- Translocation : Transport of substances in plants over longer distances through the vascular tissue (Xylem and Phloem) is called translocation.
- Means of transport : The transport of material into and out of the cells is carried out by a number of methods. These are diffusion, facilitated diffusion and active transport.
- Diffusion : Diffusion occurs from region of higher concentration to region of lower concentration across the permeable membrane. It is passive and slow process. No energy expenditure takes place.
- Facilitated diffusion : The diffusion of hydrophilic substances along the concentration gradient through fixed membrane transport protein without involving energy expenditure is called facilitated diffusion. For this the membrane possesses aquarporins and ion channels. No energy is utilized in this process.
Methods of Facilitated Diffusion :
- Some carrier or transport proteins allow diffusion only if two types of molecules moves together.
- Symport: both molecules cross the membrane in the same direction.
- Antiport: both molecule moves in opposite direction.
- Uniport: one type of molecule moves across the cell membrane.
Active transport :
- Active transport is carried by the movable carrier proteins (pumps) of membrane.
- Active transport uses energy to pump molecules against a concentration gradient from a low concentration to high concentration (uphill-transport).
- It is faster than passive transport.
Water potential :
- Water molecule possesses kinetic energy.
- The greater the concentration of water in a system, the greater is its kinetic energy or water potential.
- Pure water has the highest water potential.
- Water always moves from higher water potential to lower water potential.
- Water potential is denoted by Ψw (Psi) and measured in Pascals (Pa). The water potential of a cell is affected by solute potential (Ψs) and pressure potential (Ψp).
- Ψ w = Ψ s + Ψ p
- Water potential of pure water at standard temperature which is not under any pressure is taken to be zero (by convention).
Osmosis :
- Osmosis is movement of solvent or water molecules from the region of their higher diffusion pressure or free energy to the region of their lower diffusion pressure or free energy across a semi-permeable membrane.
- Water molecules move from higher water potential to lower water potential until equilibrium is reached.
Plasmolysis :
- Process of shrinkage of protoplasm in a cell due to exosmosis in hypertonic solution.
- Turgor pressure: a plant cell placed in hypotonic solution, water enters into it due endosmosis and the cytoplasm exert pressure against the cell wall called turgor pressure.
- Imbibition: Imbibition is the phenomenon of adsorption of water or any other liquid by the solid particles of a substance without forming a solution.
Some examples of Imbibition :
- If a dry piece of wood is placed in water, it swells and increases in its volume.
- If dry gum or pieces of agar-agar are placed in water, they swell and their volume increases.
- When seeds are placed in water they swell up.
