Structure and Functions of Cell Membrane
Cell membrane also called plasma membrane or Biomembrane.
Bio membrane separates the interior of the cell from the outside
environment of cell.
The Biomembrane consists of a Phosphate lipid bilayer that is semi-permeable.
The cell membrane regulates the transport of materials entering into the
cell and exiting the cell.
Introduction - Plasma membrane
1. Cell membrane is a dynamic, quasi- fluid. It is protective thin film like partition
which separates inside and outside the cytoplasm of the cell.
2. Thickness of cell
membrane visible only under electron microscope is about 70-100 Å, Plasma membranes thickness range
from 5–10 nm.
3. Prokaryotic cells don’t have internal membranous partition. They have
only extracellular membrane.
4. In Eukaryotes cell inside the cytoplasm, cell membrane covers various cell organelles like nucleus, endoplasmic reticulum mitochondria,
lysosomes, , golgibodies etc.
5. Both extracellular and intracellular membrane of the cell referred term
as bio-membranes or biological membranes.
6. In prokaryotic cell, cell membrane bears respiratory enzyme. Respiratory enzymes are located on plasma membrane.
7. Eukaryotic cell, cell membrane
lacks respiratory enzyme.
8. In Eukaryotic cell membrane lacks the respiratory chain activity.
9. In Eukaryotic cell membrane lacks the glycolytic activity.
8. Chemically cell membrane primarily composed of –
a. Proteins; 60-75%
b. Lipids; 20-40%
c. Carbohydrates; 1-5%
9. Cell membrane are renewed, formed, deformed, folded and extended.
Cell Membrane Proteins
Proteins embedded within the phospholipid bilayer. Proteins are form
major portion of Biological membrane-
1. Structural proteins
2. Carrier proteins
3. Enzyme
1. Structural Proteins
1. Structural proteins are Lipophilic; fat friendly.
2. Structural proteins form the backbone
of cell membrane and,
3. Structural proteins provide to plasma membrane elasticity and
mechanical stability
2. Carrier Proteins
Carrier Proteins transport materials against concentration gradients means from an area of low concentration to an area of high concentration.
Carrier Proteins transport materials by active process.
During active transport, the flow of substances moves against
the concentration gradient.
Types of protein in Plasma Membrane
Based on protein location in plasma membrane, membrane proteins are
differentiated into two main groups-
1. Extrinsic proteins or peripheral proteins
2. Intrinsic proteins or Integral proteins
1. Extrinsic Proteins or Peripheral
1. Extrinsic proteins are peripheral proteins
2. It present on outer and inner surface of phospholipid bilayer
3. It serves in transport of molecule and as receptor for hormone
4. It involve in cell recognition
5. Peripheral protein permits interaction with surrounding and with
polar surface of lipid.
6. Peripheral proteins are loosely associated with polar heads of lipid
molecule.
2. Intrinsic proteins or Integral
1. Intrinsic proteins are integral proteins
2. Integral proteins penetrate partially or wholly with their
polar head from surface of membrane
3. Intrinsic membrane proteins have amino acids with non polar side
chains and are attached either to glycoproteins or to phospholipids
3. Enzymes
Enzymes are catalytic proteins which regulate cell activities.
3. Lipids
Important Bio membrane lipids are-
a. Phospholipids
b. Sterol, Cholesterol
c. Glycolipids
d. Sphingolipids
e. Cerebrosides
1. Lipids form structure
framework and fluid membrane
2. Lipid form permeable barrier to ions and polar molecules
3. Membrane lipids are both hydrophilic and hydrophobic ends referred as
Amphipathic molecules.
Hydrophilic polar ends - Water loving
in form of phosphate head and directed towards the outside.
Hydrophobic non-polar ends - Water
repelling contains two tails of fatty acids which usually occur towards centre,
inner side of membrane.
The polar (Hydrophilic) portion consists of a phosphate group and glycerol, while the nonpolar (Hydrophobic) portion consists of fatty acids.
4. Carbohydrates presents in the cell membrane are Oligosaccharides, example;
Hexose
Carbohydrates inform of two combinations, confined the external surface
–
a. Glycolipids – Carbohydrate attached to lipids
b. Glycoproteins- Carbohydrate attached to
proteins
Structure of Cell Membrane
Fluid Mosaic Model
Fluid Mosaic Model given to explain the
molecular structure of cell membrane.
