What is the difference between C3 and C4 plants?
The names C3 and C4 refer to the number of carbon atoms in the first stable product formed during carbon dioxide fixation. Both pathways use the Calvin cycle to make organic compounds. The key distinction is that C4 plants concentrate carbon dioxide around that cycle.
The process and its underlying mechanism
1. Where the C3 pathway begins In C3 plants, the enzyme Rubisco attaches carbon dioxide to a five-carbon molecule called RuBP. The resulting intermediate splits into three-carbon compounds. Wheat, rice and many trees use this pathway. The name C3 does not mean that the leaf contains only compounds with three carbon atoms. It describes the first stable product of fixation. Later metabolism produces sugars, starch and a wide range of other organic molecules with different sizes, functions and chemical structures. 2. Why Rubisco can encounter a problem Rubisco can react with oxygen instead of carbon dioxide. In the resulting process, called photorespiration, the plant spends energy and loses some previously fixed carbon. This becomes especially relevant when temperatures rise and stomatal closure reduces carbon dioxide inside leaves. Photorespiration is not the same process as ordinary mitochondrial respiration. It reflects the enzyme’s interactions with two competing gases under particular conditions, although describing it as an entirely useless mistake would also oversimplify its biological role. 3. Concentrating carbon dioxide in C4 plants In the C4 pathway, initial fixation occurs through PEP carboxylase in mesophyll cells. In most C4 species, four-carbon acids move into bundle sheath cells and release carbon dioxide there. Rubisco consequently operates in a more concentrated carbon dioxide environment. Maize, sorghum and sugar cane are familiar examples. This arrangement is often associated with Kranz anatomy. Nevertheless, a single standard leaf diagram does not represent every anatomical and biochemical variation found across the diversity of C4 plants.
Environmental conditions and biological limits
4. The effect of heat and strong light Carbon concentration can reduce losses from photorespiration in hot, brightly lit environments. Water-use efficiency may increase if a C4 plant can gain the same amount of carbon with less stomatal opening. Efficiency does not mean that the plant uses no water, however. A fast-growing C4 crop with a large leaf area may still have substantial total water consumption. A ratio measured at the level of an individual leaf and the water budget of an entire ecosystem are different quantities. 5. Why the C3 pathway persists The C4 concentration system requires additional ATP. Under cool or low-light conditions, the benefit obtained for this energy cost may decline, allowing C3 plants an advantage. Carbon dioxide concentration and nutrient availability also influence the comparison. C4 is therefore not a universally superior replacement for C3. It is an adaptation that works well in particular environments. Ranking species productivity solely by this classification overlooks the environmental conditions and other features that determine their actual growth and performance. 6. Distinguishing C4 from CAM C4 plants generally separate initial fixation from the Calvin cycle between different cells. CAM plants separate carbon uptake at night from daytime use instead. The presence of four-carbon intermediates in both systems does not make the pathways identical. Nor does being C3 or C4 alone determine drought tolerance. Root depth, leaf surfaces, the timing of development and tissue resistance all matter. Those characteristics need to be considered together when comparing how different plants cope with limited water.