What is Granum? – Definition & Function


Granum are a necessary part of the process of photosynthesis. In this article you will learn exactly what grana are: their structure and function.

What are Grana?

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 house chloroplasts 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. There can be as few as 3 or as many as 120 thylakoids in grana, or groups of granum, and between 50 and 70 grana in a plant cell. The shape of the thylakoids allow for optimum surface area, maximizing the amount of photosynthesis that can happen.

Thylakoids contain chlorophyll, the pigment used by plants for photosynthesis. Within the thylakoid membrane we find two photosystems, or protein complexes. Photosystems are responsible for absorbing the sunlight to be used in the creation of sugars.

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.


Granum act to increase the surface area of the thylakoids, which results in more light being absorbed into the chlorophyll. When you increase the surface area, you allow more materials to attach to the surface, or in this case, react in the different photosystems in the thylakoid membrane. This allows the plant to take in more sunlight and increase production of sugars through photosynthesis.

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.

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