In this post you will find the detailed information about cell membrane structure in animal cell.
Cell membrane or biological membranes are thin quasified pliable film like structures that occur both extracellularly or around the protoplast of animal cell.
The following are the terms below regarding animal cell membrane which are discussed briefly.
- Membrane appearance
- Membrane composition
- Fluid mosaic model
- Functions of cell membrane
- Passive transport
- Active transport
- Bulk transport
- Pinocytosis and Phagocytosis
- Membrane Transport
- Sodium Potassium Exchange pump
The membrane is about 7.5nm thick. Because of this dimension, they are not visible under optical microscope. However, their presence was inferred long before their observation due to plasmolysis, bursting of animal cells placed in hypotonic solution.
Cell membranes are visible under electron microscope. Under the microscope, membrane looks like trilaminar means made of three layers. There is a middle electron layer which is transparent and covered by two electron dense layer on both sides of transparent layer. It contains number of particles with different sizes.
Membrane is made up of three types of component- proteins, carbohydrates and lipids. The lipids are generally phospholipids or phosphoglycerides. It consist of hydrophilic polar head and hydrophobic non-polar tail. Having the presence of both hydrophobic and hydrophilic, they are called amphipathic. Phospholipids form a bilayer in contact with water. Other lipids present in the membrane are sphingolipids, cerebrosides, gangliosides and cholesterol. Cholesterol provides stability and rigidity to the animal cell.
Membrane has number of proteins which functions as enzyme. E.g. ATP-ase helps in synthesis and breakdown of ATP, Cyt P-450 helps in hydroxylation. Receptor protein act as neurotransmitters to receive information.
Carbohydrates present in the membrane are oligosaccharides. The latter may be branched or unbranched. Glycolipids and glycoproteins are associated with the external surfaces of proteins and lipids.
Fluid Mosaic Model
It is the most accepted model of cell membrane given by Singer and Nicolson in 1972. According to fluid mosaic model, cell membranes have a quasified consistency. Lipid bilayer is continuous and having other lipids in it. e.g cholesterol is present in animal membranes. Proteins occur like a mosaic both in the interior of lipid bilayer as well as on the two surfaces. They are called as proteins icebergs in a sea of lipids.
Proteins often have a tendency to shift position.Extrinsic and Intrinsic are the two categories of membrane proteins.
It occurs inside the lipid bilayer passing to its different depths. Transmembrane proteins are embedded in the membrane of intrinsic proteins forming a channel to allow the water, ions and some small sized solutes. Some of the channels have gate mechanism to provide entry to specific substances. Intrinsic proteins develop hydrophobic interactions with phospholipid molecules. Because of this, intrinsic proteins cannot be easily separated from the membrane without disrupting.
It is also called peripheral proteins which are found on both the sides of membrane. These proteins are available in higher amount at inner surface of membrane than outer side of the membrane. They are also attached to the polar heads of intrinsic proteins with the help of weak electrostatic bond and ionic bonds. Therefore, peripheral proteins are easily displaced by sonication, mild detergents and hypotonic solutions with chelates.
Functions of Cell membrane
- Holding cell contents: It holds the semifluid protoplasmic content.
- Protection: It helps in protecting the cell from an injury.
- Organelles: It is also covers various organelles like nucleus, mitochondria, golgi apparatus, endoplasmic reticulum etc.
- Compartmentalization : it separates the cells from external environment and cell organelles from cytosol.
- Cell Recognition: Using glycolipids and glycoproteins , cell membranes can also distinguish between similar and dissimilar cells.
- Antigens: Antigens in cell membranes helps in determining blood grouping, immunological response, and transplant acceptance or rejection.
- Cell junctions: They help in keeping animal cell together.
- Microvilli: They have evaginations which helps in increasing absorption.
- Endocytosis: Membrane develops membrane vesicles to ingest the materials in bulk.
- Exocytosis: Membrane helps in excreting out the waste substances.
- Selective permeability: It allows the entry of only selected substances.
- Retentivity: Membranes are not leaky. They do not allow outward passage of absorbed substances.
- Electron transport: Electron transport chains occurs in the membrane of mitochondria.
- Osmosis: Membrane contains special water channels by which osmosis takes place.
- Carrier Proteins: It helps in active transport and located at the membranes.
