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What are the factors affecting the rate of diffusion?

The factors affecting the rate of diffusion are

(1) the gradient of concentration, 

(2) permeability of the membrane separating them, 

(3) temperature and

(4) pressure.


What are porins? What role do they play in diffusion?

The porins are proteins that form huge pores in the outer membranes of the plastids, mitochondria and some bacteria.

Function: It allows molecules up to the size of small proteins to pass through.


Describe the role played by protein pumps during active transport in plants.

There are different proteins in the membrane, which play a major role in both active as well as passive transport. Active transport is carried out by membrane – protein. In protein pumps energy is used to carry substances across the cell membrane. These pumps can transport substances from a low concentration to a high concentration (‘uphill’ transport). Transport rate reaches a maximum when all the protein transporters are being used or are saturated. The carrier protein is very specific in what it carries across the membrane. These proteins are sensitive to inhibitors that react with protein side chains


Explain why pure water has the maximum water potential.

The water potential is greater if the concentration of the water is greater in the system. In pure water system the concentration of water is highest so the kinetic energy or water potential is maximum.


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.


(a) Diffusion and Osmosis



1. It is the movement of particles, molecules or ions from the region of their higher free energy to the region of their lower free energy.

1. It is the movement of solvent or water from the area of its higher free energy or chemical potential to the area of its lower free energy or chemical potential through a semi-permeable membrane.

2. It can occur in any medium.

2. It occurs only in the liquid medium.

3. The diffusing molecules may be solids, liquids or gases.

3. It involves the movement of solvent molecule only.

4. It does not require a semi-permeable membrane.

4. A semi-permeable membrane is required for the operation of osmosis.

5. It is purely dependent upon the free energy of the diffusing substance.

5. It depends upon the free energy or chemical potential of the solvent present on the two sides of the semi-permeable membrane.

(b) Transpiration and Evaporation



(i) This is a physiological process that occurs in plants.

(i) This is a physical process that occurs from any free surface.

(ii) It involves living tissues.

(ii) It may involve both living and non-living surfaces.

(iii) It is controlled by environmental factors as well as by plant factors such as osmotic pressure of the cells, thickness of cuticle, number and position of stomata, etc.

(iii) It is influenced only by environmental factors.

(iv) It moistens the surface of leaves and young stems and protects them from the burning Sun.

(iv) It causes dryness of free surfaces.

(v) It is comparatively a slow process.

(v) It is comparatively a faster process.

(c) Osmotic Pressure and Osmotic Potential

Osmotic pressure

Osmotic potential

1. The maximum pressure, which is developed in a solution, when it is separated from its pure solvent by a semi-permeable membrane, is called osmotic pressure.

1. The confined solution has osmotic pressure, but it does not literally exert any pressure. The pressure is exerted only when it is confined by a membrane. Thus, the term osmotic potential is preferred to the term osmotic pressure.

(d) Imbibition and Diffusion



1. Imbibition is a type of diffusion by which movement of water takes place along a diffusion gradient. An adsorbent is required for imbibition to occur. Swollen seeds produce a large pressure developed by imbibition

1. It is the movement of particles, molecules or ions from the region of their higher free energy to the region of their lower free energy.

2. It occurs in solids

2. It can occur in any medium.

3. Dry plant material or seeds act as adsorbent to imbibe water and swell

3. The diffusing molecules may be solids, liquids or gases.

4. The two factors involved in imbibition are,
(i) water potential gradient between the surface of the adsorbent and the liquid imbibed and
(ii) Affinity between the adsorbent and the imbibed liquid

4. It does not require a semi-permeable membrane. It is purely dependent upon the free energy of the diffusing substance.

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



1. The apoplastic movement of water occurs exclusively through the intercellular spaces and the walls of the cells.

1. In symplastic movement, the water travels through the cells – their cytoplasm;

2. Movement through the apoplast does not involve crossing the cell membrane. 

2. Water has to enter the cells through the cell membrane,

3. This movement is dependent on the gradient.

3. Movement is again down a potential gradient.

4. The apoplast does not provide any barrier to water movement and water movement is through mass flow.

4. intercellular movement is through the plasmodesmata.

(f) Guttation and Transpiration.



Due to root pressure water is pushed to a small height in the stem this excess of water collects in the form of droplets around special openings of veins. Such water loss in liquid phase is known as guttation.

Transpiration is the evaporative loss of water by plants. The water is lost through the stomata of the leaf.



Briefly describe water potential. What are the factors affecting it?

The fundamental concept of understanding the movement of water is called water potential. Water molecules possess kinetic energy and the greater the concentration of water greater the kinetic energy. This kinetic energy with which the movement of water takes place is called water potential. It is represented by the Greek symbol Psi or ψ w and is expressed in Pascals (pa).

The factors affecting water potential is solute potential and pressure potential. The solute potential is the magnitude of lowering the water potential due to dissolution of solute in pure water it is represented by ψ s and it is always negative.

The pressure potential is a pressure greater than the atmospheric pressure applied to pure water it is represented as ψ p and it is usually positive.


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

A pressure greater than atmospheric pressure when applied to pure water or a solution, its water potential increases. This pressure is called pressure potential and it is represented as ψ p it is usually positive.


(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.


