Photosynthesis Uses Light Energy to Synthesise Organic Molecules

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 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.

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.

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.

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.