NCERT Solutions for Class 11 Biology Transport in Plants [Old Book]

NCERT
SOLUTIONS
CLASS - 11th

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Class : 11th
Subject : Biology
Chapter : 11
Chapter Name : Transport In Plants

Q1 What are the factors affecting the rate of diffusion?

Answer. 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.

Page : 193, Block Name : Exercise

Q2 What are porins? What role do they play in diffusion?

Answer. 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.

Page : 193, Block Name : Exercise

Q3 Describe the role played by protein pumps during active transport in plants.

Answer. 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 speci c protein pumps. The protein pumps are made up of speci c proteins called
transmembrane proteins. These pumps rst make a complex with the substance to be transported
across the membrane, using the energy derived from ATP. The substance nally gets liberated into
the cytoplasm as a result of the dissociation of the protein—substance complex.

Q4 Explain why pure water has the maximum water potential.

Answer. Water potential quanti es 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 V. 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.

Q5 Differentiate between the following:
(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.

Answer. (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.

Q6 Brie y describe water potential. What are the factors affecting it?

Answer. Water potential quanti es 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 and is expressed in Pascals (Pa). The water potential of pure water is always taken as zero at
standard temperature and pressure. Water potential (Ww) is expressed as the sum of solute
potential (I-PS) and pressure potential (Wp).
q.Jw = + q.Jp
When some solute is dissolved in water, the water potential of pure water decreases. This is
termed as solute potential (Ws), which is always negative. For a solution at atmospheric pressure,
Ww = Vs. 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 Wp 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.

Q7 What happens when a pressure greater than the atmospheric pressure is applied to pure water
or a solution?

Answer. 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.

Q8 (a) With the help of well-labelled diagrams, describe the process of plasmolysis in plants,
giving appropriate examples.
(b) Explain what will happen to a plant cell if it is kept in a solution having higher water potential.

Answer. (a) Plasmolysis can be de ned 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.

Q9 How is the mycorrhizal association helpful in absorption of water and minerals in plants?

Answer. 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 don't
germinate and establish in the absence of mycorrhizal.

Q10 What role does root pressure play in water movement in plants?

Answer. 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 ow of water molecules from the
roots to the shoots.

Q11 Describe transpiration pull model of water transport in plants. What are the factors
in uencing transpiration? How is it useful to plants?

Answer. 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 accid. 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.

Q12 Discuss the factors responsible for ascent of xylem sap in plants.

Answer. 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.

Q13 What essential role does the root endodermis play during mineral absorption in plants?

Answer. 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.

Q14 Explain why xylem transport is unidirectional and phloem transport bi-directional.

Answer. 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.

Q15 Explain pressure ow hypothesis of translocation of sugars in plants

Answer. According to the pressure ow 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 nally removed from the sink cells through active
transport.

Q16 What causes the opening and closing of guard cells of stomata during transpiration?

Answer. 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 modi ed 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 micro brils 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 micro brils
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 micro brils get arranged longitudinally.