cell

cell The cell is the fundamental unit of all living things. The simplest forms of life are single-celled organisms; these include both ‘prokaryotes’ — bacteria, which have a simple internal structure — and the much more complex ‘eukaryotes’ (pro, before or preceding; eu, good, normal, and karyon, a kernel). Higher organisms, such as man, are sophisticated communities in which groups of eukaryotic cells carry out specialized functions and communicate with each other. Prokaryotes are usually about one thousandth of a millimetre in diameter. Eukaryotic cells are much larger, typically around one to two hundredths of a millimetre. There are about 100 million million cells in the human body. Both prokaryotes and eukaryotes usually multiply by dividing in two, although in multicellular organisms cell division is under strict control.

It was the invention of the microscope, in the seventeenth century, that allowed scientists the first glimpses of individual cells. In particular, the Dutchman, Antoni van Leeuwenhoek described the extraordinary variety of motile single-celled organisms (which he called ‘animalcules’) present in pond water. The word ‘cell’ (from the Latin cella, ‘a small room’) was first coined in 1665, by the English physicist Robert Hooke, to refer to the microscopic structure of cork. Technical improvements in microscopy in the eighteenth and nineteenth centuries allowed more precise observation. It gradually became apparent that cells had a complicated internal structure, and that some features (for example, what we now refer to as the nucleus) were common to most cells, even though the appearance of the cells themselves varied enormously. This in turn hinted that a common basic organization might underlie all living matter.

The first simple distinction had been between nucleus and cytoplasm — the rest of the cellular substance — but by the end of the nineteenth century the principal internal components of cells that we are familiar with today (sub-cellular structures or organelles) had been identified. These included the endoplasmic reticulum (an extensive network of membranes within the cell), mitochondria (cylindrical, membrane-limited structures) and the Golgi complex (a stack of flattened membrane sacs, named after the Italian anatomist who described those and other intracellular structures in 1898, and later shared a Nobel prize with Spaniard Ramón y Cajal). The true complexity of the internal structure of cells, however, only became apparent in the 1950s, when cells were examined with the newly-invented electron microscope, which had much greater resolving power than the conventional light microscope — magnifying 20–30 000 times. It was around this time that the field now known as cell biology began to come to prominence, with the goal of understanding how the various organelles acted together to allow the cell to carry out its many functions. As well as simply observing cell structure, cell biologists now began to take cells apart and purify the different organelles using high-speed centrifugation. It was also shown that the purified organelles could be made to work in isolation, which allowed a detailed study of their functions, and the identification of the mechanisms underlying them.

Our current view of the cell is as an organism-in-miniature. The blueprint is contained in the DNA, packaged into chromosomes in the nucleus. Parts of the DNA sequence are replicated into ‘messenger’ RNA, which exits the nucleus and specifies the sequences of the cell's proteins, which are constructed in the cytoplasm. The power-houses of the cell are the mitochondria, which use nutrients taken up from outside to generate ATP, the energy currency of the cell. (Plant cells have additional organelles, the chloroplasts, which contain chlorophyll, the molecule responsible for capturing the energy of sunlight and initiating the process of photosynthesis. This results in the production of carbon-containing molecules for use by the cell and the generation of oxygen, which is essential for the continuation of life on earth.)

Many cells are responsible for secreting substances which will have external effects. In the pancreas, for instance, some cells secrete enzymes into the gut, where they digest our food, whereas other cells secrete insulin into the bloodstream, which instructs cells in the rest of the body to take up glucose. Both the digestive enzymes and the insulin are packaged into the endoplasmic reticulum and are then transported to the surface of the cell via the Golgi apparatus. Thus although each organelle is a discrete structure, there is extensive communication between organelles. This intracellular trafficking system demands that there be strict controls on the movement of proteins between organelles, and that individual proteins be ‘tagged’ for delivery to particular destinations. Without this control, the organization of the cell would quickly disintegrate.

A single higher organism contains a huge variety of cell types: compare, for example, a neuron with a lymphocyte, or a skeletal muscle cell with a liver cell (hepatocyte). All of these cells were produced from a single fertilized egg, by processes including cell division, migration, differentiation, and death. We are only just beginning to understand how these processes are orchestrated to produce the complete organism. One aspect that is crucial to the development and maintenance of multicellular organisms is communication between cells. Cells are continually signalling to their neighbours through the release of molecules that are detected by specialized receptors on the surface of other cells. In the brain, for example, neurons ‘talk’ to each other by means of small molecules. These molecules, or ‘neurotransmitters’, are packaged in small sacs within the neurons, and are released when an electrical impulse passes to the end of its axon. The neurotransmitter then binds to receptors on the neighbouring neurons and changes the electrical properties of these neurons, making them more or less likely to initiate an electrical impulse themselves. In other parts of the body, neurons communicate in similar fashion with muscle cells, causing them to contract, or with glandular cells, causing them to secrete. Many drugs work by blocking or mimicking the action of these neurotransmitters. Again, some cells release molecules which travel in the blood: messengers which communicate with remotely distant cells that have the appropriate receptors on their surface.

Once tissues and organs have been formed it is essential that cell division be strictly controlled in order to maintain normal function. Many proteins are now known which control cell division, often in response to external stimuli. Mutations in these proteins can result in uncontrolled cell division. This can lead eventually to the formation of tumours, which can be life threatening.

We are now familiar with the idea that cells are produced by the division of progenitor cells. This idea, of course, begs the question as to how the first cell was produced. It has been shown that simple organic molecules can form under conditions believed to be similar to those that existed on earth in its early history. How these molecules became assembled into proteins, and more particularly how the self-replicating ‘blueprint’ molecules such as DNA came about, are fundamental unanswered questions.

Michael Edwardson

See also cell membranes; cell signalling.