Transport in Plants Question Answers: NCERT Class 11 Biology

Hydrophilic substances are those which are soluble in water like glucose, sodium ions and chloride ions etc. They cannot pass through the biological membrane due to the hydrophobic nature of the lipids which makes up the cell membrane. Transport of these substances are facilitated by the transporter Proteins.

• Uniport facilitated diffusion
• Antiport facilitated diffusion
• Symport facilitated diffusion

When two different types of molecules move across the membrane by the process of facilitated diffusion. It is carried out by the carrier proteins embedded in the membrane. When both kinds of molecules move in the same direction, i.e., either from outside to inside or from inside to outside of the cell, is known as Symport. When they move in opposite direction, is known as Antiport. When a particular type of molecule moves across a membrane independent of other molecules, the diffusion is called Uniport.

A B C D E = Yes; F and G = No

The solute potential or osmotic potential is the potential of solution that allows the water to enter the solution by diffusion or osmosis due to the presence of the solute in it.

Solute potential is always negative because as we know on adding solute water potential decreases. So, when a solute is dissolved in water, the water potential of pure water decreases or starts assuming a negative value. And the term solute potential takes place of osmotic pressure, expressed in bars with a negative sign.

(a) Salt solution is hypertonic thus cause water to move out of the cell thus leading to exosmosis. When tradescantia leaf epidermal peel would place in the salt solution then the cells would have shrink when seen under the microscope.

The water potential is more in the soil than in the cytoplasm of the root hair. Thus water passes across the semi- permeable membrane of the root hair cell in the root through the process osmosis.

Casparian strips are situated in the endodermal cell walls of the plant’s roots.

The function of Casparian strips is to prevent the water movement from pericycle to cortex and thus establishing a positive hydrostatic pressure.

Guttation is the process of the loss of water in the form of water droplets from margins of leaves. Usually it occurs through stomata like pores called hydathodes.

Transpiration is the process of loss of water in the form of water vapours by arial part of the plant or stomata. Or, Transpiration is the loss of the water in the form of vapours from the living tissues of aerial parts of the plant.

Transpiration is also called necessary evil in the plant because it is beneficial as well as harmful for the plants. When because of transpiration, a loss of huge amount of water take place then this will lead to wilting of leaves or losing the plant turgidity.

Antiport

Three physical properties of water which helps in ascent of water in xylem are:

1. Adhesion: The ability of water to stick to a surface.
2. Cohesion: The ability of water molecules to stick to each other is called cohesion.
3. Surface tension: Tendency of a liquid to occupy the smallest possible surface area.

The plant will start wilting, if the potted plant is not watered. This happens because of excessive loss of water due to transpiration. Yes, it is reversible. This can be reversed by watering the plant in time.

Passive diffusion is the type of molecular movement which is highly selective and requires special membrane proteins but does not require energy.

1. Cells shrink in hypertonic solution and swell in hypotonic solution.
2. Imbibition is a special type of diffusion when water is absorbed by dead or non- living things.
3. Most of the water flow in roots occurs via the Apoplast pathway.

1. These types of guard cells are found in dicots.
2. Stomata in figure: (i) show higher water content because it is turgid.
3. Potassium plays an important role in the opening and closing stomata.

Uniport: When a single substance moves in a single direction across the cell membrane.

Antiport: When two substances move in the opposite direction across a cell membrane.

Symport: When two substances move in the same direction across a cell membrane.

No, they do not need energy directly.

Pressure and concentration

Osmosis is a special type of diffusion of solvent molecules from low concentration of the solution to the higher concentration of the solution, when the two are separated by the semipermeable membrane. And pressure also plays important role. 

Exosmosis

Urea is more concentrated in comparison to the sap present in the plant cell, so the plant loses the water from the cell sap to outside, this is known as exosmosis.

Imbibition Pressure

Transpiration pull

The movement of water within the leaf is transmitted to the water filled in the xylem elements. This exerts a pull and the water, filled in the xylem, comes in the state of tension, which is known as transpirational pull.

The plant cell cytoplasm is surrounded by both cell wall and cell membrane, because the cell wall is freely permeable to water and substances in the solutions but membrane is selectively permeable.

