Cells and energy

Any living cell has a constant flow of chemicals though the cell wall. These chemicals are needed as raw materials to keep the `factories' in the cell working and the waste materials must be removed. For the cell to keep all these processes working it requires a constant flow of energy. The flow of energy and how it is converted from one form to another can be understood by learning about the physical laws of energy conversion, a subject called thermodynamics.

Energy and cell processes

Energy is usually considered to be in one of two states; it can be kinetic energy (*) or potential energy (*).

Potential energy is stored energy. Examples include the energy stored in water behind a dam and the energy stored in the chemical bonds of coal. In each case this energy can be released; either by opening the dam or by burning the coal.

Kinetic energy is energy of motion. Examples include the energy of a car moving at 60 miles per hour or a falling ball. Also, the energy of water falling. Water released from a dam gains kinetic energy as it looses potential energy. We say that the energy is converted from one form to another.

There are several forms of energy that we can detect around us; there is heat, light, chemical energy and mechanical energy. There are many ways in which energy can be converted from one form into another. One example is a hydroelectric power station where the potential energy of water in a high reservoir is harnessed to produce electrical energy. In between the potential energy is converted to kinetic energy and then the turbine converts that to electrical energy. Two laws of physics help us to understand these processes.

The first of these laws is called the first law of thermodynamics and says that what ever processes or conversions take place, energy is conserved. That is, energy is neither created nor destroyed, it just changes form. For example, if we strike a ball with a bat, some of the energy is given to the ball. This process is not 100% efficient but the remaining energy doesn't disappear -- some energy goes to make the `thwack' noise, some heats the ball and bat a little and so on. None vanishes!

The second of these laws is called the second law of thermodynamics and says that systems always become more disordered. In practical terms it means that when energy changes from one form to another there is always some inefficiency which results in some energy being lost as heat. Heat is just the random motion of atoms and molecules, heat is disorder. So the second law says that in the process of converting energy between different forms, for example converting the chemical energy of gasoline into the motion of a car, some energy will be converted to heat.

Cells and disorder

We have just seen how the second law of thermodynamics tells us that the universe gets more disordered with time. Now it seems that cells are an exception to this -- they are little pockets of order.

The answer to this puzzle is that cells keep their incredible order at the expense of a constant flow of energy and the release of heat to their environment. We can see this throughout the chain of energy flow from the sun to us. To start with the sun is powered by nuclear reactions with not only release the light we see but also much larger amounts of heat. The plants convert energy from the sun (solar energy) into chemical energy but also release heat in the process. Finally, when animals such as ourselves eat plants we release the stored chemical energy to fuel our life processes and keep the order in our cells. At the same time these processes release heat which is our body heat.

The chemical processes that keep cells supplied with energy are called metabolic pathways. There are many such pathways involving a wide variety chemicals which are too complex to discuss here. However, we can note that it is only the constant supply of energy to our cells that stops them decaying to disorder.