11. TRANSPORT IN PLANTS
Plants do not have interstitial fluid and circulatory system. But they need to move various substances (water, minerals, organic nutrients, growth regulators etc.) over long distances.Direction of transport
- Unidirectional transport: E.g. Transport of water and minerals in xylem (from roots to the stems, leaves etc.).
- Multidirectional transport: E.g. Transport of photosynthates (organic compounds), Transport of mineral nutrients etc.
MEANS OF
TRANSPORT
1. Diffusion
It is the slow movement of gases, liquids and solutes from higher concentrated region to lower concentrated region without the energy expenditure.
It may be from one part of the cell to the other or from cell to cell, or over short distances.
It is not dependent on a ‘living system’.
It is the only means for gaseous movement in a plant body.
Factors affecting diffusion rates:
- Concentration gradient.
- Permeability of the membrane.
- Temperature and pressure.
- Size or density. Smaller substances diffuse faster.
- Solubility in lipids of the membrane. Substances soluble in lipids diffuse through the membrane faster.
2. Facilitated Diffusion
It is the diffusion of hydrophilic substances with the help of membrane protein channels and without expenditure of ATP energy.
It also needs a concentration gradient.
Transport rate reaches a maximum when all the protein transporters are being used (saturation).
Some protein channels are always open; others can be controlled. Some are large sized. E.g. Porins.
Porins form huge pores in the outer membranes of plastids, mitochondria & some bacteria. Molecules having size of small proteins can pass through them.
An extracellular molecule binds to the transport protein. Then it rotates and releases the molecule inside the cell. E.g. water channels – made up of 8 types of aquaporins.
Passive uniports, symports and antiports
- Uniport: A molecule alone moves across a membrane through transport or carrier protein.
- Symport: Two molecules together cross the membrane in same direction.
- Antiport: Two molecules move in opposite directions.
3. Active Transport
It is the transport of molecules against a concentration gradient (from lower concentrated region to higher concentrated region) with the expenditure of energy.
It is carried out by membrane-proteins.
Pumps are proteins that use energy to transport substances across cell membrane (‘uphill’ transport).
Transport rate reaches a maximum when all the protein transporters are being used (saturated).
The carrier protein is very specific. These are sensitive to inhibitors that react with protein side chains.
Comparison of Different Transport Processes
|
Property |
Simple diffusion |
Facilitated
transport |
Active transport |
|
Requires special membrane
proteins |
No |
Yes |
Yes |
|
Highly selective |
No |
Yes |
Yes |
|
Transport saturates |
No |
Yes |
Yes |
|
Uphill transport |
No |
No |
Yes |
|
Requires
ATP energy |
No |
No |
Yes |
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