The C4 pathway and Kranz anatomy · Biology Class 11–12 | StemPy Lab
Course 9.3·Lesson 2 of 3
The C4 pathway and Kranz anatomy
C4 plants such as maize and sugarcane have Kranz anatomy: large bundle sheath cells with many chloroplasts form a wreath round each vascular bundle. In the mesophyll, PEP carboxylase joins carbon dioxide to PEP, a 3-carbon compound, to give OAA, a 4-carbon compound. The carbon dioxide is then released in the bundle sheath and enters the Calvin cycle. C4 plants lack photorespiration and do well at high temperature.
about 25 min1 simulation1 practice question
Lesson 2 of 3Show all lessonsHide
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Plants of hot, dry places
Maize, sugarcane and sorghum grow well in hot, dry, bright places. They have a different way of fixing carbon dioxide, with a special leaf. These are the C4 plants. They are called C4 because the first stable product of fixation is a 4-carbon compound, oxaloacetic acid (OAA), and not PGA.
Kranz anatomy
Two parts. On the left, a cross section of a maize leaf near a vein. A ring of large bundle sheath cells, full of dark green chloroplasts, surrounds a vascular bundle in the centre, and smaller mesophyll cells lie around the outside of the ring like a wreath. Labels name the Kranz anatomy, the bundle sheath cells, the mesophyll cells and the vascular bundle. On the right, the pathway. In a mesophyll cell, PEP, a 3-carbon compound, takes up carbon dioxide with the enzyme PEP carboxylase to give OAA, a 4-carbon compound, which becomes malic acid. An arrow carries malic acid into a bundle sheath cell. There it gives off carbon dioxide and a 3-carbon pyruvate. The carbon dioxide enters the Calvin cycle with RuBisCO and makes sugar. An arrow returns the pyruvate to the mesophyll cell, where it becomes PEP again, with ATP. A note says RuBisCO is absent from the mesophyll cells.
Cut across a leaf of maize and look near a vein. Kranz is a German word for a wreath. The veins are surrounded by a ring of large cells, the bundle sheath cells. Round the ring lie the mesophyll cells, like the petals of a flower.
The bundle sheath cells have:
a large number of chloroplasts,
thick walls that gas cannot cross,
no intercellular spaces.
A C3 leaf has no such sheath of large green cells.
The pathway
The C4 pathway uses two kinds of cell. It is also called the Hatch and Slack pathway.
In a mesophyll cell. The first acceptor of carbon dioxide is PEP, phosphoenol pyruvate, which has three carbon atoms. The enzyme is PEP carboxylase (PEPcase). PEP and carbon dioxide give OAA, with four carbon atoms.
OAA is changed to malic acid, or to aspartic acid, and is carried to the bundle sheath cells.
In a bundle sheath cell. The 4-carbon acid is broken down. It sets carbon dioxide free, and leaves a 3-carbon pyruvate.
The carbon dioxide enters the Calvin cycle. The enzyme RuBisCO is found in the bundle sheath cells. It is absent in the mesophyll cells. The cycle makes sugar as in C3 plants.
Pyruvate goes back to the mesophyll cell. There it is turned into PEP again, using ATP.
So there are two places of carbon fixation. PEP carboxylase fixes carbon dioxide first, in the mesophyll. RuBisCO fixes it again, in the bundle sheath. The result is that carbon dioxide is concentrated round RuBisCO.
Why this helps
C4 plants:
have no photorespiration (you will see why in the next lesson),
do well at high temperatures, with a temperature optimum of 30 to 45 degrees Celsius,
respond to high light,
make more biomass for the same area of leaf.
C3 and C4 compared
First stable product: PGA (3C) in C3. OAA (4C) in C4.
First acceptor: RuBP (5C) in C3. PEP (3C) in C4.
First enzyme: RuBisCO in C3. PEP carboxylase in C4.
Calvin cycle in: mesophyll in C3. Bundle sheath in C4.
Kranz anatomy: no in C3. Yes in C4.
Photorespiration: present in C3. Absent in C4.
Best temperature: 20 to 25 °C in C3. 30 to 45 °C in C4.
You should find that at 25∘C the C3 plant is near its best, and the C4 plant works at a lower rate. At 40∘C the C4 plant is near its best and the C3 plant has almost stopped.
Try it in Python
The program puts a C3 plant and a C4 plant side by side, and counts the carbon atoms of OAA.
You should see that the Calvin cycle runs in the mesophyll cells of a C3 plant and in the bundle sheath cells of a C4 plant. PEP and carbon dioxide give an OAA with 4 carbon atoms.
Practise with new numbers
Common mistakes
Saying that C4 plants have no Calvin cycle. They have it, in the bundle sheath cells.
Putting RuBisCO in the mesophyll of a C4 plant. It is absent there. PEP carboxylase works there.
Mixing up PEP and RuBP. PEP has 3 carbon atoms and is the first acceptor in C4. RuBP has 5 and is the acceptor in the Calvin cycle.
Forgetting that the pyruvate returns. It goes back to the mesophyll to make PEP again.
Calling C4 and C3 two different kinds of cycle. The Calvin cycle is the same. C4 adds a step before it.
Remember these
C4 plants: maize, sugarcane, sorghum. First stable product OAA (4C).
Mesophyll: PEP (3C) + carbon dioxide, with PEP carboxylase, gives OAA.
Bundle sheath: carbon dioxide is set free, RuBisCO and the Calvin cycle run there.
Kranz anatomy: ring of large bundle sheath cells with many chloroplasts and thick walls.
No photorespiration, high temperature optimum, high productivity.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
Why are maize and sugarcane called C4 plants?
Describe the bundle sheath cells of a C4 leaf.
Name the enzyme that fixes carbon dioxide first in a C4 plant, and say where it works.
What happens to pyruvate after the carbon dioxide has been set free?
Give two advantages that C4 plants have.
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