Light Reactions

The light reactions are the first stage of photosynthesis, occurring in the thylakoid membranes of chloroplasts. This process is responsible for converting light energy into chemical energy in the form of ATP and NADPH.

Vocabulary: Thylakoid membranes are flattened sacs within chloroplasts where light reactions take place.

Two key components of the light reactions are Photosystem II (PSII) and Photosystem I (PSI). PSII absorbs light energy and splits water molecules, releasing oxygen and generating electrons. These electrons are then passed along an electron transport chain, creating a proton gradient across the thylakoid membrane. PSI also absorbs light energy and uses the electrons received from the electron transport chain to generate NADPH.

Example: The splitting of water molecules in PSII can be represented as: 2H₂O → 4H⁺ + 4e⁻ + O₂

ATP synthesis is another crucial process in the light reactions. ATP synthase, an enzyme complex, uses the proton gradient generated during the light reactions to produce ATP through chemiosmosis.

Highlight: The light reactions produce ATP and NADPH, which are essential for the Calvin cycle.

Calvin Cycle (Dark Reactions)

The Calvin cycle, also known as the dark reactions, takes place in the stroma of chloroplasts. This process uses the ATP and NADPH produced during the light reactions to fix carbon dioxide and produce glucose.

The Calvin cycle consists of three main stages: carbon fixation, reduction and sugar production, and regeneration of RuBP (ribulose-1,5-bisphosphate).

In the carbon fixation stage, carbon dioxide is combined with RuBP using the enzyme rubisco. This reaction produces an unstable six-carbon molecule that quickly breaks down into two molecules of PGA (3-phosphoglycerate).

Vocabulary: Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) is the enzyme responsible for carbon fixation in the Calvin cycle.

During the reduction and sugar production stage, PGA is converted into G3P (glyceraldehyde-3-phosphate) using ATP and NADPH from the light reactions. Some G3P molecules are used to regenerate RuBP, while others are used to produce glucose and other organic molecules.

Highlight: The role of ATP and NADPH in Calvin cycle is crucial for the conversion of carbon dioxide into glucose.

The final stage involves the regeneration of RuBP, which allows the Calvin cycle to continue. ATP is used to convert some G3P molecules back into RuBP, ensuring a continuous supply of the carbon-fixing substrate.