Photosynthesis Uses Light Energy to Synthesise Organic Molecules

Photosynthesis Uses Light Energy to Synthesise Organic Molecules

Overview of Photosynthesis

  • Photosynthesis is a process that converts light energy, usually from the sun, into chemical energy in the form of glucose. This process takes place in green plants, algae, and certain types of bacteria.
  • The energy is stored in the bonds of glucose, an organic compound, and this energy can be used by the organism when needed.
  • The overall balanced chemical equation for photosynthesis is 6CO2 + 6H2O + Light Energy -> C6H12O6 + 6O2. It represents the conversion of carbon dioxide and water into glucose and oxygen using light energy.

Photosynthesis: A Two-Stage Process

  • Photosynthesis consists of two main stages known as light-dependent reactions (or Light phase) and light-independent reactions (or Dark phase or Calvin cycle).
  • During light-dependent reactions, light energy is absorbed by chlorophyll in the chloroplasts and is used to synthesise ATP from ADP and Pi, and split water molecules into hydrogen and oxygen in a process called photolysis.
  • ATP and the hydrogen freed by the photolysis of water are used in the light-independent reactions to convert the carbon dioxide into glucose.

Light-Dependent Reactions

  • These reactions occur in the thylakoid membranes of the chloroplasts.
  • Chlorophyll molecules absorb light energy and use it to excite electrons, elevating them to higher energy levels.
  • These high energy electrons are passed along a series of electron carriers in an electron-transport chain.
  • Energy from the electrons is used to pump hydrogen ions across the thylakoid membrane creating a concentration gradient.
  • The movement of hydrogen ions back across the membrane powers the enzyme ATP synthase, generating ATP.

Light-Independent Reactions

  • These reactions occur in the stroma of chloroplasts.
  • ATP and NADPH generated in the light-dependent reactions are used to convert carbon dioxide into glucose in a cycle of reactions known as the Calvin Cycle.
  • The molecule that initially combines with carbon dioxide is ribulose bisphosphate (RuBP). The enzyme that catalyses this reaction is RuBisCO, the most abundant protein on earth.
  • Through a series of reactions, RuBP is regenerated allowing the cycle to continue.

Importance of Photosynthesis

  • Photosynthesis is crucial for life on earth as it provides the oxygen that aerobic organisms need for respiration.
  • It also locks up significant amounts of carbon dioxide from the atmosphere, helping to reduce greenhouse gas levels and mitigate climate change.
  • Photosynthesis also provides the basis for the majority of food chains on the planet, as plants, the primary producers, are eaten by herbivores, which in turn are eaten by carnivores.