9.3 - Transpiration

Question 1

Where does photosynthesis mainly take place in plants, and what does it require?

Answer 1

Photosynthesis mainly takes place in the leaves of green plants. It requires carbon dioxide (CO₂) and water (H₂O) for the process to occur.

Question 2

How is water transported to the leaf for photosynthesis?

Answer 2

Water is transported to the leaf from the roots through the xylem, which ensures that the leaf has enough water for photosynthesis.

Question 3

How does carbon dioxide enter the leaf for photosynthesis?

Answer 3

Carbon dioxide (CO₂) enters the leaf by diffusing into the leaf cells from the air spaces within the leaf. This happens down a concentration gradient.

Question 4

What is the role of oxygen in photosynthesis, and how does it move out of the leaf?

Answer 4

Oxygen is a waste product of photosynthesis. It diffuses out of the leaf cells into the air spaces, moving down its concentration gradient.

Question 5

What other substance moves out of the leaf during photosynthesis, and how does it leave?

Answer 5

Water evaporates from the surfaces of the leaf cells into the air spaces during photosynthesis. This process is part of transpiration, where water is lost from the plant.

Question 6

What is the role of leaves in photosynthesis, and how are they adapted to this function?

Answer 6

Leaves have a large surface area to capture sunlight and carry out photosynthesis. They are covered with a waxy cuticle that makes them waterproof, preventing rapid water loss by evaporation while still allowing gas exchange to support photosynthesis.

Question 7

How does carbon dioxide enter the leaf, and how does oxygen exit the leaf?

Answer 7

Carbon dioxide moves into the leaf from the air through microscopic pores called stomata. Oxygen moves out of the leaf by diffusion, both gases moving down their concentration gradients.

Question 8

What are stomata, and how are they controlled?

Answer 8

Stomata are tiny pores, usually found on the underside of the leaf, that allow gases (carbon dioxide and oxygen) to move in and out. The stomata are surrounded by guard cells, which control their opening and closing.

Question 9

What happens when the stomata open for gas exchange?

Answer 9

When the stomata open for gas exchange, water vapor also escapes from the leaf by diffusion. This loss of water vapor from the leaves and stems is known as transpiration.

Question 10

Why is transpiration an inevitable consequence of gaseous exchange?

Answer 10

Transpiration occurs because, during the opening of stomata for gaseous exchange (carbon dioxide in and oxygen out), water vapor also diffuses out, leading to water loss from the plant.

Question 11

How do stomata regulate water loss, and why must they remain open during the day and night?

Answer 11

Stomata open and close to control water loss. During the day, they need to be open to allow carbon dioxide in for photosynthesis. At night, when photosynthesis stops, the stomata still need to be open to allow oxygen in for cellular respiration.

Question 12

How much water can plants lose through transpiration?

Answer 12

An acre of corn can lose 11,500–15,000 litres of water through transpiration daily. A single large tree can lose more than 700 litres of water per day.

Question 13

What is the transpiration stream, and how does it work?

Answer 13

The transpiration stream is the movement of water from the roots through the xylem to the leaves. Water enters the roots by osmosis, moves up the xylem, and then moves across membranes in the leaf by osmosis and diffusion, where it evaporates and exits through the stomata into the air.

Question 14

How does water move from the xylem into the mesophyll cells of the leaf?

Answer 14

Water moves from the xylem into the mesophyll cells of the leaf by osmosis and through the apoplast pathway (across cell walls). Once in the mesophyll cells, water evaporates into the air spaces of the leaf.

Question 15

How does water vapor move out of the leaf?

Answer 15

Water vapor moves from the air spaces in the leaf into the external air through the stomata, following a concentration gradient. This diffusion of water vapor out of the leaf is part of the transpiration stream.

Question 16

What happens when water evaporates from the mesophyll cells in the leaf?

Answer 16

The evaporation of water from the mesophyll cells lowers the water potential of the cell. This causes water to move into the cell from adjacent cells by osmosis through both the apoplast and symplast pathways.

