NCERT Solutions for Class 11 Biology Chapter 11 - Transport in Plants

How transportation or movement in plants takes place is the crux of the chapter' Transport in Plant.' Here, regarding plants, students will learn about the Means of Transport, Relationship shared between Plant and Water, How Water is Transported to Long Distance, How Mineral Nutrients are Absorbed and Transported, Transpiration, Transport: Flow from Source to Sink, and Phloem, Plants receive miscellaneous inorganic elements (ions) and salts from the surroundings in which they inhabit, particularly from soil and water. The presence of xylem and phloem in a vascular system in higher plants helps in translocation in them.

In this process, Phloem in the transportation of food sucrose (primarily) from the source to the sink. Furthermore, with the help of the pressure-flow hypothesis, the process of translocation in phloem has been explained in this chapter. Let us dig a bit deeper into the topics of this chapter to discover more knowledge about them.

• Medium of Transport
• Relationship between plant and water
• How is long distance transportation of water done?
• Transpiration
• How are Mineral and Nutrients absorbed and transported?
• Transport of Phloem - Flow from Source to Sink

Multiple substances and elements, viz minerals, water, hormones, organic solutes, gases, and more are transported by plants through various modes, like diffusion, facilitated diffusion, etc. Based on this, the different transport processes are compared to each other through which a concept of plant-water relation is derived in brief. All these concepts and subtopics have been summarised under chapter 11 of class 11 Biology.

It is also said here that the concept of water potentials is radical to understanding water movement. Other topics covered are plasmolysis, osmosis, long-distance water transport, imbibition, water absorption by plants, transpiration, The chapter emphasizes the concept of how water is transported upwards in plants to reach the last leaf, for which a clear understanding of root pressure, guttation, and a few other theories is required.

Q1. What are the factors affecting the rate of diffusion?
Answer:

The factors affecting the rate of diffusion are as follows

1.The density of the substance- The rate of diffusion of a substance is inversely proportional to the square root of its density.

2. Permeability of the medium- The rate of diffusion decreases with the permeability of the medium.

3. Temperature- A rise in temperature increases the rate of diffusion.

4. Diffusion pressure gradient- The rate of diffusion is directly proportional to the diffusion pressure gradient.

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

Answer:

Porins are proteinaceous hydrophilic channels present in the outer membrane of plastids, mitochondria, and some bacteria. They allow the passage of large biomolecules, even small-sized proteins as per diffusion gradient. Aquaporins are water channels for the diffusion of water molecules across the plasma membrane.

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

Answer:

Protein pumps are carrier proteins that take part in the transport of solutes across the cell membrane with the help of energy in the form of ATP even against a concentration gradient. On being activated with energy, the protein pump picks up solute particles from outside and transports them to the inner side into the cytoplasm. There are a number of protein pumps such as Na- K pump and Ca pump.

Q4. Explain why pure water has the maximum water potential.

Answer:

Water potential is the quantitative measure of 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

Answer:

b.Transpiration and Evaporation

Answer:

c.Osmotic Pressure and Osmotic Potential

Answer:

d.Imbibition and Diffusion

Answer:

e.Apoplast and Symplast pathways of movement of water in plants.

Answer:
 

f.Guttation and Transpiration.

Answer:

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

Answer:

Water potential is the quantitative measure of water to move from one part of the plant to the other part 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 pressure of pure water is taken as zero at standard temperature and pressure. A solution has less water potential due to less water concentration. The water potential of a cell is affected by solute and pressure potential.

Solute potential- The magnitude of lowering of water potential due to the dissolution of solute is called solute potential.

Pressure potential- 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.

The relation between water potential and pressure potential is as follows:

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

Answer:

When a pressure greater than the atmospheric pressure is applied to pure water or a solution, the water potential of pure water or a solution increases 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-labeled diagrams, describe the process of plasmolysis in plants, giving appropriate examples.

Answer:

Plasmolysis refers to the shrinkage of the cytoplasm of the cell away from its cell wall under the influence of a hypertonic solution. Plasmolysis 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). It will cause 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 plasmolyzed. This process can be observed in an onion peel kept in a highly concentrated salt solution.

Q8. (b) Explain what will happen to a plant cell if it is kept in a solution having higher water potential.

Answer:

When a plant cell is placed in a solution having higher water potential, the water diffuses into the cell. The entry of water into the plant cell exerts pressure on the rigid cell wall. As a result of its rigid cell wall, the plant cell bursts.

