Passive transport is a type of diffusion in which ions and particles or molecules of various substances move through the plasma membrane from a region of higher to lower concentration by diffusion.
How is the transport of molecules takes place in passive transport:
Transport of molecules takes place along electrochemical or concentration gradient without consuming any energy (energy independent). This can be described as the transporting of ‘material downhill’. The energy for this movement is thermal molecular motion.
(1) Simple Diffusion:
It is transport of metabolites across the plasma membrane along the concentration gradient without the use of a carrier molecule.
The plasma membrane has numerous tiny pores. These pores are presumed to be of two types;
(i) These pores are 7Å in diameter and permanent in nature. The fine aqueous channels either through a protein or between clustered integral proteins. These extend through the entire lipid bilayer. Some pores are charged positively, while others have negative charge. These pores act as valvular openings. The hydrophilic or the small molecules could cross the membrane through these channels.
Because of their kinetic energy, ions and molecules are constantly bouncing around inside or outside the cell and occasionally they strike the Plasma membrane. If they hit solid regions, they simply bounce back. But if they happen to hit a pore, they pass through to the other side. This random, thermally induced movement of ions or small molecules from a region of higher concentration to low concentration is known as diffusion.
(ii) Statistical pores are not stable. These keep on appearing and disappearing. These are formed as gaps in the highly fluid lipid bilayer because of random thermal movement of membrane phospholipids. Substances soluble in lipids readily pass through these pores. For this reason the hydrophobic substances of both low and high molecular weight are readily transported through the plasma membrane, but only small molecules of hydrophilic substance can pass through it.
The rate of diffusion of molecules across the plasma membrane depends on:
(i) Size of molecules.
(ii) Concentration gradient across the membrane.
(iii) Hydrophobicity of molecules or their solubility in lipids.
In the partition coefficient, Colander and Barlund demonstrated that the rate of diffusion of substances depends on their solubility in lipids. The permeability (p) of molecules across the membrane is represented by:
[ where P = permeability of molecule, D = diffusion coefficient K = partition coefficient, T = thickness of membrane ]
During simple diffusion substances move from aqueous solution into the hydrophobic interior of phospholipid bilayer. Partition coefficient (K) is the concentration of solute in oil, divided by the ‘concentration’ of solute in aqueous phase. It is an equilibrium constant and it denotes relative affinity of molecules for lipids verses water.
Diffusion Constant (D) is proportional to the ability of a species to dissolve m hydrophobic interior of phospholipid membrane.
(2) Facilitated Diffusion:
The facilitated diffusion is a carrier assisted transport mechanism without the use of energy. Specific carrier protein called permease acts as a passive conveyer belt that permits the substance to pass through the plasma membrane along the concentration gradient. The concentration gradient acts as the driving force.
The transporter molecule projects out on either side of the plasma membrane. It loosely combines with the solute molecule on one side of the membrane (where concentration of substance is higher), and transported across die membrane.
The characteristics bf facilitated diffusion are;
(i) The rate of transport of molecules across the membrane is far greater than expected from simple diffusion (Maximum transport).
(ii) Facilitated diffusion proteins or permeases are very specific and each transports only a single species of ions or molecules or group of closely related molecules.
(iii) With an increase in the concentration gradient there is a corresponding increase in the rate of transport.
Kinetics of Facilitated Diffusion:
If a substances S is present initially outside the plasma membrane, its transport inside can be represented by the following equation:
S out + permease Km ⇌ (S – Permease complex) V max → S
[ where S = is substance, Km= binding constant for substance to permease, Vmax = maximum rate of transport ]
If C is the concentration of S outside, the rate of Its transport can be calculated by the following equation:
V = Vmax/+C/Km
The facilitated diffusion is helpful in the transport of polar nutrients like glucose and amino acids, which can enter the cell more rapidly than by simple diffusion. But this process is stereospecific and can transport only one of the isomers.
The diffusion of solute through a differentially permeable membrane is called dialysis. If a cellophane bag containing sugar solution is immersed in a beaker of pure water, the sugar molecules diffuse out of the cellophane into the water till the concentration of sugar on both the sides becomes equal. The solute molecules continue to pass through the membrane but there is no change in the concentration.
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