Every leaf is a tiny solar-powered factory. It takes in air and water, catches sunlight, and ships out sugar and oxygen — quietly running the reaction that keeps almost all life on Earth alive.
Photosynthesis is the process plants, algae, and some bacteria use to convert light energy into chemical energy. They build glucose (a simple sugar) from carbon dioxide and water, releasing oxygen as a by-product. It's the reason the atmosphere has oxygen, the reason plants grow, and the foundation of nearly every food chain on the planet.
Sunlight, Water, CO₂
Light energy is captured by chlorophyll; water comes up from the roots; carbon dioxide enters through tiny leaf pores.
Glucose + Oxygen
Glucose stores chemical energy for the plant; oxygen is released into the air as a waste product.
Chloroplasts
Inside plant cells, organelles called chloroplasts house the chlorophyll and run the two main stages.
The Overall Equation
CHEMICAL SUMMARYPhotosynthesis can be summed up in one balanced chemical equation. Six molecules of carbon dioxide plus six molecules of water, powered by light, produce one molecule of glucose and six molecules of oxygen.
This is an endothermic reaction — it stores energy rather than releasing it. The energy comes from sunlight and ends up locked inside the chemical bonds of glucose.
The oxygen released comes from splitting water molecules, not from carbon dioxide. That detail was discovered using isotope tracers in the 1930s and is a classic exam question.
Stage 1: The Light-Dependent Reactions
NEEDS LIGHTThe first stage happens in the thylakoid membranes inside the chloroplast. These flattened sacs are packed with chlorophyll and other pigments that absorb light energy.
When light hits chlorophyll, it excites electrons to a higher energy level. Those electrons travel through a series of proteins called the electron transport chain, and their energy is used to pump hydrogen ions across the membrane. The resulting flow of ions back across the membrane drives an enzyme called ATP synthase, which produces ATP — the cell's energy currency.
Meanwhile, water molecules are split in a reaction called photolysis. This replaces the electrons lost by chlorophyll and releases oxygen gas as a by-product. Another energy carrier, NADPH, is also produced.
What the light reactions produce
- ATP — chemical energy for the next stage.
- NADPH — a reduced electron carrier.
- Oxygen (O₂) — released into the atmosphere.
Thylakoid membranes inside chloroplasts. These membranes are stacked into structures called grana, which increase the surface area available for light capture.
Stage 2: The Calvin Cycle (Light-Independent Reactions)
BUILDS SUGARThe second stage takes place in the stroma — the fluid-filled space surrounding the thylakoids. It doesn't need light directly, which is why it's sometimes called the dark reactions, but it depends on the ATP and NADPH made in stage one.
In the Calvin cycle, carbon dioxide is fixed into an organic molecule by an enzyme called RuBisCO. Through a series of reactions, the carbon atoms are rearranged to build G3P (glyceraldehyde-3-phosphate), which is then used to make glucose and other carbohydrates.
The three phases of the Calvin cycle
- Carbon fixation: CO₂ is attached to a 5-carbon sugar (RuBP) by RuBisCO.
- Reduction: ATP and NADPH are used to convert the fixed carbon into G3P.
- Regeneration: Most of the G3P is used to regenerate RuBP so the cycle can continue.
For every three molecules of CO₂ fixed, the cycle produces one net molecule of G3P. It takes two G3P molecules to make one glucose.
Inside a Leaf: The Big Picture
Zoom out from the chloroplast, and you see how the whole leaf is built for photosynthesis.
- Stomata: tiny pores on the underside of leaves that let CO₂ in and O₂ out.
- Mesophyll cells: the main photosynthetic tissue, packed with chloroplasts.
- Veins (xylem and phloem): xylem delivers water from the roots; phloem carries glucose away to the rest of the plant.
- Cuticle: a waxy outer layer that reduces water loss.
What Affects the Rate of Photosynthesis?
Photosynthesis doesn't run at the same speed all the time. Several factors control how fast it goes, and each has an optimum.
| Factor | Effect | Optimum |
|---|---|---|
| Light intensity | More light → faster rate, up to a plateau | Bright but not scorching |
| CO₂ concentration | More CO₂ → faster rate, up to a saturation point | Around 1% in air (higher than normal) |
| Temperature | Enzymes work faster when warm, denature when too hot | ~25–35 °C for most plants |
| Water availability | Too little → stomata close, CO₂ can't enter | Adequate soil moisture |
| Chlorophyll content | More chlorophyll → more light absorbed | Healthy, green leaves |
Why Photosynthesis Matters
Without photosynthesis, life as we know it wouldn't exist. Here's why it's the most important chemical reaction on Earth.
- Oxygen supply: virtually all atmospheric oxygen comes from photosynthesis — mostly from ocean phytoplankton.
- Food chains: plants are producers; every animal either eats plants or eats something that does.
- Carbon cycle: photosynthesis pulls CO₂ out of the air, helping regulate the planet's climate.
- Fossil fuels: coal, oil, and natural gas are ancient photosynthesis stored underground.
Photosynthesis vs. Cellular Respiration
They're often taught together because they're almost mirror images of each other.
| Feature | Photosynthesis | Cellular Respiration |
|---|---|---|
| Energy | Stores energy (endothermic) | Releases energy (exothermic) |
| Reactants | CO₂ + H₂O + light | Glucose + O₂ |
| Products | Glucose + O₂ | CO₂ + H₂O + ATP |
| Location | Chloroplasts | Mitochondria |
| Who does it | Plants, algae, some bacteria | Almost all living cells |
The products of photosynthesis are the reactants of respiration, and vice versa. Together they cycle carbon and oxygen through the biosphere.