A cell is the basic unit of life, as we understand it. Whilst the overall workings of all cells are very similar, there is no such thing as the conveniently termed ‘typical cell’ but cells within the two main groups of organisms, the prokaryotes (mainly bacteria) and the eukaryotes (higher animals and plants), have many chemical and physical features in common.The prokaryotic cell
Cells with genetic material and cell chemicals all enclosed within a cell wall, and having no defined organelles or nucleus, are called prokaryotes. Organisms in this group are small in size and are mainly bacteria.
Ribosomes in these small cells are also small and ‘free floating’.
DNA in prokaryotic cells is in the form of a circular strand and not as a chromosome. Sometimes small rings of DNA called plasmids are present but none of the DNA is supported on histone protein.
Extracellular matrix material is not associated with these cells.
Prokaryotic cells have a fairly rigid cell wall but this is not made of cellulose as it is in plants.
The eukaryotic Cell
This type of cell is found in all higher animal and plant cells and contains membrane bound organelles and a well defined nucleus. The cell contents contained within the outermost membrane in this type of cell are divided into two main parts, the nucleus and cytoplasm. The nucleus contains the genetic material and the operating instructions. The cytoplasm contains the ‘machinery’ to carry out the instructions to communicate and make products. This is carried out by organelles.
About 20 organelles are known but they are not divided evenly across, for example, the 200 specialist cells found in humans. For details of cell organelles, click on ‘cells unpacked’.
Specialist cells nearly always depend on the exaggeration of a property common to all cells to create their own specialist role.
Cells in eukaryotes are generally larger than those found in prokaryotes and their ribosomes, many of which are membrane bound on the endoplasmic reticulum, are also larger.
DNA in the nucleus is in linear form, is supported on histone protein and is in the form of chromosomes.
Extracellular matrix covers the plasma membrane in most eukaryotic cells to some degree. In plants this takes the form of a cell wall made mainly of cellulose and which can be hard and rigid or soft and flexible. Animal cells do not have a cell wall. The boundary of the cell is the plasma membrane. Extracellular matrix and cell adhesion molecules are often attached to the outer side of this.
Think of cells of higher animals and plants (with a few differences) as membranous sacs containing a dynamic set of liquids, a multitude of membranes, a forest of filaments and an inventory of instructions.
Think of a cell as being like a human being, having a large measure of independent control but needing to link socially with the local community of cells to work, communicate and exchange things with them.