FACTORS AFFECTING RATE OF DIFFUSION

FACTORS AFFECTING RATE OF DIFFUSION

Rate of diffusion of substances through the cell membrane is affected by the following factors:

1. Permeability of the Cell Membrane

Rate of diffusion is directly proportional to the permeability of cell membrane. Since the cell membrane is selectively permeable, only limited number of substances can diffuse through the membrane.

2. Temperature

Rate of diffusion is directly proportional to the body temperature. Increase in temperature increases the

rate of diffusion. This is because of the thermal motion of molecules during increased temperature.

3. Concentration Gradient or Electrical Gradient of the Substance across the Cell Membrane

Rate of diffusion is directly proportional to the concentration gradient or electrical gradient of the diffusing

substances across the cell membrane. However, facilitated diffusion has some limitation beyond certain

level of concentration gradient.

4. Solubility of the Substance

Diffusion rate is directly proportional to the solubility of substances, particularly the lipid-soluble substances. Since oxygen is highly soluble in lipids, it diffuses very rapidly through the lipid layer.

5. Thickness of the Cell Membrane

Rate of diffusion is inversely proportional to the thickness of the cell membrane. If the cell membrane is thick, diffusion of the substances is very slow.

6. Size of the Molecules

Rate of diffusion is inversely proportional to the size of the molecules. Thus, the substances with smaller

molecules diffuse rapidly than the substances with larger molecules.

7. Size of the Ions

Generally, rate of diffusion is inversely proportional to the size of the ions. Smaller ions can pass through the membrane more easily than larger ions with the same charge. However, it is not applicable always. For instance, sodium ions are smaller in size than potassium ions. Still, sodium ions cannot pass through the membrane as easily as potassium ions because sodium ions have got the tendency to gather water molecules around them. This makes it difficult for sodium ions to diffuse through the membrane.

8. Charge of the Ions

Rate of diffusion is inversely proportional to the charge of the ions. Greater the charge of the ions, lesser is the rate of diffusion. For example, diffusion of calcium (Ca++) ions is slower than the sodium (Na+) ions.

SPECIAL TYPES OF PASSIVE TRANSPORT

In addition to diffusion, there are some special types of passive transport, viz.

1. Bulk flow

2. Filtration

3. Osmosis.

BULK FLOW

Bulk flow is the diffusion of large quantity of substances from a region of high pressure to the region of low pressure. It is due to the pressure gradient of the substance across the cell membrane.

Best example for bulk flow is the exchange of gases across the respiratory membrane in lungs. Partial pressure of oxygen is greater in the alveolar air than in the alveolar capillary blood. So, oxygen moves from alveolar air into the blood through the respiratory membrane. Partial pressure of carbon dioxide is more in blood than in the alveoli. So, it moves from the blood into the alveoli through the respiratory membrane .

FILTRATION

Movement of water and solutes from an area of high hydrostatic pressure to an area of low hydrostatic

pressure is called filtration. Hydrostatic pressure is developed by the weight of the fluid. Filtration process is seen at arterial end of the capillaries, where movement of fluid occurs along with dissolved substances from blood into the interstitial fluid. It also occurs in glomeruli of kidneys.

OSMOSIS

Osmosis is the special type of diffusion. It is defined as the movement of water or any other solvent from

an area of lower concentration to an area of higher concentration of a solute, through a semipermeable

membrane. The semipermeable membrane permits the passage of only water or other solvents but

not the solutes. Osmosis can occur whenever there is a difference in the solute concentration on either side of the membrane. Osmosis depends upon osmotic pressure.

Osmotic Pressure

Osmotic pressure is the pressure created by the solutes in a fluid. During osmosis, when water or any other solvent moves from the area of lower concentration to the area of higher concentration, the solutes in the area of higher concentration get dissolved in the solvent. This creates a pressure which is known as osmotic pressure. Normally, the osmotic pressure prevents further movement of water or other solvent during osmosis.

Reverse Osmotic Pressure

Reverse osmosis is a process in which water or other solvent flows in reverse direction (from the area of

higher concentration to the area of lower concentration of the solute), if an external pressure is applied on the area of higher concentration.

Colloidal Osmotic Pressure and Oncotic Pressure

The osmotic pressure exerted by the colloidal substances in the body is called the colloidal osmotic pressure. And, the osmotic pressure exerted by the colloidal substances.