Photosynthesis

Plants are like tiny kitchens powered by sunlight. In a process called photosynthesis, a plant takes in light from the sun, carbon dioxide from the air, and water from the soil. Inside its leaves, the plant uses that light energy to turn the carbon dioxide and water into glucose, which is a kind of sugar the plant uses as food to grow. Oxygen is made too, and the plant lets it go back into the air for us to breathe. This lesson follows that energy on its journey. A cow eats grass and takes in the energy the grass stored. Then the energy reaches you when you drink milk or eat food from the farm. So the next time you eat, remember that the energy in your meal started as sunlight captured by a plant. Photosynthesis is the first step in feeding almost every living thing on Earth.

Many living things make their own food through photosynthesis. Plants do it, and so do algae and some tiny one-celled organisms. Photosynthesis runs on energy from sunlight. The plant uses this energy to rearrange the atoms in carbon dioxide and water, turning them into sugars and oxygen. Carbon dioxide and water are the inputs, the raw materials the plant takes in from its surroundings. Carbon dioxide comes from the air, and water is soaked up from the soil by the roots. Sugars and oxygen are the outputs, the things photosynthesis makes. The oxygen floats out into the air, and the sugars feed the plant right away or get stored for later, so the plant can use them to grow.

A tiny soybean seed grows into a big, heavy plant. Where does all that extra stuff come from? Many people guess it comes from the soil, but that is mostly wrong. In this lesson, students grow soybean seeds in clear CD cases for two weeks and measure how the mass of the bean and the water changes. The big surprise is that most of a plant's new mass comes from carbon dioxide, a gas in the air. During photosynthesis, the plant pulls carbon dioxide in through its leaves and joins it with water to build sugars. Those sugars become the building blocks for new leaves, stems, and roots.

Energy enters an ecosystem when sunlight is changed into chemical energy. This change happens during photosynthesis, and it is done by photosynthetic organisms like plants and algae. Photosynthesis also moves matter around. Photosynthetic organisms take in carbon dioxide from the air and water from the soil, then use them to build sugars. At the same time, they release oxygen back into the air. Other living things all around the ecosystem depend on this. Animals eat the plants to get the stored energy, and they breathe in the oxygen the plants gave off.