Parts of the body

In a healthy person, the different parts of the body work together perfectly all the time. That we can walk, for example, without thinking about it is made possible through accurate co-ordination by the nervous system. Similarly, every activity – be it the digestion of food, or the beating of the heart – can be traced ultimately to perfect co-operation between cells.

How the cells know what other cells are doing is still one of the biggest mysteries of the body. How do healthy cells tell each other to stop growing and why do cancer cells not do that? It was not until the seventeenth century that the Dutchman Van Leeuwenhoek, with the help of his primitive microscope, discovered that the body was made up of cells. Scientists have since discovered that cells are made up of even smaller components known as organelles, which together ensure that the cell functions properly. Using an electron miscroscope, scientists can even examine the genetic information carried in a cell’s DNA.

That the body functions as a whole despite having millions of cells is because all of them have specialized activities. For example, the liver cells produce bile, the skin cells produce hair and the cells of the muscles are responsible for power and movement. This means that organs that are composed of a great many different cells – for example the intestines • can carry out very complicated tasks, such as the digestion of food.

This specialization, with each part of the body having its own function, allows for maximum efficiency. However, it also has disadvantages: it means that every organ is indispensable and if one organ is defective its work cannot be taken over by other parts of the body.

Cells

In the mid-seventeenth century a Dutch draper, Antonie van Leeuwenhoek (1632-1723), spent much of his spare time studying animal tissues with a primitive home-made microscope. He observed many types of living matter and saw they were composed of tiny units of various shapes and sizes which he called cells. Building on van Leeuwenhoek’s work the German anatomist Theodor Schwann (1810-1882) proposed, in 1838, the theory that all animals – indeed all living systems – were composed of cells and cell products. The development of microscopy, and particularly the powerful electron microscopes, has greatly advanced the study of the cell in recent years and confirmed its position as the fundamental unit of life. There are two main types of cell: procaryotic and eucaryotic. Procaryotic cells have no discernible internal structure and are found in simple organisms such as bacteria and algae. Eucaryotic cells, on the other hand, are much more complex and in their various forms are the building blocks of all multicellular plants and animals.

Inside the cell

In a eucaryotic cell are various structures known as organelles, just as larger organisms are made up of collections of cells called organs. Various differences exist between plant and animal cells but both contain a vital organelle, the nucleus. This controls all aspects of the cell’s life and is central to the process of cell reproduction.

Within the nucleus are long molecules of DNA (deoxyribonucleic acid), twisted into spirals that are visible under the microscope as chromosomes. Coded into the DNA are instructions – genes – that determine the size, shape and function of the cell or, in a multicellular being, of different organs. The nucleus is enclosed by a membrane which has small pores in it, permitting communication between the DNA inside the nucleus and the other parts of the cell. Outside the nucleus, and inside the cell’s ‘skin’, the cell membrane, is a jelly-like semi-transparent material called cytoplasm. In the cytoplasm float various other organelles, each with its specific job. The sausage-shaped mitochondria, for example, are the main power-producers, taking energy-rich glucose and breaking it down to release the energy which is then used to drive cellular chemical processes. The early microscopists described the cytoplasm as an homogenous, randomly-flowing ‘ground substance’ of no detailed structure. Electron microscopy has revealed the cell as a highly-organized system of membranes, vacuoles, sacs and compartments, containing various organelles, every bit as complex in its substructure as a multicellular organism.

Animal and plant cells

Animal cells lack the rigid walls of plant cells, which are thickened with cellulose, but are more densely-packed with organelles. However, plant cells contain organelles named chloroplasts on which all animal life ultimately depends. Chloroplasts contain the green pigment chlorophyll which traps the energy in sunlight and converts it to chemical energy. We all survive, one way or another, by eating plant material or eating animals that have eaten plants.

Cell reproduction

To reproduce, a cell splits into two new cells, both of which contain all the genetic characteristics of the original cell. This genetic information lies in the nucleus of the cell, in the form of chromosomes. All human body cells (except red blood cells which have no nucleus) have 23 pairs of chromosomes, which are in fact long strands of DNA molecules. This DNA material directs all the functions of the cell. A genetic characteristic like hair colour, for instance, is determined by the composition of DNA at a particular point on a pair of chromosomes. Cell division – called mitosis – starts with the duplication of DNA material. (It is only after this duplication that chromosomes can be seen under the microscope; otherwise the DNA molecules are dissolved in the nucleus.) Instead of 46 chromosomes the nucleus of a human cell now contains 92 in the form of two identical sets of chromosome pairs. Each set then moves to opposite poles of the nucleus, and the cell divides. A different kind of cell division called meiosis produces the male and female germ cells, the sperm and ovum. The nucleus of each cell divides twice but the pairs of chromosomes only once, with the result that each cell contains half the normal number of chromosomes. When fertilization takes place, both sets of chromosomes of the egg and the sperm combine to firm the first cell, which now has a full complement of 46 chromosomes, of the new baby. In this way, a genetically unique individual is created.