Photosynthesis Part 4 (Overcoming CO2 limitations & limiting factors) - (Week 3)

Question 1

What molecule other than CO2 can Rubisco bind to?

Answer 1

Oxygen - acts as a ‘competing’ molecule

Question 2

What process occurs when Rubisco fixes O2 instead of CO2?

Answer 2

Photorespiration

Question 3

When does photorespiration occur? Why is this a disadvantage to the organism?

Answer 3

• When there is low [CO2] rubisco will less likely bind to CO2 (& more likely to O2)
• When temp is higher CO2 is less soluble so O2 is more abundant
• Photorespiration takes up energy & does not produce much energy (not very beneficial to organism)

Question 4

What are Carbon Concentrating Mechanisms (CMMs)?

Answer 4

Mechanisms that cells have that increase the concentration of carbon for rubisco

Question 5

In what way do C4 plants have CMMs?

Answer 5

• Its anatomy allows it to have an interplay between mesophyll cells & bundle of sheath cells (arranged around vascular bundle)
• PEPC has high affinity for bicarbonate which prevents photorespiration
• CO2 produced is released next to rubisco, increasing its fixation (enters Calvin cycle - carbon reaction)

Question 6

In what way do CAM plants have CMMs?

Answer 6

• When CO2 enters cells it is quickly converted to bicarbonate
• At night nocturnal acidification occurs where CO2 is converted into malate & stored as malic acid
• Malic acid is then converted back into CO2 in close proximity to rubisco

Question 7

Why is it important that CAM plants have their CMMs?

Answer 7

It is important for plants in extremely hot conditions

Question 8

Define ‘limiting factor’

Answer 8

The factor which is the shortest supply is likely to be the one which is determining the rate of photosynthesis

Question 9

Give 3 main factors that affect the rate of photosynthesis

Answer 9

• Light intensity
• CO2 concentration
• Temperature

Question 10

Which factor is the 1st to affect the rate of photosynthesis?

Answer 10

Insufficient CO2 concentration ([CO2] becomes a limiting factor)

Question 11

Which factor is the 2nd to affect the rate of photosynthesis?

Answer 11

Temperature (becomes a limiting factor when CO2 is in excess)

Question 12

What factor is the 3rd to affect the rate of photosynthesis?

Answer 12

Light intensity (becomes a limiting factor when there is excess CO2 & increased temp)

Question 13

What happens to the rate of photosynthesis when the light intensity is too high?

Answer 13

• Rate decreases

- Reactive O2 species are made & photo damage occurs (damage to chlorophyll)

Question 14

Describe the photosynthetic response to bright light

Answer 14

• Increases the production of ATP, NADPH & O2
• More power to reduce & phosphorylate GP thus increasing TP production

(RuBP levels high & GP low)

Question 15

Describe the photosynthetic response to bright light

Answer 15

• Increase in GP but not enough to convert to TP

- Levels of TP & RuBP decrease

Question 16

Describe the photosynthetic response to high [CO2]

Answer 16

• More carbon is fixed by Rubisco so there is increased amount of GP & TP
• RuBP levels are low

Question 17

Describe the photosynthetic response to low [CO2]

Answer 17

• RuBP accumulates as carbon fixation is limited

- GP & TP are not formed

Question 18

Describe the photosynthetic response to temperature

Answer 18

• Temp. has a huge effect on photosynthesis which is dependent on enzymes
• Increase in carboxylation rates w/ temp.
• Decrease in affinity of Rubisco for CO2 as temp. rises
• Increased temp. also reduces CO2 uptake

(Plant also starts to close its stomata - to prevent water loss)