Plants Make Their Own Food
Unlike animals, plants do not need to eat other organisms to get energy. Instead, they are able to make their own food using sunlight. This remarkable process is called photosynthesis, from the Greek words for "light" and "putting together."
Photosynthesis takes place mainly in the leaves of a plant. Leaves are flat and wide to catch as much sunlight as possible. Inside leaf cells are tiny green structures called chloroplasts, and these are where photosynthesis actually happens. Chloroplasts contain a green pigment called chlorophyll, which absorbs light energy from the sun.
The Photosynthesis Equation
To carry out photosynthesis, a plant needs three things: sunlight, water, and carbon dioxide. Water is absorbed from the soil through the plant's roots and travels up the stem to the leaves. Carbon dioxide enters the leaf through tiny pores called stomata on the underside of the leaf.
Using the energy from sunlight, the plant converts water and carbon dioxide into glucose (a sugar the plant uses for energy and growth) and oxygen (which is released into the air as a by-product). The summary equation is:
Carbon dioxide + Water → Glucose + Oxygen (in the presence of sunlight and chlorophyll)
The oxygen we breathe is a by-product of photosynthesis. Forests, grasslands, and ocean algae are constantly releasing oxygen into the atmosphere, making Earth's air breathable for animals and humans.
What Affects Photosynthesis?
Three main factors determine how fast photosynthesis occurs — these are called limiting factors. The rate of photosynthesis increases when there is more light, a higher concentration of carbon dioxide, or a warmer temperature (up to a point — too much heat damages the enzymes involved).
Gardeners and farmers use this knowledge to grow healthier crops. Greenhouses are kept warm and sometimes have extra carbon dioxide pumped in to help plants grow faster. You might have noticed that plants placed near a window grow better than those kept in dark corners — this is because they get more light for photosynthesis.
To remember what photosynthesis needs and produces, try this: "CO₂ and H₂O go IN, O₂ and glucose come OUT." Light and chlorophyll make it happen.
Photosynthesis and the Food Chain
Photosynthesis is the foundation of almost every food chain on Earth. Plants and other photosynthetic organisms are called producers because they produce their own food from non-living materials. Animals that eat plants are called consumers. Even meat-eating animals ultimately depend on plants, because the animals they eat also ate plants at some point.
Without photosynthesis, there would be no food chains, no oxygen in the air, and no life as we know it. It is one of the most important chemical processes on the planet.
Students often mix up photosynthesis and respiration. Remember: photosynthesis stores energy in glucose (plants do this), while respiration releases energy from glucose (all living cells do this, including plant cells).
Real world: greenhouse crop farming
Commercial tomato growers keep greenhouses warm (25°C) and pump in extra CO₂ (up to three times atmospheric concentration). Both changes increase the rate of photosynthesis, producing faster plant growth and higher yields. Farmers directly manipulate limiting factors to maximise their crop — an applied understanding of photosynthesis.
Real world: deforestation and CO₂ levels
Tropical rainforests absorb approximately 2.5 billion tonnes of CO₂ per year through photosynthesis. When forests are cleared, this "carbon sink" is lost and the stored carbon is released as CO₂. Understanding the role of photosynthesis in the global carbon cycle is central to debates about deforestation and climate change policy.
Real world: algae biofuels
Scientists are developing algae farms as a renewable fuel source. Algae photosynthesise rapidly, converting CO₂ and sunlight into oils that can be refined into biofuels. Per hectare, algae can produce 30 times more fuel than land-based crops. The commercial application relies entirely on the photosynthesis process.
Practice Problems
What three things does a plant need to carry out photosynthesis?
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Where exactly in the leaf does photosynthesis take place?
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Why are plants called 'producers' in a food chain?