Ch 10 Transpiration, transport and support in plants

Question 1

Process of transpiration

Answer 1

1. Water diffuses out of the mesophyll cells to form a water film of the cell surface.
2. Water on the water film evaporates to form water vapour. The water vapour moves into the air space among the mesophyll cells.
3. Water vapour in the air space diffuses through the stomata to the atmosphere.

Question 2

Creation of transpiration pull

Answer 2

1. When mesophyll cells near the air space lose water to the air space, their water potential lowers.
2. Water moves from neighbouring cells into these cells by osmosis. This is repeated across the layer of mesophyll cells.
3. Water moves out of the xylem vessels by osmosis to replace the water loss in mesophyll cells.
   A water potential gradient is created along a chain of cells across the leaf. This causes water to flow from the xylem vessels to the mesophyll cells near the air space. Transpiration pull is created to pull water up the xylem vessels from roots.

Question 3

Adaptations of leaves to prevent excessive water loss

Answer 3

1. The epidermis is covered with a waxy cuticle which is almost impermeable to water. This reduces water loss through leaf surfaces.
2. In the leaves of most dicotyledonous plants, there are only a few or no stomata on the upper epidermis. Stomata are mainly found on the lower epidermis. As the leaves of dicot plants are usually oriented horizontally, their upper epidermis faces the sun and is hotter than the lower epidermis. The small number of stomata on the upper epidermis helps reduce water loss by evaporation.
3. Guard cells are present to control the opening and closing of the stomata.

Question 4

Adaptations of xerophytes to reduce water loss through transpiration

Answer 4

1. Reduced leaves
2. Rolled leaves
3. Sunken stomata

Question 5

Stomatal density

Answer 5

Average no. of stomata / Area of microscopic field of view (mm2)

Question 6

Measurement of the rate of transpiration using a bubble potometer

Answer 6

Assumption: the rate of water uptake is the same as the rate of transpiration
Cut a leafy shoot from a plant and fit it into a bubble potometer under water: prevent air bubbles from entering the xylem vessels and blocking water uptake
Seal off all connections with vaseline: ensure no leakage of water

Question 7

Measuring the amount of water absorbed and lost by a plant using a weight potometer

Answer 7

add a thin layer of oil to the water surface to prevent evaporation of water
change in volume: amount of water absorbed by the leafy shoot
change in mass: amount of water lost by the leafy shoot

Question 8

Effect of light intensity on the rate of transpiration

Answer 8

1. The rate of transpiration is low in darkness
   - In darkness, stomata are closed. Only a very small amount of water vapour can diffuse out of the leaves to the atmosphere.
2. The rate of transpiration increases when the light intensity increases.
   - As light intensity increases, the stomata open wider. The cross-sectional area for the diffusion of water vapour increases. Water vapour diffuses out of the leaves more rapidly through the stomata.
   *Level off: some other factors limit the rate of transpiration

Question 9

Effect of temperature on the rate of transpiration

Answer 9

The rate of transpiration increases when the temperature increases
- As temperature increases, the rate of evaporation of water from surfaces of mesophyll cells increases. This increases the concentration gradient of water vapour between the air space in the leaves and the surrounding air. Water vapour diffuses out of the leaves more rapidly through the stomata.
*Level off: some other factors limit the rate of transpiration

Question 10

Effect of air movement on the rate of transpiration

Answer 10

1. The rate of transpiration is low in still air.
   - In still air, water vapour that has diffused out of the leaves accumulates around the stomata. This decreases the concentration gradient of water vapour between the air space in the leaves and the surrounding air. The rate of diffusion of water vapour out of the leaves decreases.
2. The rate of transpiration increases when the wind speed increases.
   - Wind blows away water vapour around the stomata. This helps maintain a steep concentration gradient of water vapour between the air space in the leaves and the surrounding air. Water vapour diffuses out of the leaves more rapidly through the stomata.
3. The rate of transpiration decreases when the wind becomes too strong.
   - Under strong wind, water may be lost from the plant too rapidly, causing most stomata to close. Less water vapour diffuses out of the leaves through the stomata.

