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Types of Cells

The kind of cells that Mendel saw down his microscope belonged to a class we now call eukaryote meaning "true nucleus". These were amongst the first cells seen as researchers such as Robert Hooke and Anton van Leeuwenhoek turned their increasing degrees of magnification onto the substructure of the living world. But, from the beginning, smaller, simpler types of cells were also seen floating around in the experimental broths. These cells were more primitive and had no obvious sub cellular components, so they were called prokayote meaning "before nucleus".

It is now recognized that these two types of cells constitute the first and most fundamental division of life on our planet. The Kingdom monera holds over 3000 individual species of single-celled, primitive organisms (many of them bacteria), which, while simple in physical structure, can never the less show an amazing diversity of chemical metabolism and life style. Some members of this Kingdom inhabit the boiling hot springs of Yellowstone park, or live inside rocks thousands of meters below the surface of the earth. They can live in concentrated acids or inside you, where they are responsible for many, many diseases.

Prokaryotic cells are small and have a membrane that separates them from the outside environment. It is through this membrane that they exchange materials and communicate. Protecting this delicate surface is a cell wall (not all prokaryotes have a wall, but most do) which may be rigid or flexible, and which gives shape and support to the cell. Inside the wall and membrane the cytoplasm is not usually divided up into compartments, however, some of the more complex prokaryotes have a surprisingly complex internal structure.

The genetic material of prokaryotes is DNA which can often be seen as a dark staining nucleoid region, often bonded to the membrane. This DNA is not separated from the protein synthesizing machinery by a membrane, and the modes and methods of transferring information from this DNA to the assembly systems is much more direct than in the other major type of cells.

All the remaining cells on earth, which comprise all the other major Kingdoms of organisms, are of the second type; eukaryotic. These cells form everything from single celled protists (like amoebae), fungi (such as mushrooms) to giant redwood trees and whales. They are much more limited than prokaryotes in the range of environmental conditions they can tolerate (no eukaryote can survive temperatures much above human body temperature), but they compensate for those limitations by the ability to form multicellular communities in which different cells can specialize in an astonishing variety of ways.

Although specialized eukaryotic cells show a huge range of internal structure, the generalized picture of a eukaryotic cell consists of a membrane bound globule of cytoplasm in which can be found a series of membrane enclose vesicles and organelles. Each of these organelles is a specialist at some task. Nuclei store and control the actions of the DNA genetic material: mitochondria take high energy food molecules and safely release that energy in usable form: the endoplasmic reticulum is the site of protein synthesis and modification. A eukaryotic cell, therefore is a collection of integrated components that together produce a product that is more than the sum of its parts.




Science@a Distance
© 2002, Professor John Blamire