Describe the process of photosynthesis - Junior Cycle Geography - Question 1

Photosynthesis is the biological process by which green plants, algae, and some bacteria convert light energy into chemical energy. This process primarily occurs in the chloroplasts of plant cells and involves two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle).

Light-Dependent Reactions

The light-dependent reactions take place in the thylakoid membranes of the chloroplasts. When sunlight strikes these membranes, it excites electrons in chlorophyll, leading to a series of reactions that produce ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). Water molecules are split in a process known as photolysis, releasing oxygen as a byproduct.

1. Photon Absorption: Chlorophyll absorbs light energy which energizes electrons.
2. Electron Transport Chain: The energized electrons are transferred through a series of proteins, generating a proton gradient.
3. ATP Synthesis: This gradient drives ATP synthesis via ATP synthase.
4. NADPH Formation: Electrons reduce NADP+ to NADPH.

Light-Independent Reactions (Calvin Cycle)

The Calvin cycle occurs in the stroma of chloroplasts and uses ATP and NADPH produced in the light-dependent reactions to convert carbon dioxide into glucose.

1. Carbon Fixation: Carbon dioxide is combined with ribulose bisphosphate (RuBP) to form 3-phosphoglycerate (3-PGA).
2. Reduction Phase: 3-PGA is converted into glyceraldehyde-3-phosphate (G3P) using ATP and NADPH.
3. Regeneration of RuBP: Some G3P molecules go on to form glucose, while others are used to regenerate RuBP, allowing the cycle to continue.

Overall, photosynthesis transforms light energy into the chemical energy of glucose, supporting the energy needs of the plant and producing oxygen as a critical byproduct.