Diffusion is the passive movement of substances from a region of higher concentration to a region of lower concentration. Diffusion of substances plays an important role in cellular transport in plants. Rate of diffusion is affected by concentration gradient, membrane permeability, temperature, and pressure. Diffusion takes place as long as there is a difference between the concentrations of a substance across a barrier. However, diffusion stops, when the concentrations of the substance on either side of the barrier become equal. The permeability of a membrane affects the rate of diffusion. Diffusion rate increases as membrane permeability increases. Changes in temperature and pressure values also affect the diffusion of substances. Pressure plays an important role in the diffusion of gases as gases diffuse from a region of higher partial pressure to a region of lower partial pressure.

Question - 2 : -
What are porins? What role do they play in diffusion?

Answer - 2 : -

Porins are types of proteins which form pores of large sizes in the outer membranes of plastids such as chloroplast, mitochondria and the membranes in bacteria. They help in facilitating the passive transport of small-sized protein molecules.

Question - 3 : -
Describe the role played by protein pumps during active transport in plants.

Answer - 3 : -

In plant cells, active transport occurs against the concentration gradient, i.e., from a region of lower concentration to a region of higher concentration. The process of active transport involves specific protein pumps. The protein pumps are made up of specific proteins called trans-membrane proteins. These pumps first make a complex with the substance to be transported across the membrane, using the energy derived from ATP. The substance finally gets liberated into the cytoplasm as a result of the dissociation of the protein–substance complex.

Question - 4 : -
Explain why pure water has the maximum water potential.

Answer - 4 : -

Water potential quantifies the tendency of water to move from one part to the other during various cellular processes. It is denoted by the Greek letter Psi or Ψ. The water potential of pure water is always taken as zero at standard temperature and pressure. It can be explained in terms of the kinetic energy possessed by water molecules. When water is in liquid form, the movement of its molecules is rapid and constant. Pure water has the highest concentration of water molecules. Therefore, it has the highest water potential. When some solute is dissolved in water, the water potential of pure water decreases.

Question - 5 : -
Differentiate between the following:

(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.

Answer - 5 : -

(a) Diffusion andosmosis

Diffusion		Osmosis
1.	Diffusion is the passive movement of particles, ions, and molecules along the concentration gradient.	1.	Osmosis is the process in which the diffusion of a solvent (water) occurs across a semi-permeable membrane.
2.	It can occur in solids, liquids, and gases.	2.	It occurs in the liquid medium.
3.	It does not require a semi-permeable membrane.	3.	It requires a semi-permeable membrane.

(b) Transpiration andevaporation

Transpiration		Evaporation
1.	It occurs in plants.	1.	It occurs from any free surface and involves living and non-living surfaces.
2.	It is a physiological process.	2.	It is a physical process.
3.	It occurs mainly through the stomatal pores on plant leaves.	3.	It is occurs through any free surface.
4.	It is controlled by environmental factors as well as physiological factors of plants such as root-shoot ratio and number of stomata.	4.	It is entirely driven by environmental factors.

(c) Osmotic pressure andosmotic potential

Osmotic pressure		Osmotic potential
1.	It is expressed in bars with a positive sign.	1.	It is expressed in bars with a negative sign.
2.	It is a positive pressure.	2.	It is a negative pressure.
3.	Its value increases with an increase in the concentration of solute particles.	3.	Its value decreases with an increase in the concentration of solute particles.

(d) Imbibition anddiffusion

Imbibition		Diffusion
1.	Imbibition is a special type of diffusion. In this process, water is absorbed by solids and colloids, causing an enormous increase in volume.	1.	Diffusion is the passive movement of particles, ions, and molecules along the concentration gradient.
2.	It usually involves water.	2.	It involves solids, liquids, and gases.

(e) Apoplast andsymplast pathways of movement of water in plants

Apoplast pathway		Symplast pathway
1.	The apoplast pathway involves the movement of water through the adjacent cell walls of the epidermis and cortex. The movement of water is restricted at the casparian strips of the root endodermis.	1.	The symplast pathway involves the movement of water through the interconnected protoplasts of the epidermis, cortex, endodermis, and root pericycle.
2.	It is a faster process of water movement and water moves through mass flow.	2.	It is a slower process of water movement.

