Questions - Answers
Very Short Answer Type Questions
1. What are porins? What role do they play in diffusion?
A: Porins are huge pores present in the
1) Plasma Membrane
2) Outer membrane of Chloroplast
3) Outer membrane of Mitochondria
4) Plasma membrane of Bacteria.
They are made of Proteins.
They help in the passive transport of hydrophilic substances only from higher concentration to lower concentration. It is called facilitated diffusion. They allow molecules upto the size of small proteins to pass through.
2. Define Water Potential? What is the value of water potential of pure water?
A: The potential energy or chemical potential of pure water is called Water Potential. It is a relative term and refers to the chemical potential of pure water to that of chemical potential of a solution. The term Water Potential was coined by R.O. Slatyr and S.A. Taylor (1960). Its symbol is ψ (Psi) derived from Greek. The water potential of pure water is Zero. It is the highest value.
3. Differentiate Osmosis and Diffusion.
A:
4. Compare transpiration and evaporation.
A: 
5. What are Apoplast and Symplast?
A: The system of cell walls, intercellular spaces, place between cell wall and plasma membrane and dead tissues are together called Apoplast. It is dead and continuous. Symplast comprises net work of protoplasm of the entire living cells of the plant inter connected by plasmodesmata. It is continuous and living.
6. How does Guttation differ from Transpiration?
A:
7. What happens when a pressure greater than atmospheric pressure is applied to pure water or solution?
A: Diffusion of water molecules through plasma membrane i.e. Osmosis is stopped.
8. Explain what will happen to plant cell if it is kept in a solution having higher water potential?
A: Endosmosis occurs. Water passes from solution of higher water potential into the cell. The size of the vaccuole increases. The cell becomes turgid.
9. What are physical factors responsible for the ascent of sap through Xylem in plants?
A: The physical factors responsible for ascent of sap are
1. Root pressure
2. Cohesion - Attraction between water molecules.
3. Adhesion - Attraction between water molecules and surface of tracheary walls.
4. Transpiration pull - It is the force behind ascent of sap.
10. Explain why xylem transport is unidirectional while that in Phloem is bidirectional?
A: Root absorbs water from soil. It is transported through xylem from root to the stem apex. Thus it is strictly unidirectional (Acropetal).
Food material produced in leaves is transported through phloem to storage organs and to the all parts including root. Thus it is multi or bidirectional. It can be upwards (acropetal) or downwards (basipetal).
11. With reference to transportation within plant cells, what are source and sink?
A: Leaf is source and root (or another non - photosynthetic part) is sink.
12. Does transpiration occur at night? Give an example.
A: Transpiration generally occurs during day time through photoactive stomata. But in some plants (succulents) like Bryophyllum, Cacti, Opuntia etc., transpiration occurs at night through scotoactive stomata (open at night).
13. Compare pH of guard cells during the opening and closing of stomata?
A: pH of the guard cells during day time is more (above 7) and during night it is less (below 7).
14. In the wake of transpirational loss, why do the C4 plants are more efficient than C3 plants?
A: C4 plants loose only half as much water as a C3 plant for the same amount of CO2 fixed. Thus C4 plants maximise CO2 availability and minimise water loss.
15. What is meant by transport saturation? How does it influence facilitated diffusion?
A: Transport rate reaches maximum when all of the protein carriers (Porins) are used. It is called Transport saturation. It indicates high rate of facilitated diffusion.
16. How does ABA bring about the closure of stomata under water stress conditions?
A: Under Water stress conditions ABA (natural antitranspirant) closes the stomata by
1) changing the permeability of plasma membrane.
2) driving out K+ ions out of guard cells.
17. Pressure potential in plant systems can be negative. Elaborate.
A: Pressure potential in plant systems can be sometimes negative - when the plants loose water by transpiration inspite of insufficient water in the soil.
Short Answer Type Questions
1. Define and explain water potential?
A: The term water potential was coined by R.O. Slatyr and Sterling A. Taylor (Australian biologists) in 1960. The chemical potential of pure water is called water potential. It is a relative value. Water potential of pure water is arbitrarily decided as ZERO. It is the highest value. Water potential of any solution is Negative (less than ZERO). More the concentration of solution water potential value is more -ve.
It's symbol is ψ or ψw (Psi is a greek term). It's measured in Pascals (Pa) or Mega pascals (MPa) or bars. The term water potential is coined in the place of Diffusion Pressure Deficit (DPD). Water potential value of soil solution is more and that of cell sap is less. So water passes from the region of higher water potential to a region of lower water potential i.e., from the soil into the roots of the plant.
2. Write short notes on facilitated diffusion.
A: Lipids from major constituents of plasma membrane. Lipid soluble substances pass through the membrane easily and move faster. But hydrophilic substances find it difficult to pass through the membranes. Their diffusion requires the help of protein channels called Porins. They are present in the plasma membrane. The diffusion of hydrophilic substances through plasma membrane with the help of Porins is called facilitated diffusion. It is passive and specific and selective. It allows the cells to select substances for uptake. The porins may be larger or smaller. They may be kept always open or their movements may be controlled.