Question 17

How does water move from the leaf to the xylem?

Answer 17

Water moves from the mesophyll cells to the xylem by osmosis. This continues in a repeating cycle across the leaf until the water reaches the xylem, which is constantly replenished by the transpiration stream.

Question 18

What is adhesion, and how does it help in the transpiration stream?

Answer 18

Adhesion is the attraction between water molecules and the carbohydrates in the cell walls of the xylem vessels. This helps water “stick” to the walls of the narrow xylem vessels, assisting its movement up the plant.

Question 19

What is cohesion, and how does it contribute to the transpiration stream?

Answer 19

Cohesion is the attraction between water molecules. Water molecules tend to stick together due to hydrogen bonding, which helps to pull the water up the xylem in a continuous stream, contributing to capillary action.

Question 20

What is capillary action, and how does it help water rise in the xylem?

Answer 20

Capillary action is the process by which water rises up narrow tubes (like xylem vessels) against gravity, driven by the combined effects of adhesion and cohesion. This helps water move up the plant from the roots to the leaves.

Question 21

What is the transpiration pull?

Answer 21

The transpiration pull is the force generated by the evaporation of water from the leaf that draws water up through the xylem from the roots. This continuous stream of water is part of the transpiration stream.

Question 22

How does the transpiration pull affect the roots?

Answer 22

The transpiration pull creates tension in the xylem, which helps to move water across the roots from the soil into the plant. This ensures a continuous flow of water from the soil to the leaves.

Question 23

What is the cohesion-tension theory?

Answer 23

The cohesion-tension theory explains how water moves from the soil, through the roots, up the xylem, and into the leaves in a continuous stream. It involves cohesion between water molecules and adhesion to xylem walls, creating a tension that pulls the water upwards against gravity.

Question 24

What evidence supports the cohesion-tension theory in plants? (3)

Answer 24

Evidence supporting the cohesion-tension theory includes:

Changes in the diameter of trees due to transpiration
Air being drawn into xylem vessels when broken rather than water leaking out
Disruption of water movement when a xylem vessel is broken and air enters.

Question 25

How does the diameter of a tree change with transpiration?

Answer 25

During the day, when transpiration is at its highest, the tension in the xylem vessels is also highest, causing the tree to shrink in diameter. At night, when transpiration is low, the tension in the xylem vessels decreases, causing the tree’s diameter to increase.

Question 26

How can changes in tree diameter be measured to support the cohesion-tension theory?

Answer 26

The circumference of a suitably sized tree can be measured at different times of the day. A decrease in diameter during the day and an increase at night provides evidence for the tension in the xylem vessels.

Question 27

What happens when a xylem vessel is broken, and why is this important evidence for the cohesion-tension theory?

Answer 27

When a xylem vessel is broken (e.g., cutting flower stems), air is typically drawn into the xylem, rather than water leaking out. This supports the idea that a continuous stream of water held together by cohesion is crucial for water transport.

Question 28

How does breaking a xylem vessel affect water movement in plants?

Answer 28

If a xylem vessel is broken and air is pulled into the xylem, the continuous water stream is disrupted, and the plant can no longer move water upwards. This shows the importance of cohesion between water molecules for the process.

Question 29

What role does transpiration play in delivering water to the plant?

Answer 29

Transpiration delivers water, along with dissolved mineral ions, to the cells where they are needed for processes like photosynthesis and cell function.

Question 30

How does transpiration help with cooling the plant?

Answer 30

The evaporation of water from leaf cell surfaces helps to cool the leaf down, preventing heat damage, especially under high sunlight conditions.

Question 31

Why can transpiration be problematic for plants?

Answer 31

Transpiration can be problematic because it results in water loss. If the water supply cannot meet the demand, especially under high-intensity sunlight when photosynthesis and transpiration are high, the plant may lose more water than it can take in.

