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## Royal Holloway, University of London

Skip to 0 minutes and 8 seconds Earlier in this activity, you learned how much energy we use in everyday life. Now I want to pose a simple question. Why do we need this energy? The answer is that we ultimately need it to do work. That may be physical work, which we do by contracting our muscles. Or it may be chemical work. The first sort of work is easier to understand. So we will start with that. Physical work, in which a force is applied to move an object, is central to our understanding and measurement of energy.

Skip to 0 minutes and 46 seconds Indeed, it is built into the very definition of a joule, which is the energy done by a force of 1 Newton as it moves its object through a distance of 1 metre. This is a nice simple system that we can keep in mind when things get complicated. At a fundamental level, energy and work are inter-convertible. So when we hear “energy,” we can think of the capacity to lift a weight. I want to introduce an important term here. When I raise the weight by doing work on it, I am giving it a kind of energy called potential energy. This is energy that an object has by virtue of its position or state.

Skip to 1 minute and 31 seconds In this case, the object has gained potential energy, because I’ve moved it further away from the Earth and it’s subject to a gravitational force that tends to pull it downwards. When I release the object, the potential energy is converted into kinetic or movement energy. As we have seen, though, most of the energy we use during a typical day isn’t used to lift weights or to move us around. It’s used to keep our metabolism ticking over, to perform the chemical housekeeping duties needed to keep us alive. It isn’t immediately obvious how this kind of chemical work is related to physical work. A good way to see the connection is with a fuel cell.

Skip to 2 minutes and 20 seconds This hydrogen-powered model car is using the potential energy stored in hydrogen and oxygen gas to drive an electric motor. This does work on the wheels and gives kinetic energy to the car. The way this car is powered is not very different from the way that humans and other animals are. We take fuel molecules– hydrogen gas for the car– carbohydrates, fats, and proteins for you and me– and react them with oxygen gas to release energy. The chemical potential energy that is released when food and oxygen molecules combine isn’t the same as gravitational potential energy, though. As we will see, chemical energy is associated with the positions of electrons in molecular electrostatic fields, not gravitational fields.

Skip to 3 minutes and 14 seconds We use chemical energy to do work with our bodies and to do metabolic work inside our bodies. In the next activity, we will take a step back and look at how energy is stored and transformed in the simplest systems so that we can properly understand what happens in biology.

# Energy and work in living things

We need energy to do work, and we get that energy from chemicals. In the same way that a hydrogen-powered car uses a fuel cell to get energy from hydrogen, we use respiration to get energy from food.