ABSTRACT
All living creatures are made of cells. Cells are small membrane-bounded compartments filled with a concentrated aqueous solution of chemicals. Functionally, cells can be defined as objects that possess at least two major components of machinery for life: information transfer, which stores, distributes, reads out, and reproduces the information that controls the processes of life, and energy transduction, which changes energy from one form to another, stores it, and distributes it to execute the processes of life. All cells can be classified as belonging to one of two classes. Prokaryotic cells are cells that lack a well-defined, membrane-enclosed nucleus (prokaryote, from the Greek for ‘before nucleus’); thus, the DNA material is not enclosed within a nucleus. Examples of prokaryotic cells include bacteria and blue-green algae (cyanobacteria). Today prokaryotic cells probably resemble the primordial cells from which all life on Earth was derived some 3 billion years ago. About 1.5 billion years ago, a second class of cells known as eukaryotic cells emerged that had a more complex internal structure than prokaryotes. Eukaryotic cells contain a nucleus (eu-karyote, from the Greek ‘good or true nucleus’) and cell organelles, which are semi-autonomous structures of defined function within the cell. Eukaryotic cells are as much as 10-20 times larger in diameter and thousands of times greater in mass than most prokaryotic cells. Organisms, which are made out of eukaryotic cells, are members of the kingdoms Protoctista, Animala, Plantae, and Fungi. To begin to understand the structure and function of cells as a whole, we now describe the various internal structures, that make up cells (Figure 12.1).
