In common with all other living plants and creatures, human beings have a basic cellular sub-structure.
The cell is the smallest living unit capable of growth, movement and reproduction.
In most animals cells that are specialised in function group together to form tissues, and in more complex creatures such as man these tissues become further aggregated to form organs.
Organs display their specialised function in an organised and collective way.
Whatever their function cells are constantly being recreated from other cells by a complex process of division known as mitosis.
Cell survival times in the human body vary from a few minutes for certain intestinal cells to about 4 months for red blood cells and many years for some nerve cells. Within humans the normal length of time for the mitotic process to complete itself is about 15 - 20 minutes.
However it is important to remember that within the human body there are many different variations in cell form and function.
A few examples are shown here.
Lymphocyte
(a type of white
blood cell)
Red
blood cells
Smooth Muscle Cells
Despite the huge variation in human
cells they all have
three common features :
| 1. all cells have an outer membrane which separates it from it's environment and from other cells. |
| 2. within the outer membrane there is a solution of proteins, electrolytes and carbohydrates known as the cytosol. This is divided up into specialist compartments known as organelles by inner membrane systems. |
| 3. the shape and fluidity is partly determined by internal protein filaments which form the cytoskeleton. (literally cell skeleton) |
The external membrane covers a cytoplasm riddled with interconnecting channels called the endoplasmic reticulum which are studded with ribosomes.
At one point on the cell's surface a pinocytic vacuole has blended with the cell membrane to discharge it's contents to the exterior.
Scattered throughout the cytoplasm are small structures known as organelles. These include mitochondria, Golgi apparatus, lysosomes, centrioles, vesicles etc.
Mitochondria are sometimes called the "powerhouse of the cell" and are more abundant in cells which have a high metabolic rate. They have many internal folds rich in enzyme systems vital to energy production.
The Golgi apparati act as "marshalling yards" for materials synthesised elsewhere in the cell.
Lysosomes contain enzymes which break down large molecules (macromolecules) by a process called hydrolysis.
Vesicles are temporary packages of material undergoing transport around the cell.
The nucleus is covered in a porous membrane and contains D-nucleic acids (DNA) and chromatin material. The nucleolus contains R-nucleic acids (RNA) which acts as a messenger to the DNA of the nucleus about cellular activity, repair and synthesis.
The rod like centrioles are involved in the process of mitosis.
Page last updated Fri Aug 13 17:15:14 BST 2004