Biology Explain the process of photosynthesis, including the main stages and their role in the overall process. - HSC - SSCE Biology - Question 2 - 2010 - Paper 1

Photosynthesis is a biochemical process that converts light energy into chemical energy in the form of glucose, using carbon dioxide and water. The process takes place primarily in the chloroplasts of plant cells and can be divided into two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle).

1. Light-Dependent Reactions

The light-dependent reactions occur in the thylakoid membranes of the chloroplasts. When light is absorbed by chlorophyll, it excites electrons, which are then transferred through a series of proteins in the electron transport chain. This process generates two key molecules:

• ATP (Adenosine Triphosphate): The energy currency of the cell.
• NADPH (Nicotinamide Adenine Dinucleotide Phosphate): An electron carrier used in the Calvin cycle.

2. Light-Independent Reactions (Calvin Cycle)

The light-independent reactions, also known as the Calvin cycle, occur in the stroma of the chloroplasts. During this stage, ATP and NADPH produced in the light-dependent reactions are used to convert carbon dioxide from the atmosphere into glucose through a series of reactions:

1. Carbon Fixation: Carbon dioxide is incorporated into an organic molecule.
2. Reduction Phase: ATP and NADPH are used to convert the organic molecule into G3P (glyceraldehyde-3-phosphate).
3. Regeneration of RuBP: The cycle regenerates ribulose bisphosphate (RuBP), allowing the cycle to continue.

Overall, photosynthesis plays a crucial role in sustaining life on Earth by providing oxygen and organic compounds that serve as food for a vast number of organisms.