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|Chapter One: An Introduction to Cells|
There are many different types of unicellular organisms, all of which fall into two general categories: prokaryotic organisms (also called bacteria) and eukaryotic organisms. In general, eukaryotic cells are more complex than prokaryotic organisms, because prokaryotes don't have membrane-bound organelles. Prokaryotes and eukaryotes also differ in the chemical composition of some of their structures as well as in the organization of their DNA. Below is a chart which highlights the important differences between the two types of cells. Although you probably aren't yet familiar with the terms on the chart, you may find it helpful to return back to this page as you learn new information in Chapter Three.
|nucleus||no membranes||surrounded by two membranes|
|chromosomes||circular without histones||linear with histones|
In addition to this classification, organisms may also be classified be their method of nutrition. Heterotrophs are organisms which obtain food from the environment, whereas autotrophs have a method of producing their own food. Also, aerobes are organisms which require oxygen to survive, whereas anaerobes do not need oxygen.
The many different types of unicellular organisms will be discussed fully in Chapter Seven, which occurs after the chapters which describe the differences mentioned in the charts above. This way, you will first have an understanding of the what the terms above mean before you learn which cells have what.