This is the first of a 5-part series on Metabolism.
Just like you, your cells need to eat. They need fuel to burn for energy so that they can perform their jobs. You supply this energy to them in the food you eat but ultimately all energy comes from the sun.
As you may or may not remember from high school chemistry, plants can directly absorb energy from the sun’s rays through their leaves. This is why plants have leaves -they are nature’s solar panels. In a process called photosynthesis, the green pigment (chlorophyll) in plant cells uses the energy from the sunlight to make glucose (C6H12O6) and oxygen (O2) out of the carbon dioxide and water molecules.
Through the magic of chemistry, the sunlight is now trapped inside the bonds of the glucose molecule. With the addition of nitrogen molecules from the soil, the plant can make protein molecules too. Fats are made and packed inside seeds to feed the seedling when it reaches new ground.
People and animals are different, we aren’t green and we don’t have leaves. We could sit under the sun all day and the ultraviolet radiation shining on our skin would at best make lots of vitamin D and at worst give us a bad sunburn. So our cells must get their energy second hand; we steal it from those clever plants. By eating their green leaves and roots we can absorb the energy they have stored. Or we can get the energy third hand and eat animals that have eaten the green plants.
To do this our meal must first be digested, absorbed and metabolized.