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Light Reaction

There are two light reactions (two kinds of pigment systems)
involved in photosynthesis, one sensitive to longer wave length (reaction
centre P 700) and the other to shorter wave length (P 680). The P is short
for pigment and the number is the specific absorption peak in nanometres
for the chlorophyll molecules in each reaction centre. These reaction
centres contain chlorophyll a, each reaction centre is associated with a
group of function light – harvesting pigments. Hence, there are two kinds
of pigment systems photosystem I and Photosystem II. Photosystem I has
a high proportion of chlorophyll (a) and is sensitive to longer wavelength.
In contrast, photosystem II sensitive to shorter wavelength, contains (P
680) and has a high proportion of chlorophyll (b) in its light harvesting
system.

Thylakoid membrane

Light reactions take place in the thylakoid membrane in the
chloroplast. The thylakoid membrane (Figure 18) is bilayer lipid-globular
membrane that contain integral membrane proteins which play an
important role in light harvesting and the light-dependent reactions of
photosynthesis. There are four major protein complexes in the thylakoid
membrane:

1. Photosystems II
2. Cytochrome b6f complex
3. Photosystems I
4. ATP synthase.

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