LIWA

Transpiration

ICSE Class 10 · Biology · Chapter 5

What this chapter covers

Key points

Questions answered

Define transpiration. Name the three types of transpiration in order of the amount of water lost.
Transpiration is the loss of water in the form of water vapour from the aerial parts of a living plant, chiefly through the stomata. In decreasing order: stomatal, cuticular, lenticular.
Why is transpiration called a "necessary evil"?
It wastes most of the water a plant absorbs and can cause wilting, so it is an "evil". It is "necessary" because the stomata must open to let carbon dioxide in for photosynthesis, and the open stomata unavoidably let water vapour out; transpiration also drives the ascent of sap, distributes minerals and cools the leaf.
Give two reasons why the guard cells bend outwards when they become turgid.
The inner wall facing the pore is thick and inelastic, while the outer wall is thin and elastic. When water enters, the outer wall stretches more than the inner wall, so each guard cell curves away from the pore.
State the role of potassium ions in the opening of stomata.
In light, potassium ions are actively pumped into the guard cells from the subsidiary cells. This raises the osmotic pressure of the guard cells, water enters them by osmosis, they become turgid, and the stoma opens.
What happens to the stomata of a plant kept in the dark? Give a reason.
They close. Without light there is no ATP from photosynthesis to keep potassium ions in the guard cells, so the ions leave, water follows by osmosis, and the flaccid guard cells close the pore.
How do (a) humidity and (b) wind affect the rate of transpiration?
(a) High humidity lowers the rate, because the difference in water vapour concentration between the leaf and the air is small. (b) Wind raises the rate, because it removes the moist air around the leaf and replaces it with drier air.
In the potted plant experiment, why is the pot covered with a polythene sheet?
To stop water evaporating from the soil and the sides of the pot, so that any loss in weight is due only to transpiration from the plant.
Why is oil poured on the water in the test tube experiment with a leafy shoot?
The oil layer prevents evaporation from the water surface, so the fall in water level and weight is due only to water drawn up and transpired by the shoot.

Past board exam questions

ICSE 2027 sample paper · 1 mark

Which of the following statements are correct? P – Cobalt ions help in the opening of stomata. Q – Strawberry leaves show guttation. R – Duplicated chromosomes are joined at centrosome. S – Conversion of ADP to ATP occurs in the light dependent phase.

  1. P and R
  2. Q and R
  3. P and S
  4. Q and S
Show answer
Q and S

ICSE 2027 sample paper · 5 marks

Give the biological / technical terms for the following: (a) Respiratory openings present on the stems of woody plants. (b) The fluid present between the cornea and lens of the eye. (c) The canal through which the testes descend into scrotum just before the birth of a male baby. (d) The type of cell division for the formation of gametes in humans. (e) The neurotransmitter stored in the axon endings of neurons.

Show answer
(a) Lenticels (b) Aqueous humour (c) Inguinal canal (d) Meiosis (e) Acetylcholine

ICSE 2027 sample paper · 1 mark

Avinash placed a potted plant on a windowsill on a hot summer morning. By afternoon, the leaves had wilted though the soil was well watered. He moved the plant indoors and the leaves regained their turgidity. Name the process responsible for the wilting of leaves.

Show answer
Transpiration (excessive transpiration, more than the water absorbed by the roots).

ICSE 2027 sample paper · 2 marks

Avinash placed a potted plant on a windowsill on a hot summer morning. By afternoon, the leaves had wilted though the soil was well watered. He moved the plant indoors and the leaves regained their turgidity. Define the above process.

Show answer
Transpiration is the loss of water in the form of water vapour from the aerial parts of the plant (mainly through the stomata of leaves).

ICSE 2027 sample paper · 2 marks

Avinash placed a potted plant on a windowsill on a hot summer morning. By afternoon, the leaves had wilted though the soil was well watered. He moved the plant indoors and the leaves regained their turgidity. Explain how the process will differ on a rainy day.

Show answer
On a rainy day the rate of transpiration will be less, because the atmosphere is humid (has more water vapour), so diffusion of water vapour from the leaf to the outside air is slow.

ICSE 2027 sample paper · 2 marks

Avinash placed a potted plant on a windowsill on a hot summer morning. By afternoon, the leaves had wilted though the soil was well watered. He moved the plant indoors and the leaves regained their turgidity. Which are the primary pores through which the above process takes place? Where are they located?

Show answer
Stomata. They are located in the epidermis of the leaves (mostly the lower surface) and of young green stems.

ICSE 2027 sample paper · 3 marks

Avinash placed a potted plant on a windowsill on a hot summer morning. By afternoon, the leaves had wilted though the soil was well watered. He moved the plant indoors and the leaves regained their turgidity. List three adaptations in plants to reduce this process.

Show answer
Any three: narrow / reduced leaves (or leaves modified into spines); thick waxy cuticle; fewer stomata; sunken stomata; leaves covered with hairs; shedding of leaves in dry season.

ICSE 2027 sample paper · 3 marks

Draw a neat, labelled diagram of a stomatal apparatus surrounded by epidermal cells.

Show answer
Diagram: two kidney-shaped guard cells enclosing a stomatal pore, surrounded by irregular epidermal cells. Labels: guard cell, stoma (stomatal pore), chloroplasts in guard cells, nucleus, thick inner wall / thin outer wall of guard cell, epidermal cells.