Plants lack interstitial fluid and a circulatory system but need to transport 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 stems, leaves, etc.).
- Multidirectional transport: E.g., Transport of photosynthates (organic compounds), mineral nutrients, etc.
Sometimes, plant hormones and other chemical stimuli are transported in a polarized or unidirectional manner from where they are synthesized to other parts.
Means of Transport
1. Diffusion
- It is the slow movement of gases, liquids, and solutes from a higher concentrated region to a lower concentrated region without energy expenditure.
- It may occur from one part of the cell to another, from cell to cell, or over short distances.
- It is not dependent on a living system and 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 without expenditure of ATP energy.
- It requires a concentration gradient.
- It is very specific, allowing the cell to select substances for uptake. It is sensitive to inhibitors that react with protein side chains.
- Transport rate reaches a maximum when all protein transporters are 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, and some bacteria, allowing molecules the size of small proteins to pass through.
- An extracellular molecule binds to the transport protein, which rotates and releases the molecule inside the cell. E.g., water channels made up of eight types of aquaporins.
Passive uniports, symports, and antiports:
- Uniport: A molecule moves alone across a membrane through a transport or carrier protein.
- Symport: Two molecules cross the membrane together in the same direction.
- Antiport: Two molecules move in opposite directions.
3. Active Transport
- It is the transport of molecules against a concentration gradient (from a lower concentrated region to a 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 the cell membrane (uphill transport).
- Transport rate reaches a maximum when all protein transporters are used (saturated).
- The carrier protein is very specific and 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 |