Photosynthesis
Photosynthesis is the process by which plants, algae, and some bacteria use light energy to turn carbon dioxide and water into glucose and oxygen.
Formula
6CO_2 + 6H_2O + \text{light} \rightarrow C_6H_{12}O_6 + 6O_2
Definition
Photosynthesis is how plants make their own food: they take in carbon dioxide from the air and water from the soil and use energy from sunlight to turn them into sugar, releasing oxygen as a leftover. Producers such as plants, algae, and cyanobacteria convert light energy into chemical energy stored in glucose inside chloroplasts, where chlorophyll absorbs light, and the overall reaction is $6CO_2 + 6H_2O + \text{light} \rightarrow C_6H_{12}O_6 + 6O_2$. The process runs in two coupled stages. In the light-dependent reactions on the thylakoid membrane, photosystem II uses photon energy to split water ($2H_2O \rightarrow 4H^+ + 4e^- + O_2$), electrons pass through an electron transport chain that pumps protons into the thylakoid lumen, ATP synthase uses the gradient to make ATP, and photosystem I reduces NADP$^+$ to NADPH. In the Calvin cycle in the stroma, rubisco fixes $CO_2$ onto ribulose-1,5-bisphosphate, and ATP and NADPH reduce the products to glyceraldehyde-3-phosphate, from which glucose and other carbohydrates are built.
Example
A bean plant on a sunny windowsill grows bigger every week though nobody feeds it, because it builds leaves and stems from sugar made out of air, water, and light. Making $1$ molecule of glucose takes $6$ molecules of $CO_2$ and $6$ of $H_2O$ and releases $6$ molecules of $O_2$; counting atoms gives $6$ carbons, $12$ hydrogens, and $18$ oxygens on each side, so the equation balances. The Calvin cycle spends $18$ ATP and $12$ NADPH per glucose, and photosynthesis stores about $2{,}870\ \text{kJ}$ per mole of glucose, yet the overall conversion efficiency of a crop plant from sunlight to biomass is typically only $1$ to $2$ percent.
Key Insight
Almost all the oxygen you breathe was made by photosynthesis, and every meal you eat traces back to a plant that captured sunlight. The oxygen released comes from the water, not the carbon dioxide, while the carbon from $CO_2$ ends up in the sugar, which is why plants are made mostly of carbon pulled from the air. Photosynthesis is a redox reaction in which water is oxidized and carbon dioxide is reduced, the reverse of cellular respiration, and because chloroplasts and mitochondria use the same chemiosmotic mechanism, a plant cell is both a solar collector and a fuel cell.