Life needs to attend to two concerns: the passing on of genetic information and the capture of energy. Both require having some control over the environment -- and that is why life forms are made up of cells. A cell is a tiny package that keeps the genetic information (the chromosomes) together and stores and transforms chemicals that provide energy. A cell regulates what comes into and goes out of itself by regulating what passes through the cell wall: it controls its interior environment. How did cells come into being? We do not know. We know that there are very simple cells (prokaryotes), cells in which the genetic material floats in the other cell contents. More complex cells (eukaroytes), cells in which the nucleus is inside its own membrane in the cell, developed from these simpler forms.

For the purposes of this course we will divide life forms into three groups: 

autotrophs, cells that can make their own food from materials in the environment,  

heterotrophs, cells that consume organic matter that has been assembled by other cells and,

detritovores, special kinds of heterotrophs that eat waste material, dead plant parts and animals, old wood and roots, and so on.

Autotrophs:   Some single celled organisms have chloroplasts in their cytoplasm (the fluid cell contents) and are able to synthesize the chemicals that they need to live and grow by using an outside energy source, sunlight. The earliest photosynthesizing cells were bacteria, and they used a type of chlorophyll to capture solar energy

Heterotrophs:   Even on the single cell level, some cells engulf and eat other cells.  They also capture organic molecules that may be floating in the water around them.  They extract energy from captured chemicals.

Decomposers   A third group is the detritovores, who extract energy as they break down dead organic materials. Single celled detritovores break down the chemicals in dead organic matter so that the chemicals can be recycled through the food chain.  Early in our planet's history they would absorb organic molecules: today they also digest fallen leaves, rotten wood, and animal bodies.  Many are still single celled bacteria, but decomposers also include fungi. Often we don't notice the decomposers, but they perform very important work.

Looking Ahead

     These methods of obtaining food have serious consequences for life strategies.  We shall see this as our life forms evolve. Organisms with chlorophyll need to find water and light, but do not produce enough energy to be able to move around very much. On earth they do their moving by slowly  growing in the direction that they want to go, or, in the reproductive stage, by producing spores or seeds that are blown or carried to new locations. Heterotrophs, on the other hand, take in a concentrated form of energy, which enables them to move around and find more organic matter to eat.  The relationships between autotrophs and the predator / prey relationships between the heterotrophs is described as a food chain.  Decomposers are often not mentioned in a food chain, but without their work the community would not cycle nutrients successfully.

     An idea that will guide us through all our environments is the idea of an ecological community. Just as a body works with the cooperation of all the different organs, so living creatures live interactively with other organisms. For example, the rabbit eats the grass, and the coyote eats the rabbit. If there are too few coyotes, the rabbits will eat almost all the grass and many will starve; if there are too many coyotes they will catch too many rabbits, and then some of the coyotes will go hungry. But the picture is more complex than this. The coyote eats frogs, too, and prairie dogs (useful if the rabbits get a plague and many die). The tunnels of the prairie dogs help to aerate the soil, which could not maintain fertility without the presence of earthworms, nematodes, and hosts of tiny invisible creatures that turn dead animal and vegetable matter into food for the grass. The rabbits eat wild flowers, too, may of which are pollinated by insects. In an ecological community a number of different animals and plants live interactively together. This makes for a flexible community that can deal with adverse weather conditions or environmental changes.

     Even though you are working on unicellular organisms today, your group should start thinking immediately about interactions between life forms -- what is there to eat on your planet, and who will eat it? On earth there is a flow of (usually) solar energy through a community: energy is first captured by plants and then turned into flesh by grazers. We will learn more about ecological communities as this course unfolds.