BIOLOGY ……………………………………………………………………..……………………………………. Dr. Fakhir
Lec. No. 2 ……….. Cell Membrane
Introduction::The 'cell membrane' (also known as the plasma membrane, cytoplasmic
membrane, or plasmalemma) is a biological membrane that separates the interior of
all cells from the outside environment. The cell membrane is selectively permeable to
ions and organic molecules and controls the movement of substances in and out of
cells. It consists of the phospholipid bilayer with embedded proteins. Cell membranes
are involved in a variety of cellular processes such as cell adhesion, ion conductivity
and cell signaling and serve as the attachment surface for several extracellular
structures, including glycocalyx, and intracellular cytoskeleton.
Lipid bilayer: The fundamental part of the cell membrane structure is the lipid
bilayer. Types of lipids present in the plasma membrane are phospholipids,
cholesterol, and glycolipids.
Membrane Proteins: Another key component of the plasma membrane is protein,
which help in selective transport of the macromolecules like sucrose, amino acids,
and ions. They remain embedded in the lipid layer. Based on the actual location of
proteins with reference to the phospholipid bilayer, two types of proteins are
identified.
1- Integral membrane proteins are intensely attached to the lipids of the bilayered
structure. Those integral proteins that traverse the phospholipid bilayer are
called transmembrane proteins.
2- Peripheral membrane proteins are indirectly or loosely attached to the
membrane. They are non-covalently connected with the lipids or ends of the
integral proteins.
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BIOLOGY ……………………………………………………………………..……………………………………. Dr. Fakhir
Carbohydrates: In addition to phospholipids and proteins, the cell membrane also
consists of carbohydrates, basically glycoproteins and glycolipids. These molecules
are exclusively arranged in the outer side of the cell membrane attaching to the
proteins or phospholipids, so the carbohydrate portions are exposed to the external
surface of the cell.
The Fluid-Mosaic Model of the Cell Plasma Membrane
The plasma membrane that surrounds these cells has two layers (a bilayer) of
phospholipids (fats with phosphorous attached), which at body temperature are like
vegetable oil (fluid). And the structure of the plasma membrane supports the old
saying, “Oil and water don’t mix.”
Each phospholipid molecule has a head (composed of glycerol and phosphates) that
is attracted to water (hydrophilic: hydro = water; philic = loving) and a tail (which
composed of fatty acids chains) that repels water (hydrophobic: hydro = water;
phobic = fearing). Both layers of the plasma membrane have the hydrophilic heads
pointing toward the outside; the hydrophobic tails form the inside of the bilayer.
Because cells reside in a watery solution (extracellular fluid), and they contain a
watery solution inside of them (cytoplasm), the plasma membrane forms a circle
around each cell so that the water-loving heads are in contact with the fluid, and the
water-fearing tails are protected on the inside.
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BIOLOGY ……………………………………………………………………..……………………………………. Dr. Fakhir
Proteins and substances such as cholesterol become embedded in the bilayer, giving
the membrane the look of a mosaic. Because the plasma membrane has the
consistency of vegetable oil at body temperature, the proteins and other substances
are able to move across it. That’s why the plasma membrane is described using the
fluid-mosaic model.
Diffusion and Transport Across Cell Membranes
Nutrients must enter the cell and waste products have to leave in order for the cell to
survive. For this and many other reasons, it is crucial that membranes be selectively
permeable. For example, the movement of ions across membranes is important in
regulating vital cell characteristics such as cellular pH and osmotic pressure.
Membrane permeability is also a key determinant in the effectiveness of drug
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BIOLOGY ……………………………………………………………………..……………………………………. Dr. Fakhir
absorption, distribution, and elimination. For example, a drug taken orally that targets
cells in the central nervous system must cross several membranes: first the barrier
presented by the intestinal epithelium, then the walls of the capillaries that perfuse the
gut, then the blood-brain barrier. Some endogenous substances and many drugs easily
diffuse across the lipid bilayer. However, the lipid bilayer presents a formidable
barrier to larger and more hydrophilic molecules (such as ions). These substances
must be transported across the membrane by special proteins.
Types of movement across membranes include:
Molecules can diffuse across a membrane down a concentration gradient without the
aid of a protein or the input of energy. Passive and active transport require membrane
transport proteins. Channel proteins carry out passive transport, but carrier proteins
can carry out passive or active transport. Active transport, or movement against a
concentration gradient requires energy. In brief, there is no need for energy in the
case of simple diffusion and passive transport, while active transport need for energy.
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BIOLOGY ……………………………………………………………………..……………………………………. Dr. Fakhir
Endocytosis/exocytosis. Large macromolecules like proteins, polysaccharides,
lipoprotein particles require more complex mechanisms to traverse membranes, and
are transported into and out of cells selectively via endocytosis and exocytosis (also
known as bulk transport). Interestingly, endocytosis and exocytosis are not only
important for the import/export of large molecules. Often, essential small molecules
that are hydrophobic or toxic (e.g., iron) travel through the bloodstream bound to
proteins, which enter and exit cells via these mechanisms.
In exocytosis, materials are exported out of the cell via secretory vesicles. In this
process, the Golgi complex packages macromolecules into transport vesicles that
travel to and fuse with the plasma membrane. This fusion causes the vesicle to spill
its contents out of the cell. Exocytosis is important in expulsion of waste materials
out of the cell and in the secretion of cellular products such as digestive enzymes or
hormones.
Endocytosis, on the other hand, is the process by which materials move into the cell.
There are three types of endocytosis: phagocytosis, pinocytosis, and receptormediated endocytosis. In phagocytosis or “cellular eating,” the cell’s plasma
membrane surrounds a macromolecule or even an entire cell (solid structures) from
the extracellular environment and buds off to form a food vacuole or phagosome. The
newly-formed phagosome then fuses with a lysosome whose hydrolytic enzymes
digest the “food” inside.
In pinocytosis or “cellular drinking,” the cell engulfs drops of fluid by pinching in
and forming vesicles that are smaller than the phagosomes formed in phagocytosis.
Like phagocytosis, pinocytosis is a non-specific process in which the cell takes in
whatever solutes that are dissolved in the liquid it envelops.
Unlike phagocytosis and pinocytosis, receptor-mediated endocytosis is an extremely
selective process of importing materials into the cell. This specificity is mediated by
receptor proteins on the surface of the plasma membrane. In receptor-mediated
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BIOLOGY ……………………………………………………………………..……………………………………. Dr. Fakhir
endocytosis, the cell will only take in an extracellular molecule if it binds to its
specific receptor protein on the cell’s surface. Once bound, the coated pit on which
the bound receptor protein is located then invaginates to form a coated vesicle.
Similar to the digestive process in non-specific phagocytosis, this coated vesicle then
fuses with a lysosome to digest the engulfed material and release it into the cytosol.
Mammalian cells use receptor-mediated endocytosis to take cholesterol into cells.
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