Photosynthesis Calculators

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Photosynthesis is the biological process by which plants, algae, and cyanobacteria convert light energy into chemical energy stored as glucose, using carbon dioxide and water. The overall equation is: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Photosynthesis occurs in two stages in the chloroplast: the light-dependent reactions (in the thylakoid membranes) capture light energy to produce ATP, NADPH, and O₂; and the light-independent Calvin cycle (in the stroma) uses that ATP and NADPH to fix CO₂ into carbohydrates. Photosynthesis is the foundation of virtually all food webs on Earth.

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Overall Equation

6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂

Carbon dioxide is fixed into glucose; water is split (photolysis) releasing oxygen; light energy is converted to chemical bond energy.

Light Reactions (Thylakoid)

  • Photosystem II (PSII): Absorbs light at 680 nm; splits water (2H₂O → 4H⁺ + 4e⁻ + O₂); excites electrons
  • Electron transport chain: Electrons flow through plastoquinone, cytochrome b6f, plastocyanin; pumps H⁺ across thylakoid membrane; generates proton gradient
  • ATP synthase (CF₁F₀): H⁺ gradient drives ATP synthesis (photophosphorylation)
  • Photosystem I (PSI): Absorbs 700 nm light; reduces NADP⁺ → NADPH

Calvin Cycle (Stroma)

Three stages: (1) Carbon fixation — CO₂ + RuBP → 2 × 3-PGA (Rubisco enzyme). (2) Reduction — 3-PGA + ATP + NADPH → G3P. (3) Regeneration — G3P + ATP → RuBP (ribulose-1,5-bisphosphate, the CO₂ acceptor). Net: 3 CO₂ → 1 G3P; 9 ATP + 6 NADPH consumed per G3P.

C3 vs. C4 vs. CAM Plants

  • C3: CO₂ fixed directly by Rubisco → 3-carbon 3-PGA; susceptible to photorespiration; most plants
  • C4: CO₂ first fixed by PEP carboxylase into 4-carbon OAA; concentrates CO₂ around Rubisco; eliminates photorespiration; corn, sugarcane
  • CAM: CO₂ fixed at night; stomata closed during day; desert plants (cacti, succulents)

Glossary

Rubisco
Ribulose-1,5-bisphosphate carboxylase/oxygenase; the enzyme fixing CO₂ in the Calvin cycle; most abundant enzyme on Earth; slow catalyst containing ~25% of leaf nitrogen.
Calvin Cycle
The light-independent reactions of photosynthesis in the chloroplast stroma; uses ATP and NADPH to fix CO₂ into G3P via Rubisco; requires 9 ATP and 6 NADPH per G3P produced.
Photorespiration
The wasteful reaction of Rubisco with O₂ instead of CO₂; releases CO₂ and consumes ATP; significant in C3 plants in hot conditions; suppressed in C4 plants by CO₂ concentration.

Frequently Asked Questions

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Six molecules of carbon dioxide and six of water are converted to one glucose molecule and six oxygen molecules using light energy captured by chlorophyll. The oxygen released comes from water splitting (photolysis) in Photosystem II — not from CO₂. This reaction represents the conversion of light energy into chemical energy stored in glucose's C-C and C-H bonds.

Stage 1 — Light reactions (thylakoid membranes): Light is absorbed by chlorophyll in Photosystem II and I. Water is split, releasing O₂. Energy drives electron flow through the ETC, pumping H⁺ across the thylakoid membrane to generate ATP (via ATP synthase). NADP⁺ is reduced to NADPH. Stage 2 — Calvin cycle (stroma): ATP and NADPH from light reactions power CO₂ fixation by Rubisco, producing glyceraldehyde-3-phosphate (G3P), which is used to synthesize glucose and other organic compounds.

Rubisco (RuBisCO = ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes carbon fixation — the addition of CO₂ to RuBP (ribulose-1,5-bisphosphate) in the first step of the Calvin cycle. The product is two molecules of 3-phosphoglycerate (3-PGA). Rubisco is the most abundant enzyme on Earth and contains ~25% of leaf nitrogen. However, it is slow (~3 CO₂/active site/second) and can also react with O₂ (causing photorespiration, a wasteful process). C4 plants evolved to concentrate CO₂ around Rubisco and suppress photorespiration.

In hot, sunny conditions, Rubisco increasingly reacts with O₂ instead of CO₂ (photorespiration), wasting energy and releasing CO₂. C4 plants (corn, sugarcane, sorghum) avoid this by using PEP carboxylase to fix CO₂ into 4-carbon malate in mesophyll cells, then transport it to bundle sheath cells where it is decarboxylated, concentrating CO₂ around Rubisco and suppressing photorespiration. This gives C4 plants 25–50% higher photosynthetic rates in hot conditions and higher water use efficiency than C3 plants, making them dominant in hot, open environments.