ATP Synthesis – Chloroplasts

ATP Synthesis – Chloroplasts

The process of photosynthesis takes place completely in the chloroplasts. They consist of a double membrane, like mitochondria, but they also have another membrane: the thylakoid membrane. This membrane is folded into little vesicles, called thylakoids, within which are little spaces known as the thylakoid lumen. The thylakoids tend to be layered forming stacks called grana.

To produce ATP the thylakoid membrane uses the same synthase particles as mitochondria. The products of photosynthesis are then store in the chloroplasts as liquid droplets and starch grains.

Chloroplasts contain chlorophyll, two types to be exact: a and b. They also contain numerous other light-absorbing accessory pigments like luteins and carotenoids. Each pigment absorbs light at a different wavelength. This enables the chloroplasts to make more use of the light energy or increase its absorption spectrum.

Different plant species contain different photosynthetic pigments and in different amounts. This is what affects the colour of leaves. For example, plants adapted to living in the shade usually contain more chlorophyll and so their leaves are darker than a plant who have adapted to bright conditions.

The action spectrum is the rate of photosynthesis with different light wavelengths. Using a graph you can clearly see which pigments are most efficient.

Chlorophyll is quite a small molecule and has a structure similar to haem except that it swaps the iron atom for magnesium. Chlorophyll arranges itself with other pigments and proteins to form photosystems. A photosystem is made from around 200 molecules of chlorophyll, about 50 accessory pigment molecules, and a few proteins and lipids. They’re located in the thylakoid membranes which position them at the best angle to catch the light. Within chloroplasts are two different kinds of photosystem, photosystem I (PSI) and photosystem II (PSII), which absorb light at different wavelengths.

Photosynthesis is, in fact, composed of two major steps:

  • light-dependent reactions
  • light-independent reactions