Class 11 Transport in plants Facilitated Diffusion
l The diffusion rate depends upon the size of the substances, i.e., smaller substances diffuse faster. The diffusion of any substance across a membrane also depends on its solubility in lipids, the major constituents of the membrane. Substances soluble in lipids diffuse through the membrane faster. The hydrophilic solutes, find it difficult to pass through the membrane. Thus their movement has to be facilitated. Membrane proteins provide sites at which such molecules cross the membrane. They do not set up a concentration gradient. No energy is utilized. Thus, the diffusion of hydrophilic substances along the concentration gradient through fixed membrane transport protein without involving energy expenditure, is called facilitated diffusion.
l Facilitated diffusion is very specific as it allows cell to select substances for uptake. It is sensitive to inhibitors which react with protein side chains. It can show saturation effect (when all the protein transporters are being used) like the active transport.
l Two major types of transport proteins are known channel proteins and carrier proteins. Channel proteins allow diffusion of an appropriate size. Some channels are always open while others can be controlled. Some channels are large allowing a variety of molecules to pass.
l Carrier proteins bind the extracellular molecule, then rotate and release the molecule inside. Water Channels are specialized to allow passage of water through them. Each water channel is surrounded by eight different types of aquaporins.
l Ion channels allow passage of their own specific ions. The porins are proteins that form large pores in the outer membrane of the plastids, mitochondria and some bacteria allowing molecules upto the size of small proteins to pass through.
l Some carrier proteins allow transport only if two types of molecules move together. This is called cotransport. It is of two types, symport and antiport. In Symport method of cotransport, both molecules cross the membrane in the same direction. In antiport method of cotransport, both molecules move in opposite direction. When a molecule moves across a membrane independent of another molecule, the process is called uniport.
Table: Comparison of different Transport Mechanism
l It is a mode of transport which uses energy, involves carrier protein and is against a concentration gradient. Since active transport occurs against a concentration gradient (uphill transport) the carrier proteins involved in it are called pumps. Pumps use energy to carry substances across the cell membrane. The rate of active transport reaches the maximum when all the protein pumps are being used in transport (saturation effect). Like enzymes, the carrier proteins are very specific. These proteins are sensitive to inhibitors that react with protein side chains.
l Facilitated diffusion and active transport show common characteristics of being highly selective, requiring proteins, liable to saturate, respond to inhibitors and under hormonal regulation. But diffusion whether facilitated or not takes place only along a gradient and does not use energy.