Why Maize and Sugarcane Thrive in the Blazing Summer Sun
Follow the Calvin cycle through carboxylation, reduction and regeneration, count the ATP and NADPH for one glucose, see how Kranz anatomy concentrates carbon dioxide in C4 plants, understand photorespiration, and apply the law of limiting factors.
What happens to ATP and NADPH after the light reaction?
The light reaction makes ATP and NADPH but no sugar. In the stroma, those products power reactions that fix carbon dioxide into carbohydrate — reactions that do not directly use light, which is why they are often called the dark reaction.
This part covers the Calvin cycle, the C4 pathway, photorespiration, and the factors that limit photosynthesis.
This part covers the Calvin cycle, the C4 pathway, photorespiration, and the factors that limit photosynthesis.
How do carboxylation, reduction and regeneration work in the Calvin cycle, and how much ATP and NADPH does one glucose need?
In the Calvin cycle, RuBisCO fixes carbon dioxide onto RuBP to form 3-phosphoglycerate, ATP and NADPH reduce this to sugar, and more ATP regenerates RuBP; making one glucose takes six turns, using 18 ATP and 12 NADPH.
Acceptor and first product. In C3 plants the CO acceptor is ribulose bisphosphate (RuBP), a 5-carbon sugar, and the first stable product is 3-phosphoglyceric acid (PGA), a 3-carbon compound.
The three phases:
- Carboxylation — CO joins RuBP, catalysed by RuBisCO (RuBP carboxylase-oxygenase), giving two molecules of 3-PGA
- Reduction — 3-PGA is reduced to carbohydrate, using **2 ATP and 2 NADPH for each CO fixed
- Regeneration — RuBP is remade, using 1 ATP for each CO, so the cycle continues
Worked example — count the cost.** Glucose has six carbons, so six CO must be fixed:
An everyday example. A sugar mill needs both electricity and raw material — here ATP supplies the power and NADPH the reducing electrons, and neither alone produces sugar.
The substance. The dark reaction does not run at night — it depends on ATP and NADPH from light, so it stops soon after dark.
Acceptor and first product. In C3 plants the CO acceptor is ribulose bisphosphate (RuBP), a 5-carbon sugar, and the first stable product is 3-phosphoglyceric acid (PGA), a 3-carbon compound.
The three phases:
- Carboxylation — CO joins RuBP, catalysed by RuBisCO (RuBP carboxylase-oxygenase), giving two molecules of 3-PGA
- Reduction — 3-PGA is reduced to carbohydrate, using **2 ATP and 2 NADPH for each CO fixed
- Regeneration — RuBP is remade, using 1 ATP for each CO, so the cycle continues
Worked example — count the cost.** Glucose has six carbons, so six CO must be fixed:
An everyday example. A sugar mill needs both electricity and raw material — here ATP supplies the power and NADPH the reducing electrons, and neither alone produces sugar.
The substance. The dark reaction does not run at night — it depends on ATP and NADPH from light, so it stops soon after dark.
What is Kranz anatomy, and how does the Hatch-Slack pathway concentrate carbon dioxide in bundle sheath cells?
C4 plants first fix carbon dioxide in mesophyll cells into a 4-carbon acid, move it into large bundle sheath cells arranged in a ring around the veins, and release it there, building up a high carbon dioxide level where RuBisCO works.
Kranz anatomy (Kranz means wreath):
- Large bundle sheath cells in layers around the vascular bundles
- These cells have many chloroplasts, thick walls impervious to gas, and no intercellular spaces
The Hatch-Slack pathway:
- In mesophyll cells, the acceptor phosphoenolpyruvate (PEP), a 3-carbon compound, takes up CO using PEP carboxylase (PEPcase)
- The first product is oxaloacetic acid (OAA), a 4-carbon acid, which becomes malic or aspartic acid
- These acids move into bundle sheath cells and release CO
- The CO enters the Calvin cycle; the 3-carbon leftover returns to mesophyll to remake PEP
Examples. Maize, sorghum and sugarcane are C4 plants; wheat and rice are C3 plants.
An everyday example. Maize and sugarcane fields stay lush in hot, sunny summers, conditions in which many C3 crops slow down.
The substance. C4 plants still use the Calvin cycle — it simply runs in bundle sheath cells instead of mesophyll cells.
Kranz anatomy (Kranz means wreath):
- Large bundle sheath cells in layers around the vascular bundles
- These cells have many chloroplasts, thick walls impervious to gas, and no intercellular spaces
The Hatch-Slack pathway:
- In mesophyll cells, the acceptor phosphoenolpyruvate (PEP), a 3-carbon compound, takes up CO using PEP carboxylase (PEPcase)
- The first product is oxaloacetic acid (OAA), a 4-carbon acid, which becomes malic or aspartic acid
- These acids move into bundle sheath cells and release CO
- The CO enters the Calvin cycle; the 3-carbon leftover returns to mesophyll to remake PEP
Examples. Maize, sorghum and sugarcane are C4 plants; wheat and rice are C3 plants.
