Half, The evolution of the $\mathrm{C}_4$ or plants photosynthetic system is probably one of the strategies for maximising the ability of $\mathrm{CO}_2$ while minimising water loss. $\mathrm{C}_4$ plants are twice as efficient as $\mathrm{C}_3$ plants in terms of fixing carbon in the form of glucose, but lose only half as much water as $\mathrm{aC}_3$ plant for the same amount of $\mathrm{C}_2$ fixed.

Xylem is involved in the one way transport of water and minerals from soil to root $\rightarrow$ stem $\rightarrow$ leaves. Several forces act in this mechanism like imbibition, root pressure and finally transpiration pull. There is continuous loss of water at the body surface of plants. Thus, it is a unidirectional process. Phloem's main function is to transport food from source to sink where source implies with the part of plant responsible for synthesising food and sink to those parts and organs that needs or consumes food for their growth and development. These source and sink parts of a plants may vary in different phases of its growth, thus the food need to travel in both upwards and downward direction. So, phloem shows bidirectional movement of substances.

1. Uniport In this process, there is movement of a molecule across a membrane, i.e., of other molecules.

II. Antiport In this process, there is a movement of two types of molecules in opposite direction.

III. Symport In this process, there is a movement of more than one molecule across the membrane in the same direction, at one time.

Property	Simple Diffusion	Facilitated Transport	Active Transport
Highly Selective	No	Yes	Yes
Uphill transport	No	No	Yes
Requires ATP	No	No	Yes

Active Transport It uses energy to pump molecules against a concentration gradient. Hence, different proteins in the membrane play a major role in active transport.

Carrier protein involved in active transport is very specific in what it carries across the membrane.

Facilitated Transport In facilitated transport special proteins help in movement of substances across the membrane without the expenditure of ATP. Facilitated transport is very specific as it allows cell to select substances for uptake.

Simple Diffusion It is a physical phenomenon which involves the movement of water from higher concentration to lower concentration. It is not a selective process and do not require energy.

Water potential is a measure of free energy associated with water per unit volume $\left(\mathrm{JM}^{-3}\right)$. The water potential of pure water $\left(\psi_w\right)$ at atmospheric pressure is zero. The unit of water potential is bars or Pascal ( $1 \mathrm{mPa}=10$ bars).

Solute Potential The addition of solutes reduce water potential (to a negative value). This reduces the concentration of water. Hence, solutions have a lower water potential than pure water, the magnitude of this lowering due to dissolution of a solute is called solute potential or $\psi_{\mathrm{s}}$.