Transport Across Biomembrane:

Biomembranes regulate transport across  cellular boundaries. This enable the cell to exist as open system. The fluid Mosaic model helps explain how biological membranes regulate the transport of molecules into and out of cell and cell organelles. 


All cells, to grow and function, take up and turn out materials through the cell membrane. The substances drawn in included 
(i) raw materials for metabolism, viz., foodstuffs, water,salts and oxygen; and

(ii) regulatory substances e.g., vitamins and hormones .These are taken from the environment __ outer world, neighbouring cells or body fluids. The materials turned out are  the products of metabolism, namely, nitrogenous wastes, secretions and carbon dioxide. The materials having small molecules pass through the cell membrane by physical processes whereas those having large molecules pass by biological processes. Intake of materials is known as absorption. Unwanted and harmful materials are kept out by the cell membrane. The cell membrane, thus, carries on  selective absorption, and is said to be selectively permeable or semipermeableHowever, the substances that pass through the biomembrane do so at different rates.

         The membranes of the cell organelles are also semipermeable. They enable the cell organelles to have environment different from that in the surrounding cytosol.

Advantage of Semipermeability:

Semeipermeability ensures that the useful molecules enter the cell, the metabolic  intermediates remain within the cell, and secretions and wastes leave the cell. This, the Semipermeability of cell membranes enables the cell to maintain homeostasis, i.e., a constant internal environment inspite of the changes outside it. 

Disadvantages of permeability:

It is good that the membranes of the cells and the cell organelles are not permeable. If they were, any material around them could enter the cell, and the cell could not remain a unit separate from its environment. 

1. Physical Processes:

The physical processes which move the materials are the cell membrane are  Diffusion, dialysis, osmosis and filtration. 

1. Diffusion 

Diffusion is a basic law of physics but it hold true for biological systems as well. 

Molecules Motion ( Brownian Movement).

Molecules are in constant random motion in all kinds matter. This phenomenon is called  Brownian movementafter an English Robery Brown ( 1773__ 1858) who first described it in 1827. The differences  between the states of matter __ solid, liquid and gas __ result from the degree of movement of their constituent molecules. In a solid, the molecules are very closely packed and the force of attraction between the molecules is strong.This allows the molecules to vibrate in a very limited space, but not to move around. They known into each other constantly, much like the passengers in a crowded bus. In a liquid,  the molecules are somewhat farther apart and the intermolecular force is weak. Therefore, the molecules move about with considerable freedom.In a gas, the molecules are very far apart and the intermolecular force is negligible. With the result, the molecules move about very freely and can be kept in place only by external barriers. Molecular movement in the three states of matter is due  to the intrinsic kinetic energy, called thermal motion, of the molecules. This energy can be increased by heating, and this will change matter from one state to another. For, instance, ice, on heating, become water, which, on further heating, changes into vapour or steam. In any substance, some molecules move faster than others. 


If a cube of sugar is placed in a beaker of water, the sugar will dissolve .In this process, the sugar molecules move out in water, collide¹ with one another or sides of the beaker, rebound² and move in new directions. By this random movement, the sugar molecules finally become uniformly distributed throughout the water in the beaker.This can be tested by tasting drops of water taken from different parts of the beaker. The sugar molecules continue to move about even after their uniform distribution in water. Now, however,  as some molecules shift from left to right, others move from right to left at the same rate. With the result, their uniform distribution is maintained. 

          If a crystal of copper sulphate is placed in a beaker containing water, copper sulphate molecules diffuse out throughout water, turning it bluish. Diffusion can be seen as a stream of spreading colour.
        Similarly, if a strong smelling gas, such as ammonia, is released in a corner of a room, its molecules quickly spread out in the air of the room, and the gas can be smelt in the farthest corner.

The natural tendency of a substance to uniformly spread in another by random movement of its particles, molecules or icons from a region of higher concentration ( more molecules per unit volume) to a region of lower concentration   ( fewer molecules per unit volume) due to their kinetic energy is called  diffusion.

