Transport in Plants
Welcome to HSLC Guru! In this article, we provide complete question answers, important notes, MCQs, fill in the blanks, true or false statements, and a glossary for Class 11 Biology Chapter 11 — Transport in Plants based on the latest ASSEB (Assam State School Education Board) syllabus. This chapter explains how plants move water, minerals, gases, and food across short and long distances. Students will learn the mechanisms of diffusion, osmosis, transpiration, ascent of sap, and the pressure-flow hypothesis. The chapter is essential for building strong concepts for board examinations and competitive entrance tests.
Summary
Means of Transport: Plants transport substances by three main processes — diffusion, facilitated diffusion, and active transport. Diffusion is the passive movement of molecules from a region of higher concentration to lower concentration without expenditure of energy; it is slow and depends on gradient, temperature, and the size of molecules. Facilitated diffusion involves transport proteins (porins, aquaporins, symporters, antiporters, uniporters) that help polar or large molecules cross the membrane down their concentration gradient without ATP. Active transport uses ATP-driven pumps to move molecules against their concentration gradient and shows saturation kinetics like enzymes.
Plant–Water Relations: Water status in plant cells is described by water potential (Ψw), which depends on solute potential (Ψs) and pressure potential (Ψp). Pure water at standard conditions has Ψw = 0; addition of solutes makes it negative. Osmosis is the diffusion of water across a semi-permeable membrane along the water potential gradient. When a cell is placed in a hypertonic solution, water leaves the cell, the protoplast shrinks away from the wall and the cell undergoes plasmolysis; in a hypotonic solution, water enters and the cell becomes turgid. Imbibition is the special form of diffusion in which water is absorbed by hydrophilic solids such as seeds and dry wood, generating large imbibition pressures.
Long-Distance Transport — Apoplast and Symplast: Water and minerals absorbed by roots move radially through the cortex by two pathways. The apoplast pathway goes through cell walls and intercellular spaces without crossing membranes; it offers least resistance. The symplast pathway goes through the cytoplasm of cells connected by plasmodesmata. At the endodermis, the Casparian strip blocks the apoplast, forcing water into the symplast. Ascent of sap — upward movement of water through xylem — is explained by transpiration pull, root pressure, capillary action, and the cohesion–tension theory of Dixon and Joly. Cohesion among water molecules and adhesion to xylem walls maintain a continuous water column pulled up by transpiration from leaves.
Transpiration, Mineral Uptake and Phloem Transport: Transpiration is the loss of water in vapour form from aerial parts; it is of three types — stomatal (most important), cuticular, and lenticular. It is affected by temperature, light, humidity, wind speed, and the number and distribution of stomata. The opening and closing of stomata depend on the turgidity of guard cells controlled by K+ ion movement. Antitranspirants like PMA, abscisic acid, low-viscosity silicone oils, and white plastic reflectors reduce water loss. Mineral nutrients are absorbed mainly as ions by active transport at the root epidermis and loaded into the xylem. Phloem transport of food (sucrose) follows the pressure-flow or Münch’s hypothesis — sugars are loaded at the source (leaves), creating low water potential that draws in water; the resulting high turgor pressure pushes the sap to sinks (roots, fruits) where sugars are unloaded and water exits.
Question and Answers
1 Mark Questions
Q1. What is diffusion?
Answer: Diffusion is the passive movement of molecules from a region of higher concentration to a region of lower concentration along the concentration gradient without expenditure of metabolic energy.
Q2. Define water potential.
Answer: Water potential (Ψw) is the chemical potential of water expressed in pressure units (bars or pascals). It is the sum of solute potential and pressure potential and determines the direction of water movement.
Q3. What is plasmolysis?
Answer: Plasmolysis is the shrinkage of the protoplast away from the cell wall when a plant cell is placed in a hypertonic solution due to exosmosis of water.
Q4. Define imbibition.
Answer: Imbibition is the absorption of water by solid colloids (such as dry seeds, wood) causing them to swell. It is a special type of diffusion accompanied by an increase in volume.
Q5. What is the apoplast pathway?
Answer: The apoplast pathway is the movement of water and dissolved substances through the non-living parts of the plant — cell walls and intercellular spaces — without crossing any membranes.
Q6. Name the scientists who proposed the cohesion–tension theory.
Answer: The cohesion–tension theory of ascent of sap was proposed by Dixon and Joly in 1894.
Q7. What are antitranspirants?
Answer: Antitranspirants are chemicals or substances that reduce transpiration without affecting other physiological processes. Examples include phenyl mercuric acetate (PMA), abscisic acid, and silicone oil.
