Class 11-science NCERT Solutions Biology Chapter 11 - Transport In Plants

The factors affecting the rate of diffusion:

i. Temperature: As the temperature increases, the rate of diffusion also increases, because an increase in the temperature causes increased kinetic energy of diffusing particles.

ii. Density of diffusing substance: The rate of diffusion is inversely proportional to the square root of the density of the diffusing substance.

iii. Medium in which diffusion occurs: The rate of diffusion decreases in a concentrated medium. Example: A gas diffuses more rapidly through a vacuum than air.

iv. Diffusion pressure gradient: Diffusion pressure gradient implies the application of concentration difference over a specific distance. Greater the diffusion pressure gradient, more rapid will be the net diffusion of molecules. 

Porins are large protein molecules embedded in the outer membranes of the plastids, mitochondria and some bacteria. They form large pores to allow large molecules to pass through. Thus, they play an important role in facilitating diffusion.

Protein pumps are carrier proteins which take part in the transport of solutes across the cell membrane with the help of energy against the concentration gradient. Energy is supplied by ATP. On being activated with energy, the protein pump picks up solute particles from the outside and transfers the same to the inner side into the cytoplasm. There are a number of pumps (e.g. Na⁺-K⁺ pump). The transport of ions across plant membranes through these protein pumps which involve energy is called active transport.

Water molecules possess kinetic energy of their own and they remain in constant random motion. The kinetic energy of water molecules in pure water is maximum. It decreases when a solute is mixed in it, and consequently, the free energy of water molecules is reduced. Thus, the water potential of water molecules is maximum in pure water as compared to that in a solution.

(a) Diffusion and Osmosis:

(b) Transpiration and Evaporation:

(c) Osmotic pressure and Osmotic potential:

(d) Imbibition and Diffusion:

(e) Apoplast and Symplast pathways of movement of water in plants:

(f) Guttation and Transpiration:

Water potential is the difference in the free energy or chemical potential per unit molal volume of water in a system and that of pure water at the same temperature and pressure. It is the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, matrix effects and surface tension. It is measured in units of pressure and is represented by the Greek letter (Psi).  Pure water at standard temperature and pressure is defined as having a water potential of 0. The addition of solutes to water lowers its potential, whereas the increase in pressure increases its potential. Example: Sea water or the solution within living cells has negative water potentials as compared to pure water. 

If two systems having water are in contact, more random movement of water molecules in the system having higher water potential or more energy will result in their net movement towards the system with low energy or low water potential.

The following factors affect water potential:

1. Solute potential (ψ_s)
2. Pressure potential (ψ_p)
3. Matric potential
4. Temperature
5. Pressure
6. Loss or gain of water 

If a pressure greater than the atmospheric pressure is applied to pure water or a solution, then its water potential increases.

(a)  Plasmolysis is the shrinkage of protoplast from the cell wall under the influence of a hypertonic solution.

When a plant cell is placed in a hypertonic solution, the plant cell loses water and hence turgor pressure, making the plant cell flaccid. Plants with cells in this condition wilt. There are some mechanisms in plants to prevent excess water loss in the same way as excess water gain, but plasmolysis can be reversed if the cell is placed in a weaker solution (hypotonic solution).

Plasmolysis only occurs in extreme conditions and rarely occurs in     nature.

It can be induced in the laboratory (e.g. by using onion epidermal cells) by immersing the cells in a strong saline or sugar solution to cause exosmosis.

(b)  Higher water potential occurs in a hypotonic or dilute solution. A plant cell present in such a solution will absorb water due to endosmosis. It will become turgid or swollen. The swollen protoplast develops a wall pressure which becomes equal to the water potential of the system that causes the endosmosis to stop.

The fungal hyphae of mycorrhiza have a very large surface area which absorbs mineral ions and water from the surrounding soil solution. Thus, the mycorrhiza association is helpful in absorption of water and minerals in plants.

