Topic 5—A: Photosynthesis and respiration- 4. Limiting factors in photosynthesis

Question 1

Optimum conditions for photosynthesis

Answer 1

• High light intensity
• Temperature (25 degrees)
• Carbon dioxide (0.4%)
• Water

Question 2

High light intensity (optimum conditions)

Answer 2

• Light is needed to provide the energy for the light-dependant reaction
• The higher the intensity of the light, the more energy it provides
• Only certain wavelengths of light are used for photosynthesis
• The photosynthetic pigments chlorophyll a, chlorophyll b and carotene only absorb the red and blue light in sunlight

Question 3

Temperature (25 degrees) (optimum conditions)

Answer 3

• Photosynthesis involves enzymes (ATP synthase, rubisco)
• If the temperature falls below 10 degrees the enzymes become inactive but if the temperature is more than 45 degrees they may start to denature.
• At high temperatures, stomata close to avoid losing too much water which causes photosynthesis to slow down because less carbon dioxide enters the leaf when the stomata is closed.

Question 4

Carbon dioxide (0.4%) (optimum conditions)

Answer 4

• Carbon dioxide makes up 0.04% of the gases in the atmosphere
• Increasing this to 0.4% gives a higher rate of photosynthesis but any higher and the stomata start to close

Question 5

Water (optimum conditions)

Answer 5

• Plants also need a constant supply of water
• Too little and photosynthesis has to stop but too much and the soil becomes waterlogged which reduces the uptake of minerals such as magnesium which is needed to make chlorophyll a

Question 6

Limiting factors of photosynthesis

Answer 6

• Light
• Temperature
• Carbon dioxide

Question 7

Light intensity (limiting factor)

Answer 7

• As light intensity increases so does the rate of photosynthesis
• But there is a saturation point (shown by a straight line on a graph) so therefore increasing light intensity after this point makes no difference because something else has become the limiting factor

Question 8

Temperature (limiting factor)

Answer 8

• The graph levels off when light intensity is no longer the limiting factor
• The graph at 25 degrees levels off at a higher point than the one at 15 degrees showing that temperature must have been a limiting factor at 15 degrees

Question 9

Carbon dioxide concentration (limiting factor)

Answer 9

• Both these graphs level off when light intensity is no longer the limiting factor
• The graph at 0.4 % carbon dioxide levels off at a higher point than the one at 0.04% so carbon dioxide must have been a limiting factor at 0.04% carbon dioxide
• The limiting factor here isn’t temperature because it’s the same for both graphs (25 degrees)

Question 10

What does it mean when the graph levels off?

Answer 10

• It means that the rate of photosynthesis is not increasing anymore

Question 11

What is a saturation point?

Answer 11

• Where a factor is no longer limiting the reaction

Question 12

What is a limiting factor?

Answer 12

• A variable that can slow down the rate of reaction

Question 13

How do farmers increase plant growth?

Answer 13

• In a glass house, carbon dioxide is added to the air
• Light can get through the glass and lamps provide light at night time
• Glasshouses trap heat energy from sunlight which warms the air and heaters and cooling systems can also be used to keep a constant temperature and air circulation systems make sure the temperature is even throughout the glasshouse

Question 14

If carbon dioxide is reduced what would the effect be on GP?

Answer 14

• GP would be reduced as not as much carbon dioxide combines with ribulose biphosphate so RuBP can’t turn into GP

Question 15

If carbon dioxide is reduced what would the effect be on TP?

Answer 15

• TP would decrease because there is no GP to be reduced to triose phosphate

Question 16

If carbon dioxide is reduced what would the effect be on RuBP?

Answer 16

• It would stay the same as there is no TP to be converted to RuBP
• Or it would increase because it isn’t being converted to GP

Question 17

If carbon dioxide is reduced what would the effect be on the light dependant phase?

Answer 17

• No ish as photoionisation and photolysis can still occur
• LD phase will run out of reactants (ADP +Pi & NADP) so products of ATP and NADPH will decrease

Question 18

If light intensity is reduced what would the effect be on GP?

Answer 18

• It would increase as RuBP & C02 to GP and it would stay as GP as it isn’t converted to TP to be converted to RuBP

Question 19

If light intensity is reduced what would the effect be on TP?

Answer 19

• It would decrease because ATP & NADP from light dependant reaction is not available so GP can’t be reduced to TP

Question 20

If light intensity is reduced what would the effect be on RuBP?

Answer 20

• Less RuBP because it’s converted to GP but not remade from TP

Question 21

If light intensity is reduced what would the effect be on the light dependant phase?

Answer 21

• Less ATP and NADPH as no photolysis or photoionisation

Question 22

Temperature (low) effect on enzymes?

Answer 22

• ATP synthase & Rubisco would be effected
• Low temperature means lower kinetic energy
• This means fewer enzyme-substrate complexes are formed so a slower rate of reaction

Question 23

Temperature (high) effect on enzymes?

Answer 23

• ATP synthase & Rubisco would be effected
• H bonds in tertiary structure change
• This changes the 3D shape of the enzyme
• This means the active site is no longer complementary to the substrate
• E-S complexes can’t be formed