The diversity of single-celled organisms is astounding. These differences are apparent in both the structure and function of the organisms. For starters, single-celled organisms range greatly in size, from less than 1 micron (one-millionth of a meter) in diameter for the smallest bacteria, to more than 100 microns for some protozoans.
The manner in which single-celled organisms acquire food varies greatly too. Some, like the amoeba, go in search of food, crawling on pseudopods (temporary bulges in the cell membrane) toward prey that they eventually engulf and digest internally. Others, including all of the different species of algae, make their own food by harnessing the sun's energy just as plants do. Structures called chloroplasts inside the algae contain the pigment chlorophyll that allows them to use the sun's energy to make carbohydrates from carbon dioxide and water.
Several types of algae are so versatile that, depending on light conditions, they change the mode by which they obtain food. When sunlight is available, Euglena can be seen gravitating toward the light, which they sense through a photoreceptor, or eyespot, at one end of the cell. If the light is too bright, the organisms will swim away toward more optimal light conditions. If no sunlight is available, however, Euglena have the ability to transition entirely to absorbing nutrients from the water. They may acquire their food this way indefinitely, assuming sufficient nutrients are available and light is not.
Single-celled organisms reproduce in some interesting and surprisingly efficient ways. While some rely on hosts or a series of host species to complete their complex life cycles, many reproduce by simply dividing in two. In fact, the biological success and remarkable diversity of single-celled organisms is due, at least in part, to their rapid rate of cell division. Growing under optimal conditions, the common bacterium Escherichia coli, for example, can double its population size in just 20 minutes. (Contrast that with humans, who take on average 20 years to produce a single offspring.) Within weeks, a population of E. coli bacteria can virtually reinvent itself. Given that the time required for one individual to pass its genes to the next generation is one of the most important factors in the evolutionary process, single-celled organisms are on the evolutionary fast track!
In recent decades, this phenomenon has pitted doctors and researchers against quickly evolving bacteria in an anxious battle over antibiotic resistance. Experts now estimate that as many as two dozen types of bacteria have evolved resistance to most, if not all, antibiotics available today. A battle that we had been winning handily for 50 years, thanks to advances in medicine, appears now to be an even match.