Properties of water that make it useful for all biological processes on the earth are

(i) Water is the major solvent through which mineral nutrients enter a plant from the soil solution.

(ii) It is an ideal solvent with neutral pH .

(iii) Water is the major constituent of protoplasm. It constitute approximately $90 \%$ of the protoplasm.

(iv) Water act as a medium for translocation of nutritive substances. Mineral nutrients are absorbed by the roots. Carbohydrates that are formed during photosynthesis are transported by water from cell to cell, tissue to tissue and organ to organ.

(v) In plants, water is involved in photosynthesis by incorporating hydrogen atom into carbohydrate and oxygen atoms are further released as $\mathrm{O}_2$.

(vi) Water act as agent for temperature control. The specific heat of water helps plant in aintaining a relative stable internal temperature.

(vii) In some plants water is necessary for pollination. Like in bryophytes and pteridophytes, they essentially require water for the fertilisation process.

Ion channels are 'gated', i.e., they may be open or closed. The $\mathrm{Na}^{+}, \mathrm{K}^{+}$, ATPase create a charge imbalance across the plasma membrane by carrying $3 \mathrm{Na}^{+}$out of the cell for every $2 \mathrm{~K}^{+}$ion carried inside making the inside negative relative to outside.

The membrane is said to be polarised. That is the reason the intracellular levels of $\mathrm{K}^{+}$are higher than extracellular levels in animals cells.

Cut pieces of beetroot do not leave colour in cold water but do so in hot water because pigment of beetroot dissolve and diffuse readily in hot water as hot temperature help in leaking of the plasma membrane and release of beetroot pigment.

In a girdled plant, when water is supplied to the leaves above the girdle, leaves may remain green for sometime because leaves can synthesise their own carbohydrate food through photosynthesis, then wilt due to non-availability of water. The system of xylem vessels from root to the leaf vein can supply the needed water, during girdling there is a possible loss of xylem vessels and the water supply cuts off, resulting in death of plant.

Various types of ions, minerals and organic compound are transported in plants in various modes, e.g.,

(i) Food substances synthesised in leaves are translocated downward towards root and stem.

(ii) Similarly food is translocated upwards to the developing leaves, buds and fruits.

(iii) Radial transport of food occurs across the stem from the cells of pith, from cortex etc, towards epidermis.

(iv) lons and minerals are transported upwards through xylem.

Since, diffusion is a slow process and allows movement of molecules for short distances only, it cannot filled and alone carry out the above mentioned movements of organic and inorganic substances. Therefore, a need arises for special long distance transport systems that permits and move substances at a much faster rate, i.e., mass or bulk flow system through conducting tissues (translocation).