- Membrane enzymes: Variety of enzymes are bound to the membrane to perform various biochemical activities such as hydrolysis and synthesis of ATP.
- Membrane Receptors: It contains different receptors for hormones, neurotransmitters, immunoglobulins and several other biochemicals.
It is the passage of biochemicals, metabolites and by-products across the biological membranes. They are of four types: passive transport, active transport, symport-antiport transport and bulk transport.
It is the movement of substances from one portion or area of a system to another without requiring the system to expend any energy, such as from the outside to the inside of a cell across the cell membrane.
Passive transport is more common along concentration gradient and electrochemical potential. Examples : Diffusion and Osmosis. Cell membrane possess narrow channels produced by tunnel proteins. They are of two types, aquaporins and ion channels. Aquaporins allows passage of water according to osmotic forces. Specific ion channel allows the ions to pass through the membrane. CO2 and O2 are believed to pass along with water.
It is the mode if transport across the cell membrane which involves expenditure of metabolic energy. Energy is mostly obtained from ATP.
Active transport is the most common method of some ion absorption in animal cells. These are Ca2+, K+,Na+, Fe2+, urate, many sugars and amino acids. Active transport usually occurs against concentration gradient. It is unidirectional and highly selective.
Cotransport allows the membrane to move the substances and other materials in symport (same direction) or antiport(opposite direction). It is sometimes called facilitated diffusion. For cotransport, carrier proteins possess extra binding sites. For example, glucose, nucleosides and some groups of amino acids pass inwardly along with Na+.
It is the mode of transport of large quantities of materials, micromolecules , macromolecules and food particles, across the membrane. Bulk transport involves chemical stimulation, folding and fusion of membrane to produce vesicles called carrier vesicles. The transport occurs both inwardly and outwardly.
Endocytosis and Exocytosis are two types of bulk transport.
It allows cells to internalize extracellular materials by means of carrier vesicles, endosomes. Endocytosis is further are of two kinds, pinocytosis and phagocytosis.
It transports the materials from inside the cells to the outside with the help of carrier vesicles called exocytotic vesicles. Secretory or excretory products are packed in vesicles by Golgi bodies.
Frequently Asked Questions
Q1.Difference between Pinocytosis and Phagocytosis.
Pinocytosis is the intake of extracellular fluid, ions and molecules by means of small vesicles or pinosomes developed through folding of cell membrane. Lysosomes may get involved in the digestion of extracellular substances.
Phagocytosis is the cell eating of large particles such as microbes, cell debris, degenerating cells etc. Pseudopodia like projections are formed for engulfing the solid materials. Lysosomes are essential for destroying and digestion of solid food materials. The undigested substances are excreted by means of ephagy or exocytosis.
Q2.Describe the mechanism of Active transport.
Active transport is mediated by carriers or transport proteins. They are integral proteins particles of the cell membrane that have the affinity for specific solute particles. A carrier combines with the solute particle on the surface and forms carrier solute complex. The complex gains energy from ATP and undergoes conformational change. In the complexed state the carrier particle transports the solute to the other side of the membrane where it is released. The carrier is freed and reverts to its original state for binding with another solute particle.
Q3.Give a brief account on Sodium Potassium Exchange pump.
It operates in most of the body cells. Large α-subunit and a small β-subunit forms the carrier proteins. The inner side of the larger subunit has three receptor sites for Na+ ions and one site for ATP-ase activity. The outer side of this subunit has two receptor sites for K+ ions. The carrier binds with three Na+ ions from cell interior and two K+ ions from extra-cellular fluid. With the help of energy obtained from ATP, the carrier undergoes conformation change, throws three Na+ ions into the extra-cellular fluid and two K+ ions into the cell interior.
The Na+-K+ exchange pump helps to maintain a higher concentration of Na+ ions in extracellular fluid than in intracellular content. In comparison to extracellular fluid, intracellular K+ concentration is greater. The Na+-K+ exchange pump is responsible for nerve cells’ resting potential. It also encourages active secondary transport.
To wrap up this post, we conclude that animal cell membrane is thin in structure but separates the cell components from the outer environment. It also allows the transfer of different substances from the membrane making it semi-permeable in nature due to the presence of various proteins and lipids.