The biological membranes have many mechanism for movement of substances to and for. Some are active and some are passive. Specific membrane proteins are also involved for special types of transport mechanisms.
Uniport This is a membrane transport system by a integral membrane protein that is involved in facilitated diffusion. These channels get open in response to a stimulus for free flow of specific molecules in a specific direction. These transport molecule with solute gradient without energy expenditure.
Symport This involves the movement of two or more different molecules or ions, across the membrane in the same direction, with no energy expenditure.
Antiport also called exchanger. This integral membrane protein is involved in secondary active transport of two or more different molecules or ions across the membrane in opposite directions, without affecting the tansport of other molecules.
Yes, plants need to adjust the type and quantity of solutes that reach the xylem. The transport proteins of endodermal cell help in maintaining and adjusting solute movement. As the minerals are present in soil as charged particles with a very low concentration as compared of roots, they, all cannot be complately passively transported across cell membranes of root hairs.
Thus, minerals are transported both by active and passive processes, to the xylem. Upon reaching xylem, they are further transported, i.e., upwards to sinks through transpiration stream. At the sink regions mineral ions are unloaded through diffusion and active uptake by receptor cells.
Some of the mineral ions moving frequently through xylem are
(i) Nitrogen travels in plants as inorganic ions $\mathrm{NO}_2$ and $\mathrm{NO}_3$ but much of the nitrogen moves in the form of amino acids and related organic compounds.
(ii) Sulphur and Phosphorus small amount of these two nutrients are carried in organic forms.
(iii) Mineral ions are frequently remobilised particularly from older senescing parts. Older dying leaves export much of their mineral content to younger leaves. Similarly, before leaf fall in decidous plants, minerals are removed to other parts.
Elements most readily mobilised are phosphorus, sulphur, nitrogen and potassium. Some elements that are structural components like calcium are not remobilised.
Wilting refers to the loss of turgidity of leaves and other soft aerial parts of a plant causing droping, folding and rolling of non-woody plants. It occurs when rate of loss of water is higher than the rate of absorption.
| Temporary Wilting | Permanent Wilting |
|---|---|
| Temporary drooping of young leaves and shoots due to loss of turgidity especially during noon. | State of permanent loss of turgidity in leaves and other parts of plant. |
| Occurs when rate of transpiration is more than water absorption due to shrinkage of roots. | Rate of transpiration is more than rate of absorption but difference is below critical level. |
| Wilting recovers as soon as water is replenished in the soil around root hairs. | Wilting is not recovered as cells do not regain their turgidity even in presence of plentiful water and atmosphere. |
| Plant regains its normal growth. | Plant eventually dies. |
Which of these is a Semipermeable Membrane (SP) and which is Selectively Permeable (SL)?
(a) Animal bladder (b) Plasmalemma (c) Tonoplast (d) Parchment membrane (e) Egg membrane
Animal bladder Semipermeable
Plasmalemma Selectively permeable
Tonoplast Selectively permeable
Parchment membrane Semipermeable
Egg membrane Semipermeable