Topic 7 Revision - Year 9

The diagram shows a cross-section through a leaf.

From which cell will most water evaporate during transpiration?

• A

• B

• C

• D

Answer explanation

The correct answer is cell D, spongy mesophyll tissue cell.

During transpiration, water evaporates from the stomata located on the surface of the leaf. Cell D (spongy mesophyll tissue cell) is located near the stomata, making it more exposed to the external environment and facilitating greater water loss.

What contributes to the wilting of plant leaves?

• the mesophyll cells lose turgor

• the phloem stops translocating

• the stomata close

• the xylem fills with air

Answer explanation

The wilting of plant leaves occurs when the mesophyll cells lose turgor. This causes the cells to become flaccid and unable to maintain their shape, resulting in the drooping of the leaves.

The diagrams show a plant in a flask of water. It is left in the light at 16 °C for six hours.

What explains the change in mass after six hours?

• absorption of water into the root hairs

• evaporation of water from the flask

• photosynthesis in the leaves of the plant

• transpiration from the leaves of the plant

Answer explanation

The change in mass after six hours is explained by transpiration from the leaves of the plant.

Transpiration is the process by which water is lost from the leaves through small openings called stomata. During transpiration, water vapor is released into the air, resulting in a decrease in the plant's mass. This process helps the plant regulate its temperature and transport nutrients.

The diagram shows cross-sections through the root and stem of the same plant.

What is tissue T?

• epidermis

• mesophyll

• phloem

• xylem

Answer explanation

The correct answer is xylem, which is responsible for transporting water and dissolved mineral ions from the roots to the rest of the plant.

HOW WATER MOVES ONCE IT ARRIVES AT THE LEAF:

At 1: There is a high water potential in xylem vessels than in the spongy mesophyll cells. So water moves from xylem vessels, across the partially membrane and into the spongy mesophyll cells via ...

Answer explanation

Once water reaches the leaf, water moves from xylem vessels to the spongy mesophyll cells through a partially permeable membrane due to the higher water potential in the xylem vessels. This process is known as osmosis.

HOW WATER MOVES ONCE IT ARRIVES AT THE LEAF:

At 2: On the surfaces of the mesophyll cells surface is a thin film of moisture and this water moisture (liquid) undergo ______ to form water vapour (gas).

A lot of these water vapour move into the intercellular air spaces and accumulate in the large air spaces near the stomata.

This is way there is a high concentration of water vapour i.e. increased air humidity inside the leaf.

Answer explanation

The correct answer is 'evaporation'.

When the thin film of moisture on the surfaces of the mesophyll cells evaporate, it forms water vapour. This water vapour then moves into the intercellular air spaces and accumulates near the stomata, resulting in an increased humidity inside the leaf.

Hence inside the leaf, there is a high concentration of water vapour molecules, i.e. high humidity inside the leaf than the atmosphere outside.

When the water vapour diffuses out through the stoma, this process is transpiration.

There are internal and external factors that affect the rate of transpiration (refer notes pages 7-9)

HOW WATER MOVES ONCE IT ARRIVES AT THE LEAF:

At 3: There is constantly a high concentration of water vapour molecules i.e. very humidity inside the leaf, than outside the leaf.

So the water vapour molecules exit the leaf through the stoma via _____ .

Answer explanation

Inside the leaf, there is a high concentration of water vapour molecules, creating higher humidity than outside the leaf.

As a result, the water vapour molecules exit the leaf through the stoma via diffusion. This is transpiration.

This process helps helps create the suction force that pulls water up the xylem vessels in plants.

The rate of transpiration is affected by the internal and external conditions of the leaf (refer notes page 7-9).

What condition will increase the rate of transpiration in a plant?

• an increase in the humidity of the atmosphere surrounding the leaf

• an increase in the surface area of the lamina of the leaf

• a decrease in the number of stomata present on the surface of the leaf

• a decrease in the temperature of the atmosphere surrounding the leaf

Answer explanation

The rate of transpiration in a plant increases when the surface area of the leaf lamina increases i.e. broad leaves.

This allows for more water to evaporate from the leaf, leading to a higher rate of transpiration.

A student set up the apparatus shown figure to investigate transpiration rate.

The student sprayed a lot of water inside the transparent bag before securely tying it.

The student predicts the results to be ...

• increasing transpiration rate

• decreasing transpiration rate

Answer explanation

Ø Increasing the humidity is predicted to cause a decrease in the rate of transpiration.

 

Ø Explanation: Spraying water inside the transparent water bag will increase the number of water vapour outside the leaf i.e. increasing humidity.

If the water vapour concentration outside the leaf is high, there is less steep concentration gradient as there is always a high concentration of water vapour inside the leaf.

So less water vapour will diffuse from the leaf if there is more water vapour in the air i.e. lower/less/slower transpiration rate.

The diagram shows two shoots at the start of an experiment on transpiration.

What are the likely readings on the spring balances after three days?

• Reading of shoot X will increase more than 30g and reading of shoot Y will decrease below 30g.

• Readings of shoot X and shoot Y will stay the same at 30g.

• Reading of shoot X will stay the same at 30g and reading of shoot Y will increase more than 30g.

• Reading of shoot X will decrease below 30g and reading of shoot Y will stay the same at 30g.

Answer explanation

In shoot X: With more leaves, there are more stomata, through which water vapour will diffuse out. This cause the shoot stem to uptake more water from the tube to replace water loss during transpiration.

In shoot Y: All the leaves have been removed, so there is no transpiration happening. Which means the shoot stem does not have to uptake any water from the tube as there is no water loss.