Chapter 10: Transpiration

Question 1

Process of transpiration

Answer 1

• Water diffuses out of mesophyll cells to form a moist water surface on their surface
• Water on the moist water suirace evaporates to form water vapour, whichj diffuses out of the air spaces among the mesophyll cells
• Water vapour accumulates in the air spaces of the mesophyll cells
• Concentration of the water vapour in the air spaces is higher than that of the atmosphere -> concentration gradient of water vapour occurs between air spaces and atrmosphere
• Water vapour in the air space diffuses through the stomata down the concentration gradient to
  the atmosphere

NOTE: water film ≠ moist water surfaces on leaf cells

Question 2

Creation of transpiration pull

Answer 2

• As water on the moist surfaces of mesophyll cells evaporates after transpiration occurs, there is loss of water from the mesophyll cells and the water potential of the mesophyll cells lowers.
• Water is drawn from neighbouring cells to the mesophyll cells by osmosis to replace water loss. This is repeated across the layer of mesophyll cells.
• Water is drawn from xylem vessels in the leaf veins by osmosis to replace the water loss in mesophyll cells. A water potential gradient is built up along a chain of mesophyll cells across the leaves and the xylem vessels. This causes water to flow from the xylem vessels to the mesophyll cells near the air space.
• Withdrawal of water produces a pulling force in xylem vessels. Suction pressure is developed. The pulling force pulls water from the roots through the stem to the leaf unidirectionally.

Question 3

Significance of transpiration

Answer 3

Creates transpiration pull to aid the transport and absorption of water-soluble minerals and water
Creates transpiration pull for the continuous upward movement of water and dissolved minerals in xylem vessels to aid the transport and absorption of water-soluble minerals and water from the roots to other parts of the plant
Helps absorption of water by the roots
Usage of the absorbed water
98%: compensate water loss
2%: photosynthesis and metabolic activities, formation of new cells for growth and providing support

Produces a cooling effect:
During transpiration,
Evaporation of water removes heat from the plant → cooling effect
Prevents overheating of plant under hot conditions

Question 4

Structure adaptations of leaves for reducing water loss

Answer 4

Epidermis is covered with waxy cuticle
Impermeable to water → prevents water loss and thus prevents dehydration
Fewer or no stomata on the upper epidermis of most dicots
Temperature at the upper epidermis is higher when the plant is under direct sunlight
**Size of stomata is adjustable **
Guard cells control the size of the stomata
When plant are deficient in water, stomata close to reduce water loss through transpiration

DSE 2013 Q6: the size of stomata reduce during the night
to reduce transpiration (water loss)
when the need for gas exchange decreases in the absence of photosynthesis

Question 5

Environmental factors affecting the rate of transpiration: Light Intensity

Answer 5

**The rate of transpiration is low at low light intensity **
In darkness, more stomata are closed.
The cross-sectional area for the diffusion of water vapour decreases.
Therefore, water diffuses out of the leaves through the stomata at a lower rate.

**The rate of transpiration increases with an increase in light intensity: **
The stomata pores increase in size when light intensity increases
The cross-sectional area for the diffusion of water vapour increases
Water vapour diffuses out of the leaves at a faster rate through the stomata

**Temperature increases with an increase in light intensity **
Rate of evaporation of water and diffusion of water vapour increases
Rate of transpiration increases

Question 6

Environmental factors affecting the rate of transpiration: Temperature

Answer 6

The rate of transpiration increases when the temperature increases:
As temperature increases, the rate of evaporation of water from the moist surfaces of mesophyll cells increases.
This increases the concentration gradient of water vapour between the air space in the leaves and the surrounding air
Rate of water vapour diffusing out of the leaves through the stomata increases
As a result, the transpiration rate increases.

Question 7

Structure of root: Epidermis

Answer 7

Outermost layer of the root
- Not covered by cuticle so that absorption of water and minerals is not blocked
- Made up of a layer of closely-packed thin-walled cells
- Protects the inner tissues from injury and infections
- Some epidermal cells have outgrowths called root hairs, which provide a large surface area for absorption of water and minerals from the soil

Question 8

Structure of root: Cortex

Answer 8

The region beneath the epidermis.
- Made up of layers of loosely-packed thin-walled cells
- Stores food in the form of starch
- Allows the passage of water and minerals across the root

Question 9

Structure of roots: Vascular bundle

Answer 9

Located in the centre of the root
- Contains two types of vascular tissues: xylem and phloem. Xylem transports water and minerals.
Phloem transports organic nutrients (mainly sugars)