An everyday example. Maize and sugarcane fields stay lush in hot, sunny summers, conditions in which many C3 crops slow down.
The substance. C4 plants still use the Calvin cycle — it simply runs in bundle sheath cells instead of mesophyll cells.
Why is photorespiration wasteful, and why are C4 plants more productive than C3 plants?
Photorespiration happens when RuBisCO binds oxygen instead of carbon dioxide, releasing carbon dioxide without making sugar or ATP; C4 plants avoid it by keeping carbon dioxide concentrated around RuBisCO, so they fix carbon more efficiently in heat and bright light.
One enzyme, two activities. RuBisCO has a higher affinity for CO, but binding depends on the relative amounts of O and CO:
- Carboxylase — RuBP + CO gives 2 PGA, as in the Calvin cycle
- Oxygenase — RuBP + O gives one PGA and one phosphoglycolate
The cost. The photorespiratory pathway releases CO and uses ATP, with no sugar, ATP or NADPH made — a wasteful loop.
Why C4 plants win:
- High CO in bundle sheath cells keeps RuBisCO working as a carboxylase, so photorespiration is nearly absent
- They tolerate higher temperatures and use higher light intensities
- They show greater productivity of biomass
An everyday example. Sorting red chillies from a heap crowded with green ones, a worker picks up the wrong colour more often — just as RuBisCO grabs oxygen more often when oxygen far outnumbers carbon dioxide.
The substance. Photorespiration rises when leaves shut their stomata in hot, dry weather, because oxygen builds up and carbon dioxide falls inside the leaf.
One enzyme, two activities. RuBisCO has a higher affinity for CO, but binding depends on the relative amounts of O and CO:
- Carboxylase — RuBP + CO gives 2 PGA, as in the Calvin cycle
- Oxygenase — RuBP + O gives one PGA and one phosphoglycolate
The cost. The photorespiratory pathway releases CO and uses ATP, with no sugar, ATP or NADPH made — a wasteful loop.
Why C4 plants win:
- High CO in bundle sheath cells keeps RuBisCO working as a carboxylase, so photorespiration is nearly absent
- They tolerate higher temperatures and use higher light intensities
- They show greater productivity of biomass
An everyday example. Sorting red chillies from a heap crowded with green ones, a worker picks up the wrong colour more often — just as RuBisCO grabs oxygen more often when oxygen far outnumbers carbon dioxide.
The substance. Photorespiration rises when leaves shut their stomata in hot, dry weather, because oxygen builds up and carbon dioxide falls inside the leaf.
How does Blackman's law of limiting factors predict the rate of photosynthesis?
Blackman's law states that when a process depends on several factors, its rate is set by the factor nearest its minimum value, so raising that factor raises the rate while raising the others has no effect.
The factors:
- Light — rate rises with intensity in dim light; in bright light other factors limit; very strong light can damage chlorophyll
- Carbon dioxide — the major limiting factor, because its level in air is very low; C3 plants respond to extra CO more than C4 plants, which saturate sooner
- Temperature — the dark reaction is enzyme-controlled; C4 plants have a higher temperature optimum than C3 plants
- Water — its effect is mainly indirect: water stress closes stomata, cutting CO supply, and wilted leaves expose less surface
Worked example — find the limit. A leaf in bright light shows no rise in rate when light is doubled, but its rate rises when CO is increased: **CO was the limiting factor.
An everyday example. Polyhouse farmers growing capsicum and tomato can enrich the air with carbon dioxide to raise yields.
The substance. Only one factor limits at a time** — once it is raised, another factor becomes limiting.
The factors:
- Light — rate rises with intensity in dim light; in bright light other factors limit; very strong light can damage chlorophyll
- Carbon dioxide — the major limiting factor, because its level in air is very low; C3 plants respond to extra CO more than C4 plants, which saturate sooner
- Temperature — the dark reaction is enzyme-controlled; C4 plants have a higher temperature optimum than C3 plants
- Water — its effect is mainly indirect: water stress closes stomata, cutting CO supply, and wilted leaves expose less surface
Worked example — find the limit. A leaf in bright light shows no rise in rate when light is doubled, but its rate rises when CO is increased: **CO was the limiting factor.
An everyday example. Polyhouse farmers growing capsicum and tomato can enrich the air with carbon dioxide to raise yields.
The substance. Only one factor limits at a time** — once it is raised, another factor becomes limiting.