(a) Plasmolysis is process, which takes when a cell is placed in a hypertonic solution. During this process water moves out of the cell and the cell membrane of a plant cell shrinks away from its cell wall. When water moves out; it is first lost from the cytoplasm and then from the vacuole. The water when drawn out of the cell through diffusion into the extracellular (outside cell) fluid causes the protoplast to shrink away from the walls. The cell is said to be plasmolysed. The movement of water occurred across the membrane moving from an area of high water potential (i.e., the cell) to an area of lower water potential outside the cell. Eg. When grapes are immersed in sugar solution for few days we can see that the solution inside the grapes has moved out to the external medium making the grapes to shrink.

(b) The process of plasmolysis is usually reversible when the plant cell is placed in solution, which has higher water potential. The water from the external solution enters into the plant cell causing the cytoplasm to build up a pressure against the cell wall called turgor pressure. Eg. When dry raisins are placed in pure water for few hours we can see that the water diffuses into the raisins making the cell turgid.


How is the mycorrhizal association helpful in absorption of water and minerals in plants?

The mycorrhizal association is a symbiotic association of a fungus with a root system. The fungal filaments form a network around the young root or they penetrate the root cells. The hyphae have a very large surface area that absorb mineral ions and water from the soil from a much larger volume of soil that perhaps a root cannot do. The fungus provides minerals and water to the roots, in turn the roots provide sugars and N-containing compounds to the mycorrhizae.


What role does root pressure play in water movement in plants?

The positive pressure created by the flow of water, which increases the pressure inside the xylem is called root pressure. This root pressure only provides a modest push in the overall process of water transport. They obviously do not play a major role in water movement up tall trees. The greatest contribution of root pressure may be to re-establish the continuous chains of water molecules in the xylem, which often break under the enormous tensions created by transpiration. Root pressure does not account for the majority of water transport; most plants meet their need by transpiratory pull.


Describe transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to plants?

The circulation of water through the xylem vessels is achieved mainly through the transpiration pull. The driving force for this process is transpiration from the leaves. This is referred to as the cohesion-tension-transpiration pull model of water transport.

Factors that influence transpiration are

(1) Water is transient in plants.

(2) Less than 1 per cent of the water reaching the leaves is used in photosynthesis and plant growth.

(3) Most of it is lost through the stomata in the leaves.

This water loss is known as transpiration.

The transpiration pull helps in the upward intake of water through roots and circulation of water through the xylem vessels to all the parts of the plant.


Discuss the factors responsible for ascent of xylem sap in plants.

The factors that responsible for the ascent of sap are

1.Cohesion – This is a mutual attraction between water molecules.

2.Adhesion – An attraction of water molecules to polar surfaces (such as the surface of tracheary elements) is known as adhesion.

3.Surface Tension – The water molecules are attracted to each other in the liquid phase more than to water in the gas phase.


What is the essential role the root endodermis plays during mineral absorption in plants?

The root endodermis has a layer of suberin in this layer which has the ability to actively transport ions only in one direction.


Explain why xylem transport is unidirectional and phloem transport bi-directional.

Xylem transports only inorganic nutrients like water and minerals. The source for water and minerals is soil; from the soil the inorganic nutrients move upward. Since this movement is in only one direction xylem transport is unidirectional.

Phloem transports mainly organic substances like food. The source in this transport is understood to be that part of the plant which synthesises the food, i.e., the leaf and sink, the part that needs or stores the food. But, the source and sink may be reversed depending on the season, or the plant’s needs. Sugar stored in roots may be mobilised to become a source of food in the early spring when the buds of trees, act as sink; they need energy for growth and development of the photosynthetic apparatus. Since the source-sink relationship is variable, the direction of movement in the phloem can be upwards or downwards, i.e., bi-directional.


Explain pressure flow hypothesis of translocation of sugars in plants.

The mechanism used for the translocation of sugars from source to sink is called the pressure flow hypothesis. As glucose is prepared at the source, which is the leaf, by the processes of photosynthesis it is converted to sucrose .The sugar is then moved in the form of sucrose into the companion cells and then into the living phloem sieve tube cells by active transport. This process of loading at the source produces a hypertonic condition in the phloem. Water in the adjacent xylem moves into the phloem by osmosis. As osmotic pressure builds up the phloem sap will move to areas of lower pressure. At the sink osmotic pressure must be reduced. Again active transport is necessary to move the sucrose out of the phloem sap and into the cells, which will use the sugar –converting it into energy, starch, or cellulose. As sugars are removed, the osmotic pressure decreases and water moves out of the phloem. This mechanism used for the translocation of sugar from source to sink is called pressure flow hypothesis.


What causes the opening and closing of guard cells of stomata during

The opening or closing of the stomata by the guard cell is due to a change in the turgidity of the guard cells. The inner wall of each guard cell, towards the pore or stomatal aperture, is thick and elastic. When turgidity increases within the two guard cells flanking each stomatal aperture or pore, the thin outer walls bulge out and force the inner walls into a crescent shape.

The opening of the stoma is also aided due to the orientation of the microfibrils in the cell walls of the guard cells. Cellulose microfibrils are oriented radially rather than longitudinally making it easier for the stoma to open. 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 stoma closes.

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