NCERT Class 11 Biology Transport in Plants Important Notes

Diffusion:

When molecules move from higher concentration to lower concentration in a random manner, the movement is called diffusion. The movement of substances through diffusion is a passive transport. Diffusion is a slow process and is not dependent on a ‘living’ system.

Factors affecting diffusion:

• Concentration gradient,

• Permeability of membrane; separating the substances

• Temperature and Pressure 

Facilitated Diffusion:

While non-polar substances diffuse through the membrane, the polar substances move with help of special proteins. This process is called facilitated diffusion. Special proteins help the substances move against concentration gradient and this is energy dependent process. The energy for this is supplied by ATP. Facilitated diffusion happens with the help of proteins which are selective in nature. Such a transport is also sensitive to inhibitors.

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. The transport protein then rotates and releases the molecule inside the cell. For example; water channels are made up of eight different types of aquaporins.

Symports and Antiports: Some carrier proteins allow diffusion only if two types of molecules move together. When both the molecules cross the membrane in the same direction, it is called symport. When the molecules move in opposite directions, it is called antiport. When a molecule moves across a membrane independent of other molecules, the movement is called uniport.

Active Transport: Energy pumps are used against a concentration 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.

PLANT-WATER RELATIONS 

Water Potential: 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. Movement happens from higher water potential to lower water potential.

Solute Potential: All solutions have a lower water potential because of the dissolved solutes. The magnitude of lowering of water potential is called solute potential (Ψs). It is important to remember that solute potential is always negative. For a solution at atmospheric pressure:

Water potential = Solute potential

Pressure Potential: 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. Pressure potential is usually positive. Pressure potential is expressed as Ψp.

Ψw = Ψs + Ψp

Osmosis: Diffusion of water across a semipermeable membrane is called osmosis. The net direction and rate of osmosis depends on pressure gradient and concentration gradient. Water moves from higher concentration to its lower concentration until equilibrium is achieved. The two chambers across the semi-permeable membrane have the same water potential at equilibrium.

Plasmolysis:

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.

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.

Plasmolysis rarely happens in nature and can only occur in extreme conditions. There can be two types of plasmolysis, viz. concave and convex plasmolysis. Convex plasmolysis is always irreversible, while concave plasmolysis is reversible.

Imbibition: When water is absorbed by solids (colloids), resulting in an enormous increase in volume, this is called imbibition. Seeds swell up because of this process.

LONG DISTANCE TRANSPORT OF WATER

Long distance transport of water in plants happens in bulk. This is called translocation.

Absorption of Water

Absorption of water from the soil happens through root hairs. Root hairs are extensions of the root epidermis and have thin walls. Water enters the root hairs because of osmosis. Presence of numerous root hairs increases the surface area and hence enhances the absorption of water. 

Movement of water into deeper root layers:

The further movement of water takes place by two distinct pathways, viz. apoplast pathway and symplast pathway.

Apoplast Pathway: The free diffusional space outside the plasma membrane is the apoplast. This is interrupted by the Casparian strip in roots, air spaces between plant cells and the cuticula of the plant. The apoplast is formed by the continuum of cell walls of adjacent cells as well as the extracellular spaces. The apoplast pathway facilitates the transport of water and solutes across a tissue or organ.

Symplast Pathway: The inner side of the plasma membrane is the symplast. The symplast pathway is made continuous because of the presence of plasmodesmata across the cell walls of adjacent cells. Small molecules; such as sugars, amino acids and ions; flow through symplast pathway. Larger molecules are also transported through this route with the help of actin structures.

The symplast pathway allows direct cytoplasm to cytoplasm flow of water and other nutrients along concentration gradient.

Water Movement up a Plant

Root Pressure: When various ions from the soil are actively transported into the vascular tissues of the roots, water also follows. This increases the pressure inside the xylem. This positive pressure is called root pressure. The root pressure can push water up to small heights in the stem.

Guttation: In some plants, under the conditions of low evaporation, water comes out from the tips of leaves. Such loss of water in its liquid phase is called guttation. Guttation takes place in smaller plants only.

Limitations of Root Pressure: 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.

Transpiration Pull: The evaporative loss of water by plants is called transpiration. Transpiration mainly occurs through stomata. Stomata are usually open during daytime and remain close during the night.

Opening and Closing of Stomata: 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 meicrobifibrils 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.

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 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 tensi

are the important physical properties of water which further help in the upward movement of water through xylem.

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.

Adhesion-cohesion and capillarity result in formation of a continuous column of water molecules inside the xylem. This water column is pulled up because of transpiration pull. Thus, the adhesion-cohesion-transpiration pull theory explains the rise of water in very tall trees.

Uptake and Transport of Mineral Nutrients

Minerals cannot be passively absorbed by roots. There are two main reasons for this.

a. Minerals are present as charged particles in soil. They cannot move across cell membranes.

b. Concentration of minerals in the soil is usually lower than the concentration of minerals in the root. 

Hence, 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.

Translocation of Mineral Ions: Minerals ions reach xylem through active or passive uptake, or a combination of both. Their further movement through the xylem is alongwith the transpiration stream. The growing regions of the plant are the main sinks for mineral elements. Mineral ions are frequently mobilized; especially from older, senescing parts. Phosphorus, sulphur, nitrogen and potassium are the most readily mobilized elements. However, elements which are structural components are not mobilized, e.g. calcium.

Some elements are also transported in organic forms in plants.

PHLOEM TRANSPORT:

Food is transported through phloem; from source to sink. Leaf usually plays the role of source and storage organs are the sinks. But there can be role reversal when new leaves emerge during early spring. Thus, movement of substances through phloem is bi-directional.

The phloem sap is mainly composed of water and sucrose, but other sugars, hormones and amino acids may also be present.

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.

NCERT Solution for Class 11 Biology Transport in Plants

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.

(c) Osmotic Pressure and Osmotic Potential

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:

(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 meicrobifibrils 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.