Porins are present not only in the plasma membrane and also seen in the outer membranes of Chloroplasts, mitochondria and also some bacteria.
3. What is meant by Plasmolysis? How is it practically useful to us?
A: The outside movement of water from the cell during which plasma membrane shrinks is called plasmolysis or exosmosis or negative osmosis or Anosmosis or reverse osmosis. It occurs when a cell is kept in hypertonic solution as follows.
1. Water is first lost from the Cytoplasm.
2. Then water from the vacuole is lost.
3. Protoplast shrinks.
4. Vacuole becomes smaller.
5. Plasma membrane separates from the cell wall, first in the corners. It is called incipient plasmolysis.
The cell that looses water is said to be plasmolysed or flaccid cell. In such cells cell sap becomes hypertonic.
6. Hypertonic solution is seen between cell wall and plasma membrane. It occurs when there is water stress or in saline environments.
The principle of plasmolysis is useful in making
(1) Jellys (2) Pickles (3) Raisins (4) Jams
4. How does ascent of sap occur in tall trees?
A: Upward movement or translocation of water with its dissolved minerals and salts from root to the shoot tip against to gravitational force is called Ascent of sap. Stephen Hales was the first to propose that it occurs through xylem tissue. Ascent of sap in taller trees occurs according to Cohesion-tension-transpiration pull model. It was proposed by Dixon.
Cohesion: The union between 2 water molecules is called cohesion. The force developed due to cohesion is called Cohesive force. It is around 30 MPa.
Adhesion: The union between water molecules and xylem walls is called adhesion. It develops adhesive force. It helps in continuous movement of water.
Transpiration pull: Water loss from the leaves when temperature is more during day time is called transpiration. It results in pull in the xylem vessels. It becomes the driving force for ascent of sap. Since the xylem of the leaf is in continuity with xylem of stem and root, water ascends upwards in the form of an unbroken water column.
5. Stomata are turgor operated values. Explain.
A: Stoma is an elliptical pore in the epidermis. It is protected by 2 kidney shaped guard cells. The inner wall of each guard cell, towards the pore is thick and outer wall towards the subsidiary cells is thin, convex and elastic. Guard cells have chloroplasts. The cell walls have cellulosic microfibrils oriented radially.
According to Levitt's K+ Pump theory
Active Efflux of H+ leads to passive in flux of K+ and Cl -. Water potential value of guard cells decreases, Endosmosis occurs. Guard cells become turgid. Outer thin walls of guard cells expand outwardly and become more convex. It forces the inner walls into a crescent shape Stoma opens. At night, Efflux of K+ and Cl - occurs. In turn influx of H+ occurs. Water potential increases. Exosmosis occurs. Guard cells become flaccid. Stoma closes. Thus the turgidity of the guard cells controls the movement of stomata. Hence we can say that stoma is a turgor operated valve.
6. Explain pressure flow hypothesis of translocation of sugars in plants.
A: Girdling experiment proves that food materials are translocated through Phloem. Food is conducted in the form of Sucrose. It occurs between source (Leaf) and sink (Root). Source is one but sink may be any non-photosynthetic organ. So the conduction of food is multidirectional. Munch (German) proposed Mass flow/ pressure flow hypothesis to explain translocation of Sugars. It occurs according to Turgor Pressure gradient (from higher T.P. to lower T.P.). It is explained in 2 steps.
1) Phloem Loading: Sucrose moves into sieve tube cells through companion cells by active transport. The solution in phloem becomes hypertonic. Water moves from xylem moves into phloem by osmosis. ψ in Phloem increases T.P. becomes more. Then sugars move from source to sink.
2) Phloem Unloading: At sink sucrose is unloaded. It results increase in Mass at sink. Sucrose is converted to starch or utilised in sink ψ increases. It pushes the water in the xylem towards source.
7. "Transpiration is a necessary evil". Explain.
A: Though excessive transpiration retards growth of the plant and plants show permanent wilting, transpiration is beneficial to plants in many ways like
1) It helps in passive absorption of minerals and water.
2) It provides necessary force to pull the water during ascent of sap.
3) It regulates the temperature of the plant body.
4) It gives cooling effect.
Hence transpiration is a necessary evil.
8. A Gardener forgot to water a potted plant for a day in summer. What will happen to the plant? Do you think it is reversible? Explain.
A: The plant shows wilting. It is reversible. If the plant is again watered, it becomes normal. That wilting showed by the plant is temporary. If the plant is not watered, it becomes wilted permanent.
9. Explain the type of molecular movement which is highly selective and requires special membrane proteins, but does not require any metabolic energy.
