Photosynthesis for Middle School Science

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Photosynthesis is the process that plants and some other organisms use to make their own food using sunlight. Instead of eating like animals do, plants take in carbon dioxide (CO₂) from the air and water (H₂O) from the soil. Then, using energy from the sun, they turn these ingredients into glucose, a type of sugar they use for energy and growth. As a bonus, they also release oxygen (O₂) into the air: something we breathe every day!

This process takes place in the chloroplasts, special parts of plant cells that contain chlorophyll, a green pigment that captures sunlight. The simple formula for photosynthesis is:

Sunlight + Water + Carbon Dioxide → Glucose + Oxygen

Photosynthesis happens in two main steps:

Light-dependent reactions: These happen when sunlight hits the chlorophyll. The plant uses that light energy to split water molecules into hydrogen and oxygen. The oxygen is released into the air.
Light-independent reactions (also called the Calvin Cycle): These use the hydrogen (from water) and carbon dioxide (from the air) to build glucose molecules.

The glucose can be used right away for energy or stored for later in the plant's roots, stems, leaves, or fruits.

Photosynthesis and Cellular Respiration

Photosynthesis and cellular respiration are like opposite sides of the same coin. Photosynthesis stores energy in glucose, while cellular respiration releases energy from glucose. Plants perform both processes. They make food through photosynthesis and then break it down through cellular respiration to get energy. Animals, on the other hand, can only do cellular respiration, so they depend on plants (or other animals that ate plants) for food and oxygen.

Which Organisms Photosynthesize?

Most green plants, algae, and some bacteria can perform photosynthesis. Trees, grass, flowers, and many houseplants are all photosynthetic. However, not all plants do it the same way. Some plants that grow in very dry areas, like cacti, use special versions of photosynthesis to save water. There are also parasitic plants, like dodder, that don’t photosynthesize at all. Instead, they steal nutrients from other plants.

Besides plants, algae in oceans and lakes also carry out photosynthesis, and they produce a huge amount of the Earth’s oxygen! So do certain bacteria, such as cyanobacteria, which played a major role in shaping our planet’s atmosphere.

Cyanobacteria and the Early Earth

More than 2.5 billion years ago, Earth’s atmosphere had almost no oxygen. That changed when cyanobacteria, tiny photosynthetic organisms, began to use sunlight to make food and release oxygen. These ancient bacteria lived in the oceans and formed large colonies called stromatolites, some of which can still be found today.

During the Great Oxygenation Event, cyanobacteria began releasing large amounts of oxygen into Earth’s atmosphere through photosynthesis. At the time, most living organisms were simple and had evolved in an oxygen-free environment. For them, oxygen was poisonous. As oxygen levels rose, many of these ancient life forms died off in one of the earliest mass extinctions in Earth's history. But this deadly change for some life created a huge opportunity for others. Oxygen allowed new, more complex organisms to evolve because it allowed for more efficient ways to release energy from food. This led to the rise of complex cells and eventually animals and humans. In the long run, the oxygen created by cyanobacteria completely transformed Earth and made life as we know it possible.

Photosynthesis isn’t just something that happens in leaves—it’s one of the most important processes on Earth. It feeds ecosystems, provides the oxygen we breathe, and even shaped the history of our planet. Every time you see a plant soaking up the sun, remember that it’s busy making food and helping keep Earth alive.