Table : Comparison of different transport processes
Property Simple diffusion Facilitated diffusion 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
- response to protein inhibitions No yes yes
PlANT - wATER RElATIONS
Water is essential for all physiological activities of the plant, and plays a very important role in all living organisms. To
comprehend plant-water relation, an understanding of the following standard terms is necessary.
Membrane permeability
Membrane permeability is the ability of a membrane to allow
passage of gases, liquids, solutes or dissolved substances through
it. On the basis of their permeability, membranes are of following
types –
● Impermeable : do not allow passage of substances, e.g.,
cuticle layer.
● Permeable : allow the passage of substances, e.g., cellulose
cell walls.
● Semipermeable : allow passage of only solvent molecules, but not the solute particles, e.g., egg membrane, parchment
membrane.
● Selectively or differentially permeable : allow passage of solute molecules only upto a certain extent, e.g.,
plasma membrane.
Imbibition
Imbibition is the phenomenon of adsorption of water or any other liquid, by the solid particles of a substance, without
forming a solution. Solid substances or adsorbents, which take part in imbibition are called imbibants, e.g., seeds, dry
wood. The liquid, usually water, which is imbibed is known as imbibate.
In case of imbibition, molecules of water or any other liquid i.e., imbibate are held in between, or over the surface
of solid substances, i.e., imbibants, through the process of capillarity, or by the force of adsorption. Hence, we can say that
imbibition is a phenomenon of adsorption.Plant imbibants are hydrophilic colloids. They have a highly negative water potential so, when they come in contact with
water (yw = O), a steep water potential gradient is established. Water diffuses rapidly into the adsorbent till equilibrium
is attained.
Amount of imbibition depends upon :
● Water potential gradient between adsorbant and water.
● affinity of adsorbant for water.
Imbibition increases the volume of the imbibant, but the increase is less than the volume of water absorbed. During
imbibition, the water molecules get tightly adsorbed, and become immobilized. They lose most of their kinetic energy