What Is Photosynthesis?
Photosynthesis is one of the most important biological processes on Earth. It is how plants, algae, and some bacteria make their own food using energy from sunlight. The word comes from Greek: photo (light) and synthesis (putting together).
Plants are called autotrophs (or producers) because they produce their own food. Animals, by contrast, are heterotrophs — they must eat other organisms to get energy.
The Ingredients and Products
Photosynthesis needs three things:
- Carbon dioxide (CO₂) — absorbed from the air through tiny pores in leaves called stomata
- Water (H₂O) — absorbed by roots from the soil
- Light energy — captured by chlorophyll inside chloroplasts
The process produces two things:
- Glucose (C₆H₁₂O₆) — a sugar the plant uses for energy, growth, and storing food
- Oxygen (O₂) — released into the air as a by-product (the oxygen we breathe!)
Word equation: Carbon dioxide + Water → Glucose + Oxygen (using light energy)
Nearly all the oxygen in Earth's atmosphere was produced by photosynthesis over billions of years. Every breath you take is made possible by plants and photosynthetic organisms.
Where Does Photosynthesis Happen?
Photosynthesis takes place in the chloroplasts, organelles found mainly in leaf cells. Chloroplasts contain the green pigment chlorophyll, which absorbs red and blue light from the sun but reflects green light — which is why plants look green.
Leaves are specially adapted for photosynthesis:
- They are broad and flat to catch maximum sunlight
- They are thin so carbon dioxide can reach all cells easily
- They have a network of veins to deliver water and carry away glucose
- They are packed with chloroplasts in the upper cell layers
Factors That Affect the Rate of Photosynthesis
Photosynthesis speeds up or slows down depending on conditions:
Light intensity — More light (up to a point) means faster photosynthesis. Carbon dioxide concentration — More CO₂ available means faster photosynthesis. Temperature — Photosynthesis speeds up as temperature rises, but becomes too slow or stops if temperatures are too high or too low, because enzymes are damaged. Water availability — Without enough water, stomata close to prevent water loss, which also stops CO₂ from entering.
Farmers and greenhouse growers sometimes increase CO₂ levels inside greenhouses deliberately — this can increase crop yields because plants photosynthesise faster.
How Glucose Is Used
The glucose made by photosynthesis is used in several ways:
- Respiration — broken down in mitochondria to release energy
- Growth — converted into cellulose for cell walls
- Storage — converted into starch (stored in roots and seeds) or other molecules
- Repair — used to make new cell components
Real world: Greenhouse farming and boosting photosynthesis
Commercial greenhouse growers deliberately increase CO₂ concentrations inside glasshouses to 1,000–1,500 ppm (compared with the natural 420 ppm outdoors). This speeds up photosynthesis and can increase crop yields by 20–30%. Tomatoes, cucumbers, and peppers grown in CO₂-enriched greenhouses grow faster and produce more fruit — a direct application of understanding what limits photosynthesis.
Real world: Deforestation and the carbon cycle
Tropical rainforests absorb around 2.5 billion tonnes of CO₂ per year through photosynthesis. When forests are cleared for agriculture or logging, those trees no longer fix carbon — and burning them releases stored carbon back into the atmosphere. This is why deforestation accounts for roughly 10–15% of global CO₂ emissions and why replanting trees is a key climate strategy.
Real world: Aquatic plants and oxygen in fish tanks
Aquarium owners add aquatic plants to fish tanks for a practical reason: healthy plants photosynthesise and release oxygen directly into the water, supplementing the pump filter. In a bright, well-planted tank you can sometimes see tiny bubbles rising from leaves — those are oxygen molecules produced by photosynthesis. If the light is dim, the plants respire more than they photosynthesise and oxygen levels drop, stressing the fish.
Where do you see this in real life? On a sunny day, look at aquatic plants in a pond or fish tank. Tiny bubbles rising from the leaves are pure oxygen — the by-product of photosynthesis happening in real time. You can speed up or slow down the bubble rate by shading the plant or moving it into brighter light — directly demonstrating that light intensity controls the rate of photosynthesis.
Practice Problems
Write the word equation for photosynthesis.
Show answer
List two ways a plant uses the glucose produced by photosynthesis.