How do C4 and CAM plants differ in the way they separate initial CO2 fixation from the Calvin cycle?
AC4 separates them in time; CAM separates them in space
BC4 separates them between mesophyll and bundle-sheath cells; CAM separates them between night and day
CBoth separate them between roots and leaves
DNeither separates them; both use only RuBisCO
In C4 plants, mesophyll cells fix CO2 into four-carbon acids that move into bundle-sheath cells, where CO2 is released for the Calvin cycle. In CAM plants, the same cells fix CO2 into acids at night and release it to the Calvin cycle during the day.
Question 2 True / False
C4 plants such as maize fix carbon without using RuBisCO.
TTrue
FFalse
Answer: False
C4 plants use PEP carboxylase for the first capture of CO2, but the CO2 is later released in the bundle-sheath cells and fixed by RuBisCO in the Calvin cycle. The C4 steps raise the CO2 level around RuBisCO.
Question 3 Short Answer
Why does a C4 grass often outgrow a C3 grass in a hot, sunny summer but not in a cool, cloudy spring?
Think about your answer, then reveal below.
Model answer: In heat, stomata partly close and RuBisCO's oxygenation rises, so C3 plants lose much fixed carbon to photorespiration. The C4 pump keeps CO2 high around RuBisCO and avoids most of that loss. In cool conditions photorespiration is low, and the extra ATP the C4 pathway spends to regenerate PEP becomes a cost with little benefit.
The advantage of C4 depends on conditions. It pays when photorespiration would otherwise be high, which is in hot, bright and dry environments.
Question 4 Multiple Choice
Which plant is an example of CAM photosynthesis?
AWheat
BPineapple
CMaize
DOak
Pineapple, many cacti and other succulents use CAM. Wheat and oak are C3 plants, and maize is a C4 plant.