A: Hydrophilic substances or molecules are transported through the membrane proteins called Porins. The movement is highly selective. Porins are specific, special proteins present in the membrane which help in the facilitated transport without the expenditure of energy. It may be uniport, symport or antiport. It may be shown as follows.
1) Molecules move through the carrier independently in one direction.
e.g.: H+
2) Two molecules move in opposite direction.
e.g.: Inward movement of H+
outward movement of Na+
3) Two molecules move in the same direction.
e.g.: H+, NO3-
H+, Cl-
H+, PO4-
10. How does most of the water move within a healthy plant body and by which path?
A: Water moves within a healthy plant body takes place through xylem by apoplast pathway.
11. Transpiration and photosynthesis, a compromise. Explain.
A: Surface of the lamina is related to the rate of transpiration and photosynthesis.
1) If the lamina is large the rate of transpiration is more. If transpiration is more it may cause insufficient supply of water which further affects photosynthesis.
2) Lamina is reduced to reduce the rate of transpiration. In such case, rate of photosynthesis will also reduce as the lamina is narrow. Thus plant has to survive by making a compromise between transpiration and photosynthesis.
12. Do different species of plants growing in the same area show the same rate of transpiration at a particular time. Justify your comment.
A: The same area may be inhabited by different species belonging to mesophytes, xerophytes, dicots and monocots. If there is a pond, hydrophytes also grow there. Through they grow in the same area the rate of transpiration is not same because
1) Mesophytes - The rate of transpiration is more.
2) Xerophytes - The rate of transpiration or amount of transpiration is less.
3) Hydrophytes show very less or no transpiration.
Exercises
1. Differentiate uphill and downhill transport?
A.
2. Compare facilitated diffusion and simple diffusion.
A.
3. What happens when two solutions of different concentrations are separated by egg membrane? State the reason.
A. Water (solvent) passes from the solution of lower concentration to the solution of higher concentration through the egg membrane. This process is called Osmosis.
It is also diffusion but taking place through a membrane.
Egg membrane is acting as semipermeable membrane. Water passes from the region of higher water potential to the region of lower water potential. This occurs until both solutions reach equilibrium.
4. Compare imbibing capacity of Pea seeds and Wheat seeds.
A. Pea seeds store proteins in its cotyledons. Wheat seeds store starch in the endosperm. The imbibing capacity of proteins is very high when compared to the starch (carbohydrates).
5. In general in a plant which path of water movement is more and why?
A. There are 2 kinds of distinct pathways of water movement in the plants.
1) Apoplast pathway
2) Symplast pathway
Apoplastic movement of water occurs through the cell walls and intercellular places. It does not involves any crossing the cell membranes. Where as symplast pathway occurs through the cytoplasm plasma membrane and plasmodesmata. Hence Apoplast path of water movement in the plants is faster.
6. Why Pinus seeds fail to germinate in the absence of Mycorrhiza?
A. Mycorrhiza is a symbiotic association of fungi with roots of higher plants. It absorbs water, minerals and supplies to the roots. Thus fungus takes shelter and food from the roots and inturn supplies water and minerals. The roots of the host have no root hairs. So host needs the help of fungus.
Pinus seeds can not germinate and establish without mycorrhiza as seeds have obligate association with mycorrhiza.
7. Which structures do you think the Pinus plant does not possess due to which its seeds fail to germinate?
A. Pinus plant does not possess root hairs. So it cannot absorb water on its own.
8. What do you think is the driving force for ascent of sap?
A. Transpiration pull is the driving force for ascent of sap.
9. Why do stomata close under water stress conditions?
A. When there is insufficient water in the soil, plants are subjected to water stress. Then Abscisic Acid (ABA) is produced more and it induces the stomatal closure.
10. How are stomata distributed in a typical monocot plant?
A. A typical monocot leaf has stomata on upper and lower epidermis. Such leaf is said to be Amphistomatic. The number of stomata is same on either side. So it can also be called iso-bilateral leaf.
11. In what form the sugars are transported through phloem?
A. Sugars are transported through phloem in the form of sucrose.
12. The inward movement of water into a plant begins either as symplast or apoplast. How does it conclude before entering into xylem?
A. The movement of water through the root layers from epidermis to cortex (endodermis) may be apoplastic or symplastic. But ultimately it is symplastic in the endodermis and pericycle, before entering into xylem.
13. Why does the root endodermis transports ions in one direction only?
A. As mineral ions are dissolved in water the root endodermis transports ions in one direction only.
14. If a ring of bark is removed from an actively growing plant, what will happen and why?
A. The part of the bark above the ring on the stem swells in a week as the food is accumulated in the tissues there, because food was not allowed to pass down due to the absence of phloem. It indicates that phloem is responsible for the conduction of food.
15. A flowering plant is planted in earthen pot and watered. Urea is added to make the plant grow faster, but after sometime the plant dies. Why?
A. When urea is added, it dissolved in the water and soil solution became hypertonic. So the plant could not absorb water instead, it lost due to exosmosis. So the plant died.
