Photosynthesis

Understand

Photosynthesis. Photosynthesis lets chlorophyll-containing plants make organic matter from carbon dioxide (CO₂) and water, using light energy. It releases oxygen (O₂).

Chloroplast. Organelle of chlorophyll-containing plant cells where photosynthesis takes place. Leaves contain many of them, but not every cell of a plant has them.

Chlorophyll. Pigment located in the thylakoid membranes of the chloroplast. It absorbs part of the light, notably in the blue and red.

Stoma. Adjustable opening in the leaf: it allows gas exchange, notably the entry of CO₂, and helps regulate water loss.

The mechanism

1. Capture the light. In the thylakoids, the light-dependent reactions use water and light energy. They produce oxygen as well as ATP and NADPH, two intermediates used afterward.

2. Incorporate the carbon. In the stroma of the chloroplast, the Calvin cycle uses CO₂, ATP and NADPH to form G3P, a small three-carbon molecule that is used, among other things, to make sugars.

3. Use the matter produced. Sugars serve as matter and as a store of chemical energy for the plant. They can be used in respiration or converted, for example into storage starch.

Overall equation and conditions

Simplified overall equation. For the equation expressed in glucose, 6 molecules of CO₂ and 6 molecules of water give 1 molecule of glucose and 6 molecules of O₂. Light supplies energy: it is not a molecule that gets consumed.

The conditions. Photosynthesis depends on the available light, the supply of CO₂ and water, and a temperature compatible with how enzymes work. If one factor becomes limiting, increasing another factor won't necessarily speed up the process.

Example: a dry day. A plant can close its stomata to limit water loss. Less CO₂ then enters the leaf: the production of organic matter can drop, even when there is plenty of light.

Don't mix up

Where does the oxygen come from?. The oxygen released during photosynthesis comes from water, not from CO₂. The carbon in the organic matter produced comes from CO₂.

Photosynthesis and respiration. The plant also respires in the light. Respiration uses organic matter to supply energy the cells can use; it doesn't replace photosynthesis. The gas exchanges measured reflect the balance of these processes.

The Calvin cycle is not a night phase. The Calvin cycle doesn't capture light directly, but depends on the products of the light reactions. The old term “dark phase” therefore doesn't mean it only takes place at night.

An overall equation is not a step. The overall equation is a simplification: glucose isn't made in a single reaction. The Calvin cycle first forms G3P, which is then used to produce other organic molecules.