Q8. What is the source–sink relationship in phloem transport?
Answer: Phloem transports sugars from the source (where sugar is produced or stored, e.g., mature leaves) to the sink (where sugar is consumed or stored, e.g., roots, fruits). The direction may reverse depending on plant needs.
Q9. Name the cells that regulate stomatal opening and closing.
Answer: Guard cells, the bean-shaped (or dumbbell-shaped in grasses) cells flanking the stomatal pore, regulate stomatal opening and closing through changes in their turgor pressure.
Q10. What is root pressure?
Answer: Root pressure is the positive hydrostatic pressure developed in the xylem of roots due to active accumulation of ions, which forces water upwards into the stem. It is responsible for guttation in some plants.
2-3 Marks Questions
Q1. Differentiate between diffusion and active transport.
Answer: Diffusion is a passive process in which molecules move down a concentration gradient without using energy; it is slow and non-selective. Active transport, on the other hand, moves molecules against the concentration gradient using ATP, is mediated by specific carrier proteins, shows saturation kinetics, and can be inhibited by metabolic inhibitors.
Q2. Differentiate between apoplast and symplast pathways.
Answer: The apoplast pathway is movement through cell walls and intercellular spaces; it does not cross membranes and offers low resistance. The symplast pathway is movement through the cytoplasm of cells connected by plasmodesmata; it crosses membranes and is slower. At the endodermis, the Casparian strip blocks apoplastic flow, forcing water into the symplast.
Q3. What are the factors affecting transpiration?
Answer: External factors include light intensity (increases transpiration), temperature (increases the rate by raising vapour pressure), humidity (decreases transpiration), wind speed (increases up to a limit), and atmospheric pressure. Internal factors include leaf area, leaf structure, number and distribution of stomata, presence of cuticle, and root–shoot ratio.
Q4. Explain the three types of transpiration.
Answer: (i) Stomatal transpiration takes place through stomata mostly on the lower surface of leaves and accounts for nearly 80–90% of total water loss. (ii) Cuticular transpiration occurs through the cuticle of leaves and young stems; it is small but significant in plants with thin cuticles. (iii) Lenticular transpiration occurs through lenticels of woody stems and contributes a very small fraction.
Q5. Briefly explain the role of guard cells in stomatal movement.
Answer: Guard cells regulate the opening and closing of stomata. When K+ ions actively enter guard cells, water follows by osmosis, making them turgid; their thin outer walls bulge, the thick inner walls bow apart and the stoma opens. When K+ ions leave, water also leaves, guard cells become flaccid and the stoma closes. Light, CO2 levels and abscisic acid influence this movement.
Q6. Explain Münch’s pressure-flow hypothesis briefly.
Answer: Proposed by Ernst Münch in 1930, this hypothesis explains translocation of food in phloem. At the source, sugars are loaded into sieve tubes, lowering water potential; water enters from adjacent xylem, raising turgor. At the sink, sugars are unloaded, water leaves and turgor falls. The pressure gradient between source and sink drives bulk flow of phloem sap from source to sink.
5-7 Marks Questions
Q1. Describe the various means of transport in plants — diffusion, facilitated diffusion and active transport — with their characteristics.
Answer: Plants use three main means to transport substances across cell membranes:
(i) Diffusion is the simplest, slowest and passive transport. Molecules move from higher to lower concentration along the gradient without ATP. It is non-specific and depends on temperature, gradient steepness, particle size and density of the medium. Gases like O2 and CO2 move through stomata by diffusion.
(ii) Facilitated diffusion uses transport proteins to help large or polar molecules (sugars, amino acids, ions) cross the lipid membrane down their gradient without ATP. It is specific and shows saturation kinetics. Channel proteins like aquaporins and porins, and carrier proteins (uniporter, symporter, antiporter) participate.
(iii) Active transport moves molecules against the concentration gradient using ATP-driven pumps. It is faster, highly selective and shows saturation kinetics like enzymes. Examples include the uptake of mineral ions by root cells. Both facilitated diffusion and active transport are protein-mediated and can be inhibited by chemicals that bind transport proteins.
Q2. Explain water potential, solute potential and pressure potential. How do they determine water movement in plant cells?
Answer: Water potential (Ψw) is the free energy of water per unit volume, measured in pascals or bars. The water potential of pure water at standard temperature and atmospheric pressure is taken as zero. The relationship is:
Ψw = Ψs + Ψp
Solute potential (Ψs) is the reduction in water potential due to the presence of solutes; it is always negative because solutes lower the free energy of water. The greater the solute concentration, the more negative the solute potential.