The Fluid Mosaic Model of Biological membrane Proposed by Singer
and Nicolson in 1972.
Principle Components of Structure of Fluid Mosaic Model
a. Phospholipids
b. Cholesterol
c. Proteins
d. Carbohydrate
1. According to fluid mosaic model, cell membrane are quasified structure; partially fluid
or partially solid, in which lipids
and proteins are arranged in a mosaic manner.
2. Intrinsic proteins are arranged in mosaic manner instead of rigid
structure.
3. Lipid molecule arrange in two layer and protein molecules embedded
among lipid molecule.
4. There is lipid bilayer in which protein molecules are embedded.
5. Lipid bilayer gives Cell membrane fluidity (CMF; substance
to flow easily and elasticity to membrane.
6. Globular Proteins
Globular proteins are spherical shape or globe shaped.
Fluid mosaic model describe the cell membrane
as “protein icebergs in the sea of phospholipids”. Proteins are able to move
within lipids bilayers.
Globular proteins are two types –
1. Extrinsic or peripheral membrane protein
2. Intrinsic, integral membrane protein
Integral Membrane Proteins
Tunnel protein
Tunnel protein are intrinsic protein, it embedded within phospholipid
bilayer. Tunnel proteins are very large molecule lie throughout phospholipids
matrix.
Tunnel proteins project on both structure and have channels for passage
of water soluble material.
Carrier protein
Carrier protein are intrinsic
protein, embedded within phospholipid bilayer. It transport both water soluble
and insoluble material by active transport.
During active transport, substances move against
the concentration gradient means from an area of low concentration to an area of
high concentration. This process called “active” because it requires the use of
energy (usually in the form of ATP).
Peripheral Proteins
It lies on the inner or outer surface of phospholipids bilayer.
7. Carbohydrate
Generally attached to the outside of membrane layer
8. Cholesterol
Attach between two phospholipids bilayer
a. Phosphate head – polar, hydrophilic
b. Fatty acid tails - non-polar, hydrophobic
9. Phospholipids molecule
Phospholipids molecules composed of amphipathic i.e Hydrophilic, Hydrophobic
nature.
1. Hydrophilic – water loving areas of those
molecules are in contact with aquous fluid both inside and outside of cell,
tend to be polar.
2. Hydrophobic – water repelling molecules,
tend to be non – polar.
Fluid mosaic model explains dynamic and functional nature of membrane.
Cell membranes expand and contract during cell movement and during
change in shape.
Cell membrane grows with growth of cell and sub- cellular organelles.
Checkout here difference between Osmosis and Diffusion
Functions of Cell Membrane/ Bio Membrane
Key functions of Bio Membrane are -
1. Compartmentalization
2. Selectively permeable
3. Protect cell from injury
4. Have carrier protein for active transport
5. Bound semi fluid content of cell
6. Provide organic connection between adjacent cell
7. Help in movement of material
8. Ability to recognized antigen
9. Hormone receptor
10. Endocytosis and Exocytosis function
11. Responsible for communication with environment
12. Responsible forming connection between cell
13. Responsible for controlling biochemical reaction
14. Responsible for selectively exchange substance
1. Compartmentalization
Main functions of bio membrane is
compartmentalization.
2. Protect from cell injury
Plasma membrane protects the cell from injury
3. Bounds semi fluids content
Plasma membrane founds semi-fluids content of cell.
4. Selectively permeability
Plasma membrane are selectively permeable. They allow only selected
substances to enter or leave the cell.
5. Flow of material
It helps to regulating the flow
of materials into and out of the cell.
6. Organic connection
Bio membrane provides organic
connection between adjacent cells.
7. Determine Antigen specificity
Substance attach to cell membrane surface determine antigen specificity.
8. Active Transport
Cell membrane has carrier protein for active transport
9. Endocytosis
Plasma membrane infolds are used for intake of material, substance
brought into cell.
Used for intake of materials, substance brought into cell by plasma
membrane.
10. Exocytosis
Plasma membrane are used for transport molecule out of cell such as secretory, excretory and waste product
are thrown out of cells.
11. Helps in cell movement
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