C3 and C4 Pathways
Use this short read to understand the exact C3/C₄ pattern NSEB has tested before you start the linked PYQs.
Photosynthesis uses light energy to make food in green plant cells; transport tissues move water and sugar where they are needed.
Track what enters, leaves and moves inside
H₂O rises from roots in xylem · sugar travels from the leaf in phloem
CO₂ enters through stomata; water arrives from roots; oxygen leaves the leaf. Sugar moves from the source leaf to use or storage sites.
The one-line difference
In C₃ plants, the first stable product of carbon dioxide fixation has three carbon atoms. In C₄ plants, carbon dioxide is first fixed into a four-carbon compound before it is supplied to the Calvin cycle.
Why C₄ helps under stress
High temperature or partial stomatal closure lowers internal carbon dioxide. RuBisCO can then act as an oxygenase, causing photorespiration. C₄ plants concentrate carbon dioxide around RuBisCO, reducing photorespiration and improving water-use efficiency in hot, bright, water-limited conditions.
The NSEB energy trap
C₄ is not universally cheaper. The carbon-concentrating mechanism uses additional energy. Its advantage is lower photorespiration under suitable conditions, not a lower energy cost for every carbon dioxide molecule fixed.
C3/C₄ stress and water-use efficiency
C₄ plants concentrate CO₂ around Rubisco, reducing photorespiration in hot, dry conditions. Their carbon-concentrating mechanism also supports higher water-use efficiency and photosynthesis at lower ambient CO₂; C₃ plants lack this spatial concentrating step.
C₃ versus C₄: the ATP clue
For the same amount of carbon fixed, C₄ plants spend additional ATP to regenerate phosphoenolpyruvate (PEP) and concentrate CO₂ around RuBisCO. In the standard comparison, C₃ fixation uses about 3 ATP per CO₂ and C₄ about 5. If two plants use the same NADPH but one requires substantially more ATP, the higher-ATP plant is the C₄ plant (as in maize versus wheat).
C₄ versus CAM: space or time
C₄ plants separate the initial PEP-carboxylase fixation in mesophyll cells from the Calvin (C₃) cycle around RuBisCO in bundle-sheath cells: this is spatial separation. CAM plants separate the steps by time: stomata usually open and CO₂ is fixed at night into organic acids, then stored carbon is released for the Calvin cycle during the day. Do not swap the day/night sequence.
Use spatial separation in C₄ plants
C₄ plants first fix CO₂ in mesophyll cells and run the Calvin cycle in bundle-sheath cells at the same time. This spatial separation concentrates CO₂ around Rubisco and limits photorespiration. The dedicated bundle-sheath compartment is therefore central; do not confuse C₄ spatial separation with CAM's temporal separation. Choose B and D.
Connect stomatal closure to the C₄ advantage
Heat and light can close stomata, lowering internal CO₂. In C₃ plants, RuBisCO then acts more as an oxygenase and photorespiration rises. C₄ plants use PEP carboxylase and a CO₂-concentrating system. Both statements are required. Choose D.
OlympIQ exam lens
- ✓Is the question testing heat, intense light, water stress, or stomatal closure?
- ✓Is it testing RuBisCO versus PEP carboxylase?
- ✓Is it testing photorespiration, carbon dioxide concentration, or water-use efficiency?
- ✓Does an option falsely claim that C₄ always uses less energy?
- ✓Is C₄ separation spatial and CAM separation temporal?
- ✓Does the ATP demand reveal the added C₄ carbon-concentrating cost?
- ✓C₄ photosynthesis separates initial fixation and the Calvin cycle spatially.
- ✓C₄ CO₂ concentration limits photorespiration when stomata reduce internal CO₂.
Do not select an option merely because C₄ sounds more efficient. Ask: efficient in which condition, and at what ATP cost?
Ready to try the pattern?
100% coverage across the 8 linked questions.
Practise 8 linked questions