The Absorption Transport and Water Loss in Plants NIOS chapter explains how plants take in water and minerals and transport them through xylem and phloem. It also covers processes like transpiration and water regulation. Students learn about the cohesion-tension theory and root pressure.
Terminal exercise solutions provide practice for important exam questions. This chapter helps in understanding plant physiology, making it easier for students to revise and prepare well for the NIOS Class 12 exam.
Absorption Transport and Water Loss in Plants NIOS
1. Name two types of passive absorption in plants.
Diffusion and Osmosis.
2. In what ways is diffusion important to a plant?
Diffusion is an effective method of transport of matter over short distances. Diffusion does not require a membrane. It helps the plant get CO2 and oxygen that can freely diffuse through the cell membrane.
3. Name various factors that affect osmosis in plants.
Concentration gradient, water potential, temperature.
4. Differentiate between turgor pressure and wall pressure.
Turgor Pressure is the pressure exerted by the protoplasm against the cell wall. Wall pressure refers to the back pressure exerted by the cell wall against the protoplasm in a turgid cell.
5. Discuss the mechanism of stomatal opening in dicot plants.
The mechanism of opening and closing of stomata in dicots and monocots is as follows:
The dicotyledonous plants have kidney-shaped guard cells. The inner walls around the stoma are thicker than the outer walls.
- When guard cells get distended by turgor pressure, the cells expand.
- The tough inner walls of the guard cells become convex.
- This pulls the inner wall apart, leaving the stomata open.
- When the turgor pressure in the guard cell decreases, the cells sag.
- The inner walls come closer.
- This causes the stomata to close.
6. Explain any four factors that affect transpiration in plants.
Many external and internal factors affect the process:
- (i) Temperature: The increase in temperature increases the rate of transpiration by increasing the rate of evaporation of water from the cell surface and decreasing the humidity of the atmosphere.
- (ii) Wind velocity: The increase in wind velocity increases the rate of transpiration by removing the water vapour of the atmosphere and lowering the relative humidity around the aerial parts of a plant.
- (iii) Light: Light has no direct effect on the rate of transpiration, but indirectly it affects the rate in two ways, firstly by controlling the stomatal opening and secondly by affecting the temperature. With an increase in intensity of light rate of transpiration increases because stomata open and the temperature increases.
- (iv) Water supply: Deficiency of water supply in the soil decreases the rate of transpiration by decreasing the rate of absorption. When the deficiency of water in the soil becomes too much, then the plants wilt and do not recover from wilting unless water is supplied to the soil. This is known as permanent wilting. When in a hot and dry summer day, the plant transpires more, causing higher water loss by the leaves than the roots are able to absorb. Even though there is enough water in the soil, the plants wilt, exhibiting temporary wilting as the plant recovers from such wilting in the late afternoon or at night.
- (v) Atmospheric pressure: Reduction of atmospheric pressure reduces the density of the external atmosphere, thus permitting more rapid diffusion of water. Plants growing at high altitudes will show a higher rate of transpiration, hence they develop xerophytic characters.
- (vi) Atmospheric humidity: Humidity means the amount of water vapour present in the atmosphere. The diffusion and evaporation of water depend on the vapour pressure gradient or the difference in water potential gradient between the atmosphere and the inside of the leaf. The more the difference, more will be the rate of transpiration.
7. Describe an experiment to demonstrate osmosis using by potato osmometer.
Experiment to demonstrate Osmosis
Experiment: To demonstrate the phenomenon of osmosis through the plant membrane with the help of a potato osmoscope.
Requirements. A large potato tuber, 10% sugar solution, beaker, water, scalpel, and pin.
Method.
- Take a large potato tuber and peel off its outer skin with the help of a scalpel.
- Cut one end to make the base flat.
- Now, make a deep hollow cavity on the opposite side.
- Pour some sugar solution to fill half of the cavity and mark the level by inserting a pin in the wall of the tuber.
- Put the potato in the beaker containing a small amount of water and allow the apparatus to stand for some time.
- Make sure that the level of water in the beaker is below the level of the sugar solution in the cavity of the potato osmoscope.
Observation and Conclusion. The level of sugar solution in the cavity rises. It is because of the movement of water molecules into the cavity from pure water in the beaker. This experiment shows the phenomenon of osmosis.
8. Discuss the cohesion-tension theory for the uptake of water in plants.
The water moves up through the xylem vessels to the leaf along the water potential gradient, as explained by the cohesion-tension theory (most acceptable). Transpiration or evaporation of water from the plant through stomata causes a pull, and water moves up like a water column due to the force of cohesion and tension created by transpiration.
9. Describe the mechanism of translocation of solutes. Name the most appropriate theory for the translocation of solutes in plants. Who proposed this theory?
Mechanism of translocation
Sugar solution in the phloem sieve tube moves along the water potential gradient created between the source (leaf) and sink (storage) cells. Here we find a mass movement of sugar solution from the leaf mesophyll to the sieve tubes of the leaf, and then to all parts of the plant.
The Münch hypothesis, or Mass flow theory, proposed by Ernst Münch, is the most acceptable model for phloem translocation.
10. Differentiate between the symplast and apoplast pathways of water movement in plants.
- The cytoplasm of the entire plant is connected through plasmodesmata, which are the protoplasmic strands forming the symplast system. Water movement through the cells takes this symplast pathway by osmosis.
- The cell wall and the intercellular spaces form the apoplast pathway, which allows water movement inside the plant by the phenomenon of capillarity and adsorption.
11. Define transpiration.
The loss of water from aerial parts of the plant in the form of water vapour is termed transpiration.
12. Name the holes in the bark through which transpiration in the bark of old trees takes place.
Lenticels.
13. Why is transpiration considered to be a necessary evil?
Stomata remain open during the day for the absorption of carbon dioxide and release of oxygen for the very important process of photosynthesis. When the stomata remain open for this important gaseous exchange, the escape of water vapour cannot be controlled. Thus, loss of water is a wasteful process that cannot be avoided because stomata must remain open to do something more important, that is, absorption of carbon dioxide during the day for photosynthesis. It is for this reason that Curtis, in 1926, referred to transpiration as a necessary evil.
14. Give one way by which desert plants prevent transpiration.
Some plants, like Cacti, retain water by reducing transpiration. This saves the plants from high temperatures and strong sunlight.
15. State one point of difference between transpiration and guttation.
| Transpiration | Guttation |
| Water is lost in the form of water vapor. | Water is lost in the form of water drops. |
| Occurs through stomata, cuticle, and lenticels. | Occurs through special pores, called hydathodes. |
| Occurs during the day and at high temperatures. | Occurs at night and early in the mornings at low temperatures. |
| Water vapour lost is pure water and does not contain minerals. | Water lost has substances dissolved in water. It contains sugars, salts, and amino acids. |
| Increased transpiration is a physical process. | It is due to increased root pressure that develops in the aerial shoot system when water absorption by roots is more and transpiration by aerial plant parts is low. |