(f) Guttation andtranspiration

Guttation		Transpiration
1.	It occurs usually at night.	1.	It occurs usually during the day.
2.	Water is lost from the leaves in the form of liquid droplets.	2.	Water is lost from the leaves in the form of water vapour.
3.	It occurs through the vein endings of leaves.	3.	It occurs through the stomata.
4.	It is an uncontrolled process.	4.	It is a controlled process.

Question - 6 : -
Briefly describe water potential. What are the factors affecting it?

Answer - 6 : -

Water potential quantifies the tendency of water to move from one part to the other during various cellular processes such as diffusion, osmosis, etc. It is denoted by the Greek letter Psi or Ψ and is expressed in Pascals (Pa). The water potential of pure water is always taken as zero at standard temperature and pressure.

Water potential (Ψw) is expressed as the sum of solute potential (Ψs) and pressure potential (Ψp).

Ψw = Ψs + Ψp

When some solute is dissolved in water, the water potential of pure water decreases. This is termed as solute potential (Ψs), which is always negative. For a solution at atmospheric pressure, Ψw = Ψs.

The water potential of pure water or a solution increases on the application of pressure values more than atmospheric pressure. It is termed as pressure potential. It is denoted by Ψp and has a positive value, although a negative pressure potential is present in the xylem. This pressure potential plays a major role in the ascent of water through the stem.

Question - 7 : -
What happens when a pressure greater than the atmospheric pressure is applied to pure water or a solution?

Answer - 7 : -

The water potential of pure water or a solution increases on the application of pressure values more than atmospheric pressure. For example: when water diffuses into a plant cell, it causes pressure to build up against the cell wall. This makes the cell wall turgid. This pressure is termed as pressure potential and has a positive value.

Question - 8 : -
(a) With the help of well-labelled diagrams, describe the process of plasmolysis in plants, giving appropriate examples.

(b) Explain what will happen to a plant cell if it is kept in a solution having higher water potential.

Answer - 8 : -

(a) Plasmolysis can be defined as the shrinkage of the cytoplasm of a plant cell, away from its cell wall and toward the centre. It occurs because of the movement of water from the intracellular space to the outer-cellular space. This happens when the plant cell is placed in a hypertonic solution (i.e., a solution having more solute concentration than the cell cytoplasm). This causes the water to move out of the cell and toward the solution. The cytoplasm of the cell shrinks and the cell is said to be plasmolysed. This process can be observed in an onion peel kept in a highly concentrated salt solution.

(b) When a plant cell is placed in a hypotonic solution or a solution having higher water potential, the water diffuses into the cell (i.e., movement is observed from higher to lower water pressure region). The entry of water in the plant cell exerts pressure on the rigid cell wall. This is called turgor pressure. As a result of its rigid cell wall, the plant cell does not burst.

Question - 9 : -
How is the mycorrhizal association helpful in absorption of water and minerals in plants?

Answer - 9 : -

Mycorrhiza is a symbiotic association of fungi with the root systems of some plants. The fungal hyphae either form a dense network around the young roots or they penetrate the cells of the roots. The large surface area of the fungal hyphae is helpful in increasing the absorption of water and minerals from the soil. In return, they get sugar and nitrogenous compounds from the host plants. The mycorrhizal association is obligate in some plants. For example, Pinus seeds do not germinate and establish in the absence of mycorrhizal.

Question - 10 : -
What role does root pressure play in water movement in plants?

Answer - 10 : -

Root pressure is the positive pressure that develops in the roots of plants by the active absorption of nutrients from the soil. When the nutrients are actively absorbed by root hairs, water (along with minerals) increases the pressure in the xylem. This pressure pushes the water up to small heights. Root pressure can be observed experimentally by cutting the stem of a well-watered plant on a humid day. When the stem is cut, the solution oozes from the cut end.

Root pressure is also linked to the phenomenon of guttation, i.e., the loss of water in the form of liquid droplets from the vein endings of certain herbaceous plants.

Root pressure is only able to transport water up to small heights. However, it helps in re-establishing the continuous chains of water molecules in the xylem. Transpirational pull maintains the flow of water molecules from the roots to the shoots.

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