Pressure potential (Ψp) is the pressure exerted by the cell wall on the protoplast (or vice versa); it is usually positive in turgid cells and adds to the water potential. In the xylem of transpiring plants, it can be negative (tension).
Water always moves from a region of higher (less negative) water potential to lower (more negative) water potential. This principle governs osmosis, water absorption by roots, ascent of sap, and the loading and unloading of phloem.
Q3. Explain the cohesion–tension theory for the ascent of sap. Discuss the contribution of root pressure, capillarity and transpiration pull.
Answer: The ascent of sap is the upward movement of water and dissolved minerals from roots to the leaves through xylem. The most accepted explanation is the cohesion–tension and transpiration pull theory proposed by Dixon and Joly (1894).
(i) Cohesion: Water molecules attract each other strongly through hydrogen bonding, giving xylem sap high tensile strength.
(ii) Adhesion: Water molecules also stick to the lignified walls of xylem vessels, preventing the column from breaking.
(iii) Transpiration pull: Loss of water from leaf mesophyll cells creates negative pressure (tension) that pulls the continuous water column upwards through xylem like a suction force.
Root pressure develops due to active uptake of ions by roots and creates a positive pressure in xylem. It contributes only in short herbaceous plants and during early morning hours; it cannot push water to the top of tall trees and is responsible for guttation. Capillary action raises water through the narrow xylem vessels by surface tension, but only by a few centimetres in tall plants. The major force responsible for ascent of sap in tall trees is therefore the transpiration pull supported by cohesion and adhesion of water molecules.
Q4. What is transpiration? Describe its types, significance and the factors affecting its rate.
Answer: Transpiration is the loss of water in the form of vapour from the aerial parts of plants. It occurs mainly through stomata, but also through cuticle and lenticels.
Types: (i) Stomatal transpiration — through stomata; accounts for 80–90% of total transpiration. (ii) Cuticular transpiration — through the cuticle of leaves; about 5–10%. (iii) Lenticular transpiration — through lenticels of woody stems; about 0.1%.
Significance: Transpiration creates the pull that lifts water and minerals up the plant; it cools the leaves through evaporative loss of latent heat; it maintains the shape and structure of cells through turgidity; and it helps in the distribution of mineral nutrients.
Factors: External factors — light, temperature, atmospheric humidity, wind velocity, available soil water, and atmospheric pressure. Internal factors — leaf area, leaf structure (cuticle thickness, hairs), number, size and distribution of stomata, water status of the plant, and orientation of leaves. High humidity and low temperature reduce transpiration, while high temperature, light and moderate wind increase it.
Q5. Describe the uptake and translocation of mineral nutrients in plants. Also explain the pressure-flow hypothesis of phloem transport.
Answer: Plants absorb mineral nutrients from the soil mainly as ions. The uptake involves both passive and active mechanisms. Initially, ions enter the apoplast of root epidermal cells passively. To enter the symplast and reach the xylem, ions must be transported actively across the plasma membrane against their electrochemical gradient using ATP-driven pumps and specific carriers. Once loaded into the xylem, mineral ions are carried upward along with water by the transpiration stream. They are unloaded near the cells where they are needed and may be remobilized through the phloem.
Phloem transport — Pressure-flow (Münch) hypothesis: Translocation of organic food (mainly sucrose) takes place through phloem. According to Münch’s pressure-flow hypothesis (1930):
(i) At the source (e.g., mature leaves), sugars produced by photosynthesis are actively loaded into sieve tubes, often via companion cells. This raises solute concentration and lowers water potential in sieve tubes.
(ii) Water moves into the sieve tubes from the adjacent xylem by osmosis, generating high turgor pressure at the source end.
(iii) At the sink (e.g., growing fruits, roots), sugars are actively unloaded into surrounding cells. Water potential in sieve tubes rises and water leaves into the xylem, lowering turgor at the sink end.
(iv) The pressure gradient between source and sink drives a bulk flow of phloem sap from source to sink. Since phloem can load and unload at either end, the direction of transport can reverse between seasons (for example, sugar moves from storage roots to growing leaves in spring).
Multiple Choice Questions (MCQs)
Q1. Diffusion is:
(a) Active and fast
(b) Passive and slow
(c) Active and slow
(d) Passive and fast
Answer: (b) Passive and slow.
Q2. Water potential of pure water at standard conditions is:
(a) Positive
(b) Negative
(c) Zero
(d) Infinite
Answer: (c) Zero.
