What is Photosynthesis?
The process by which plants, algae, and some bacteria convert light energy into chemical energy, storing it as glucose using carbon dioxide and water.
Photosynthesis is one of the most important chemical processes on Earth. Without it, almost no life as we know it could exist. Every time you eat a piece of fruit, a vegetable, or even meat, you are ultimately depending on energy that was originally captured from sunlight by a plant.
Think of photosynthesis as a plant's way of making its own food. Unlike animals, plants do not need to hunt or find food — they manufacture it themselves using raw materials from their environment.
The three essential ingredients for photosynthesis are:
- Carbon dioxide (CO₂) — absorbed from the air through tiny pores in leaves called stomata
- Water (H₂O) — absorbed from the soil through the roots and transported up the stem
- Light energy — captured from sunlight (or artificial light) by a green pigment inside leaf cells
Think of a plant like a solar-powered kitchen. The sunlight is the electricity supply, carbon dioxide and water are the ingredients, and glucose is the meal being cooked. Chlorophyll acts like the solar panel, collecting the energy needed to power the whole process.
MYP Key Concept — Transformation: Photosynthesis is a perfect example of transformation — light energy from the sun is transformed into chemical energy stored in glucose. As you work through this topic, look for how matter and energy are constantly being transformed in living systems. The related concept of systems also applies: a leaf is a beautifully organised system where each part works together to make photosynthesis happen as efficiently as possible.
The Word Equation and Symbol Equation
Scientists summarise photosynthesis using chemical equations. You need to know both the word equation and the balanced symbol equation.
Word equation:
Carbon dioxide + Water → Glucose + Oxygen
Symbol equation:
Let's break this down:
- 6CO₂ — six molecules of carbon dioxide (from the air)
- 6H₂O — six molecules of water (from the soil)
- C₆H₁₂O₆ — one molecule of glucose (the food the plant makes)
- 6O₂ — six molecules of oxygen (released as a by-product)
The light energy arrow shows that light is required for the reaction to happen — it is an energy input that drives the chemical change. Photosynthesis requires a continuous input of light energy to proceed, which is why it only occurs when light is available.
A very common mistake is to write the equation backwards, or to say that plants breathe in oxygen. Plants actually release oxygen as a by-product of photosynthesis. During the day, the rate of photosynthesis is much greater than the rate of respiration, so the net effect is oxygen being released.
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