Why Maize and Sugarcane Beat Wheat on a Hot, Bright Day

The ATP and NADPH made in the light reactions are not stored for long. In the stroma of the chloroplast they are spent building sugar from carbon dioxide — and plants such as maize and sugarcane use an extra trick to do this efficiently in heat and bright light.

This lesson covers the Calvin cycle, the C4 pathway, photorespiration and the factors that limit photosynthesis.

The Calvin cycle fixes carbon dioxide in three stages — carboxylation, reduction and regeneration — using ATP and NADPH from the light reactions to turn carbon dioxide into sugar.

1. Carboxylation:

- The enzyme RuBisCO joins carbon dioxide to the 5-carbon acceptor RuBP (ribulose bisphosphate)
- The product splits into two molecules of 3-carbon 3-PGA (3-phosphoglyceric acid), the first stable product — which is why these plants are called C3 plants

2. Reduction:

- 3-PGA is reduced to glyceraldehyde-3-phosphate using ATP and NADPH
- Some of it leaves the cycle to form glucose

3. Regeneration:

- The remaining molecules are rearranged, using ATP, to rebuild RuBP so the cycle can continue

The balance sheet. Making one glucose molecule needs six turns of the cycle, fixing six carbon dioxide molecules and using 18 ATP and 12 NADPH.

An everyday example. Wheat and rice growing in the fields of Punjab are C3 plants, fixing carbon dioxide directly through the Calvin cycle.

The substance. The Calvin cycle is often called the dark reaction but does not need darkness — it runs in daylight, because it depends on a steady supply of ATP and NADPH from the light reactions.

C4 plants such as maize and sugarcane have Kranz anatomy — large bundle sheath cells full of chloroplasts around their veins — and first fix carbon dioxide in mesophyll cells into a 4-carbon acid, which later releases carbon dioxide inside the bundle sheath for the Calvin cycle.

Kranz anatomy:

- Bundle sheath cells form a wreath around each vascular bundle
- These cells are large, with many chloroplasts, thick walls that gases cannot pass, and no intercellular spaces

The C4 pathway:

- In mesophyll cells, PEP carboxylase fixes carbon dioxide onto the 3-carbon PEP (phosphoenolpyruvate), forming 4-carbon oxaloacetic acid
- This becomes other 4-carbon acids, such as malic acid, which move into the bundle sheath cells
- There they release carbon dioxide and a 3-carbon molecule; the carbon dioxide enters the Calvin cycle
- The 3-carbon molecule returns to the mesophyll to regenerate PEP

C3 versus C4 plants:

- First stable product — 3-PGA versus oxaloacetic acid
- Kranz anatomy — absent versus present
- Carbon-fixing enzymes — RuBisCO only versus PEP carboxylase and RuBisCO
- Photorespiration — present versus nearly absent
- Optimum temperature — lower versus higher
- Examples — wheat and rice versus maize, sugarcane and sorghum

An everyday example. Sugarcane fields in Maharashtra and Uttar Pradesh thrive under intense heat and sunlight because sugarcane is a C4 plant.

The substance. C4 plants still use the Calvin cycle — the C4 pathway only concentrates carbon dioxide around RuBisCO in the bundle sheath.

Photorespiration is a wasteful pathway in which RuBisCO binds oxygen instead of carbon dioxide, so RuBP is broken down and carbon dioxide is released without making sugar or ATP.

The dual role of RuBisCO:

- Carboxylase — joins carbon dioxide to RuBP, forming two 3-PGA
- Oxygenase — joins oxygen to RuBP, forming one 3-PGA and one 2-carbon phosphoglycolate
- The same active site can bind either gas, and the one at higher relative concentration wins

What happens in photorespiration:

- Phosphoglycolate is processed through the chloroplast, peroxisome and mitochondrion
- It increases when leaves are hot and stomata close, letting oxygen build up inside

Significance:

- It lowers the efficiency of photosynthesis in C3 plants
- C4 plants avoid it by keeping carbon dioxide high around RuBisCO in the bundle sheath

An everyday example. Wheat crops in north India suffer in an unusually hot spell during grain filling, partly because photorespiration rises with temperature.

The substance. RuBisCO is the most abundant enzyme on Earth — plants make it in huge amounts, partly to make up for how slowly it works and how often it grabs oxygen instead.

Blackman's law of limiting factors states that when a process depends on several factors, its rate is set by the factor that is nearest its minimum value — the limiting factor.

How it works. If light is bright but carbon dioxide is scarce, adding more light does nothing; the rate rises only when carbon dioxide is increased, until another factor becomes limiting.

External factors:

- Light — the rate rises with light intensity at low levels but levels off at high intensity; very strong light can damage chlorophyll
- Carbon dioxide — the major limiting factor; C3 plants respond to higher levels more than C4 plants, which saturate sooner
- Temperature — the enzyme-controlled dark reactions are more sensitive to temperature than the light reactions; C4 plants have a higher optimum
- Water — shortage closes stomata, cutting carbon dioxide intake

Internal factors. The number, size, age and orientation of leaves, their chlorophyll content, and the carbon dioxide concentration inside the leaf.

An everyday example. Greenhouse growers raise carbon dioxide levels for tomatoes and bell peppers to get higher yields, because carbon dioxide is often limiting for these C3 crops.

The substance. At any moment only one factor limits the rate — improving any other factor gives no benefit until that one is raised.