C4 photosynthesis is a form of photosynthesis in which plants initially incorporate inorganic carbon into four-carbon organic acids before delivering it to the Calvin–Benson cycle. This biochemical carbon-concentrating mechanism raises the concentration of carbon dioxide around Rubisco, thereby suppressing photorespiration. Unlike C3 photosynthesis, it usually divides initial carbon capture and subsequent carbon assimilation between two specialized leaf cell types. The designation “C4” refers to the four carbon atoms in the first products of fixation, not to the number of stages in photosynthesis. (pmc.ncbi.nlm.nih.gov)
Discovery and terminology
The pathway became established through experiments on sugarcane during the 1960s. In their 1966 paper, Marshall Davidson Hatch and Charles Roger Slack exposed leaves to carbon dioxide labeled with radioactive carbon-14. After approximately one second, more than 93 percent of the fixed radioactivity occurred in malate, aspartate, and oxaloacetate, rather than predominantly in the three-carbon compounds expected from the conventional pathway. With longer exposure, the label appeared in three-phosphoglycerate and sugars. These observations supported a route in which four-carbon acids precede carbohydrate formation. The alternative name “Hatch–Slack pathway” commemorates this work. (pmc.ncbi.nlm.nih.gov)
Biochemical mechanism
In the typical pathway, atmospheric carbon dioxide enters a leaf through stomata and reaches mesophyll cells. Carbonic anhydrase facilitates its conversion to bicarbonate. The enzyme phosphoenolpyruvate carboxylase (PEPC) combines bicarbonate with the three-carbon compound phosphoenolpyruvate, producing four-carbon oxaloacetate. Unlike Rubisco, PEPC does not catalyze a competing oxygenation reaction. Oxaloacetate is subsequently converted into malate or aspartate, depending on the species and biochemical route. (pmc.ncbi.nlm.nih.gov)
These compounds move into bundle sheath cells, where decarboxylation releases carbon dioxide. Rubisco then fixes this locally concentrated carbon dioxide through the Calvin–Benson cycle, which produces precursors for carbohydrates. The remaining three-carbon compounds return to the mesophyll, where phosphoenolpyruvate is regenerated. In many species, pyruvate phosphate dikinase performs this regeneration using ATP. Thus, the C4 cycle supplies carbon dioxide to the Calvin–Benson cycle rather than replacing it. Transport across cellular and organelle membranes is integral to maintaining the cycle. (pmc.ncbi.nlm.nih.gov)
Three conventional biochemical subtypes are distinguished by their principal decarboxylating enzymes: NADP-dependent malic enzyme, NAD-dependent malic enzyme, and phosphoenolpyruvate carboxykinase. They differ in transported metabolites, decarboxylation locations, and the distribution of energetic requirements between cells. These categories are useful descriptions, but individual plants can operate more than one decarboxylation route; the subtypes are not always mutually exclusive. (pmc.ncbi.nlm.nih.gov)
Leaf anatomy and cellular organization
Most terrestrial C4 plants possess Kranz anatomy, named from the German word for “wreath.” Photosynthetically active bundle sheath cells surround the vascular bundles, and mesophyll cells form an adjoining outer layer. This arrangement places the initial carbon-capturing cells close to the cells where carbon dioxide is released and refixed. Relatively dense leaf venation and extensive connections between the two cell types support rapid metabolite exchange. Their chloroplasts can differ in structure and biochemical specialization. (pmc.ncbi.nlm.nih.gov)
Kranz anatomy is nevertheless not an absolute requirement. Certain species of Bienertia and Suaeda perform C4 photosynthesis within individual cells. They maintain spatially separated biochemical compartments and differently specialized chloroplast populations inside a single cell. These exceptions demonstrate that effective compartmentation, rather than a universally required two-cell arrangement, is the central organizational principle. (annualreviews.org)
Physiological advantages and costs
Rubisco can react with oxygen as well as carbon dioxide. Its oxygenation reaction initiates photorespiration, which consumes resources and releases some previously fixed carbon. This limitation becomes especially important at high temperatures or when restricted gas exchange lowers internal carbon dioxide availability. By concentrating carbon dioxide near Rubisco, C4 plants reduce these losses and can sustain high assimilation rates in warm, strongly illuminated environments. (annualreviews.org)
Carbon concentration has an energy cost. A common idealized accounting requires about five ATP and two NADPH per carbon dioxide assimilated, compared with three ATP and two NADPH for the Calvin–Benson cycle without photorespiratory losses. Actual costs vary with the biochemical subtype and carbon dioxide leakage. Consequently, C4 photosynthesis is not universally more efficient: its additional ATP requirement can outweigh its benefits under cool conditions or limited light. (pmc.ncbi.nlm.nih.gov)
C4 plants commonly achieve greater carbon assimilation relative to water lost through transpiration, because the concentrating mechanism permits efficient fixation at lower stomatal conductance. They also often require less Rubisco, improving photosynthetic nitrogen-use efficiency. These physiological advantages do not imply that every C4 species tolerates severe drought or outperforms every C3 species in every environment. (pmc.ncbi.nlm.nih.gov)
Evolution and ecological distribution
C4 photosynthesis is a prominent example of evolution producing similar complex traits independently. Comparative research identifies more than 60 independent origins among flowering plants. Its development involved modifications to existing enzymes, cellular organization, and gene expression, rather than the invention of an entirely new carbon-assimilation cycle. Some C3–C4 intermediate plants concentrate carbon dioxide released by photorespiration in bundle sheath cells, providing evidence for possible transitional configurations. (pmc.ncbi.nlm.nih.gov)
C4 grasses are important components of warm grasslands and savannas. Their abundance reflects interactions among temperature, atmospheric carbon dioxide, water availability, disturbance, and evolutionary history; C4 metabolism is not simply synonymous with desert adaptation. C4 species also occur outside grasses, including several lineages of herbaceous flowering plants. (pmc.ncbi.nlm.nih.gov)
Agriculture and comparison with CAM
Important C4 crops include maize, sugarcane, and sorghum. Their productivity has encouraged research into introducing C4 traits into C3 crops such as rice. Engineering this system requires coordinated changes in anatomy, enzyme localization, metabolite transport, and cellular energy supply—not merely the insertion of a single enzyme. (pmc.ncbi.nlm.nih.gov)
C4 photosynthesis differs from crassulacean acid metabolism (CAM). Both use initial fixation into organic acids, but typical C4 plants separate carbon capture and release spatially, whereas CAM plants separate them mainly in time, capturing much of their carbon dioxide at night and releasing it internally during the day. (pmc.ncbi.nlm.nih.gov)
References
- Photosynthesis by sugar-cane leaves: A new carboxylation reaction and the pathway of sugar formationpmc.ncbi.nlm.nih.gov
- The Path from C3 to C4 Photosynthesispmc.ncbi.nlm.nih.gov
- Finding the C4 sweet spot: cellular compartmentation of carbohydrate metabolism in C4 photosynthesispmc.ncbi.nlm.nih.gov
- Update: Improving the Efficiency of Photosynthetic Carbon Reactionspmc.ncbi.nlm.nih.gov
- The metabolite transporters of C4 photosynthesispmc.ncbi.nlm.nih.gov
- Regulation and Evolution of C4 Photosynthesisfenix.ciencias.ulisboa.pt
- Recruitment of pre-existing networks during the evolution of C4 photosynthesispubmed.ncbi.nlm.nih.gov
- Nature's green revolution: the remarkable evolutionary rise of C4 plantspmc.ncbi.nlm.nih.gov
- Photosynthesispmc.ncbi.nlm.nih.gov