Q3. The cohesion–tension theory for the ascent of sap was proposed by:
(a) Münch
(b) Curtis
(c) Dixon and Joly
(d) Priestley
Answer: (c) Dixon and Joly.
Q4. Plasmolysis occurs when a cell is placed in:
(a) Hypotonic solution
(b) Isotonic solution
(c) Hypertonic solution
(d) Pure water
Answer: (c) Hypertonic solution.
Q5. The Casparian strip is found in:
(a) Epidermis
(b) Cortex
(c) Endodermis
(d) Pericycle
Answer: (c) Endodermis.
Q6. Pressure-flow hypothesis of phloem translocation was given by:
(a) Dixon
(b) Münch
(c) Curtis
(d) Hales
Answer: (b) Münch.
Q7. Which of the following is an antitranspirant?
(a) IAA
(b) PMA (phenyl mercuric acetate)
(c) Gibberellin
(d) Cytokinin
Answer: (b) PMA.
Q8. The opening of stomata is mainly due to entry of:
(a) Na+
(b) Ca2+
(c) K+
(d) Cl-
Answer: (c) K+.
Q9. Imbibition takes place in:
(a) Living cells only
(b) Hydrophilic colloids
(c) Hydrophobic colloids
(d) Mineral ions
Answer: (b) Hydrophilic colloids.
Q10. The major force responsible for ascent of sap in tall trees is:
(a) Root pressure
(b) Capillarity
(c) Transpiration pull
(d) Atmospheric pressure
Answer: (c) Transpiration pull.
Fill in the Blanks
Q1. The water potential of pure water is taken as ________.
Answer: Zero.
Q2. Movement of water through cell walls and intercellular spaces is called the ________ pathway.
Answer: Apoplast.
Q3. The phloem transport of food is explained by ________ hypothesis.
Answer: Münch’s pressure-flow.
Q4. The loss of water in liquid form from leaves is called ________.
Answer: Guttation.
Q5. Stomata generally remain open during the ________ in most plants.
Answer: Day.
True or False
Q1. Active transport requires ATP. (True/False)
Answer: True.
Q2. Water potential of a solution is always greater than that of pure water. (True/False)
Answer: False.
Q3. The Casparian strip is impermeable to water and prevents apoplastic flow at the endodermis. (True/False)
Answer: True.
Q4. Lenticular transpiration is the major form of transpiration in herbaceous plants. (True/False)
Answer: False.
Q5. Phloem transport always occurs from roots to leaves. (True/False)
Answer: False.
Glossary
| Term | Meaning |
|---|---|
| Diffusion | Passive movement of molecules from higher to lower concentration without ATP. |
| Facilitated Diffusion | Protein-mediated passive transport across membranes down the gradient. |
| Active Transport | Movement of molecules against the gradient using ATP. |
| Water Potential (Ψw) | Free energy of water per unit volume; sum of solute and pressure potentials. |
| Solute Potential (Ψs) | Reduction in water potential due to dissolved solutes; always negative. |
| Pressure Potential (Ψp) | Hydrostatic pressure exerted on water in a cell; usually positive in turgid cells. |
| Osmosis | Movement of water across a semipermeable membrane along the water potential gradient. |
| Plasmolysis | Shrinkage of protoplast away from the cell wall in a hypertonic solution. |
| Imbibition | Absorption of water by hydrophilic colloids causing swelling. |
| Apoplast | Continuous system of cell walls and intercellular spaces. |
| Symplast | Continuous system of cytoplasm interconnected by plasmodesmata. |
| Casparian Strip | Suberin-impregnated band on radial walls of endodermal cells blocking apoplast. |
| Ascent of Sap | Upward movement of water and minerals through xylem. |
| Root Pressure | Positive pressure developed in roots due to active ion uptake. |
| Transpiration | Loss of water in vapour form from aerial parts of plants. |
| Guttation | Loss of water in liquid form through hydathodes due to root pressure. |
| Stomata | Small pores on leaf surfaces flanked by guard cells for gas and water exchange. |
| Antitranspirants | Substances that reduce water loss without affecting other processes. |
| Cohesion | Mutual attraction between water molecules due to hydrogen bonding. |
| Adhesion | Attraction of water molecules to xylem vessel walls. |
| Pressure-Flow Hypothesis | Münch’s theory of phloem transport from source to sink by bulk flow. |
| Source | Site of sugar production or storage from which sugars are exported. |
| Sink | Site of sugar consumption or storage where sugars are imported. |