C4 Photosynthesis Flashcards
How was the C4 pathway discovered?
By feeding 14C labelled carbon dioxide to sugarcane. First products seen were C4 (malate and oxaloacetate), not phosphoglycerate (C3).
What are the advantages of the C4 pathway?
C4 plants are more resistant to low carbon dioxide availability and photorespiration.
Why do stomata need to be opened less in C4 plants and why is this an adavantage?
Carbon dioxide usage is more efficient, meaning less needs to be taken in and the stomata don’t need to open as frequently.
Advantageous as less water is lost when stomata are opened for gas exchange.
Why is the C4 pathway common in tropical plants?
Tropical plants live in hot environments are are at more risk of water stress and oxygenation than C3 plants.
Why are plants that live in hot environments at a higher risk of oxygenation than C3 plants?
Rubisco’s ability to distinguish between carbon dioxide and oxygen worsens with temperature increase.
Why hasn’t C4 photosynthesis evolved in all plants?
The C4 pathway consumes more ATP - and it would not be worth the energetic cost to plants that live in colder environments and are at a lower risk of oxygenation.
Describe the typical leaf anatomy of a C3 plant.
Palisade mesophyll cells are found underneath the upper epidermis of the leaf and are full of chloroplasts containing all of the photosynthetic enzymes.
Describe the typical leaf anatomy of a C4 plant.
All the photosynthetic enzymes are concentrated in bundle sheath cells which surround the vein. Bundle sheath cells are surrounded by mesophyll cells which have fewer photosynthetic enzymes.
How does the C4 pathway prevent high levels of photorespiration?
Carbon dioxide is concentrated near the Calvin cycle enzymes, by allowing carbon dioxide to be spatially transferred from mesophyll cells to bundle sheath cells.
What is the energetic cost of moving one molecule of carbon dioxide from the mesophyll cells to bundle sheath cells?
2 ATP
Why and when did C4 photosynthesis evolve?
Evolved between 7 and 30 million years ago in response to low carbon dioxide and high temperatures.
What is the advantage of using PEP carboxylase for the initial step of the Calvin cycle?
It uses hydrogencarbonate instead of carbon dioxide and has no oxygenase activity.
How does PEP carboxylase distinguish between hydrogencarbonate and oxygen?
Using the fact that hydrogencarbonate is trigonal planar and oxygen is bent.
How is hydrogencarbonate generated?
By carbonic anhydrase
Give an overview of the C4 pathway.
- PEP (3C) is carboxylated to form oxaloacetate (4C)
- Oxaloacetate is reduced to form malate (4C)
- Malate is transported to the bundle sheath cells
- Malate is broken down to give pyruvate and carbon dioxide – this is done by NADP malic enzyme
- CO2 released feeds into the Calvin cycle and pyruvate travels back into the mesophyll cells where the cycle can start again.
- Pyruvate is converted back into phosphoenolpyruvate using ATP:
