Miss lee - Transpiration

Question 1

What is transpiration

Answer 1

Transpiration is the loss of water vapour (by evaporation) from the surface of leaves and stems of a plant.
-Transpiration is a consequence of gas exchange as the stomata need to already be open to allow CO2 in and O2 out. So water moves out of the leaf from an area of high WP to Low WP

Question 2

How is the rate of Transpiration controlled

Answer 2

• 1) Waxy cuticle (=waterproof layer)
• 2) Guard cells can open or close stomata
• 3) Very few stomata on upper surface of leaf

Question 3

What is the transpiration stream

Answer 3

Transpiration stream: the flow of water (in continuous columns), up the xylem vessels from roots to leaves.

Question 4

What are the two states stomata can be in

Answer 4

Turgid – Open
• Water moves into the vacuoles by osmosis
• Outer wall is more flexible than the inner wall, so to cell bends and opens the stoma
Flaccid – Closed
• Water moves out of the vacuoles by osmosis
• Outer wall is more flexible than the inner wall, so to cell bends back and closes the stoma

Question 5

When do stomata open

Answer 5

• Stomata open during the day and close during the night.
• High water potential outside the stomata
• Low concentrations of CO2 cause stomata to open. High CO2 causes stomata to close.
• Light causes stomata to open.

Question 6

When does stomata close

Answer 6

• HIGH CO2 concentration inside the leaf

* Low light intensity (e.g. darkness)

Question 7

What is the function of stomata

Answer 7

When stomata are open they allow gas exchange between the leaf and the outside environment.
Carbon dioxide can enter the leaf through the stomata and oxygen and water vapour can diffuse out of the stomata.
Transpiration is mainly controlled by the opening and closing of the stomata.

Question 8

How are guard cells adapted

Answer 8

• Unevenly thickened (cell) wall – wall beside the pore is thicker.
• Able to change shape/bend
• Transport proteins/ion channels in the plasma membrane.
Absorption of K+ ions by the guard cells.
• K+ ions decrease the water potential hence water enters by
osmosis and guard cells can become turgid.
• Presence of chloroplasts & mitochondria to provide ATP energy.

Question 9

What is the rate of transpiration

Answer 9

•
• •
The rate at which transpiration occurs refers to the amount of water lost by plants over a given time period.
Plants regulate the rate of transpiration by opening and closing of stomata.
There are, however, a number of external factors that affect the rate of transpiration, namely: temperature, light intensity, humidity, and wind.

Question 10

How does temperature affect the rate of transpiration

Answer 10

Temperature affects the transpiration rate in two ways.
Firstly, at warmer temperatures water molecules move faster, and the rate of evaporation from stomata is therefore much faster.
Secondly, the water-holding capacity of warm air is greater than that of cold air.

Question 11

How does light intensity affect the rate of transpiration

Answer 11

• At high light intensity, the rate of photosynthesis increases.
• As photosynthesis increases, the amount of stored glucose in the guard cells increases.
• This lowers the water potential of the leaf (i.e. the contents of the leaf are less dilute).
• Asthewaterpotentialdecreases,morewater enters the guard cells making them more turgid.
• The turgor pressure of the guard cells leads to an opening up of stomata resulting in transpiration.

Question 12

How does humidity affect rate of transpiration

Answer 12

• The amount of water vapour in the air is referred to as the humidity.
• Wateralwaysmovesdownaconcentrationgradient. Therefore when the humidity is high (lots of water vapour in the air) the water potential gradient between the inside of the leaf stomata and the atmosphere is shallow and the rate of transpiration will be low.
• However, if the atmosphere is dry, there will be a steep water concentration gradient between the humid inside of the stomata and the outside air and the rate of transpiration will therefore be fast.

Question 13

How does the wind speed affect transpiration

Answer 13

• When water is lost from the leaf it forms a thin layer outside the leaf.
• This reduces the water potential between the leaf and the atmosphere outside.
• When there is wind, this layer is blown away, thus maintaining the water potential gradient across the leaf.