- C4 plants, such as maize, sorghum, sugarcane, and millet, have a unique photosynthetic pathway different from C3 plants.
- C4 plants initially produce acids with four carbon atoms, while C3 plants produce a 3C acid called GP.
- C4 plants, like maize, exhibit a leaf anatomy known as “Kranz” anatomy, with two rings of cells around vascular bundles.
- C4 plants are more efficient at carbon dioxide uptake and can remove CO2 from the atmosphere at concentrations as low as 0.1 ppm.
- Hatch and Slack discovered the Hatch-Slack pathway, a new carbon pathway found in C4 plants.
- The chloroplasts in bundle sheath cells of C4 plants differ in form from those in mesophyll cells, making them dimorphic.
- The unique biochemical pathway of C4 plants allows for improved CO2 fixation and increased photosynthetic efficiency.
The Hatch-Slack Pathway #
- The Hatch-Slack pathway is a carbon fixation pathway that occurs in certain plants, known as C4 plants.
- It involves the transport of carbon dioxide (CO2) and hydrogen (H) from mesophyll cells to bundle sheath cells.
- Carbon dioxide is initially fixed in the mesophyll cells using an enzyme called phosphoenolpyruvate (PEP) carboxylase.
- The fixed carbon dioxide is then converted to malate, a 4-carbon (4C) acid, which is transported to the chloroplasts of bundle sheath cells.
- In the bundle sheath cells, malate is converted to pyruvate, a 3C acid, by removing CO2 and H.
- Pyruvate is then returned to the mesophyll cells and used to regenerate PEP with the addition of phosphate from ATP.
- The net result of the Hatch-Slack pathway is the movement of carbon dioxide and hydrogen from mesophyll cells to bundle sheath cells, requiring energy from ATP.
- In the bundle sheath cells, CO2 is refixed by RuBP carboxylase in the C3 pathway, and reduced NADP is used to reduce GP to sugar.
- The main advantage of the Hatch-Slack pathway is the increased efficiency of carbon dioxide fixation, making C4 plants more photosynthetically efficient.
- C4 plants are adapted to regions of high light intensity and are often found in drier subtropical and tropical regions.
Key Terms #
- Hatch-Slack pathway: A pathway for transporting CO2 and H from mesophyll cells to bundle sheath cells in C4 plants.
- Mesophyll cells: Cells found in the leaves that surround bundle sheath cells and are involved in initial carbon dioxide fixation.
- Bundle sheath cells: Cells found in the leaves that surround vascular tissues and are involved in the Calvin cycle for sugar production.
- PEP carboxylase: An enzyme used in the Hatch-Slack pathway for fixing carbon dioxide in mesophyll cells.
- RuBP carboxylase: An enzyme used in the C3 pathway for fixing carbon dioxide in bundle sheath cells.
- ATP: Adenosine triphosphate, a molecule that provides energy for cellular processes.
- GP: Glyceraldehyde-3-phosphate, an intermediate in the Calvin cycle involved in sugar production.
- NADP: Nicotinamide adenine dinucleotide phosphate, a coenzyme involved in redox reactions in photosynthesis.
| Differences | Mesophyll Chloroplasts | Bundle Sheath Chloroplasts |
|---|---|---|
| Size of Grana | Large | No grana or very few and small |
| Light-dependent reactions | Favored, plenty of ATP, reduced NADP, and O2 | Occur at very low rate, little ATP, NADP, or O2 |
| RuBP Carboxylase | Virtually no presence | High concentration |
| CO2 Fixation | Occurs in cytoplasm by PEP Carboxylase | Occurs efficiently as in C3 plants |
| Starch | Little starch | Abundant starch grains |
sheath chloroplasts in C4, plants.
Significance of the C₄ Pathway #
- C₄ plants have evolved in arid regions and are adapted to thrive in environments with limited water availability. They have a superior carbon dioxide-fixing mechanism that allows them to conserve water and maintain photosynthesis even in drought conditions.
- Due to their efficient carbon fixation, C₄ plants experience faster growth and increased dry mass compared to plants following the C₃ pathway. This makes them desirable crop plants, especially in regions where water resources are scarce.
- C₄ plants also exhibit a higher tolerance to high temperature and heat stress. This characteristic is particularly advantageous in tropical and subtropical regions, where temperatures can be consistently high.
- The efficient photosynthetic process of C₄ plants leads to higher crop yields. Cultivation of C₄ crops, such as maize, sugarcane, and sorghum, can therefore contribute to food security and sustainable agriculture.
| Comparison | C3 Plants | C4 Plants |
|---|---|---|
| Representative species | Most crop plants, e.g. cereals, tobacco, beans | Maize, sugarcane |
| Light intensity for maximum rate of photosynthesis | 10,000-30,000 foot candles | Not saturated at 10° lux |
| Effect of temperature rise from 25°C to 35°C | No change in rate or lower rate | 50% greater at 35°C |
| Point at which no more CO2 can be taken up | 40-60 ppm CO2 | Around zero ppm CO2 |
| Water loss per g dry mass produced | 450-950 | 250-350 |
| Carbon dioxide fixation | Occurs once | Occurs twice, first in mesophyll cells, then in bundle sheath cells |
| Carbon dioxide acceptor | RuBP, a 5C compound | RuBP, a 5C compound |
| Carbon dioxide-fixing enzyme | RuBP carboxylase, which is inefficient | PEP carboxylase which is very efficient |
| First product of photosynthesis | A 3C compound, GP | A 4C compound, oxaloacetate |
| Leaf anatomy | Only one type of chloroplast | “Kranz” anatomy, i.e. two types of cell, each with its own type of chloroplast |
| Efficiency | Less efficient photosynthesis than C4 plants. Yields usually much lower. | More efficient photosynthesis than C3 plants but use more energy. Yields usually higher. |
Practice questions #
- Discuss photosynthesis in C4 plants [8]
- The differences between C3 and C4 plants [8]
- Why is it an advantage that bundle sheath chloroplasts lack grana? [3]
- The malate shunt is, in effect, a carbon dioxide and hydrogen pump. What is the advantage of this? [2]
- What would be the effect of lowering oxygen concentrations on
- (a) C3 photosynthesis,
- (b) C4 photosynthesis? Explain your answers. [6]