Half

In plants, translocation of food is carried out by the phloem from the source to the sinks. But the process is reversed during the season like spring because during that term the food stored in sinks is supplied to the parts of the plant. Thus, the direction of movement in the phloem can be upward or downwards and we can say that it is bidirectional. The translocation in the xylem of mineral ions are unidirectional because the flow occurs from the roots to the top of the plants.   

Diffusion is the passive movement of substances from a region of higher concentration to a region of lower concentration. Diffusion of substances plays an important role in cellular transport in plants. Rate of diffusion is affected by concentration gradient, membrane permeability, temperature, and pressure. Diffusion takes place as long as there is a difference between the concentrations of a substance across a barrier. However, diffusion stops, when the concentrations of the substance on either side of the barrier become equal. The permeability of a membrane affects the rate of diffusion. Diffusion rate increases as membrane permeability increases. Changes in temperature and pressure values also affect the diffusion of substances. Pressure plays an important role in the diffusion of gases as gases diffuse from a region of higher partial pressure to a region of lower partial pressure.

Root pressure is the positive pressure that develops in the roots of plants by the active absorption of nutrients from the soil. When the nutrients are actively absorbed by root hairs, water (along with minerals) increases the pressure in the xylem. This pressure pushes the water up to small heights. Root pressure can be observed experimentally by cutting the stem of a well-watered plant on a humid day. When the stem is cut, the solution oozes from the cut end.

Root pressure is also linked to the phenomenon of guttation, i.e., the loss of water in the form of liquid droplets from the vein endings of certain herbaceous plants.

Root pressure is only able to transport water up to small heights. However, it helps in re-establishing the continuous chains of water molecules in the xylem. Transpirational pull maintains the flow of water molecules from the roots to the shoots.

In tall trees, water rises with the help of the transpirational pull generated by transpiration or loss of water from the stomatal pores of leaves. This is called the cohesion-tension model of water transport. During daytime, the water lost through transpiration (by the leaves to the surroundings) causes the guard cells and other epidermal cells to become flaccid. They in turn take water from the xylem. This creates a negative pressure or tension in the xylem vessels, from the surfaces of the leaves to the tips of the roots, through the stem. As a result, the water present in the xylem is pulled as a single column from the stem. The cohesion and adhesion forces of the water molecules and the cell walls of the xylem vessels prevent the water column from splitting.

In plants, transpiration is driven by several environmental and physiological factors. The external factors affecting transpiration are wind, speed, light, humidity, and temperature. The plant factors affecting transpiration are canopy structure, number and distribution of stomata, water status of plants, and number of open stomata. Although transpiration causes water loss, the transpirational pull helps water rise in the stems of plants. This helps in the absorption and transport of minerals from the soil to the various plant parts. Transpiration has a cooling effect on plants. It helps maintain plant shape and structure by keeping the cells turgid. Transpiration also provides water for photosynthesis.

Transpirational pull is responsible for the ascent of water in the xylem. This ascent of water is dependent on the following physical factors:

• Cohesion – Mutual attraction between water molecules

• Surface tension – Responsible for the greater attraction between water molecules in liquid phase than in gaseous phase

• Adhesion – Attraction of water molecules to polar surfaces

• Capillarity –Ability of water to rise in thin tubes.

These physical properties of water allow it to move against gravity in the xylem.

In plants, nutrients are absorbed through the active and passive transports. The endodermal cells of the roots containing suberin allow only selected minerals to pass through them. The transport proteins present in the membranes of these cells act as check points for the various solutes reaching the xylem.

During the growth of a plant, its leaves act as the source of food as they carry out photosynthesis. The phloem conducts the food from the source to the sink (the part of the plant requiring or storing food). During spring, this process is reversed as the food stored in the sink is mobilised toward the growing buds of the plant, through the phloem. Thus, the movement of food in the phloem is bidirectional (i.e., upward and downward).

The transport of water in the xylem takes place only from the roots to the leaves. Therefore, the movement of water and nutrients in the xylem is unidirectional.

According to the pressure flow hypothesis, food is prepared in the plant leaves in the form of glucose. Before moving into the source cells present in the phloem, the prepared food is converted into sucrose. Water moves from the xylem vessels into the adjacent phloem, thereby increasing the hydrostatic pressure in the phloem. Consequently, the sucrose moves through the sieve cells of the phloem. The sucrose already present in the sink region is converted into starch or cellulose, thereby reducing the hydrostatic pressure in the sink cells. Hence, the pressure difference created between the source and the sink cells allows sugars to be translocated from the former to the latter. This starch or cellulose is finally removed from the sink cells through active transport.