Exam tip
What earns full marks on the Calvin cycle and C4 plants?
Answer C3 versus C4 comparisons point by point — acceptor, first product, enzyme, cell type, photorespiration.
- Calvin cycle: carboxylation, reduction, regeneration; RuBP acceptor; 3-PGA first product
- One glucose: 18 ATP and 12 NADPH
- C4: PEP acceptor; OAA first product; PEPcase in mesophyll; RuBisCO in bundle sheath
- Kranz anatomy: large bundle sheath cells, many chloroplasts, no intercellular spaces
- Photorespiration: RuBisCO as oxygenase; no ATP or sugar; nearly absent in C4
- Limiting factors: the factor nearest its minimum sets the rate
The trap. Saying C4 plants lack the Calvin cycle. They run it in bundle sheath cells.
- Calvin cycle: carboxylation, reduction, regeneration; RuBP acceptor; 3-PGA first product
- One glucose: 18 ATP and 12 NADPH
- C4: PEP acceptor; OAA first product; PEPcase in mesophyll; RuBisCO in bundle sheath
- Kranz anatomy: large bundle sheath cells, many chloroplasts, no intercellular spaces
- Photorespiration: RuBisCO as oxygenase; no ATP or sugar; nearly absent in C4
- Limiting factors: the factor nearest its minimum sets the rate
The trap. Saying C4 plants lack the Calvin cycle. They run it in bundle sheath cells.
Did you know
Why do leaves pack in so much of one slow enzyme?
RuBisCO is the most abundant protein in the biosphere, and a large share of the soluble protein in a green leaf is this single enzyme.
The reason is its slowness. Each RuBisCO molecule fixes only a handful of carbon dioxide molecules every second — far slower than most enzymes — and it also wastes effort grabbing oxygen.
To fix enough carbon to grow, plants simply make enormous amounts of it. C4 plants take another route: by crowding carbon dioxide around RuBisCO, they get far more work out of every molecule.
The reason is its slowness. Each RuBisCO molecule fixes only a handful of carbon dioxide molecules every second — far slower than most enzymes — and it also wastes effort grabbing oxygen.
To fix enough carbon to grow, plants simply make enormous amounts of it. C4 plants take another route: by crowding carbon dioxide around RuBisCO, they get far more work out of every molecule.
Exam relevance
How are the Calvin cycle, C4 pathway and limiting factors tested in NEET?
The dark reaction and limiting factors complete Photosynthesis in Higher Plants in NEET Biology, and they reward precise recall of acceptors, products and enzymes.
What gets asked. The three phases of the Calvin cycle, ATP and NADPH needed per glucose, primary acceptors and first stable products in C3 and C4 plants, features of Kranz anatomy, the dual activity of RuBisCO, why C4 plants show little photorespiration, and graphs on limiting factors.
Question types. Statement-based questions, match-the-column lists, graph-based questions and assertion-reason questions.
The trap that costs marks. Confusing PEP, the C4 acceptor, with OAA, the first product.
What gets asked. The three phases of the Calvin cycle, ATP and NADPH needed per glucose, primary acceptors and first stable products in C3 and C4 plants, features of Kranz anatomy, the dual activity of RuBisCO, why C4 plants show little photorespiration, and graphs on limiting factors.
Question types. Statement-based questions, match-the-column lists, graph-based questions and assertion-reason questions.
The trap that costs marks. Confusing PEP, the C4 acceptor, with OAA, the first product.
Key takeaways
What must you be able to do from this part?
- Calvin cycle: RuBisCO fixes CO onto RuBP giving 3-PGA; reduction uses ATP and NADPH; regeneration remakes RuBP; 18 ATP and 12 NADPH per glucose
- C4 pathway: PEPcase fixes CO into OAA in mesophyll; 4-carbon acids release CO in Kranz bundle sheath cells
- Photorespiration: RuBisCO acting as oxygenase wastes energy; nearly absent in C4 plants, which tolerate heat and bright light
- Limiting factors: rate set by the factor nearest its minimum; light, CO, temperature and, indirectly, water
A plant fixes molecules of carbon dioxide through the Calvin cycle. Work out the ATP and NADPH used, and how many glucose molecules can form.
- C4 pathway: PEPcase fixes CO into OAA in mesophyll; 4-carbon acids release CO in Kranz bundle sheath cells
- Photorespiration: RuBisCO acting as oxygenase wastes energy; nearly absent in C4 plants, which tolerate heat and bright light
- Limiting factors: rate set by the factor nearest its minimum; light, CO, temperature and, indirectly, water
A plant fixes molecules of carbon dioxide through the Calvin cycle. Work out the ATP and NADPH used, and how many glucose molecules can form.