Monocular compound microscopes can be used in studying energy transformations that take place inside a living cell.

Energy, defined as the capacity to do work, may exist in active (or kinetic) form or in stored (or potential) form. For work actually to be done, the energy must become kinetic, which is to say that there must be motion of some sort. For example, a car parked on a steep hill has potential energy by virtue of its position; when its brake is released, the car rolls down the hill, converting its potential energy into kinetic energy of motion. Similarly, a lump of coal contains potential energy, which is released as kinetic energy when the coal burns.

Energy can occur in many different forms. Light is energy, and so is electricity; there is heat energy, mechanical energy, chemical energy and many more. All these involve motion-motion of photons, as in light; motion of electrons, as in electricity; or motion of atoms and molecules, as in heat. All forms of energy are at least partly interconvertible. Accord¬ing to the First Law of Thermodynamics also called the Law of Con¬servation of Energy, when energy is converted from one form into another, no energy is either gained or destroyed (for this law to be strictly valid, matter must be considered a form of energy).

Living cells, when studied using monocular compound microscopes, draw primarily on chemical energy; they do work by utilizing the energy stored in complex organic molecules. Every such molecule represents an amount of potential energy equal to the amount of energy necessary to synthesize it originally. Living cells, then, are transducers that turn chemical energy into other forms of energy, or do the reverse, or move energy from one chemical com¬pound to another.

A living organism is a storehouse of potential chemical energy, when studied using monocular compound microscopes, which can be used when necessary to do work. But as this stored energy is converted into other forms less and less of the stored energy remains in reserve.. A source of usable energy outside the organism must be available to replenish its supply. For many organisms, that outside energy source is other organisms; one living thing obtains new supplies of energy rich molecules by eating other living things. Since all these energy conversions, according to the First Law of Thermodynamics, are ac¬complished without reduction of the total amount of energy, ft might seem at first glance that the same energy could be passed continuously from organism to organism and that no source of energy outside the system composed of all living things would be required. But a little further thought shows that this is not true, because energy is con¬stantly passed from organisms to nonliving matter, as when you throw a rock or move a pencil or when heat from your body warms the air; such energy is lost to the life system. Furthermore, the molecules of substances that leave the body retain some energy, and this energy, too, may be lost. But there is another basic reason why energy is con¬stantly being lost from the living system. The Second Law of Thermodynamics says that every energy transformation results in a reduction in the usable, or free, energy of the system; or, to pit it another way, there is a steady decrease in the amount of energy avail¬able to do work.

Photosynthesis

The ultimate energy source for most living things is sunlight, and the organisms that transform the light energy into chemical energy are primarily the green plants. The process by which this transformation is carried out is, of course, called photo¬synthesis. There is still much about the process that is not known even with the advent of the latest microscopes, but in the last decades enormous strides have been made in analyzing the chemical pathways involved. Photosyn¬thesis is so fundamental to life that at least understanding the broad outlines of the process and some of the principles of energy transformation in living systems exemplified by it is needed.