9.3 Transpiration

Question 1

Importance of a waxy cuticle

Answer 1

Makes leaves waterproof so it prevents leaf cells from losing water rapidly and constantly due to evaporation

Question 2

What happens at the stomata

Answer 2

CO2 diffuses into the plant and O2 diffuses out the plant. Both move down the concentration gradient.

Question 3

How is the stomata open and closed

Answer 3

Controlled by guard cells which surround the stomata controlling whether the stomata is open or closed

Question 4

What happens when the stomata is open

Answer 4

Gas exchange can occur between the air outside the leaf and the air inside the leaf. Water vapour is also lost and moves out via diffusion

Question 5

Transpiration

Answer 5

The loss of water vapour from a plant to its environment by evaporation and diffusion. It is a consequence of gas exchange

Question 6

Advantages of transpiration

Answer 6

• Provides a mean of cooling via evaporative cooling
  -Transpiration stream helps with the uptake of mineral ions
• Turgor pressure of the cells provides support to the leaves in the cells

Question 7

Transpiration stream

Answer 7

Movement of water from the roots to the leaves. The evaporation of water from the leaves reducing the water potential as well as cohesive and adhesive forces cause water to move up the xylem

Question 8

Pathway of water

Answer 8

• Roots take up water from the soil
• water is drawn up the stem to the leaves
  -veins carry the water from the stem to the leaves
  -water evaporates from the leaves

Question 9

How does water show cohesive and adhesive forces

Answer 9

cohesive- forms hydrogen bonds with other water molecules
adhesive- forms hydrogen bonds with the carbohydrates in the xylem

Question 10

Capillary Action

Answer 10

Movement of water through narrow tubes against the force of gravity due to cohesion and adhesion

Question 11

Transpiration pull

Answer 11

Water is drawn up the xylem in a continuous stream to replace the water lost by evaporation in the leaf. This results in tension in the xylem which helps move water across the roots from the soil

Question 12

What does changes in the diameter of trees during the day show

Answer 12

Evidence for the cohesion-tension theory. When transpiration is at the highest during the day tension in the xylem is the highest swell which causes the diameter of the tree to decrease. During the night transpiration decreases so less tension causing the diameter of the tree to increase

Question 13

How does a xylem vessel being cut show evidence for the cohesion-tension theory

Answer 13

Air is drawn in so the plant can no longer move water as the continuous stream of water molecules held by cohesive forces are broken

Question 14

How does the stomata change shape when the turgor is low

Answer 14

When the turgor is low the guard cells close the stomata

Question 15

What happens when the environmental conditions are favourable

Answer 15

Guard cells pump in solute by active transport increasing their turgor

Question 16

How does air movement affect the rate of transpiration

Answer 16

Wind can sweep water molecules away from the leaf which maintains the concentration gradient which increases the rate of transpiration

Question 17

How does temperature affect the rate of transpiration

Answer 17

Increasing the temperature causes an increase in the kinetic energy of water molecules. This causes an increase in the rare of transpiration as water molecules move out the leaf (down the concentration gradient) at a faster rate. However if it gets too hot the stomata will close to minimise water loss so rate of transpiration would decrease

Question 18

How does light intensity affect the rate of transpiration

Answer 18

Stomata close in the dark which reduces the rate of transpiration. Stomata open when there is light even if it’s at a low intensity. Once stomata are open an increase in light intensity would have no effect on the rate of transpiration

Question 19

How does humidity affect the rate of transpiration

Answer 19

If the air outside the leaf is very humid the air has a high concentration of water. This reduces the concentration gradient which reduces the rate of transpiration.