Essay on the Membrane Transport system of Structural Cell

Every living cell must acquire from its surroundings the raw materials, needed for biosynthesis of organic matter and for energy production.

It must release to its environment the byproducts of metabolism. Plasma membrane being selectively permeable controls the inflow and outflow of materials in and out of the cells.

The transport across the membrane occurs broadly in three ways: Passive transport, active transport and bulk transport.

Passive transport: This is energy independent mode of transport where the cell never spends any energy for transporting substances in and out of the cell.

Here the transport occurs along a concentration gradient and continues till equilibrium is reached. This can be of two types: simple diffusion and facilitated diffusion.

(i) Simple diffusion: In this mode no membrane protein is involved in transport. The increase in substrate concentration on one side of membrane increases the flux across the membrane. Water, oxygen, nitrogen and methane arc transported by simple diffusion.

(ii) Facilitated diffusion: Membrane bound proteins called permeases are involved in this transport. The transport occurs along the concentration gradient till equilibrium is reached. Increasing the substrate concentration on one side of the membrane will increase the flux to some extent but the flux will remain constant even if substrate concentration is increased.

This is due to the fact that all the permeases are saturated with substrates after a point. Glucose permease is an example of the facilitated diffusion system. This system shows substrate specificity.

The glucose permease is specific to D-glucose and for L-glucose the flux reduces drastically. Besides there is a family of integral proteins, the aquaporins, that provides channels for rapid movement of water molecules across the membrane

(iii) Active transport: This transport occurs against a concentration gradient and results in accumulation of solutes on one side of the membrane. As solutes can be transported from lower to higher concentration this is also referred to as uphill transport and solutes are said to be pumped in or pumped out.

This is a thermodynamically unfavorable (endergonic) process and occurs only when coupled directly or indirectly to an exergonic or energy yielding process like absorption of sunlight, an oxidation reaction or ATP break down. Active transport is of two types. In primary active transport the uphill transport is coupled directly to an exergonic reaction like ATP hydrolysis, substrate oxidation or photo excitation.

The examples of primary active transport are H⁺ ATPase pump in plant membrane (thylakoid), Na-K ATPase pump of animal plasma membrane. In secondary active transport the uphill transport (endergonic) of a solute is coupled to downhill transport (exergonic) flow of another solute previously pumped in by primary active transport.

Here the energy source is Ion gradient. Examples of secondary active transport are transport of Amino acids and sugars (Na²-driven) in intestine, lactose (H⁺-driven) in bacteria.

The transport system in biological membrane can be uniport, transporting a single substrate at a time or cotransport system, transporting two substrates simultaneously. When two substrates move in same direction simultaneously the process is symport and when two substrates move in opposite direction simultaneously the process is antiport. Glucose permease is a Uniport system while chloride-bicarbonate exchanger on erythrocyte membrane is an antiport system. Bulk Transport

This occurs mainly through membranous vesicles also called carrier vesicles. The formations of these vesicles depend upon chemical stimuli. These vesicles enclose the material to be transported and act in response to chemical stimuli to release the transport materials in their appropriate destination.

When these vehicles transport materials out of the cell, the process is called exocytose and the inward transport is called endocytosis. The exocytose vesicles formed in the cytoplasm move to the inner face of plasma membrane and fuse with it, then releasing its content to outside.

Similarly the endocytosis vehicles are formed by invagination of plasma membrane enclosing the substances from outside the cell. The vehicles pinch off the membrane and move to cytoplasm. Endocytosed vesicles may fuse with primary lysosomes if digestion of the transported substances is needed.

Another phenomenon of bulk fluid transport is known as pinocytosis. This is also known as ‘cell drinking’. Here vesicles of 100-200 nm diameters are formed by the invagination of cell membrane enclosing fluid and substances like sugars, amino acids dissolved in it. Similarly phagocytosis is called ‘cell eating’. It is the transport of solid matters like food, foreign particles and pathogens across the membrane. Cellular Movement

The cell shows various kinds of movements like:

1. Ciliary movement: This movement of cell is due to the action of cilia or flagella. This movement is responsible for locomotion in free living organisms like chlamydomonas and reproductive units like zoospores and gametes.

2. Amoeboid movement: This is a creeping movement by naked mass of protoplasm through the formation of cytoplasmic projections called pseudopodia. This movement is observed in some slime molds and few asexual aplanospores.

3. Cytoplasm streaming Movement:

It is a vital movement occurring in the cytoplasmic matrix of eukaryotic cells. This movement is not found in prokaryotic cells. This movement is also known as cyclosis and it is of two types’ rotation and circulation.

In rotation the cytoplasm matrix continuously move in one direction either clockwise or anticlockwise around a central vacuole, e.g., Hydrilla leaf, Vallisnaria cells and Chara cells. In circulation different regions of cytoplasm matrix move around vacuoles in different directions, e.g., stamina hairs of Tradescantia shoot hairs of Begonia etc.

The rate of cyclosis depends upon the viscosity of cytoplasm, metabolic rate and of respiration, influence of drugs, poisons, hormones, and temperature.