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MSF2 - Photosynthesis > C4 Photosynthesis > Flashcards

Flashcards in C4 Photosynthesis Deck (29)
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1
Q

How was the C4 pathway discovered?

A

By feeding 14C labelled carbon dioxide to sugarcane. First products seen were C4 (malate and oxaloacetate), not phosphoglycerate (C3).

2
Q

What are the advantages of the C4 pathway?

A

C4 plants are more resistant to low carbon dioxide availability and photorespiration.

3
Q

Why do stomata need to be opened less in C4 plants and why is this an adavantage?

A

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.

4
Q

Why is the C4 pathway common in tropical plants?

A

Tropical plants live in hot environments are are at more risk of water stress and oxygenation than C3 plants.

5
Q

Why are plants that live in hot environments at a higher risk of oxygenation than C3 plants?

A

Rubisco’s ability to distinguish between carbon dioxide and oxygen worsens with temperature increase.

6
Q

Why hasn’t C4 photosynthesis evolved in all plants?

A

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.

7
Q

Describe the typical leaf anatomy of a C3 plant.

A

Palisade mesophyll cells are found underneath the upper epidermis of the leaf and are full of chloroplasts containing all of the photosynthetic enzymes.

8
Q

Describe the typical leaf anatomy of a C4 plant.

A

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.

9
Q

How does the C4 pathway prevent high levels of photorespiration?

A

Carbon dioxide is concentrated near the Calvin cycle enzymes, by allowing carbon dioxide to be spatially transferred from mesophyll cells to bundle sheath cells.

10
Q

What is the energetic cost of moving one molecule of carbon dioxide from the mesophyll cells to bundle sheath cells?

A

2 ATP

11
Q

Why and when did C4 photosynthesis evolve?

A

Evolved between 7 and 30 million years ago in response to low carbon dioxide and high temperatures.

12
Q

What is the advantage of using PEP carboxylase for the initial step of the Calvin cycle?

A

It uses hydrogencarbonate instead of carbon dioxide and has no oxygenase activity.

13
Q

How does PEP carboxylase distinguish between hydrogencarbonate and oxygen?

A

Using the fact that hydrogencarbonate is trigonal planar and oxygen is bent.

14
Q

How is hydrogencarbonate generated?

A

By carbonic anhydrase

15
Q

Give an overview of the C4 pathway.

A
  1. PEP (3C) is carboxylated to form oxaloacetate (4C)
  2. Oxaloacetate is reduced to form malate (4C)
  3. Malate is transported to the bundle sheath cells
  4. Malate is broken down to give pyruvate and carbon dioxide – this is done by NADP malic enzyme
  5. CO2 released feeds into the Calvin cycle and pyruvate travels back into the mesophyll cells where the cycle can start again.
  6. Pyruvate is converted back into phosphoenolpyruvate using ATP:
16
Q

What is the role of NADP malate dehydrogenase in the C4 pathway?

A

Reduces oxaloacetate to malate using NADPH

17
Q

Why is there no net loss of NADPH during C4 photosynthesis?

A

The NADPH used to reduce oxaloacetate to malate is regenerated in the Calvin cycle reactions.

18
Q

What is the role of NADP malic enzyme?

A

Breaks down malate to give pyruvate and carbon dioxide.

19
Q

What happens to the pyruvate that is released during malate breakdown in C4 photosynthesis?

A

Returns to the mesophyll cells where it can be converted into PEP again.

20
Q

What happens to the carbon dioxide that is released during malate breakdown in C4 photosynthesis?

A

Fed into the Calvin cycle.

21
Q

How is pyruvate converted into phosphoenolpyruvate in mesophyll cells?

A

Using ATP and is catalysed by pyruvate orthophosphate dikinase.

22
Q

Give the reaction for the conversion of pyruvate to phosphoenolpyruvate.

A

Pyruvate + ATP + Pi -> PEP + AMP + PPi

23
Q

Why are two molecules of ATP needed for each molecule of carbon dioxide fixed in C4 photosynthesis?

A

Need two molecules of ATP to regenerate ATP from AMP

24
Q

How can C4 plants make more ATP?

A

Through cyclic electron flow

25
Q

Why is the extra energy cost justified in C4 plants?

A

By the reduced risk of photorespiration

26
Q

Why is there a push to engineer C4 rice?

A

Want to be able increase rice yields - C4 plants are highly productive

27
Q

Why may it be possible to engineer C4 plants?

A

C3 plants already have the enzymes needed for C4 plants

28
Q

What would need to be changed in order to engineer a C3 plant into a C4 plant?

A

Need to change the expression pattern of the genes for the enzymes, so that they appear in the correct cell types and change aspects of C3 leaf anatomy.

29
Q

Why are researchers optimistic about being able to reengineer crops into C4 plants?

A

Because the C4 pathway has evolved on multiple occasions, suggesting it is relatively easy to evolve.