Every living thing is made from at least one cell. The cell is a self-contained packet of life, surrounded by a nanoscopically thin membrane which isolates it from the rest of the its surroundings. Inside the membrane, the cell is filled with a liquid known as the cytoplasm. This is where the cell’s metabolism occurs, so it is filled with sugars, proteins and other biochemicals. Also present is some kind of genetic material in the form of DNA. The field of cell biology investigates every aspect of this tiny world, and in so doing has revealed much that all cells have in common, and much that is specific to different types of organism.
The word ‘cell’ was coined by the great English scientist Robert Hooke, who was first to discover these tiny structures. In 1665, Hooke peered at many life forms through what was then a cutting-edge microscope. When he looked at a slice of cork he found it divided into tiny compartments (opposite). He likened these to the living quarters of a monk, known then as a cell.
The outer covering of a cell is called its plasma membrane. There are similar membranes inside many cells, and they all share a structure based on the properties of chemicals called lipids. As a class of chemicals, lipids are perhaps better understood as animal fats and vegetable oils.
Each lipid molecule has a hydrophobic side that repels water, and a hydrophilic side that dissolves in water. A cell membrane is made of a double layer of these molecules – the hydrophilic parts face outwards to form the inner and outer surface of the membrane, while sandwiched between them, the hydrophobic parts mingle with each other. This creates a barrier that is surprisingly strong when constructed at the tiny dimensions of a cell. Water can move freely across the membrane, but most larger molecules must be actively transported into and out of the cell. For this purpose, the cell membrane is studded with pores and pumps made by complex protein molecules (see here).
The smallest and simplest cells belong to bacteria and their cousins the archaea. These organisms are the prokaryotes, and their cells are very different from those of other organisms, such as plants, fungi and animals, which are known as the eukaryotes.
Most prokaryotic cells are somewhere between 1µm (1 micrometre – a millionth of a metre) and 5µm long. They are mostly limited to this size range by their plasma membranes (some bacteria have two membranes around the cell), which are less fluid and flexible than in other cell types. Some cells have a long twisted tail-like extension called a flagellum, which spins like a corkscrew to propel the cell. There are smaller hair-like extensions called pilli, which cling to surfaces. Internally, the cell appears quite spartan: a tangle of DNA molecules floats freely in the cytoplasm, and the only other structures visible are ribosomes – sites where the genetic code is read and processed (see here).
The cells in your body share the same structures as those in all animals and many single-celled eukaryotes, such as amoebae. The animal cell is around 20 times the length of a prokaryotic cell (and with a considerably larger volume) – mostly thanks to a cell membrane that is strengthened by the inclusion of cholesterols. An animal cell may use a flagellum (plural: flagella) to move, and it may also be equipped with cilia – shorter extensions that are similarly mobile. These are used for locomotion as well as for drawing a nutrient-rich current over the cell.
The larger size of eukaryotic cells means they cannot rely on passive diffusion alone to distribute material in the cytoplasm. Instead, there is a network of microtubules to transport useful molecules around. The cell is also filled with an array of structures called organelles, each controlling an aspect of its metabolism. The final distinction from bacterial cells is that the DNA is held in a nucleus.
As another example of an eukaryote, a plant cell shares many features with the animal cell. There is a nucleus containing the genetic material, and a similar collection of organelles working away inside the cell membrane. One major difference is the inclusion of chloroplasts – the structures in which photosynthesis takes place.
The other big difference is that a plant cell has a cell wall that surrounds the cell membrane. While the flexible membrane is able to change volume as water flows in and out of the cell, the wall is stiff and unchanging. A plant cell wall is made from fibres of cellulose, a polymer of sugar molecules linked into a chain.
Fungal cells could be seen as a halfway house between plants and animals (although they are closer relatives of animals). The cells do not photosynthesize and so lack chloroplasts, but they do have a cell wall. Instead of cellulose, this wall is made from chitin, a polymer also found in seashells and insect bodies.
The most familiar organisms are obviously made up of many cells – often numbering in the trillions – working together. This multicellular mode of life is in contrast to the unicellular mode, where bacteria and a range of eukaryotic organisms can survive as single cells.
However, the distinction is not completely clear-cut. Many unicellular organisms form colonies – a throat infection is a colony of bacteria living in your larynx. There are many cases documented where colonies of unicellular organisms exhibit a division of labour with certain cells specializing in tasks that support the wider colony.
A multicellular body takes this further, with genetically identical cells specializing in several ways to ensure the survival of the whole. One of the simplest examples of this is the sponge, an animal that uses nine cell types to provide its bodily structure, feed, defend and reproduce.
Sponges and other simple sea life represent the first stage in animal evolution, where collections of different cells work together to make a body.
