|Table of contents|
|Cell Membrane or Plasma Membrane or Plasmalemma|
|Difference between Diffusion and Osmosis|
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Each cell (prokaryotic as well as eukaryotic) is surrounded by a covering called plasma membrane or plasmalemma or cell membrane. Most cell organelles in eukaryotic cells (e.g., Mitochondria, Plastids, Golgi apparatus, Lysosomes, Endoplasmic reticulum, Peroxisomes, Vacuoles etc). are enclosed by subcellular unit membranes. These membranes, thus, compartmentalise the cell.
Plasma membrane is a living, ultra-thin, elastic, selectively permeable membrane. Chemically, it is composed of phospholipids, proteins, oligosaccharides and cholesterol.
Trilamilar or 3-layered Structure
J.D. Robertoson noted trilamilar or 3-layered structure for all membranes he studied. Based on his findings, he proposed the 'unit membrane hypothesis' in 1959.
Fluid Mosaic Model
In 1972, S.J. Singer and G. Nicolson proposed fluid mosaic model to explain the structure and functions of plasma membrane. According to this model, the plasma membrane is made up of a phospholipid bilayer and two types of protein molecules 'floating about' in the fluid phospholipid bilayer.
The two types of proteins are:
(i) Intrinsic proteins which are embeded in the phospholipid matrix incompletely or completely, and
(ii) Extrinsic proteins which occur superficially either on the outer surface or on the inner surface of the phospholipid layer.
In other words, the membrane is a viscous fluid with phospholipids and protein molecules arranged as a mosaic.
Oligosaccharide molecules are present on the exposed surface of the plasma membrane. They are associated with proteins as well as lipid molecules forming glycoproteins and glycolipids respectively. Cholesterol molecules are inserted between the phospholipid molecules of plasma membrane of animal cells to stabilize the membrane.
Presence of lipids and proteins provides flexibility to the plasma membrane. Proteins present in the membrane serve as:
Fluid Mosaic Structure of the Plasma Membrane
(i) Enzymes catalysing chemical reactions within the membrane.
(ii) Transport proteins (permeases) for movement of water soluble ions.
(iii) Pumps for active transport of materials and
(iv) Receptor proteins (e.g., glycoproteins on the cell surface) to recognize and bind specific molecules such as hormones.
Fluid mosaic model is also described as "a number of protein icebergs floating in the sea of lipids'.
(i) Impermeable membrane: If the membrane does not allow passage of substances (solvent and solute) through it.
(ii) Permeable membrane: If the membrane allows free passage of solute and solvent through it.
(iii) Semipermeable membrane: If the membrane allows passage to solvents but prevents the passing of solutes.
(iv) Selectively permeable membrane: If the membrane allows the passage of solvent and few selected solutes.
Semipermeability ensures that
1. The useful molecules enter the cell,
2. The metabolic intermediates remain within the cell and
3. The secretions and wastes leave the cell.
Thus, semipermeability of cell membranes enables the cell to maintain homeostasis, i.e., a constant internal environment inspite of the changes outside it.
The substances generally drawn in the cell include :
(i) Raw materials for metabolism, viz. food stuffs, water, salts and oxygen; and
(ii) Regulatory substances, e.g., vitamins and hormones.
The substances generally turned out of the cells include :
(i) The products of metabolism, namely, nitrogenous wastes and carbon dioxide; and
Following mechanisms are involved in the entry or exit of various materials across p.m.
(A) Physical processes. (B) Biological processes.
A. Physical Processes: These processes are slow and do not expend energy. These occur down the concentration gradient and do not use carrier proteins. Physical processes include. (i) Diffusion, (ii) Osmosis.
B. Biological processes: These processes are rapid and often use energy in the form of ATP. These can occur down as well as against the concentration gradient and often use carrier proteins. Biological processes include:-
1. Mediated transport
(i) Facilitated transport / diffusion (ii) Active transport
2. Endocytosis (Pinocytosis and Phagocytosis)
The process by which a substance uniformly spreads into another substance by random movement of its particles from a region of higher concentration to a region of its lower concentration due to their kinetic energy is called diffusion.
It is faster in gaseous phase than in liquid phase or solid phase.
(i) Diffusion helps in the distribution of various substances throughout the cytoplasm of the cell without much delay.
(ii) It helps in the exchange of respiratory gases (oxygen and carbon dioxide) between the body cells and their environment.
(iii) Various materials such as gases, liquids and solids dissolve in the medium, i.e., air or liquid by diffusion.
(iv) Loss of water in vapours form from the aerial parts of the plants (transpiration) occurs through diffusion.
(v) Flowers of plants spread aroma through diffusion. It attracts insects and other animals for pollination.
The diffusion of water or solvent through a semipermeable membrane from a solution of lower concentration of solutes to a solution of higher concentration of solutes to which the membrane is relatively impermeable, is called osmosis.
(1) Endosmosis: It is the entry of water molecules into the cells through semipermeable plasma membrane when surrounded by hypotonic solution.
(2) Exosmosis: It is the exit of water molecules from the cells through semipermeable plasma membrane when surrounded by hypertonic solution.
Requirements: Funnel fitted with a semipermeable membrane, beaker, sugar solution, water.
Procedure: Take sugar solution in a funnel fitted with a semipermeable membrane (fish bladder or egg membrane) upto mark 'A' and place it in an inverted position in a beaker filled with clean water as shown in figure. After some time, observe the level of sugar solution in the funnel.
Result: You would find that the sugar solution has risen from level 'A' to a new level 'B'.
