Photosynthesis is the central biological process that transforms light energy into chemical energy stored in organic molecules. It links energy flow with carbon cycling, providing both biomass and oxygen that support most ecosystems.

Terrestrial carbon-cycle schematic showing how photosynthesis moves carbon from atmospheric $CO_2$ into organic carbon (biomass), while respiration and decomposition return $CO_2$ to the atmosphere. The labeled arrows emphasize that photosynthesis and respiration are complementary processes that continuously cycle carbon through living and nonliving reservoirs. Source

Core concept of photosynthesis

Energy transformation

Photosynthesis converts energy from sunlight into chemical potential energy stored mainly in the bonds of carbohydrates. Cells can later access this stored energy by breaking those bonds during metabolism.

Which organisms photosynthesise

Photosynthesis occurs in photosynthetic organisms, including:

• Plants

• Algae

• Many photosynthetic bacteria

These organisms are often the base of food webs because they can build organic molecules from inorganic sources using light.

Inputs and outputs: what goes in, what comes out

Reactants and products

The overall process uses carbon dioxide (CO₂), water (H₂O), and light energy to produce carbohydrates and oxygen (O₂).

Annotated overall photosynthesis equation showing the stoichiometry of reactants ($6CO_2$ and $6H_2O$) and products ($C_6H_{12}O_6$ and $6O_2$), with light energy as the driving input. This diagram reinforces matter conservation by making the atom counts explicit on both sides of the reaction. Source

This overall equation is a simplified summary: organisms make a variety of carbohydrates, and glucose may be further converted to other sugars or polymers.

Matter conservation and atom “tracking”

Photosynthesis rearranges atoms; it does not create or destroy matter.

• Carbon atoms from CO₂ become part of carbohydrate backbones.

• Hydrogen atoms from water contribute to forming energy-rich organic molecules.

• Oxygen gas (O₂) released is produced as a by-product of splitting inputs; in oxygenic photosynthesis, the released O₂ is associated with water as the original oxygen source.

Photosynthesis as redox chemistry (high-level)

At an overview level, photosynthesis can be understood as a redox process:

• CO₂ is reduced to form carbohydrates (gains electrons/energy).

• H₂O is oxidised as part of providing electrons for building carbohydrates.

Biological significance

Foundation of ecosystems

Photosynthesis supplies organic carbon (biomass) that supports consumers.

• Primary producers convert inorganic carbon into organic molecules.

• Heterotrophs obtain energy and carbon indirectly by consuming producers or other consumers.

Oxygen and cellular metabolism

Oxygen produced by photosynthesis contributes to oxygen availability in many environments. This supports organisms whose metabolism uses oxygen to extract energy efficiently from organic molecules.

Building materials as well as “fuel”

Carbohydrates made via photosynthesis serve multiple cellular roles:

• Short-term energy storage and transport (e.g., soluble sugars)

• Long-term energy storage (e.g., starch in many plants)

• Structural components (e.g., cellulose in plant cell walls)

Essential requirements for photosynthesis (overview)

For photosynthesis to proceed, photosynthetic cells must have:

• A source of light energy

• Access to CO₂ and H₂O

• Cellular machinery that captures light and uses that energy to assemble carbohydrates

Because photosynthesis depends on environmental inputs, its overall productivity is constrained by resource availability (especially light, water, and CO₂), which in turn shapes where photosynthetic organisms can grow.