Photosynthesis: Making Food from Light
Plants build sugar out of air and water using sunlight. Almost every calorie you have ever eaten started here.
Builds on: 0.1 What Is Alive, and What a Cell Is0.3 Enzymes: Why Biology Happens at All
Where your food comes from
A tree does not grow out of the soil. Weigh the soil in a pot before and after growing a sapling and you will find it barely changed — van Helmont did roughly this experiment in the 1600s and was baffled by the result. Nearly all the added mass came from carbon dioxide in the air and water. A log is, in a real sense, solidified air.
The reaction that does it is photosynthesis: light energy is used to join CO₂ and water into glucose, releasing oxygen as waste.
Two things about that equation deserve more attention than they usually get. First, the oxygen is waste — a by-product the plant does not want, which happens to be what the rest of us breathe. Every oxygen molecule in the atmosphere was exhaled by something photosynthetic. Second, the reaction runs energetically uphill: it stores energy rather than releasing it. That is what makes glucose worth eating.
Where it happens
In the chloroplast, using the green pigment chlorophyll. Chlorophyll absorbs red and blue light strongly and reflects green — which is the entire reason plants look green to you. Leaves are radiating the one part of the spectrum they decided not to use.
The process runs in two stages:
- The light-dependent reactions, in the stacked membranes. Light splits water, releasing O₂ and loading energy onto carrier molecules.
- The Calvin cycle, in the fluid around them. That energy is spent fixing CO₂ into sugar. It needs no light directly, only the carriers the first stage charged up — which is why calling it the “dark reaction” is a misnomer worth avoiding.
Limiting factors
Photosynthesis needs light, CO₂ and a workable temperature. Here is the rule that matters, and it is one of the most transferable ideas in biology: the rate is set by whichever requirement is scarcest. Everything else is in surplus and raising it changes nothing.
Put a plant in dim light and doubling the CO₂ does nothing at all — light is the limiting factor. Turn the lights up and the rate climbs, until CO₂ or temperature becomes the new ceiling. It is precisely the limiting-reagent logic from Catalyst, and commercial greenhouses run on it: they pump CO₂ to about three times atmospheric level, because once light and warmth are handled, CO₂ is what is left holding the crop back.
Temperature behaves differently from the other two, and the reason is the previous lesson. The Calvin cycle is run by enzymes, so warmth speeds it up to an optimum and then denatures them. Light and CO₂ merely plateau; temperature has a cliff.
For the first two billion years there was essentially no free oxygen on Earth. Then photosynthetic bacteria filled the atmosphere with their waste — and to the anaerobic life of the time, oxygen was a poison. It was probably the largest mass extinction in history, caused by pollution, from organisms that could not have known. It also made aerobic respiration, and therefore you, possible.
⚗️ Lab — The Limiting Factor Bench
Pondweed in a beaker. The oxygen bubbles tell you the rate.
- Drop the light to 10% and then raise CO₂ all the way. Nothing happens — light is the ceiling.
- Now raise the light. The moment CO₂ becomes the scarcest input, the bar switches.
- Push the temperature past 40 °C. Unlike the other two, it does not plateau — it collapses.