Photosynthesis

Photosynthesis

Light, water and carbon dioxide in; sugars and oxygen out
Inputs	Light, carbon dioxide, water
Outputs	Carbohydrates, oxygen
Location	Chloroplasts (in plants and algae)
Key pigment	Chlorophyll

Photosynthesis is the process by which plants, algae and some bacteria convert light energy into chemical energy stored in carbohydrates. It is the entry point for almost all energy in the biosphere, and the source of the oxygen in the atmosphere.

The overall reaction, for the oxygen-producing form, is:

6 CO2 + 6 H2O + light → C6H12O6 + 6 O2

This summary hides the important detail: the oxygen released comes from the water, not from the carbon dioxide. This was established in the 1930s and confirmed by tracing heavy oxygen isotopes.

In the thylakoid membranes of the chloroplast, chlorophyll absorbs light, mostly in the blue and red parts of the spectrum, which is why leaves reflect green and appear that colour. The absorbed energy excites electrons, which pass down a chain of carriers.

The electrons lost by chlorophyll are replaced by stripping them from water, splitting it and releasing oxygen as a by-product. Meanwhile the electron transport chain pumps protons across the membrane, and the resulting gradient drives ATP synthase, which manufactures ATP. The stage yields ATP and NADPH, and no sugar at all.

In the stroma, the enzyme RuBisCO attaches carbon dioxide to a five-carbon sugar, and the ATP and NADPH from the first stage are spent reducing the product to carbohydrate. Three turns of the cycle fix three CO₂ molecules and yield one three-carbon sugar; the rest of the material is recycled to keep the cycle running.

RuBisCO is probably the most abundant protein on Earth and is also strikingly inefficient. It fixes only a few molecules of CO₂ per second, and it confuses carbon dioxide with oxygen. When it takes O₂ instead, the plant must spend energy clearing up the useless product, a process called photorespiration. Plants compensate by making enormous quantities of the enzyme.

Two adaptations reduce photorespiration in hot or dry conditions:

C4 photosynthesis

Carbon is first fixed into a four-carbon compound in one cell type, then delivered to another where RuBisCO works in a CO₂-rich environment. Maize, sugarcane and many tropical grasses use it.

CAM

Plants such as cacti open their stomata only at night, storing carbon as an acid and releasing it internally by day. This allows the leaf to stay closed during the heat, at the cost of a slower growth rate.

Oxygenic photosynthesis evolved in cyanobacteria at least 2.4 billion years ago and produced the Great Oxidation Event, filling the atmosphere with a gas that was toxic to most life then existing. It was among the largest changes any organism has made to the planet, and it made possible aerobic respiration, which yields far more energy per unit of food than the alternatives, and with it large, active animals such as the octopus.

Chloroplasts are descended from free-living cyanobacteria taken up by another cell and retained; they keep their own DNA and divide independently. Photosynthesis also produced, over hundreds of millions of years, the buried carbon now burned as fossil fuel.

• Octopus
• Coffee