Cell

I

Introduction

Cell, the smallest unit of an organism that can function independently. All living organisms are made of cells and nothing less than a cell can truly be said to be alive. Some microscopic organisms, such as bacteria and protozoa, are single cells whereas animals and plants are composed of many millions of cells built into tissues and organs. Although viruses and cell-free extracts are able to perform many individual functions of a living cell, they lack the capacity shown by cells of independent survival, growth, and replication, and are therefore not considered to be living. Biologists study cells to learn how they are made from molecules and how individual cells cooperate to make an organism as complex as a human being. Before we can fully understand how a healthy human body functions, how it develops and ages, and what goes wrong with it in disease, we need to understand the cells of which it is made.

II

Historical Origins of Cell Biology

The idea that cells are the fundamental building blocks of living organisms—sometimes termed “the cell theory”—is now universally accepted as true. However, this concept did not spring into existence fully formed, as the result of a single discovery. It took many years for the present view of cells to emerge, and there were many false turns and misapprehensions on the way. The word “cell” comes from the Latin cellula, meaning a small room or cubicle, and was first used by Robert Hooke in his book Micrographia, published in 1665. Hooke was describing the air-filled spaces of dead cells in a slice of cork (bark from an oak tree) and certainly did not realize the general importance of his discovery. Nor did many other talented microscopists of the 17th and 18th centuries, such as Antoni van Leeuwenhoek, Nehemiah Grew, Marcello Malpighi, and Jan Swammerdam, who also saw cells in plant or animal tissues, or as free-living organisms. Indeed it was not until 1839 that the combined insight of a botanist, Matthias Schleiden, and a zoologist, Theodor Schwann, led them to pronounce that “…all organisms are composed of essentially like parts, namely of cells”.

The cell theory was still far from complete and many curious notions remained, for example, about the origins of cells. It required the work of many other biologists, such as Bartholemy Dumortier and Robert Remak, to establish the fact that all cells are produced as a result of the division of existing cells. This notion, which has powerful implications for both cell biology and the origins of life, was famously articulated by German biologist Rudolph Virchow in the phrase “Omnis cellula e cellula”, that is, “all cells come from cells”.

Even then, many misconceptions still existed regarding the nature of the cell membrane, the cytoplasm, and the hereditary material, and it was not until well into the 20th century that the contemporary view of cells emerged. Indeed, even today, the amazingly complex internal structure and chemistry of living cells is still not fully understood and contains many secrets yet to be discovered.

III

Looking at Cells Under the Microscope

The invention of the microscope in the 17th century made cells visible for the first time and, for hundreds of years afterwards, all that was known about cells was discovered using this simple device. Light microscopes are still crucial to the work of cell biologists, although they have improved out of all recognition from the primitive instruments used by Hooke and Leeuwenhoek. Contemporary light microscopes incorporate sophisticated state-of-the-art devices, such as laser light sources, fluorescent optics, and computer-assisted image processing, which reveal detail at the very limit of resolution (down to 0.1 microns or micrometres (µm), each µm being a millionth of a metre). For even higher magnification, electron microscopes, invented in the 1930s, extend this limit by using beams of electrons instead of beams of light as the source of illumination. They greatly extend our ability to see the fine details of cells, and even make some of the larger molecules visible, although they cannot be used with living specimens.

What do you see if you look at cells under a light microscope? If you examine a very thin slice of a suitable plant or animal tissue, for example, you will see it is divided into thousands of small cells. These may be either closely packed or separated from one another by a material known as the extracellular matrix. Each cell will be about 20 µm in diameter. Under the right conditions, the cells in your section will show signs of life, with particles moving around inside them and individual cells slowly changing shape and dividing.

To see more of the internal structure of a cell you need to use special tricks, since cells are not only small but also transparent and colourless (being about 70 per cent water). One approach is to stain the cells in your section with dyes or specific molecular probes that colour particular components. Alternatively, you can exploit the fact that cell components differ slightly from one another in refractive index (just as glass differs from water) and these small differences can be made visible by means of special lenses. In either case, the contrast and resolution of the image can be stored, enlarged, and further enhanced by electronic processing.

IV

General Features of Cells

The microscope shows us that cells exist in many different sizes and shapes. Some of the smallest bacterial cells are short cylindrical objects less than 1 µm in length. At the other extreme, nerve cells have complex shapes including many long thin extensions, and may reach lengths of several metres (those in the neck of a giraffe provide a dramatic example). Between these extremes, plant cells are typically 20-30 µm long, polygon-shaped with box-like boundaries defined by rigid cell walls. Most cells in animal tissues are compact in shape, 10-20 µm in diameter with an irregular and often richly folded surface.

Despite their many differences in appearance and function, all cells have a surrounding membrane (termed the plasma membrane) enclosing a water-rich substance called the cytoplasm. All cells carry out multiple chemical reactions that enable them to grow, produce energy, and eliminate waste, together termed metabolism (from a Greek word meaning “change”). All cells contain hereditary information, packed into a central nucleus and encoded in molecules of deoxyribonucleic acid (DNA), which directs the cell's activities and enables it to reproduce, passing on its characteristics to its offspring. These and other similarities too numerous to mention, and including many identical or nearly identical molecules, demonstrate that all modern cells are related to one another. In other words, there must have been an unbroken continuity between modern cells—and the organisms they compose—and the first primitive cells that appeared on Earth.