How does photosynthesis work step by step?

1the willow puzzle

Nearly four centuries ago, Jan van Helmont grew a five pound willow in 200 pounds of soil for five years. It reached 169 pounds. The soil lost about two ounces.

2air and water

So plants do not eat soil. Van Helmont guessed water, which is part of it. But about half of dry wood is carbon, and that came from carbon dioxide in the air.

3the chloroplast

The work happens in the leaf. Its cells hold tiny green compartments called chloroplasts, each a miniature sugar factory packed with the green pigment chlorophyll.

4two zones

Inside are two zones. Stacks of flat membrane sacs, called thylakoids, catch the light. The fluid around them, the stroma, is where sugar gets built.

5light hits

Stage one, the light reactions, runs in the thylakoid membrane. Light hits photosystem two, and water arrives from inside the thylakoid to be split apart.

6oxygen from water

Splitting water frees oxygen. That is where the oxygen comes from: water, not carbon dioxide. In 1941 Ruben and Kamen tagged water with heavy oxygen eighteen, and the tag turned up in the oxygen gas.

7electrons to NADPH

The electrons from water slide down a chain of carriers, get a second boost of light at photosystem one, and end up loaded onto a carrier called NADPH.

8ATP turbine

The leftover hydrogen ions pile up inside, then rush out through ATP synthase, a tiny turbine that spins out ATP, the cell's energy currency.

9the Calvin cycle

Stage two, the Calvin cycle, runs in the stroma. The enzyme rubisco grabs a carbon dioxide and bolts it onto a five carbon sugar, which splits into two three carbon pieces.

10sugar out

ATP and NADPH from stage one power the conversion into G three P, a sugar. Each carbon dioxide costs three ATP and two NADPH, and six turns make one glucose.

11the whole idea

Here is the whole thing. The light reactions turn light and water into oxygen, ATP and NADPH; the Calvin cycle spends them to turn carbon dioxide into sugar.