayer and
can drift from place to place across the surface of the
membrane like the ice cubes in a bowl of punch.
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Plasma membrane Structure
Functions of glyco calyx:
• The glycoproteins and glycolipides form a viscous layer that
lubricates and protects the cell membranes.
• Because it is sticky, glycocalyx can help anchor of the cell in place
and also participates in the locomotion of specialized cells.
• Glycoproteins are recognized as normal (self) or abnormal
(foreign) by cells involved with the immune response. The
characteristics of the glycocalyx are genetically determined. For
example 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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