You are made of roughly 100 trillion of them, and their structure is what distinguishes you from a banana tree. Once they were one of the biggest mysteries in science, but now their basic functioning is largely understood. They are, of course, cells. So what are they, what do they do, and how do they do it?
Cells are the basic unit of all living organisms (viruses and prions don't count as 'living'.) They range from a few micrometres to the size of an ostrich egg. They may be formless and blobby like amoebas, or stringy and structured like nerves.
Kingdoms - Types of Cell
The highest division of biological organisms is the Empire, which divides cells with a nucleus - eukaryotes - from cells without - prokaryotes. The next level is the Kingdom1. Kingdoms are distinguished by the characteristics of the cells they contain. The most familiar system has five Kingdoms - animals, plants, fungi, prokaryotes2 and protoctists3. The differences are:
- Prokaryotes do not contain a nucleus (although they do have a nuclear region, where most of their genetic material is stored) or organelles.
- Plant cells contain chloroplasts and a large central vacuole.
- Fungi absorb their food externally and cannot photosynthesise.
- Animals have no cell wall.
- Protoctists are any organisms that do not fall into any of the above categories.
Organelles are specialised regions of a cell that have a specific function. They are analogous to organs within a multi-cellular body. They are divided by cytologists4 into two main types5; membrane-bound organelles and non-membrance bound organelles. Members of the former category are often highly-adapted pieces of membrane and are found only in eukaryotes, whereas the latter are small chemical systems in their own right and are found in all cells.
The basis of all cells is the cell membrane. This is a double layer of phospholipid molecules. Phospholipids have a hydrophilic6 phosphor 'head' end and two hydrophobic7 fatty acid tails, making them look a little like twin-tailed tadpoles. Since the tails avoid going into solution in water (oil and water do not mix), they spontaneously form a double layer (bilayer) in aqueous solution with the heads facing outwards on either side. Thus, the tails are in contact with other oily tails, but not with water molecules.
Cells have a wide variety of other molecules either embedded in the cell membrane or passing right through it. These fats and proteins fill a number of functions, including transport of large molecules through the membrane and identification of the cell to the immune system.
Within the cell membrane is the cytoplasm. This is largely water containing high concentrations of ions and biological molecules such as proteins and RNA (Ribonucleic Acid). This fluid is known as cytosol. Together with the organelles listed below, it makes up the cytoplasm.
The nucleus is the largest and most complex organelle. Its presence is the defining characteristic of eukaryotic cells. Its primary function is the storage of the nuclear DNA - it's a sort of genetic cupboard. It is also the site of RNA transcription (the first step in the process of protein manufacture) and DNA reproduction. The DNA is stored as thin strands in a slightly denser region of the nucleus known as the nucleolus. The nucleus is surrounded by a nuclear membrane, punctuated by pores to allow RNA to leave the nucleus. The nucleus of some species also contains a centromere, a pair of small, rod-like structures involved in cell division. Under normal circumstances most eukaryotic cells only have one nucleus, except during cell division.
Endoplasmic reticulum (ER) is a series of connected layers of membranes, a little like a vertical accordion. There are two types, Rough ER - which contains ribosomes - and smooth ER, which is used purely for medium-scale transport. The ER is attached to the nucleus, and fills the majority of the cell.
The cytoskeleton is a series of small rods throughout the cell that give the cell structure and allow small-scale movement of proteins to specific destinations within the cell - rather like a cross between a girder and a conveyor belt.
Ribosomes are the sites of RNA translation, where RNA from the nucleus is used to manufacture proteins. In prokaryotes, they are free-floating in the cytoplasm, whereas in eukaryotes they are attached to the ER. Ribosomes consist of several RNA and protein molecules, and are some of the smallest cell components visible through an ordinary light microscope.
The Golgi Apparatus
The Golgi Apparatus (or Golgi body) is a series of layered sacks of membrane near the cell membrane. The layers are known as cisternae8. Vesicles bud off from these and are used to transfer large quantities of materials out of the cell.
The Cell Wall
The cell wall is a rigid structure surrounding the cell membrane. It is found in all classes of cell except animal cells, although it is made from different materials in each of the Kingdoms: chitin in fungi, cellulose in plants and murein in bacteria. In bacteria, a similar structure is known as the capsule. Its purpose is both defensive and structural.
Chloroplasts are found only in plant cells, and are the site of photosynthesis. They contain their own DNA, different to that found in the nucleus. They consist of a series of grana - stacks of plate-shaped components where the chlorophyl is stored - surrounded by a double membrane. Their similarity to simple prokaryotes9 has lead many scientists to theorise that they are the remains of ancient prokaryotic photosynthetic organisms that entered into a symbiotic relationship with a eukaryotic cell10.
Mitochondria are found in all fungal, animal and plant cells. Like chloroplasts, they contain their own DNA and are believed to be descended from prokaryotes that entered into ancestral eukaryotic cells. Mitochondria are the site of cellular respiration, where ATP11 is manufactured. Their structure is a double membrane, with the inner membrane containing large folds, meaning that in cross-section they look like a toothy mouth.