Root pressure helps in the upward movement of water in herbaceous plants.

In tall trees, water rises with the help of the transpirational pull generated by transpiration or loss of water from the stomatal pores of leaves. This is called the cohesion-tension model of water transport. During daytime, the water lost through transpiration (by the leaves to the surroundings) causes the guard cells and other epidermal cells to become flaccid. They in turn take water from the xylem. This creates a negative pressure or tension in the xylem vessels, from the surfaces of the leaves to the tips of the roots, through the stem. As a result, the water present in the xylem is pulled as a single column from the stem. The cohesion and adhesion forces of the water molecules and the cell walls of the xylem vessels prevent the water column from splitting.

Factors affecting transpiration:

1. Temperature: The rate of transpiration increases with the increase in atmospheric temperature because the temperature increases the rate of water evaporation from the cell surface, opens the stomata and decreases the relative humidity of the atmosphere.
2. Relative humidity: Relative humidity is the percentage of water vapour present in the air at a given time and temperature relative to the amount required to be present to make the air saturated at that temperature. The rate of transpiration is inversely proportional to the relative humidity.
3. Light: In most plants, the stomata open in the presence of light and close in darkness. The rate of transpiration increases in the presence of light and decreases in darkness.
4. Wind: Transpiration is lower in still air because the water vapour accumulates around the transpiring organs. The movement of air increases the rate of transpiration by removing the saturated air around the leaves.

Importance of transpiration:

1. Ascent of sap: Ascent of sap mostly occurs due to the transpiration pull exerted by the transpiration of water.
2. Turgidity: Transpiration maintains the shape and structure of plant parts by keeping the cells turgid.
3. Removal of excess water: Plants absorb far more amount of water than actually required by them. Thus, transpiration removes excess of water.
4. Distribution of mineral salts: Mineral salts are mostly distributed by the rise of sap. 

The transpiration driven ascent of xylem sap depends mainly on the following physical properties of water:

 i. Cohesion: It is the mutual attraction between water molecules.

 ii. Adhesion: It is the attraction of water molecules to polar surfaces (such as the surface of treachery elements).

 iii. Surface tension: Water molecules are attracted to each other in the liquid phase more than to water in the gas phase.

 iv. Root pressure: It is positive pressure which pushes sap from below due to the active absorption by roots.

 v. Transpiration pull: Transpiration in aerial parts brings the xylem sap under negative pressure or tension due to their continuous withdrawal of water. 

Endodermal cells have thickenings in their radial walls due to the deposition of lignin and suberin which stops apoplastic movement of minerals and water. Thus, the endodermis allows only symplastic movement of minerals in plants.

Water is absorbed by the roots from the soil, so it moves only in the upward direction through the xylem. Hence, the water transport is unidirectional. Food is formed in leaves and is required by both the root system and the shoot system of the plant. Food is transported by the phloem. Phloem sap moves upwards and downwards making the food transport bidirectional so that it may reach to every part of the plant.

In plants, food is continuously prepared in the mesophyll cells of the leaves in the form of glucose. Before moving into the source cells present in the phloem, the prepared food is converted into sucrose. Water moves from the xylem vessels into the adjacent phloem, thereby increasing the hydrostatic pressure in the phloem which results in the movement of sucrose through the sieve cells of the phloem. The sucrose already present in the sink region is converted into starch or cellulose, thereby reducing the hydrostatic pressure in the sink cells. Hence, the pressure difference created between the source and the sink cells allows sugars to be translocated from the former to the latter.

Opening and closing of stomata:

When water supply is adequate, the stomata tend to open during the day time in response to light and to close at night. The immediate cause of the opening or closing of the stomata is a change in the turgidity of the guard cells. When the guard cells become turgid, their thin walls get extended and thick walls become slightly concave so that the stomatal aperture opens. When the guard cells lose turgor, due to water loss (or water stress), the elastic inner walls regain their original shape, the guard cells become flaccid and the stomata close.