Question 11

Effect on relative humidity on the rate of transpiration

Answer 11

The rate of transpiration decreases when the relative humidity of the surrounding air increases.
- As the air space in the leaves is saturated with water vapour, a higher relative humidity of the surrounding air decreases the concentration gradient of water vapour between the air space and the surrounding air. Less water vapour in the air space diffuses out to the atmosphere through the stomata.

Question 12

Relationship between rates of transpiration and water absorption

Answer 12

1. The rate of transpiration increases from 0600-1400. Light intensity increases. This causes the stomata to open wider. Temperature increases. This increases the rate of evaporation of water. Water vapour diffuses out of the leaves more rapidly through the stomata.

Question 13

Process of absorption of water by roots

Answer 13

1. Water is lost from the leaves continuously by transpiration. This creates transpiration pull.
2. Water is drawn up the xylem vessels from the roots to the leaves by the transpiration pull.
3. Water in the cortex cells near the xylem vessels of the roots enters the xylem vessels. This decreases the water potential of these cortex cells. A water potential gradient is set up across the whole cortex.
4. In the cortex, water travels from cell to cell. Water moves along a water potential gradient through the cytoplasm of cells by osmosis. Water moves along the same water potential gradient through the vacuoles of cells by osmosis. Water travels from one cell to another freely through the cell wall.
5. As water is drawn away from the root hair cells to the neighbouring cortical cells, the water potential of the root hair cells becomes lower. As the water potential of the soil water is higher than that of the root hair cells, water in the soil enters the root hair cells by osmosis.

Question 14

Process of absorption of minerals by roots

Answer 14

1. Minerals dissolve in soil. The concentration of minerals in soil is lower than that in root hair cells.
2. Minerals are absorbed into the root against a concentration gradient by active transport.
3. Water potential of root hair cell decreases, facilitating water absorption.

Question 15

Adaptation of roots for absorption of water and minerals

Answer 15

1. The root is highly branched and there are numerous root hairs on it. These provide a large surface area for the absorption of water and minerals.
2. Root hairs are long and fine. They can easily grow between soil particles. This helps absorb water and minerals around them.
3. The epidermis of the roots is not covered by cuticle. It is made up of one layer of thin-walled cells. This allows water and minerals to pass through the epidermis into the root easily.
4. Root hair cells contain many mitochondria, ensuring enough energy is supplied to absorb minerals from the soil by active transport.

Question 16

Adaptations of xylem to transport water and minerals, and provide support for plants

Answer 16

1. Hollow tube allows passage of water with little resistance
2. Cell walls are thick and lignified, prevents vessels from collapsing under the great tension of transpiration pull

Question 17

Adaptations of phloem to transport organic nutrients

Answer 17

1. Sieve tube contains little cytoplasm and no nucleus, allows organic nutrients to move along with little resistance
2. Sieve plate has pores, allows organic nutrients to pass through
3. Companion cells has dense cytoplasm and many organelles, supports metabolism of the sieve tube.

Question 18

Process of transport of water and minerals

Answer 18

1. Water is lost from leaves through transpiration. Transpiration pull is created.
2. Water with dissolved minerals is drawn up the xylem vessels by transpiration pull as a continuous stream.
3. Water in the soil is absorbed into the root by osmosis. Dissolved minerals are absorbed by active transport.

Question 19

Translocation

Answer 19

1. Organic nutrients are synthesized in the leaves by photosynthesis.
2. They are transported by phloem to other parts of the plant for direct use, growing region or storage organs

Question 20

Support in plants by turgidity of thin-walled cells

Answer 20

• When water supply is adequate, thin-walled cells in the stems and leaves gain water by osmosis. The cells become turgid and press against each other. The turgidity of the cells makes the whole stem strong enough to stand upright.
• When water supply is inadequate, thin-walled cells in the stem and leaves lose water. The cells become flaccid and can no longer support the plant. The plant wilts. If the plant can take up enough water shortly, the cells will become turgid and the plant will stand upright again.

Question 21

How xylem cells and phloem cells contribute to the support of a plant

Answer 21

• Xylem cells has thickened cell wall, which provides rigidity to the plant
• Cell type P provides turgidity to the plant when there is ample supply of water