We all need photosynthesis. All of us living on Earth's surface, that is. Turning light into food energy is the foundation of almost all food chains on Earth and is the livelihood of plants and algae. These organisms housechloroplasts within their cells, organelles responsible for manufacturing energy through the process of photosynthesis. It is within chloroplasts that we find many grana.
Grana (plural of 'granum') are stacks of structures called thylakoids, which are little disks of membrane on which the light-dependent reactions of photosynthesis take place. Stacked into grana, the shape of the thylakoids allow for optimum surface area, maximizing the amount of photosynthesis that can happen.
Within the chloroplast the grana resemble stacks of green pancakes, and are not uniform in their height or distribution. The grana are connected to each other by way of lamellae, or membrane that bridge the grana but also participate in the photosystem 1 stage of photosynthesis. All parts within the chloroplast are surrounded by a liquid suspension called stroma.
Thylakoids that make up grana contain important light-absorbing pigments, such as chlorophyll. When light strikes these pigments, they split water, releasing oxygen as a byproduct in the process of photolysis.
The freed electrons from this reaction arrive at photosystem 2 and are transferred down an electron transport chain to photosystem 1. Here they are further excited by light absorption, and go through another electron transport chain.In the end, an electron carrier called NADP+ receives the electrons and is reduced to NADPH, creating ATP, the energy currency of cells and the ultimate product of these reactions. The high surface area created by the stacking of thylakoids into grana maximize the efficiency of photosystems 1 and 2, and therefore maximize the output of ATP.