Where photosynthesis happens and its pigments

Photosynthesis happens in green leaves and other green parts. Inside the leaf, the mesophyll cells are packed with chloroplasts, which line up along the cell walls to catch the most light.

The chloroplast divides the work. Its membranes (the grana and stroma lamellae) trap light and make ATP and NADPH: these are the light reactions. The fluid stroma holds the enzymes that use ATP and NADPH to make sugar: the carbon reactions. These are sometimes called "dark reactions", but they do not need darkness; they simply do not use light directly, and they still depend on the products of the light reactions.

Separate the pigments of a leaf on paper (paper chromatography) and four bands appear:

• Chlorophyll a: bright or blue-green.
• Chlorophyll b: yellow-green.
• Xanthophylls: yellow.
• Carotenoids: yellow to yellow-orange.

Chlorophyll a is the chief pigment, like the engine of a car: without it nothing moves, whatever else is there. It absorbs light most strongly in the blue and red parts of the spectrum, and photosynthesis is also fastest in blue and red light. But the two graphs do not match exactly, so some photosynthesis happens at other colours too.

That is the job of the accessory pigments: chlorophyll b, xanthophylls and carotenoids. They absorb other colours of light and pass the energy on to chlorophyll a. They also protect chlorophyll a from being damaged by strong light (photo-oxidation).