Plant Transport

NCERT Solution

Question 1: What are the factors affecting the rate of diffusion?

Answer: Factors affecting diffusion are; Concentration gradient, Permeability of membrane; separating the substance, Temperature and Pressure.

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

Answer: Carrier proteins form porin channels in the outer membranes of plastids, mitochondria and some bacteria. The porin channels allow the molecules up to the size of small proteins to pass through molecule bound to the transport protein and thus allow facilitated diffusion.

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

Answer: Energy pumps are used against a concentrations gradient; in case of active transport. Active transport is carried out by membrane proteins. Pumps are proteins which use energy to carry substances across the cell membrane. The rate of transport reaches the maximum when all the protein transporters are being used or are saturated.

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

Answer: Water molecules possess kinetic energy. A system with higher concentration of water has a higher kinetic energy or water potential (Ψw). Hence, pure water has the highest water potential.

Question 5: Differentiate between the following:

(a)Diffusion and Osmosis

Answer: Osmosis is a type of diffusion. When diffusion happens across a semi-permeable membrane, it is called osmosis. Semi-permeable membrane is not necessary in all cases of diffusion.

(b)Transpiration and Evaporation

Answer: Evaporative loss of water from plants is called transpiration, while conversion of water into vapour at any temperature is called evaporation.

Answer: The pressure which needs to be applied to prevent the inward flow of water across a semi-permeable membrane. In other words, the minimum pressure needed to negate the osmosis is called osmotic pressure. On the other hand, the ability of a solution to suck in water from across a semi-permeable membrane is called osmotic potential.

(d) Imbibition and Diffusion

Answer: Random movement of molecules to attain concentration equilibrium is called diffusion. When osmosis happens in a way that solids (colloids) take up water, it is called imbibition.

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

Answer:

Apoplast Pathway	Symplast Pathway
The free diffusional space outside the plasma membrane is called apoplast.	The inner side of plasma membrane is called symplast.
Formed by continuum of cell walls.	Is made continuous because of plasmodesmata.
Water and solutes are usually transported by this.	Smaller molecules are transported through this.

(f)Guttation and Transpiration.

Answer: Exudation of water from smaller plants; under low evaporation conditions; is called guttation. Evaporative loss of water from plants is called transpiration. In guttation, water comes out in liquid form; while in transportation, water comes out in gaseous form.

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

Answer: Water molecules possess kinetic energy. A system with higher concentration of water has a higher kinetic energy or water potential (Ψw). Pure water has the highest water potential. Solutions have lower water potential than pure water. Solute potential and pressure potential are the two factors which affect water potential.

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

Answer: If a pressure; greater than atmospheric pressure is applied to pure water or a solution, its water potential increases. This happens because of pressure potential which develops due to increased pressure.

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

Answer: When a plant cell is placed in hypertonic solution, the plant cell loses water and hence it loses the turgor pressure. This makes the cell flaccid. The plant cells wilt in this condition. Further water loss results in plasmolysis. At this point, the pressure decreases to an extent where the protoplasm of the cell peels away from the cell wall. This leaves gaps between the cell wall and the membrane. This can also lead to cytorrhysis or complete collapse of the cell wall. Plasmolysis can be reversed by putting the cell in hypotonic solution.

Question 9: Explain what will happen to a plant cell if it is kept in a solution having higher water potential.

Answer: A hypotonic solution has higher water potential. When a plant cell is placed in hypotonic solution, the cell gains water and thus gains turgor pressure. This leads to the cell getting swollen. But the rigidity of the cell wall, prevents the cell from bursting.

Question 10: How is the mycorrhizal association helpful in absorption of water and minerals in plants?

Answer: Mycorrhiza is a symbiotic association of a fungus with a root system. The hyphae form a network around young roots and thus increase the surface area. This helps in getting access to more water and minerals for the plants.

Question 11: What role does root pressure play in water movement in plants?

Answer: Root pressure can only provide a modest push. Hence root pressure does not play a major role in water movement in tall plants. Root pressure contributes towards reestablishment of continuous chains of water molecules in the xylem; which often break under enormous tensions created by transpiration pull.

Question 12: Describe transpiration pull model of water transport in plants. What are the factors influencing transpiration? How is it useful to plants?

Answer: Transpiration creates a suction force inside the xylem. This suction force is called transpiration pull. This is powerful enough to pull the water column from beneath. Adhesion, cohesion and surface tension are the important physical properties of water which further help in the upward movement of water through xylem.

Factors Affecting Transpiration: Temperature, light, humidity, wind speed, number and distribution of stomata, number of stomatal aperture with guard cells open, water status of the plant, canopy structure, etc.

Transpiration helps the plants in following ways:

Supplies water which is required for photosynthesis.
Has cooling effect on leaves.
Helps in maintaining the turgidity and shape of plant parts.

Question 13: Discuss the factors responsible for ascent of xylem sap in plants.

Answer: Cohesion: Mutual attraction between water molecules is called cohesion.

Adhesion: Attraction of water molecules to polar surfaces is called adhesion.

Surface Tension: Any liquid has a tendency to occupy the least possible surface area. This property is called surface tension.

The above mentioned properties impart high tensile strength to water. The high tensile strength imparts an ability to resist a pulling force and high capillarity. The ability to rise in tubes is called capillarity. The thin tubes of xylem work like capillary tubes.

Question 14: What essential role does the root endodermis play during mineral absorption in plants?

Answer: Minerals need to be actively absorbed by the epidermal cells. Specific proteins in the membranes of root hairs actively pump ions from the soil to the epidermal cells.

Question 15: Explain why xylem transport is unidirectional and phloem transport bi-directional.

Answer: Water transported through xylem is utilised in photosynthesis and most of the water is lost through transpiration. Renewed demand for water is once again supplied through the same channel. Hence, transport through xylem is unidirectional. In case of phloem transport, food is transported from source to sink. Leaves are the usual source and storage organs are the usual sink. But the storage organs become source when new buds emerge during early spring. In that case, a reverse flow of food is required. Hence, movement through phloem is bi-directional.

Question 16: Explain pressure flow hypothesis of translocation of sugars in plants.

Answer: The pressure flow or mass flow hypothesis:

When glucose is prepared at the source, it is converted to sucrose.

The sucrose moves into the companion cells and then into the living phloem sieve tube cells; through active transport. This process of loading at the source produces a hypertonic condition in the phloem.

Water; from the adjacent xylem; moves into the phloem, by osmosis. This results in an increase of osmotic pressure. It forces the phloem sap to areas of lower pressure, i.e. towards the sink . The osmotic pressure must be reduced at the sink.

Active transport moves the sucrose out of the phloem sap into the cells in the sink. Once the sugar is removed, the osmotic pressure decreases and water moves out of the phloem.

Question 17: What causes the opening and closing of guard cells of stomata during transpiration?

Answer: A change in the turgidity of guard cells results in closing or opening of stomata. The inner wall of the guard cell; towards the stomatal aperture; is thick and elastic. An increase in turgidity results in the thin outer walls to bulge out. This forces the inner wall into a crescent shape and results in opening of stoma. The orientation of the microfibrils in the cell walls of the guard cells also helps in opening of stomata. These microbifibrils are radially oriented and thus make it easy for the stoma to open up. A loss in turgidity of the guard cells, leads to resumption of shape of the elastic inner wall of the guard cell and the stoma closes.