Independence of Diffusion:

Diffusion of one substance is independent of the diffusion of another substance provided the two do not react.
1. Stricke
2. Move back.

Factors Affecting Rate of Diffusion.

The rate of diffusion is affected by many factors. It is directly proportional to 
(i) the concentration of the diffusing molecules, 
(ii) the temperature of the medium,  and 
(iii) the cross_ sectional area of the diffusion pathway.It is inversely proportional to 
(i) the molecular weight of the diffusing molecules, 

(ii) density of medium, and 

(iii) the distance over which the molecules are to diffuse.

Diffusion through Membranes. 

Diffusion can also occur through membranes __ permeable or semipermeable. A permeable membrane allows all types of molecules to pass through it.A semipermeable membrane allow some molecules of the  lipid  bilayer, or through special channels in protein molecules or channels linked  by  protein molecules. Diffusion through a semipermeable membrane can be illustrated with a simple experiment. 

      Take a beaker fitted with a semipermeable membrane at its middle.Put 15% salt solution in part A of the beaker and pure water in the part B. The molecules of both salt ( solute) and water ( solvent) are in constant, rapid, random movement and frequently collide with one another. Through each molecule moves randomly, yet the diffusion of a population of molecules is directional. Due to this motion, salt molecules pass or diffuse through the membrane into the part B. Water molecules diffuse in both the part A, because  of their higher concentration in the former ( 100%) than in the latter ( 85%) . As the part A gains water and the part B receives salt  solution, the fluid level remains the same in both the parts. The mutual exchange of salt and water molecules finally results in the formation of salt solution of the same strength in both the parts. The salt and the water molecules  then continue to diffuse in both directions at the same rate so that no change occurs in their concentration on any side of the membrane. The salt and water molecules are now said to be in  a state of dynamic equilibrium on the two sides of the membrane. 

Concentration Gradient. 

The difference in the concentrations of materials on the two sides of a membrane is called a  concentration gradient. In nonliving systems,  molecules always move " down" the concentration gradient, i.e., from a region of higher concentration to a region of lower concentration. But in living cells, as will be seen later, this is not always  the case.

Alternative Definition. 

Diffusion may be redefined as the movement of the solute and solvent molecules through a partially permeable membrane  separating two fluids having the same hydrostatic pressure but different concentrations of the solutes. In other words, a substance diffusion down its concentration gradient. 

Mode of Diffusion:

Diffusion through a semipermeable membrane occurs in two ways__ 
(i) The fat __ soluble substance pass through the biomembranes simply by dissolving in lipid matrix. 

( ii) The water and water___ soluble substances and ions, such as K +, CI ‐ and HCO ‐3, diffuse through the biomembranes via protein channels or protein __ linked channels.  Only the materials having molecules smaller than the channel diameter ( 8__ 10A°) can diffuse.

Diffusion of charged Molecules. 

Two factors influence the direction in which ions will diffuse: concentration and electric charge. An ion will usually diffuse down its concentration gradient. Ions will also diffuse through a membrane from the side richer in ions of similar charges  (analogous ions) to the side richer in ions of opposite charge ( counter ions) .The difference of electrical charges between the two sides of a membrane is known as the electrical gradient .Potassium ( K+) ions and sodium ( Na+) ions diffuse from the more electro positive side to the more electronegative side of the membrane. Actually, the ions diffuse down their electrochemical gradient, i.e., the resultant of their concentration and electrical gradients.

Diffusion of Gases.

The net diffusion of gases always occurs down their pressure gradient. If two mixture of gases ( say CO2 and O2) are separated by a semipermeable membrane, each gas diffusion from the side having higher partial pressure to the side having lower partial pressure of that gas. In internal respiration, O2 diffuses from the tissue fluid into the cells because its partial pressure is higher in the former than in the latter.The CO2 diffuses from the cells into the tissue fluid as its partial pressure is higher in the former than in the latter. Cell membrane is permeable to both the gases.


Diffusion of uncharged molecules occurs down the concentration gradient, diffusion of charged molecules takes place down the electrochemical gradient, and diffusion of gases results down the partial pressure gradient. 

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