Direct answer: The reactants of photosynthesis in the net process are carbon dioxide (CO2), water (H2O), and light energy. In the light-dependent reactions, the key inputs are light and H2O (with NADP+ and ADP + Pi as additional reactants that become NADPH and ATP). In the Calvin cycle, the key input is CO2 (using ATP and NADPH made by the light reactions).
Overall reactants in the net equation
Photosynthesis is often summarized by a net chemical equation that tracks atoms entering and leaving the plant/algal cell:
\[ 6\,\mathrm{CO_2} + 6\,\mathrm{H_2O} + \text{light} \rightarrow \mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2} \]
In this net view, the reactants of photosynthesis are the molecules on the left side: carbon dioxide and water, with light providing energy to drive an otherwise nonspontaneous reduction of carbon.
Stage-by-stage reactants
The net equation hides the internal division of labor inside the chloroplast. A clear way to identify reactants is to separate the process into (1) light-dependent reactions and (2) the Calvin cycle (light-independent reactions).
| Stage | Main location in chloroplast | Key reactants (inputs) | Key products (outputs) |
|---|---|---|---|
| Light-dependent reactions | Thylakoid membranes | Light, \( \mathrm{H_2O} \), \( \mathrm{NADP^+} \), \( \mathrm{ADP} + \mathrm{P_i} \) | \( \mathrm{O_2} \), ATP, NADPH |
| Calvin cycle | Stroma | \( \mathrm{CO_2} \), ATP, NADPH | G3P (sugar precursor), \( \mathrm{ADP} + \mathrm{P_i} \), \( \mathrm{NADP^+} \) |
How the reactants fit together
- Water supplies electrons. Splitting \( \mathrm{H_2O} \) during the light-dependent reactions provides electrons and releases \( \mathrm{O_2} \) as a byproduct.
- Light supplies energy. Photons energize electrons in chlorophyll, enabling ATP formation and reduction of \( \mathrm{NADP^+} \) to NADPH.
- Carbon dioxide supplies carbon. \( \mathrm{CO_2} \) is fixed in the Calvin cycle and reduced (using ATP and NADPH) into carbohydrate.
A frequently used Calvin-cycle bookkeeping result is that producing one net G3P molecule (a 3-carbon sugar precursor) requires:
\[ 3\,\mathrm{CO_2} + 9\,\mathrm{ATP} + 6\,\mathrm{NADPH} \rightarrow \text{G3P} + 9\,(\mathrm{ADP}+\mathrm{P_i}) + 6\,\mathrm{NADP^+} \]
This reinforces that \( \mathrm{CO_2} \) is the key molecular reactant in carbon fixation, while ATP and NADPH are energetic and reducing-power reactants supplied by the light-dependent reactions.
Visualization: inputs and outputs across the chloroplast
Common pitfalls
- Light is a reactant in the energetic sense. It provides energy but is not a molecule that contributes atoms to glucose.
- Oxygen gas comes from water. The \( \mathrm{O_2} \) released during photosynthesis is produced when \( \mathrm{H_2O} \) is split in the light-dependent reactions.
- Carbon in sugar comes from CO2. The Calvin cycle incorporates carbon dioxide into organic molecules.
Summary
The reactants of photosynthesis at the whole-process level are \( \mathrm{CO_2} \), \( \mathrm{H_2O} \), and light energy. At the mechanistic level, the light-dependent reactions consume light and water to generate ATP and NADPH, and the Calvin cycle consumes \( \mathrm{CO_2} \) using ATP and NADPH to build carbohydrate.