BharatNotes What is Photosynthesis?
Photosynthesis is the process by which green plants, algae and cyanobacteria capture light energy and use it to synthesise glucose from carbon dioxide and water, releasing oxygen as a by-product. It is the principal mechanism by which solar energy is fixed into chemical energy and made available to almost all life on Earth.
The overall reaction is commonly summarised as:
6CO2 + 6H2O + light energy -> C6H12O6 (glucose) + 6O2
The process takes place in the chloroplast and unfolds in two coupled stages.
Two Stages of the Process
| Stage | Location | Inputs | Outputs |
|---|---|---|---|
| Light-dependent reactions | Thylakoid membranes | Water, light | Oxygen, ATP, NADPH |
| Light-independent reactions (Calvin cycle) | Stroma | CO2, ATP, NADPH | Glucose |
In the light-dependent reactions, chlorophyll absorbs light (mainly violet, blue and red wavelengths, reflecting green), and water molecules are split to release oxygen while energy is stored as ATP and NADPH. In the Calvin cycle (light-independent), that stored energy drives the fixation of CO2 into sugar within the stroma. Both stages are required: the second depends entirely on the products of the first.
C3, C4 and CAM Pathways
Plants are grouped by how they first fix carbon — an adaptation to climate and water stress:
| Pathway | First product | Typical plants | Key trait |
|---|---|---|---|
| C3 | 3-carbon compound | Rice, wheat | Most common; prone to photorespiration |
| C4 | 4-carbon compound | Maize, sugarcane | Spatial CO2 concentration in bundle-sheath cells; efficient in heat |
| CAM | 4-carbon (malic acid) | Cacti, pineapple | Stomata open at night; conserves water in arid zones |
C4 and CAM plants evolved mechanisms to minimise photorespiration — the wasteful uptake of oxygen instead of CO2 by the enzyme RuBisCO — making them better suited to hot, dry conditions.
Significance for the Earth System
Photosynthesis sits at the heart of the global carbon and oxygen cycles. Ocean phytoplankton produce at least 50% of the oxygen in the atmosphere and account for roughly half of global photosynthesis, delivering an annual net primary productivity of about 47.5 petagrams of carbon, against around 56.4 PgC for land plants (Current Biology / phytoplankton biogeochemistry literature). The oceanic carbon sink absorbs over 25% of annual anthropogenic CO2 emissions, comparable to the land-plant sink. This makes photosynthesis directly relevant to carbon sequestration, blue-carbon ecosystems, afforestation and climate mitigation — and explains why ocean warming and stratification, which cut nutrient supply to phytoplankton, are a climate concern.
UPSC Angle
For aspirants, the high-yield points are: the location and outputs of the two reaction stages; the C3/C4/CAM comparison (a frequent confused-pair zone); the role of RuBisCO and photorespiration; and the carbon-cycle linkage that ties pure biology to GS3 environment and climate change. Anchor the concept to its ecosystem role rather than memorising the equation in isolation.
