Photosynthesis Reactions

Photosynthesis Reactions


  • Photosynthesis is a process used by plants and other organisms to convert light energy, usually from the Sun, into chemical energy that can later be released to fuel the organism’s metabolic activities.

  • This process involves two main stages: the light-dependent reactions occur in the thylakoid membranes, and the light-independent reactions (or Calvin cycle) which takes place in the stroma.

  • Photosynthesis is summarised by the following equation: 6CO2 + 6H2O –> C6H12O6 + 6O2 (carbon dioxide + water –> glucose + oxygen)

Light-Dependent Reactions

  • In the light-dependent reactions, energy absorbed from light drives the creation of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), which are necessary for the light-independent stage.

  • The photosystem II (PSII) and photosystem I (PSI) absorb photons, leading to the photolysis of water, the release of oxygen and the production of ATP and NADPH.

  • Electron transport chains (ETCs) are used to pump protons across the thylakoid membrane, and the subsequent flow of protons back across the membrane drives the formation of ATP, a process called chemiosmosis.

Light-Independent Reactions (Calvin Cycle)

  • In the Calvin Cycle, carbon dioxide is fixed into an organic molecule through a process called carbon fixation. This is facilitated by the enzyme Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco).

  • The fixed carbon is then reduced to carbohydrate using ATP and NADPH produced in the light-dependent reactions.

  • The cycle is completed when the original acceptor molecule ribulose bisphosphate (RuBP) is regenerated, allowing the cycle to continue.

Factors Affecting Photosynthesis

  • Several factors influence the rate of photosynthesis including light intensity, carbon dioxide concentration, and temperature.

  • The rate of photosynthesis generally increases with light intensity to a certain point (known as the light saturation point), beyond which further increases in light intensity do not increase the rate of photosynthesis.

  • Similarly, the rate of photosynthesis increases with higher carbon dioxide concentrations and optimal temperatures, but can decrease if these conditions become too extreme. Temperature affects the activities of enzymes involved in photosynthesis, particularly Rubisco.

  • Any factor that limits the rate of photosynthesis under a particular set of conditions is known as a limiting factor.