C3, C4 and CAM photosynthesis
The three ways plants fix carbon dioxide, why Rubisco's weakness led to C4 and CAM, how they differ in water and nitrogen use, and which crops use which.
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Every green plant fixes carbon dioxide with the same final machinery, the Calvin cycle, driven by the enzyme Rubisco. What differs is how CO2 is delivered to Rubisco. Plants that feed it straight from the air are C3; plants that first capture CO2 in a four-carbon acid and concentrate it are C4; and plants that capture it at night and use it by day are CAM.
The differences explain why maize and sugarcane out-yield wheat in hot sunshine, why pineapple survives on little water, and why many of the world's worst weeds thrive in the tropics. A comparison table of the three pathways is one of the most reliable exam questions in plant physiology.
The problem: Rubisco's weakness
Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) adds CO2 to a five-carbon sugar, ribulose bisphosphate, giving two molecules of a three-carbon acid, 3-phosphoglycerate (3-PGA). That three-carbon first product gives C3 plants their name.
But Rubisco evolved when the atmosphere had far more CO2 and little oxygen, and it cannot fully tell the two apart. It also reacts with O2. That oxygenation produces a two-carbon compound that the plant must recycle through chloroplasts, peroxisomes and mitochondria, a costly salvage pathway called photorespiration that releases previously fixed CO2 and consumes energy.
Photorespiration gets worse as:
- temperature rises, because CO2 becomes less soluble relative to O2 and Rubisco's preference for CO2 declines;
- stomata close in dry conditions, so CO2 inside the leaf falls while O2 from photosynthesis builds up;
- light is intense, driving the cycle hard.
In warm conditions it can cut a C3 crop's net photosynthesis by a quarter or more. C4 and CAM are two evolutionary answers: both use a different enzyme, PEP carboxylase, which has no affinity for oxygen, to capture CO2 first.
C3 photosynthesis
- First stable product: 3-PGA (three carbons).
- Carboxylating enzyme: Rubisco only.
- Leaf anatomy: ordinary; photosynthesis in mesophyll cells.
- CO2 compensation point (the CO2 level at which fixation just balances respiration and photorespiration): high, around 40–100 µmol/mol.
- Optimum temperature: about 15–25 °C for many species.
- Light: saturates at moderate light intensities.
About 85–90 per cent of plant species are C3, including wheat, rice, barley, oats, soybean, groundnut, potato, cotton, most vegetables and fruits, and almost all trees.
C4 photosynthesis
C4 plants add a CO2 pump:
- In mesophyll cells, PEP carboxylase fixes CO2 (as bicarbonate) onto phosphoenolpyruvate (PEP), forming oxaloacetate, a four-carbon acid, quickly converted to malate or aspartate.
- The four-carbon acid moves into bundle sheath cells arranged in a tight ring around the veins (Kranz anatomy, from the German for "wreath").
- There it is decarboxylated, releasing CO2 at a concentration several times that of the air, right where Rubisco sits.
- The remaining three-carbon compound returns to the mesophyll and is converted back to PEP, using ATP.
Because Rubisco works in CO2-rich surroundings, photorespiration is nearly eliminated. The cost is extra energy: roughly 5 ATP per CO2 fixed instead of 3 in C3.
C4 plants are grouped into subtypes by the decarboxylating enzyme: NADP-malic enzyme (maize, sugarcane, sorghum), NAD-malic enzyme (some millets and amaranths) and PEP carboxykinase (some tropical grasses).
Consequences:
- CO2 compensation point near zero (0–10 µmol/mol).
- Higher optimum temperature, about 30–40 °C, and no light saturation at full sunlight in many species.
- Higher water-use efficiency: PEP carboxylase captures CO2 even with stomata partly closed.
- Higher nitrogen-use efficiency: less Rubisco is needed, and Rubisco is the largest single store of leaf nitrogen.
Only around 3 per cent of plant species are C4, yet they account for roughly a quarter of the land's primary production, largely through tropical grasslands and savannas. C4 crops include maize, sugarcane, sorghum, pearl millet, finger millet, foxtail millet and amaranth; many tropical forage grasses; and several notorious weeds, such as purple nutsedge (Cyperus rotundus) and many Echinochloa grasses.
CAM photosynthesis
Crassulacean acid metabolism is named after the stonecrop family (Crassulaceae), where it was first studied. CAM plants use the same two carboxylations as C4 plants, but separate them in time within the same cell:
- At night, stomata open in the cool, humid air. PEP carboxylase fixes CO2, and the product is stored as malic acid in the large vacuole. The leaf becomes measurably acid by dawn.
- By day, stomata close to save water. Malic acid is decarboxylated, and the released CO2 is fixed by Rubisco in the Calvin cycle using the day's light.
CAM plants lose very little water: their transpiration ratio can be a tenth of a C3 plant's. The cost is slow growth, because the vacuole limits how much CO2 can be stored each night. Examples include pineapple, agave, prickly pear, aloe, many orchids including vanilla, and epiphytic bromeliads. Many are facultative: they behave as C3 plants when well watered and switch to CAM under drought or salt stress.