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

Answer:

Mycorrhiza refers to the symbiotic association of fungi with the root systems of higher plants. The hyphae of fungi form a dense network around the young roots and they can even penetrate the cells of the roots. The mycorrhizal hyphae help the plants in getting nutrients especially phosphorous. Due, to this reason, the mycorrhizal association is obligated in some plants. For example, Pinus seeds don't germinate and establish in the absence of mycorrhizal associations.

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

Answer:

Root pressure is a positive pressure that develops in the xylem sap of the root of plants. Root pressure is the manifestation of active water absoorptionsThe root pressure theory was given by Priestley in 1916 to establish the role of root pressure in the movement of water. The roles played by root pressure in the water movement of plants are as follows:

1.Root pressure is able to transport water up to small heights.

2.Root pressure helps in re-establishing the continuous chains of water molecules in the xylem.

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

Answer:

Transpiration pull model of water transport in plants-

Transpiration is the loss of water in the form of water vapor from aerial parts of plants. In tall trees, the water rises with the help of the transpirational pull generated by transpiration. This is called the cohesion-tension model of water transport. According to this model, the water is mostly pulled from roots to leaves due to the driving force of transpiration from the leaves. The water molecules remain attached to one another by cohesion force. The water molecule does not break in vessels and tracheid due to adhesive force between their walls and water molecules. On account of tension created by transpiration, the water column of a plant is pulled up passively from roots to great heights.

Factors influencing transpiration-

The external factors affecting transpiration are wind, speed, light, humidity, and temperature. The physiological factors affecting transpiration are canopy structure, number and distribution of stomata, water status of plants, and the number of open stomata, etc. The advantages of transpiration are as follows:

1. It creates a transpirational pull for absorption and transport in plants.

2. It supplies water for photosynthesis.

3. It cools the leaves and maintains their shape and size.

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

Answer:

The various factors responsible for the ascent of sap are as follows:

1. Capillarity- It is the limited rise of water in narrow tubes or capillaries due to forces of cohesion amongst the molecules of water and their property of adhesion to other substances.

2. Imbibition- It is the ability of hydrophilic colloids to attract and hold water on the surface and inside their interspaces.

3. Root pressure- It is the positive pressure that pushes sap from below due to active absorption by roots.

4. Transpiration pull- Transpiration in aerial parts brings the xylem sap under negative pressure or tension due to the continuous withdrawal of water by them. The water column does not break due to its high tensile strength related to the high force of cohesion and adhesion.

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

Answer:

The endodermis of roots contains suberin which allows only selected minerals to pass through them. The transport proteins present in the membranes of endodermal cells act as checkpoints for the various solutes reaching the xylem.

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

Answer:

Leaves are the site of food production for plants as they carry out photosynthesis. The major function of phloem is to conduct the food from the source to the sink. Since, here the source is single and sink can be multiple i.e. roots, branches, flower, etc. so the conduction of food takes place in multiple directions. During spring also, the process of food conduction reverses and the food stored in the sink is mobilized toward the growing buds of the plant, through the phloem. Since the food is being conducted in many directions. the movement of food in the phloem is called bidirectional (i.e., upward and downward). The transport of water, on the other hand, takes place only from the roots to the leaves. Therefore, the movement of water and nutrients in the xylem is unidirectional.

Q15. Explain the pressure-flow hypothesis of the translocation of sugars in plants.

Answer:

The pressure-flow hypothesis is the most accepted theory for the translocation of sugar from source to sink. Glucose is prepared at the source by photosynthesis which is converted into disaccharides sucrose which further moves into companion cells and then into sieve tube cells by active transport. Loading of phloem at the source creates a water potential gradient that facilitates 

Fig- Diagramatic representation of mechanism of translocation.

the mass movement in the phloem. Sieve tube cells of phloem form a long column with holes in their wall called sieve plates. Cytoplasmic strands pass through the hole in the sieve plates to form a continuous filament. Hydrostatic pressure developed in sieve tube cells moves the sap in the phloem. At the sink, incoming sugar is actively moved out of the phloem as complex carbohydrates. The loss of solute produces a high water potential in the phloem and water passes out and returns into the xylem.

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

Answer:

Stomata are the tiny pores present on the surfaces of leaves that help in the exchange of gases. A stoma consists of bean-shaped (in dicot) or dumbbell-shaped (in monocots) guard cells that are surrounded by the modified epidermal cells called subsidiary cells. The opening and closing of the stomata are mainly due to change in the turgidity of guard cells. The inner walls of the guard cells are thick and elastic, while the outer walls are thin. Numerous microfibrils are present in the cell walls of guard cells to facilitate the opening and closing of the stomata.

Stomatal opening- 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.

Stomatal closing- 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.