Photosynthesis

In higher plants and algae, the light reactions occur on the thylakoid membranes of the chloroplasts. The thylakoid membranes are inner membranes of the chloroplasts which are arranged like flattened sacs. The thylakoids are often stacked on top of one another, like a roll of coins. A stack of thylakoids is referred to as a granum.

The light reactions of higher plants require photosynthetic pigments, chlorophyll-a, chlorophyll-b, and various types of carotenoids. These pigments are associated with special proteins which are embedded in the thylakoid membranes. Chlorophyll-a and chlorophyll-b strongly absorb light in the red and blue regions of the spectrum. Most carotenoids strongly absorb blue light. Thus, plant leaves are green simply because their photosynthetic pigments absorb blue and red light but not green light.