The three compared
| Feature | C3 | C4 | CAM |
|---|---|---|---|
| First stable product | 3-PGA (3 C) | oxaloacetate (4 C) | oxaloacetate (4 C), stored as malate |
| Primary carboxylase | Rubisco | PEP carboxylase, then Rubisco | PEP carboxylase (night), Rubisco (day) |
| Separation of steps | none | in space (two cell types) | in time (night and day) |
| Leaf anatomy | ordinary | Kranz | succulent, large vacuoles |
| Photorespiration | high in heat | negligible | low |
| CO2 compensation point (µmol/mol) | about 40–100 | about 0–10 | about 0–5 at night |
| Optimum temperature | 15–25 °C | 30–40 °C | variable, often hot by day |
| Water used per gram of dry matter | about 450–950 g | about 250–350 g | about 50–125 g |
| Growth rate | moderate to high | high | low |
Values are typical ranges; they vary with species and conditions.
How much water does the pathway save?
A crop produces 15 t/ha of above-ground dry matter. Using transpiration ratios from the table, how much water does it transpire?
- C4 crop at 300 g water per g dry matter: 15 t × 300 = 4,500 t of water per hectare. Since 1 mm over 1 ha is 10 t, that is 450 mm.
- C3 crop at 600 g per g: 15 × 600 = 9,000 t = 900 mm.
Under the same weather, the C4 crop needs about half the water for the same biomass. This is one reason sorghum and millets are favoured in dry, hot regions. (Soil evaporation comes on top of these figures.)
Why it matters for agriculture
- Crop choice by climate. C4 cereals dominate hot, sunny lowlands; C3 cereals dominate cooler seasons, higher latitudes and highlands. Wheat is a cool-season crop partly for this reason.
- Rising CO2. Higher atmospheric CO2 suppresses photorespiration, so C3 crops gain more from it ("CO2 fertilisation") than C4 crops, whose pump already saturates Rubisco. Heat and drought can cancel the gain.
- Weeds. C4 weeds in C3 crops (for example nutsedges and barnyard grass in rice or vegetables) are especially competitive in hot weather.
- Breeding. International projects are trying to engineer C4 traits into rice to raise its yield potential, a very long-term goal; others aim to improve Rubisco or to shorten photorespiration with synthetic bypass pathways.
Key points
- All plants use Rubisco and the Calvin cycle; the pathways differ in how CO2 reaches Rubisco.
- Rubisco also fixes O2, causing photorespiration, which is worst in hot, dry, bright conditions.
- C3: first product 3-PGA; most crops and trees; high CO2 compensation point.
- C4: PEP carboxylase in mesophyll, Rubisco in bundle sheath (Kranz anatomy); nearly no photorespiration; high water and nitrogen efficiency; maize, sugarcane, sorghum, millets.
- CAM: CO2 fixed at night and stored as malic acid; used by day with stomata closed; pineapple, agave, cacti; very high water-use efficiency.
- Rising CO2 benefits C3 crops more than C4 crops.
What the exams ask
- School-leaving and entrance papers: examples of C3, C4 and CAM plants, the first product of each, and Kranz anatomy.
- Degree and research-fellowship entrances: photorespiration and why it occurs, the C4 cycle step by step, CO2 compensation points, subtypes of C4, the energy cost, and a full comparison table.
- Professional and research examinations: implications for crop choice, climate change and elevated CO2, water and nitrogen use efficiency, and efforts to engineer photosynthesis.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Is photorespiration completely useless?
Not entirely: it recycles carbon that would otherwise be lost and may protect the photosynthetic machinery from damage under excess light. But in crops it costs much more than it returns, which is why reducing it is a breeding goal.
Can a plant switch between C3 and CAM?
Yes. Some succulents and the common ice plant behave as C3 plants when well watered and switch to CAM under drought or salinity. A few species have intermediate C3–C4 traits as well.
Why don't all crops become C4 in hot climates?
C4 photosynthesis evolved independently many times but requires a coordinated set of anatomical and biochemical changes. Breeders cannot simply select it into a C3 crop; it would need extensive genetic engineering.
Sources
- Taiz, L., Zeiger, E., Møller, I. M. and Murphy, A. Plant Physiology and Development, 6th edition. Sinauer, 2015. Chapters 8 and 9.
- Sage, R. F. "The evolution of C4 photosynthesis." New Phytologist 161, 341–370 (2004).
- Still, C. J., Berry, J. A., Collatz, G. J. and DeFries, R. S. "Global distribution of C3 and C4 vegetation: carbon cycle implications." Global Biogeochemical Cycles 17, 1006 (2003).
- Lüttge, U. "Ecophysiology of Crassulacean Acid Metabolism (CAM)." Annals of Botany 93, 629–652 (2004).
- Ehleringer, J. R. and Cerling, T. E. "C3 and C4 photosynthesis." In Encyclopedia of Global Environmental Change, Vol. 2. Wiley, 2002.
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