C4 photosynthesis | C4 Plants - Biology Upstem Academy

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.

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

Differences between mesophyll and bundle
sheath chloroplasts in C4, plants.

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.

The differences between C3 and C4 plants

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]