Photosynthesis Revision of structured questions

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose. This energy is then stored as potential energy and used by organisms for various metabolic activities.

The process of photosynthesis can be divided into two main stages: the light-dependent reactions and the light-independent reactions (also known as the Calvin cycle).

Light-dependent reactions: These reactions occur in the thylakoid membrane of the chloroplasts and require light energy. The key steps involved in this stage are:

1. Absorption of light: Chlorophyll and other pigments in the thylakoid membrane absorb photons of light. This energy is used to excite electrons in chlorophyll.

2. Electron transport chain: Excited electrons from chlorophyll are passed along a series of electron carriers, losing energy in the process. This energy is used to pump hydrogen ions (H+) across the thylakoid membrane, creating a proton gradient.

3. ATP synthesis: The proton gradient created is used to drive ATP synthase, an enzyme that synthesizes ATP from ADP and inorganic phosphate. This process is called chemiosmosis.

4. Generation of reducing power: Excited electrons from chlorophyll are also used to reduce NADP+ to NADPH, another molecule that stores energy.

Light-independent reactions (Calvin cycle): These reactions occur in the stroma of the chloroplasts and do not require light directly. The key steps involved in this stage are:

1. Carbon fixation: CO2 from the atmosphere is captured and added to a 5-carbon molecule called ribulose bisphosphate (RuBP), forming an unstable 6-carbon molecule. This process is catalyzed by the enzyme rubisco.

2. Reduction: The 6-carbon molecule is quickly split into two 3-carbon molecules called 3-phosphoglycerate (PGA). ATP and NADPH from the light-dependent reactions are used to convert PGA into a 3-carbon sugar called glyceraldehyde-3-phosphate (G3P).

3. Regeneration: Some G3P molecules are converted back into RuBP, while others are used to synthesize glucose and other organic molecules.

Overall, photosynthesis is a crucial process for sustaining life on Earth as it provides oxygen for respiration and serves as the primary source of energy for most living organisms.