Movement of Substances : Gas Exchange in Plants

Question 1

what happens in photosynthesis?

Answer 1

• plants absorb carbon dioxide and release oxygen
• 6CO2 + 6H2O -> C6H12O6 + 6O2

Question 2

what happens in respiration?

Answer 2

• C6H12O6 -> 6O2 -> 6CO2 + 6H2O

Question 3

what two processes do plants do?

Answer 3

• respiration
• photosynthesis

Question 4

what happens when photosynthesis is faster than respiration?

Answer 4

• during the day, at higher light intensities, there will be a net uptake of carbon dioxide and net release of oxygen
• we say oxygen is excreted from the leaves

Question 5

what happens when respiration is faster than photosynthesis?

Answer 5

• in the dark and at low light intensities, there will be a net uptake of oxygen and net release of carbon dioxide
• we say carbon dioxide is excreted from the leaves

Question 6

where does gas exchange occur?

Answer 6

• between the air spaces in the leaves and the atmosphere
• carbon dioxide and oxygen diffuse, moving through the stomata

Question 7

label the diagram.

Answer 7

• *bio rev image

Question 8

label the diagram.

Answer 8

• *bio rev image

Question 9

how is the leaf specialised for gas exchange?

Answer 9

• The leaf is thin, so the diffusion distance is short. It also has a large surface area, increasing the number of stomata, so that carbon dioxide can diffuse quicker.
  *Stomata allow gases to diffuse into / out of the air spaces of the leaf. This provides a short diffusion distance to the mesophyll cells.
• Gases include moving into/out of the leaf include water vapour (lost in
  transpiration), oxygen (absorbed at night for respiration) and carbon dioxide (absorbed for photosynthesis).
• The guard cells form the stomata and can change shape, altering the size of the stomata.
• The stomata close to reduce water loss (and gas exchange). This happens when the plant has lost too much water - the guard cells become flaccid, so the cell walls are not held apart and the stomata shrink

Question 10

describe an experiment to investigate gas exchange in plants.

Answer 10

• This investigation uses sodium hydrogen-carbonate indicator to show the changes in carbon
  dioxide concentration in the air surrounding leaves in different light conditions.
• Leaves of, for example, privet are placed in three of four boiling tubes containing a small volume
  (2 cm3) of sodium hydrogen carbonate indicator (red / orange when in balance with atmosphere).
• One tube is placed in bright light, one is wrapped in foil, one is wrapped in tissue paper (weak light) and the tube with a dead leaf is also left in bright light. The tubes are then left for 40 minutes.
• *bio rev image
• The diagram above shows the expected results.