Thylakoid

Lettuce_Thylakoid.jpg

== Thylakoid ==

A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen.

Structure

Thylakoids are arranged in stacks known as grana (singular: granum). Each granum is connected by intergranal or stroma thylakoids, also known as lamellae. The thylakoid membrane contains integral and peripheral membrane proteins, including the photosystems, cytochrome b6f complex, and ATP synthase, which are essential for the light-dependent reactions.

Function

The primary function of thylakoids is to capture and convert light energy into chemical energy during photosynthesis. This process occurs in two main stages: the light-dependent reactions and the Calvin cycle. The light-dependent reactions take place in the thylakoid membrane and involve the absorption of light by chlorophyll and other pigments, leading to the production of ATP and NADPH. These energy carriers are then used in the Calvin cycle to synthesize glucose from carbon dioxide and water.

Photosystems

Thylakoids contain two types of photosystems: Photosystem I (PSI) and Photosystem II (PSII). These photosystems are complexes of proteins and pigments that play a crucial role in the light-dependent reactions. PSII absorbs light and uses the energy to extract electrons from water molecules, releasing oxygen as a byproduct. The electrons are then passed through the electron transport chain to PSI, which further energizes them using light energy. The high-energy electrons are finally used to reduce NADP+ to NADPH.

ATP Synthesis

The thylakoid membrane also houses the ATP synthase enzyme, which synthesizes ATP from ADP and inorganic phosphate. This process is driven by a proton gradient generated across the thylakoid membrane during the electron transport chain. The flow of protons back into the thylakoid lumen through ATP synthase provides the energy needed for ATP synthesis.

Importance in Photosynthesis

Thylakoids are essential for the conversion of light energy into chemical energy, which is vital for the survival of plants, algae, and cyanobacteria. The ATP and NADPH produced in the thylakoid membrane are used in the Calvin cycle to produce glucose, which serves as an energy source for the organism.

See also

• Chloroplast
• Photosynthesis
• Photosystem
• ATP synthase
• Calvin cycle
• Grana
• Stroma