Using Light to Make Organic Molecules

Core Concepts

The three phases of the cycle

In carbon fixation, the enzyme Rubisco attaches CO₂ to ribulose-1,5-bisphosphate (RuBP), a five-carbon molecule, producing two molecules of 3-phosphoglycerate (3-PGA). In reduction, ATP and NADPH are spent to convert each 3-PGA into G3P — this is where the energy from the light reactions is actually invested. In regeneration, most of the G3P is rearranged and recombined to rebuild RuBP, using more ATP, so the acceptor is ready to fix another CO₂. The net result per three turns: three CO₂ enter, one G3P leaves, and three RuBP are regenerated.

The carbon math: three turns per sugar

Counting carbons makes the cycle's stoichiometry clear. One turn fixes one CO₂ (one carbon) onto RuBP (five carbons), yielding two 3-PGA (six carbons total). After reduction, six G3P are produced; five of them (fifteen carbons) are recycled into three RuBP (fifteen carbons), and the sixth G3P (three carbons) is the net product. So three turns fix three CO₂ and produce one net G3P. Two G3P are later joined to make one glucose (six carbons). Students who trace the carbons rarely lose points on Calvin cycle questions.

Rubisco and photorespiration

Rubisco is the enzyme that fixes CO₂, but it is not perfectly specific: when oxygen is abundant and CO₂ is scarce — the situation in hot, dry conditions when stomata close to save water — Rubisco fixes O₂ instead. This process, photorespiration, consumes O₂ and releases CO₂ while wasting ATP and NADPH, and it produces no sugar. Photorespiration is not the same as cellular respiration; it produces no ATP at all. Plants have evolved ways to suppress it by concentrating CO₂ around Rubisco, which leads to the C4 and CAM strategies.

C4 and CAM adaptations

C4 plants, such as corn and sugarcane, first fix CO₂ into a four-carbon compound in mesophyll cells, then transport it to bundle-sheath cells, where CO₂ is released in high concentration around Rubisco. The Calvin cycle runs only in the bundle sheath, so photorespiration is minimized. CAM plants, such as cacti and succulents, open their stomata at night, fix CO₂ into four-carbon acids stored in vacuoles, and release the CO₂ during the day when the Calvin cycle can run — all while keeping stomata closed in the heat of the day. Both strategies are solutions to the same problem: getting CO₂ to Rubisco without losing too much water.

The fate of G3P: sugars and starch

G3P is the export product of the cycle, but most of it is used inside the chloroplast. Pairs of G3P combine into fructose-6-phosphate and then glucose, which is exported as sucrose for transport through the plant or stored as starch in the chloroplast. Excess G3P also feeds the synthesis of amino acids, lipids, and other organic molecules. In this way the Calvin cycle is not only the source of sugar but the gateway to essentially all plant biomass.

Common Confusions