The invention of the electron microscope in the 1930s revealed that the interior of cells was more complex than anyone had previously suspected. Until then the only thing clearly visible inside eukaryotic cells was the nucleus. However, the greater resolution offered by the new technology showed this in much more detail, and also revealed several distinct structures, named organelles. The nucleus was shown to have not one but two membranes around it, both riddled with pores that allow genetic material to be hauled in and out. The store of genetic material is massed in a central region called the nucleolus.
The other organelles include the endoplasmic reticulum, which is a transport network of tubes. The Golgi apparatus prepares material for release from the cell, while the lysosome is used to collect and destroy unwanted material. Plant cells also have chloroplasts, while all eukaryotes have mitochondria, which act as the power plants of cells.
The Golgi apparatus parcels up chemicals into vesicles which then merge with the cell membrane, releasing the chemical outside the cell
The mitochondria (singular: mitochondrion) are the places in a cell where respiration occurs, releasing energy from glucose and other fuels in manageable amounts, for use in metabolism. Every eukaryotic cell has some mitochondria, and those that use a lot of energy, such as muscle cells, are packed with hundreds.
A mitochondrion has an outer membrane similar to the plasma membrane of the overall cell, with another inside that is folded in on itself. creating many narrow dead-ended channels called cristae. Respiration takes place on the membrane walls of the cristae. The reactions that occur here gradually oxidize a glucose molecule, and the energy released at each step is captured for future use by a chemical called ADP (adenosine diphosphate). An input of energy allows another phosphate to be added to this in order to create ATP (adenosine triphosphate). The ATP molecules then head out into the cell, and can release their stored energy whenever needed by reverting to ADP.
The green-coloured organelles called chloroplasts are the sites of photosynthesis inside plant cells. Those parts of a plant that do not photosynthesize, such as the roots, lack chloroplasts, as do photosynthetic prokaryotes (although the process is largely similar there, it takes place in the cytoplasm along with all the other metabolism).
A chloroplast has its own outer membrane, containing further membranous discs known as thylakoids. Chlorophyll molecules are bonded to individual thylakoids, which are stacked up into structures called grana. When light shines onto them, the chlorophylls convert some of its energy into a supply of ATP molecules (adenosine triphosphate – see here). This is the first stage of photosynthesis and is named the ‘light reaction’. The ATPs then fuel the second stage, or ‘dark reaction’. This takes place in the stroma, the space between the grana, and involves the combination of carbon dioxide and water to produce glucose sugars.
When the first single-celled organisms (initially called ‘animalcules’) were discovered by the pioneers of microscopy, biologists assumed that these organisms developed spontaneously from the decaying remains of other life forms. It was well understood that multicellular life arose through biogenesis – one life creating another – but unicellular life was thought to be abiogenic, arising from non-living material.
In 1838, however, German physiologist Theodor Schwann (with contributions from others) used the growing evidence against abiogenesis to propose what became known as ‘cell theory’. Schwann’s theory had three parts: first, all organisms are composed of one or more cells; second, the cell is the simplest form of life; and finally, all new cells arise from older ones. These three simple rules became the foundations of modern biology and were crucial in figuring out the genetic process.
Theodor Schwann’s inspiration for cell theory began when he noticed the shared similarities between nerve, muscle and plant cells.
In accordance with cell theory, every new cell arises from an older cell splitting in two. This cell division allocates the contents of the original cell more or less equally, and thus requires the nucleus to divide in two as well. Early studies of this process revealed that the nucleus was filled with a coloured material, which was named chromatin. In 1888, Heinrich Wilhelm Gottfried von Waldeyer-Hartz saw that prior to division, the diffuse chromatin formed into fibres that were then divided between the two new cells. He named these ‘chromosomes’.
Today, we know that chromosomes are the scaffold structures holding the DNA molecules that make up the genome. Human cells have 46 chromosomes, but numbers vary wildly from species to species. Most of the time, individual chromosomes are too narrow to see, existing as a nebulous mass of chromatin, with the DNA coiled around spindle-like proteins called histones. Only during cell divisions will the coils thicken into structures visible through microscopes.
The main process of cell division – the one used to grow the bodies of individuals – is called mitosis. This name, derived from the Greek word for weaving, was coined because mitosis involves a network of threadlike microtubules that are seen to pull the chromosomes into two groups and heave them to either ends of the cell prior to the division.
Mitosis occurs over several complex stages, but to summarize, the first step is to duplicate each chromosome strand. This results in the X-shaped structure commonly associated with chromosomes, which is in fact a pair of chromatids – identical copies of the chromosome connected together. Once the chromosomes have duplicated, the nuclear membranes dissolve, allowing the chromosomes to line up across the cell. The microtubules then pull the chromatids apart, temporarily doubling the number of chromosomes in the cell. Finally, a new plasma membrane develops down the middle of the cell, allowing it to cleave in two.
Mitosis, the main form of cell division, involves several phases which work to divide up the cell contents. The result is two daughter cells that are genetically identical to the parent.