A vacuole is essentially a water-filled 'hole' in a cell. It is surrounded by a double membrane, and is used as a storage area by the cell - a handy ways of having your outsides in. Many cell types contain vacuoles, although plants are noted for having a single large vacuole, often taking up the majority of the internal space of the cell.
Vacuoles can also be used as lysomes or peroxisomes. These are sealed areas within the cell that contain chemicals capable of breaking down cell components. Lysomes are used to destroy other cell components by joining their cell membranes to make a single blister. This is used for digestion; breakdown of old organelles; and by the immune system to destroy foreign cells after they have been ingested. Lysomes are formed by budding off from the Golgi apparatus.
Peroxisomes are structuarally similar to lysomes, but function to break down toxins. Peroxisomes do not bud from the Golgi apparatus, instead growing and splitting to reproduce. They may also be formed directly from the endoplasmic reticulum.
The flagellum is a long, tail-like structure found in some bacteria. Its purpose is propulsion; at its base is a group of proteins forming a 'motor' that causes the flagellum to rotate, driving the bacterium forwards through the water it inhabits. The flagellum is believed to be a modified 'injector'.
Pili are folds in the cell membrane possessed by some bacteria. Their purpose is to increase the surface area of the cell. This increases the rate at which ions can diffuse into or out of the cell.
The majority of the processes of life take place within cells. Their primary purposes are the production of the biological molecules needed for the continued existence of the cell, and the reproduction of the cell.
The majority of the components of a cell are made of proteins. These unbranched long-chain molecules are produced by the ribosomes and transferred throughout the cell by the Golgi apparatus and ER to where they are required. Most enzymes are proteins12.
Cell reproduction requires the duplication of DNA. This takes place within the nucleus by one of two processes, mitosis or mieiosis. The former leads to new cells genetically identical to the parent; the latter to gametes13.
The majority of cells are specialised. Single-celled creatures and stem cells14 are the only truly non-specialist cells - and even some single-celled creatures form colonies with specialised members.
In multicellular organisms, cells of one type come together to form tissues. Several types of tissue that have a common purpose make up an organ, and organs work together in systems.
Cells can be specialised in a number of ways. These include:
Red blood cells have a 'deflated football' shape (technically known as a biconcave disc) to give the maximum possible surface area : volume ratio, allowing efficient diffusion of gasses. Nerve cells are the longest and thinnest cells, allowing nerve impulses to travel long distances without the slowing and damping effects of crossing between cells. Those epithelial cells that line the lungs are very thin, again to allow easy exchange of oxygen from the lungs to the bloodstream and carbon dioxide in the opposite direction.
In sexually reproducing organisms, it is necessary for the gametes (reproductive cells) to have half the number of chromosomes as the somatic (non germ-line) cells. Male gametes (spermatozoa) are frequently small and motile, whereas female gametes (oocytes) are large and contain significant stores of energy to fuel the development of the embryo.
It is widely accepted that mitochondria and chloroplasts are descended from primitive cells that took up residence within precursors to modern cells, and have since become specialised and simplified to the point where they are no longer capable of independent survival and are not classed as cells. Less extreme examples of symbiosis include lichens (plants and algae growing together as a single organism) and colonies of single-celled organisms such as sponges.
Some cells - particularly those found in glands - have become 'factories' for the production of certain chemicals.
White blood cells come in many varieties, and are the body's defence against infection. They include 'killer' T-cells that can identify foreign particles in a body. These are perhaps the most specialised of all cells, as each recognises a single pathogen. Other white blood cells are capable of engulfing and destroying foreign cells.
Chloroplasts (see above) are one example of an organelle contained within a cell and giving the cell a specialised ability - photosynthesis. Muscle cells may contain large numbers of mitochondria to allow large-scale energy generation to facilitate cell contraction. Another example would be cilia, hair-like projections from a cell that can be moved to propel the cell. Adaptations of cilia include the propulsion of mucus through the respiratory system and the movement of eggs through the Fallopian tubes to prevent ectopic pregnancies.
Cnidarians (such as jellyfish and corals) have evolved specialised cells known as nematocytes which are capable of injecting venom into prey. This contrasts with, for example, scorpion stings, which are composed of many cells acting together as an organ or system.
Perhaps one of the most inspiring facets of modern biology is the extent to which we can break living organisms down into their component parts and understand, at least at a basic level, most of how they work right down to the level of atoms and molecules. Many of the above structures are visible through microscopes that can be purchased for a few tens of pounds (though staining the cells can help, as it can be hard to make out clear organelles against a clear background), allowing even amateur scientists to see for themselves. At the other end of the scale, the most powerful electron microscopes have pushed the boundaries of our understanding of biology to the point where it becomes almost indistinguishable from chemistry.