Explanation and Conclusion: Sugar solution in the funnel and water in the beaker are separated by a semipermeable membrane. The fitted membrane is permeable to small water molecules but is relatively impermeable to large sugar molecules dissolved in water. Due to difference in the concentration of solute on the two sides of semipermeable membrane, water molecules have moved from the solution having lower concentration of solutes (e.g., water in this experiment) to the solution having higher concentration of solutes [e.g. sugar solution] due to osmosis has risen to new level 'B'.
1. Isotonic Solution
Isotonic solution is one in which the concentration of water and solutes is the same as in the cytoplasm of the red blood cells. 0.9% salt solution and 5% glucose solution are isotonic for red blood cells.
2. Hypotonic Solution
Hypotonic solution is one in which the concentration of solutes is less and concentration of water is more as compared to inside the red blood cells. 0.66% salt solution and 0.2% glucose solution are hypotonic for red blood cells.
3. Hypertonic Solution
Hypertonic solution is one in which the concentration of solutes is more and the concentration of water is less as compared to in the cytoplasm of the red blood cell. 1.25% salt solution and 10% glucose solution are hypertonic for red blood cells.
1. Fresh water unicellular organisms (e.g., Amoeba, Paramecium) continuously gain water in their bodies due to osmosis. These organisms have mechanisms (e.g., contractile vacuoles) to throw out excess of water from their bodies.
2. Most plant cells have the tendency to gain water due to osmosis.
3. Absorption of water by the plant roots from the soil through root hairs is also an example of osmosis.
4. Certain plant movements (e.g., seismonastic movements in 'touch-me-not' plant) occur due to loss or gain of water.
5. Stomata are present in the leaves. They open and close at different times of the day due to osmotic movements of water.
6. In plants, cells, tissues and soft organs (leaves, young shoots, flowers) maintain turgidity or stretched form due to osmotic absorption of water.
Diffusion can occur both in air and liquid (water)
Osmosis occurs only in liquid medium
It involves movement of molecules (Solids, liquids or gases) from the region of their higher concentration to the region of their lower concentration.
It involves movement of solventmolecules only from the region of their higher concentration to the
It can occur without or through a semipermeable
It always takes place through a
It equalizes the concentration of diffusable molecules throughout the medium.
It does not equalize the concentration
It is dependent upon the kinetic energy of the molecules of diffusing substance only.
Though it is the diffusion of solvent
Type of transport of materials across the plasma membrane with the help of carrier proteins is called mediated transport.
In this case, transport proteins (e.g. permeases) assist molecules to diffuse through the membrane down the concentration gradient, i.e., from the region of higher concentration to the region of lower concentration across the membrane. It is, therefore, also termed as facilitated diffusion. No cellular energy is used in such transport. A carrier protein combines with a specific substance (e.g., glucose) to be transported and moves it down the concentration gradient from one side of membrane to another through a channel formed by it.
In liver and red blood cells, facilitated transport moves glucose across the cell membrane by specific carrier protein molecule in both directions, depending upon whether glucose concentration is higher inside or outside the membrane.
In this case, carrier proteins move substances against the concentration gradient, i.e., from lower concentration to higher concentration. This "uphill" transport involves work and always requires energy provided by ATP (adenosine triphosphate).
Mechanism of Active Transport of Materials :
(i) The carrier protein has a binding site for ATP in addition to the binding site for the substrate. As the ATP molecule binds to the carrier protein, it is hydrolyzed to ADP.
(ii) The energy so set free brings the substrate binding site of the carrier protein to the surface of the membrane. The substrate present in the medium joins the carrier protein at substrate binding site to form carrier-substrate complex.
(iii) The substrate bond carrier protein undergoes conformational change and carries the substrate through a channel in it to the cytoplasmic side of the membrane.
(iv) Now, the form of binding site changes and the substrate is released. The carrier protein regains its original form and is ready to transport another molecule of substrate.
There are many active transport systems in the cell. Among these, sodium-potassium exchange pump is prominent. It maintains sodium and potassium gradients between cells and the surrounding extracellular fluid.
Importance of Active Transport
The Na K exchange pump plays following roles :
(i) It helps in maintaining a positive charge on the outside of the membrane and negative charge on the inside (resting potential),
(ii) It helps in nerve impulse conduction,
(iii) It helps in muscle contraction,
(iv) It helps in urine formation in kidney tubules,
(v) It helps in salt excretion in marine birds, and
(vi) It helps in controlling water contents of the cell.
Animal cells can also actively take in and turn out materials in masses much larger than in the hither to described processes by utilizing energy. Such materials include macromolecules, lipid droplets and solid particles. Items of this size cannot cross the phospholipid bilayer by diffusion or with the help of transport proteins. Special processes are involved in the transport of such large quantities of materials.
These include endocytosis (phagocytosis) and exocytosis.
The term endocytosis refers to invagination of a small region of plasma membrane, and ultimately forming an intracellular membrane-bound vesicle. Endocytosis is not shown by plant cells because of their rigid cell wall and internal turgor pressure. Depending upon the intake of fluid droplet or solid particles, endocytosis is of two types :
The non-specific intake of a tiny droplet of extracellular fluid by a cell through the cell membrane which cannot otherwise pass through it. It is also, therefore, termed as cell drinking. It was first observed in Amoeba. In this process, a small region of plasma membrane invaginates and the fluid droplet passes into the pocket so formed. This pocket is called caveola. The pocket deepens and finally
nips off as a fluid-filled vacuole called pinosome or pinocytotic vesicle.
Phagocytosis is the intake of solid particles by a cell through cell membrane. It is also
called cell eating. Phagocytosis is the major feeding method in many unicellular organisms (e.g., Amoeba) and simple metazoa (e.g., sponges).