All forms of life are dependant on cells. The cells, whether from unicellular or multicellular life forms provide structure and energy to these life forms. The cell also provides a means by which the organism is capable of reproduction.

Cells vary in size depending on the organism, and the type of cell involved. Since the cells can vary between 1 – 100 micrometers (a micrometer is one millionth of a meter), then without the invention of the microscope the cells would have been impossible to see.

Today, using such modern instruments as Scanning, or Transmission types of electron microscope, scientists are able to observe even the smallest of cell structures.

Introduction to Cell Biology.

There are two types of cellular life. Prokaryotes and Eukaryotes often referred to as unicellular or muticellular organisms.

-          Prokaryotes. These are simple organisms which do not have a cell nucleus. The DNA is coiled up and contained within the nucleoid region of the cytoplasm within the cell. Such organisms include bacteria and archeans. All prokaryotes are able to live in extreme environments, ranging from underwater hot thermal vents to being buried deep in icy locations.

-          Eukaryotes.  Include such life forms as animals, plants. All of which posses cells which contain a cell nucleus where the DNA is located.

Within the prokaryote organisms are many species of bacteria, and it is now estimated that 95% of all human body cells are in fact bacteria primarily living in the digestive tract.

Eukaryote cells also contain other components within the cell walls, each having a specific task in the functioning of the cell. These small structures are called organelles –

-          Nucleus.  The location of the DNA.

-          Mitochondria. Provide the enzymes necessary for converting food to energy.

-          Endoplasmic reticulum. Responsible for the transportation of material out of, and into the cell.

-          Golgi apparatus responsible for the transport of proteins.

-          Ribosomes. The site in the cell of the transcription process ‘DNA makes RNA makes protein’.

Within the nucleus of the eukaryote cell there is the DNA which carries the genetic code. Stretches of DNA known as genes are carried in chromosomes. In the human body there are 46 such chromosomes, 23 having been passed on by the mother and 23 passed on by the father. The X and Y chromosomes determine the gender of the new offspring.

Nearly all prokaryotes reproduce by the cell dividing into two, so each offspring is identical to the parent. Eukaryote cells undergo a similar type of cell division in a process called mitosis, but reproduce sexually by a process in which the sex cells from each parent are fused together.

Cells within the human body have different life spans depending upon type and cell function. Cells within the pancreatic system often last for years, cells in the immune system usually last for about 6 weeks, whilst the cells of the digestive tract last only for a few days.

If a cell becomes infected it will usually self destruct in a system called apoptosis. This process controls the degree of natural cell division taking place (mitosis). If the apoptosis process fails then the cells will continue to divide in an uncontrolled manner resulting in the development of cancer.

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