The light-catching steps store their energy in small energy-carrying molecules. In the fluid of the chloroplast, an enzyme uses that energy to build carbon dioxide into sugar.
The enzyme is RuBisCO, short for ribulose bisphosphate carboxylase/oxygenase. Like every enzyme, it is a folded protein with an active site that fits the molecules it works on. RuBisCO's active site holds a five-carbon sugar carrying two phosphate groups, ribulose bisphosphate, and joins a carbon dioxide molecule to it. Chemists call this fixing carbon. The new six-carbon molecule splits at once into two three-carbon pieces.
Energy from the light-catching steps turns the pieces into three-carbon sugars. Most of those are rebuilt into the five-carbon sugar, ready for more carbon dioxide, so the steps run round in a cycle.
Melvin Calvin, Andrew Benson and James Bassham worked out the cycle at Berkeley from 1946 into the 1950s. They fed Chlorella carbon dioxide made with radioactive carbon-14 for a few seconds, plunged the algae into hot methanol to stop every reaction, and separated the molecules by chromatography on paper. After the shortest feeds, the carbon-14 turned up first in the three-carbon pieces. Calvin won the 1961 Nobel Prize in Chemistry.
A slow enzyme
Catalase, the enzyme in liver that breaks down hydrogen peroxide, gets through millions of molecules a second. A RuBisCO molecule fixes about three carbon dioxide molecules a second. Leaves make up for it in numbers: RuBisCO can be half the soluble protein in a leaf. In a warm leaf, about one time in four, its active site picks up an oxygen molecule in place of carbon dioxide, which is what the "oxygenase" in its name means, and the plant spends energy clearing up the product.
Sugar, starch and cellulose
The three-carbon sugars build glucose and other sugars. A plant burns some of them for energy and builds the rest into bigger molecules.
| Molecule | What it is, and where you find it |
|---|---|
| starch | coiled chains of glucose, stored in leaves, potatoes, rice and seeds |
| cellulose | straight chains of glucose, every other unit flipped, in cell walls, wood and cotton |
| sucrose | glucose joined to fructose: table sugar, carried round the plant in its sap and stored in sugar cane and sugar beet |
A leaf builds up starch in its chloroplasts during the day and breaks it back down to sugar at night, so the plant keeps getting sugar in the dark.
Quick check
That's the end of lesson 36
In this lesson you found out:
- What goes into photosynthesis and what comes out
- How chlorophyll catches light
- Where the oxygen you breathe comes from
- How an enzyme builds carbon dioxide into sugar