Granum

Granum

A granum (plural: grana) is a stack of thylakoid discs found within the chloroplasts of plant cells and algae. These structures are critical components of the photosynthetic apparatus, playing a key role in the light-dependent reactions of photosynthesis.

Structure

Grana are composed of multiple thylakoids, which are membrane-bound compartments. Each thylakoid is a flattened sac, and the membranes contain chlorophyll and other pigments that capture light energy. The thylakoid membranes are the site of the light-dependent reactions, where solar energy is converted into chemical energy in the form of ATP and NADPH.

The grana are interconnected by stroma thylakoids, also known as lamellae, which are unstacked thylakoid membranes that connect the grana stacks. This arrangement allows for efficient energy transfer and distribution of the products of the light-dependent reactions throughout the chloroplast.

Function

The primary function of grana is to facilitate the light-dependent reactions of photosynthesis. These reactions occur in the thylakoid membranes and involve the absorption of light by chlorophyll, the splitting of water molecules (photolysis), and the generation of ATP and NADPH. These energy carriers are then used in the Calvin cycle, which takes place in the stroma of the chloroplast, to synthesize glucose from carbon dioxide and water.

The stacking of thylakoids into grana increases the surface area available for light absorption, enhancing the chloroplast's ability to capture light energy. This structural organization is crucial for maximizing the efficiency of photosynthesis.

Importance in Photosynthesis

Grana play a vital role in the photosynthetic process by housing the photosystems and electron transport chains necessary for the light-dependent reactions. Photosystem II, located in the grana, initiates the process by capturing photons and using the energy to extract electrons from water molecules, releasing oxygen as a byproduct. The electrons are then passed through an electron transport chain, leading to the production of ATP and NADPH.

The spatial arrangement of grana and stroma thylakoids ensures that the products of the light-dependent reactions are efficiently distributed to the Calvin cycle, where they are used to fix carbon and produce sugars.

See Also

• Chloroplast
• Photosynthesis
• Thylakoid
• Calvin cycle

References

• Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular Biology of the Cell. Garland Science.
• Taiz, L., & Zeiger, E. (2010). Plant Physiology. Sinauer Associates.