Mass flow hypothesis was first proposed by Ernst Munch (1930). It explains the transport of solute or sugars, i.e., sucrose in the phloem tissue.

It involves the following three steps

(a) Phloem loading

(b) Translocation of solutes

(c) Phloem unloading

Loading of sieve tubes takes place in leaves. Photosynthetic cells make sugars, particularly sucrose, and other organic solutes. Companion cells use energy to collect solutes by active transport. As solute concentration increases in the companion cells, water enters by osmosis.

A pressure is created, which pushes the solutes through plasmodesmata into the sieve tubes. It is an active process.

Translocation of sucrose takes place through phloem column. Increase in hydrostatic pressure inside sieve tubes is greatest at the source and lowest at the sink. It pushes sucrose etc., from source to sink.

Unloading of the sieve tubes takes place at the sink sites.

It results in active transport of sugars out of the pholem converting them into complex sugars. Loss of solutes from the phloem create a high water potential, and water moves out of phloem, returning back to xylem.

Sinks are the regions where solutes i.e., sugars or food are being used, e.g., roots, fruits, storage organs and regions of growth.

(a) The guard cells are bean-shaped which are found in dicot plants.

(b) The guards cells in figure (i) are turgid as, they pull the inner wall of the cell outside thus, they have more water. (ii) cells are flacid, this condition results when cells lose water and close stomatal pore.

(c) The $\mathrm{K}^{+}$ions when move from neighbouring cells to guards cells, lowering their water potential as a result the water moves inside making them turgid and thus opening stomata.

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.