Overall equation for photosynthesis6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂

Same reaction as OpenStax prints it, without the light term6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

The reverse process, aerobic cellular respirationC₆H₁₂O₆ + 6O₂ → 6H₂O + 6CO₂

What the two stages actually exchangelight-dependent reactions → ATP + NADPH + O₂; Calvin cycle uses ATP + NADPH + CO₂ → G3P

What the summary equation is and is not

Photosynthesis is a multi-step process requiring specific wavelengths of visible sunlight, carbon dioxide and water as substrates, and it releases oxygen while producing glyceraldehyde-3-phosphate, or G3P. The balanced equation is a bookkeeping summary of the whole pathway, not a description of a single reaction. OpenStax is explicit that the equation is deceptively simple: in reality the process runs through many steps with intermediate reactants and products, and glucose itself is assembled from two three-carbon G3P molecules. Reading the equation as one reaction is the root of most of the confusion that follows, because it hides both the two-stage structure and where the released oxygen actually comes from.

Where students go wrong

The most common error is sourcing the oxygen from carbon dioxide because both molecules contain oxygen atoms. The oxygen released comes from the splitting of water in the light-dependent reactions, which is why six waters appear on the left. The second error is balancing carelessly: the alt text of the OpenStax figure states six carbon dioxide and six water molecules give one sugar molecule and six oxygen molecules, and any version with a single oxygen molecule on the right is simply unbalanced. A third is treating photosynthesis as the opposite of respiration in a strict chemical sense, when it is better described as the reverse summary reaction.

Where this leads next

The natural next page is the cellular respiration equation, because the two summaries are written as reverses of each other and students are routinely asked to compare them: one stores energy in carbon-carbon bonds, the other releases it. Within photosynthesis itself the split is between the light-dependent reactions, which produce ATP, NADPH and oxygen, and the Calvin cycle, which spends that ATP and NADPH to fix carbon dioxide into G3P. And because chloroplasts carry out photosynthesis and animal cells do not have them, this is also the fact that anchors the plant cell and animal cell comparison.

Common mistakes

  • Saying the oxygen released comes from carbon dioxide. It comes from water split during the light-dependent reactions.
  • Writing a single O₂ on the product side. The balanced equation carries six, matching the six water molecules consumed.
  • Treating glucose as the direct product. The immediate product is G3P, and glucose is assembled from two three-carbon G3P molecules.
  • Reading the summary as one chemical step. It compresses a two-stage pathway with many intermediates into a single line of bookkeeping.

Related concepts

  • Cellular respiration equation: Aerobic cellular respiration is summarized as C₆H₁₂O₆ + 6O₂ → 6H₂O + 6CO₂, the reverse of the photosynthesis summary.
  • Plant cell vs animal cell: Both are eukaryotic, so both have a plasma membrane, nucleus, ribosomes, mitochondria and the endomembrane system.
  • Krebs cycle: The Krebs cycle, also called the citric acid cycle or TCA cycle, is an eight-step closed loop in the mitochondrial matrix.

Sources