The tiny pores present on the surfaces of leaves, called stomata, help in the exchange of gases. Each stoma consists of bean-shaped or dumbbell-shaped guard cells. The epidermal cells surrounding the guard cells are modified to form subsidiary cells. The opening and closing of the guard cells is caused by a change in their turgidity. The inner walls of the guard cells are thick and elastic, while the outer walls are thin. The numerous microfibrils present in the guard cells facilitate the opening and closing of the guard cells.

At the time of the opening of the stomata, the turgidity of the guard cells increases. As a result, the outer walls bulge and the inner walls become crescent-shaped. The stomatal opening is facilitated by the radial arrangement of the microfibrils.

At the time of the closing of the stomata, the guard cells lose their turgidity, the outer and inner walls retain their original shapes, and the microfibrils get arranged longitudinally.

Porins are types of proteins which form pores of large sizes in the outer membranes of plastids such as chloroplast, mitochondria and the membranes in bacteria. They help in facilitating the passive transport of small-sized protein molecules.

In plant cells, active transport occurs against the concentration gradient, i.e., from a region of lower concentration to a region of higher concentration. The process of active transport involves specific protein pumps. The protein pumps are made up of specific proteins called trans-membrane proteins. These pumps first make a complex with the substance to be transported across the membrane, using the energy derived from ATP. The substance finally gets liberated into the cytoplasm as a result of the dissociation of the protein–substance complex.

Water potential quantifies the tendency of water to move from one part to the other during various cellular processes. It is denoted by the Greek letter Psi or Ψ. The water potential of pure water is always taken as zero at standard temperature and pressure.

It can be explained in terms of the kinetic energy possessed by water molecules. When water is in liquid form, the movement of its molecules is rapid and constant. Pure water has the highest concentration of water molecules. Therefore, it has the highest water potential. When some solute is dissolved in water, the water potential of pure water decreases.

(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

Water potential quantifies the tendency of water to move from one part to the other during various cellular processes such as diffusion, osmosis, etc. It is denoted by the Greek letter Psi or Ψ and is expressed in Pascals (Pa). The water potential of pure water is always taken as zero at standard temperature and pressure.

Water potential (Ψw) is expressed as the sum of solute potential (Ψs) and pressure potential (Ψp).

Ψw = Ψs + Ψp

When some solute is dissolved in water, the water potential of pure water decreases. This is termed as solute potential (Ψs), which is always negative. For a solution at atmospheric pressure, Ψw = Ψs.

The water potential of pure water or a solution increases on the application of pressure values more than atmospheric pressure. It is termed as pressure potential. It is denoted by Ψp and has a positive value, although a negative pressure potential is present in the xylem. This pressure potential plays a major role in the ascent of water through the stem.

The water potential of pure water or a solution increases on the application of pressure values more than atmospheric pressure. For example: when water diffuses into a plant cell, it causes pressure to build up against the cell wall. This makes the cell wall turgid. This pressure is termed as pressure potential and has a positive value.

(a) Plasmolysis can be defined as the shrinkage of the cytoplasm of a plant cell, away from its cell wall and toward the centre. It occurs because of the movement of water from the intracellular space to the outer-cellular space. This happens when the plant cell is placed in a hypertonic solution (i.e., a solution having more solute concentration than the cell cytoplasm). This causes the water to move out of the cell and toward the solution. The cytoplasm of the cell shrinks and the cell is said to be plasmolysed. This process can be observed in an onion peel kept in a highly concentrated salt solution.

(b) When a plant cell is placed in a hypertonic solution or a solution having higher water potential, the water diffuses into the cell (i.e., movement is observed from higher to lower water pressure region). The entry of water in the plant cell exerts pressure on the rigid cell wall. This is called turgor pressure. As a result of its rigid cell wall, the plant cell does not burst.

Mycorrhiza is a symbiotic association of fungi with the root systems of some plants. The fungal hyphae either form a dense network around the young roots or they penetrate the cells of the roots. The large surface area of the fungal hyphae is helpful in increasing the absorption of water and minerals from the soil. In return, they get sugar and nitrogenous compounds from the host plants. The mycorrhizal association is obligate in some plants. For example, Pinus seeds do not germinate and establish in the absence of mycorrhizal.