Photosynthesis can be divided into two stages:
- Light Reactions : Light energy (absorbed from the sun) is converted to chemical energy, which is temporarily stored in ATP and the energy carrier molecule NADPH.
- Dark Reaction/Calvin Cycle : Organic compounds are formed using CO2 and the chemical energy stored in ATP and NADPH.
The Light Reactions occur in the grana and the Dark Reactions take place in the stroma of the chloroplasts.
The first stage of photosynthesis includes the light reactions, so named because they require light to happen. The light reactions begin with the absorption of light in chloroplasts, organelles found in the cells of plants and algae. Most chloroplasts are similar in structure. Each chloroplast is surrounded by a pair of membranes. Inside the inner membrane is another system of membranes called thylakoids that are arranged as flattened sacs. The thylakoids are connected and layered to form stacks called grana (singular, granum). Surrounding the grana is a solution called the stroma. The photosynthetic pigments in chloroplasts are located in the thylakoid membranes. The primary light absorbing pigments in higher plants are the chlorophylls.