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The following points highlight the three methods of transport in plants. The methods are: 1. Diffusion 2. Facilitated Diffusion 3. Active Transport.
Method # 1. Diffusion:
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Movement by diffusion is passive and slow. It occurs along the concentration gradient, i.e., from region of higher concentration to region of lower concentration provided the cell membrane is permeable to it. No energy expenditure takes place.
Method # 2. Facilitated Diffusion:
Particles which are lipid soluble can easily pass directly through the cell membrane as it is mainly made of it. The hydrophilic solutes, find it difficult to pass through the membrane. Their movement has to be facilitated.
For this the membranes possess aquaporin’s and ion channels. Eight different types of aquaporin’s or water channels have been recorded. Aquaporin’s are membrane proteins for passive transport of water soluble substance.
They do not set up a concentration gradient. No energy is utilized. The diffusion of hydrophilic substances along the concentration gradient through fixed membrane transport protein without involving energy expenditure, is called facilitated diffusion. This diffusion is very specific as it allows cell to select substances for uptake. It is sensitive to inhibitors as well as show saturation effect.
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Two major types of transport proteins are known viz., carrier proteins (also called carriers, transporters) and channel proteins. Carrier proteins bind the particular solute to be transported and deliver the same to the other side of the membrane. Channel proteins allow diffusion of the solutes of an appropriate size may diffuse.
Some carrier proteins allow transport only if two types of molecules move together. This is called cotransport. It is of two types (Fig. 11.2).
In symport method of cotransport, both molecules cross the membrane in the same direction at the same time. In anti-port method of cotransport, both molecules move in opposite direction. When a molecule moves across a membrane independent of other molecule, the process is called uniport.
Ion channels allow passage of their own specific ions. They are often gated— voltage gated, mechanical gated, ligand gated. The gates open under specific conditions, e.g., K+ channels in nerve conduction. Certain pores called porins are present in the outer membrane of plastids, mitochondria and some bacteria.
They are large protein pores which allow even small sized proteins to pass through. Normally passage is allowed for only small sized particles. Passage of some important solutes is connected with the occurrence of transport or carrier proteins,
Method # 3. Active Transport:
In active transport, the movable earner proteins are called pumps. They employ ATP energy for transport across the membrane. It is uphill transport, i.e., against concentration gradient and is faster than passive transport.
The rate of active transport reaches the maximum when all the protein pumps are being used in transport (saturation effect). Carrier proteins are highly specific like enzymes. They are also sensitive to inhibitors that react with protein side chains.