Question 32

Why is it difficult to directly measure transpiration?

Answer 32

It is hard to condense and collect all evaporated water without also collecting water from other surfaces. Additionally, it is difficult to separate water vapor from transpiration and water vapor produced by respiration.

Question 33

How do scientists estimate transpiration rates if direct measurement is difficult?

Answer 33

Scientists measure water uptake instead, as around 99% of the water taken up by a plant is lost through transpiration, making it a reliable indicator of transpiration rate.

Question 34

What is a potometer, and why is it used?

Answer 34

A potometer is a device used to measure the rate of water uptake in plants, providing an indirect way to measure transpiration.

Question 35

Why must all joints in a potometer be sealed with waterproof jelly?

Answer 35

To ensure that all measured water loss is due to transpiration from the leaves and stem, not from leaks in the apparatus.

Question 36

What is the formula for calculating the rate of water uptake using a potometer? What are the units?

Answer 36

Rate of water uptake = distance moved by air bubble / time taken for air bubble to move that distance. Units: cm s⁻¹

Question 37

How do plants control the rate of transpiration?

Answer 37

Plants control transpiration mainly by opening and closing their stomatal pores, a process driven by changes in turgor pressure in the guard cells.

Question 38

What happens to guard cells when turgor pressure is low?

Answer 38

When turgor pressure is low, the asymmetric walls of the guard cells close the stomatal pore, reducing water loss.

Question 39

How do guard cells open the stomatal pore?

Answer 39

When environmental conditions are favorable, guard cells actively pump in solutes, increasing turgor pressure. This causes them to expand lengthwise due to cellulose hoops preventing width expansion, leading to a bean-shaped structure that opens the pore.

Question 40

Why do guard cells become bean-shaped when they open?

Answer 40

The inner wall of the guard cell is less flexible than the outer wall, so when turgor increases, the cells bend outward, creating a bean-shaped opening.

Question 41

How do plants respond to water scarcity to conserve water?

Answer 41

When water is scarce, hormonal signals from the roots trigger turgor loss in guard cells, causing the stomata to close and reducing water loss.

Question 42

What are the three main ways factors can affect the rate of transpiration?

Answer 42

Factors can: Influence the opening/closing of stomata Affect the rate of evaporation from leaf cell surfaces Alter the diffusion gradient between the air spaces in the leaves and the surrounding air

Question 43

How does light intensity affect transpiration?

Answer 43

1. Light is required for photosynthesis, causing stomata to open for gas exchange. 2. In darkness, most stomata close, reducing transpiration. 3. Increasing light intensity leads to more open stomata, increasing the rate of water vapor diffusion and evaporation. 4. Higher light intensity = Higher transpiration rate.

Question 44

How does relative humidity affect transpiration?

Answer 44

High relative humidity lowers transpiration because it reduces the water vapor potential gradient between the inside of the leaf and the external air. Dry air has the opposite effect, increasing the diffusion gradient and the transpiration rate.

Question 45

What are the two ways temperature affects transpiration?

Answer 45

Increased kinetic energy – Higher temperature increases the movement of water molecules, increasing evaporation from spongy mesophyll cells into air spaces. Lower relative humidity – Warm air holds more water before reaching saturation, decreasing external water potential and increasing the diffusion gradient. Both factors increase the transpiration rate.

Question 46

How does air movement affect transpiration?

Answer 46

A layer of still air around a leaf traps water vapor, reducing the diffusion gradient. Wind removes this humid air, increasing the diffusion gradient and transpiration rate. More air movement = Higher transpiration rate.

Question 47

What is the effect of still air on transpiration?

Answer 47

A long period of still air allows water vapor to accumulate around the stomata, reducing the diffusion gradient and lowering transpiration.

Question 48

How does soil-water availability affect transpiration?

Answer 48

If soil is dry, the plant experiences water stress and reduces transpiration to conserve water. If soil is well-watered, transpiration can continue at a normal or increased rate.