Equation for photosynthesis (overall balanced form)
The overall balanced chemical equation that summarizes oxygenic photosynthesis (the form used by plants, algae, and cyanobacteria) is:
This is a net equation: it summarizes the overall inputs and outputs, even though photosynthesis occurs through multiple biochemical steps inside chloroplasts.
What the coefficients mean
The coefficients (the numbers in front of each chemical formula) give mole ratios between reactants and products. Interpreting them as “amounts”:
| Species | Role in the equation | Stoichiometric coefficient | Meaning as a mole ratio |
|---|---|---|---|
| CO2 | Carbon source (fixed into carbohydrate) | 6 | 6 mol CO2 are required per 1 mol glucose formed |
| H2O | Electron source (split during light reactions) | 6 | 6 mol H2O are consumed per 1 mol glucose formed (net) |
| C6H12O6 | Carbohydrate product (energy stored in bonds) | 1 | 1 mol glucose is produced as the net carbohydrate output |
| O2 | Byproduct released from water splitting | 6 | 6 mol O2 are produced per 1 mol glucose formed |
Why “light energy” appears but is not balanced like a molecule
Light provides energy to drive energetically unfavorable steps (notably the reduction of carbon in CO2). Energy is not counted as matter, so it does not have atoms to balance; it indicates that the reaction requires an external energy input.
Checking that the equation is balanced (conservation of atoms)
A chemical equation is balanced when each element has the same number of atoms on both sides.
| Element | Left side atom count | Right side atom count | Balanced? |
|---|---|---|---|
| C | \(6 \times 1 = 6\) (from 6 CO2) | \(6\) (in C6H12O6) | Yes |
| H | \(6 \times 2 = 12\) (from 6 H2O) | \(12\) (in C6H12O6) | Yes |
| O | \(6 \times 2 + 6 \times 1 = 18\) | \(6 + 6 \times 2 = 18\) | Yes |
Equivalent “expanded water” form
Some textbooks present an equivalent net form that makes water cancellation explicit:
Canceling \(6\,H_2O\) from both sides gives the shorter form \(6\,CO_2 + 6\,H_2O \rightarrow C_6H_{12}O_6 + 6\,O_2\) (with light energy implied).
How the overall equation connects to chloroplast processes
Photosynthesis can be summarized in two coordinated stages:
Light-dependent reactions (thylakoid membranes): use light to split water, releasing O2 and producing ATP and NADPH.
Calvin cycle (stroma): uses ATP and NADPH to reduce CO2 into carbohydrate (net represented as glucose in the overall equation).
Using the equation for photosynthesis in stoichiometry
Because the coefficients are mole ratios, they allow quick conversions between amounts. Two common examples:
1) CO2 required for a given amount of glucose
From \(6\,CO_2 : 1\,C_6H_{12}O_6\), producing \(n\) moles of glucose requires \[ n_{CO_2} = 6 \times n_{glucose}. \]
2) O2 produced from a given amount of CO2
From \(6\,CO_2 : 6\,O_2\), the ratio is \(1:1\). Therefore, \[ n_{O_2} = n_{CO_2} \] (for the same extent of reaction under idealized conditions).
Numerical example
If \(12\) mol of CO2 are fixed, the amount of glucose formed (net) follows from the ratio \(6:1\):
The corresponding oxygen produced is \(n_{O_2} = 12\) mol (because \(6\,CO_2\) and \(6\,O_2\) have the same coefficient).
Key interpretation cautions
The overall equation for photosynthesis is a useful chemical summary, but living cells do not directly “manufacture” free glucose as a single step. The chloroplast produces carbohydrate through many intermediates, and glucose may be built from smaller sugars, stored as starch, or exported as sucrose depending on the organism and conditions.