cell

Typical plant and animal cell. Plant and animal cells share many structures, such as ribosomes, mitochondria, and chromosomes, but they also have notable differences: plant cells have chloroplasts, a large vacuole, and a cellulose cell wall. Animal cells do not have a rigid cell wall but have an outside cell membrane only.

A typical animal cell. Some animal cells are specialized for a particular function and may look very different (for example nerve cells and sperm cells). However, most animal cells have a nucleus containing their genetic information (red blood cells are an exception) and are made up of cytoplasm surrounded by a thin membrane. The cytoplasm contains the cell organelles, such as the mitochondria.

A typical plant cell. Plant cells differ from animal cells in that they have chloroplasts (where photosynthesis takes place), a large vacuole, and a cellulose cell wall.

The nucleolus is found in the nucleus. It contains the nucleolar organizers, the regions on some chromosomes containing genes that code for ribosome synthesis. An average, healthy cell can produce up to 10,000 ribosomes per minute.

A cross-section of a mitochondrion (above) and a 3-D cutaway. Mitochondria have an outer membrane and an inner membrane. The inner membrane is much folded to form christae. Mitochondria have their own DNA and ribosomes.

In biology, the basic unit of a living organism. It is the smallest unit capable of independent existence. In organisms, other than the smallest ones, the body of the organism is made up of several cells or many cells. A single cell, therefore, is the smallest unit that shows characteristic features of life, such as reproduction, growth, respiration, response to environmental stimuli, and the ability to take in mineral salts. Viruses are particles that are not cells. A virus can only reproduce by ‘taking over’ a cell from another organism. This cell often dies as a result of making many new virus particles.

Some organisms are composed of only one cell, including many bacteria and some fungi, such as yeast. Single-cell organisms are termed unicellular, while plants and animals which contain many cells are termed multicellular organisms. Organisms such as human beings consist of billions of cells. In organisms made of many cells, groups of cells are specialized to carry out specific functions and are organized into tissues and organs.

Cells always have a cell membrane around them and cytoplasm inside, and normally a nucleus. There are differences between plant cells and animal cells; for example, plant cells have a cell wall made of cellulose outside their cell membrane. This helps to explain why plants look so different from animals.

In plants and animals, cells divide by mitosis or by meiosis when the organism carries out sexual reproduction. In both forms of cell division, the chemical carrying inherited information, DNA, has to be copied before division. Rarely, mistakes occur, causing mutations, but it is normally done accurately. Copying the DNA results in the duplication of structures called chromosomes in the nucleus. In cell division, the duplicated chromosomes are separated from each other into daughter cells. New cells produced by mitosis are needed to replace cells that die and some cells live only a short time, such as white blood cells which live for only a few days. Within the human body, about 3 billion cells die every minute. Many cells may be lost in normal activities - for example, human skin cells are constantly being worn off and have to be replaced by new cells produced by mitosis. The new cells needed during growth are also produced by mitosis.

In respiration a cell uses chemicals that it has taken in, as nutrients, and breaks them down to release energy that the cell can use for its life processes.

The cytoplasm of all cells contains ribosomes, which carry out protein synthesis, and DNA, the coded instructions for the behaviour and reproduction of the cell and the chemical machinery for the translation of these instructions into the manufacture of proteins. Viruses lack this translation machinery and so have to parasitize cells in order to reproduce themselves.

Eukaryotic cells In eukaryotic cells, found in protozoa, fungi, and higher animals and plants, DNA is organized into chromosomes and is contained within a nucleus. Each eukaryotic cell has a surrounding membrane, which is a thin layer of protein and fat that restricts the flow of substances in and out of the cell and encloses the cytoplasm, a jellylike material containing the nucleus and other structures (organelles) such as mitochondria. The nuclei of some cells contain a dense spherical structure called the nucleolus, which contains ribonucleic acid (RNA) for the synthesis of ribosomes. The only cells of the human body which have no nucleus are the red blood cells.

Plant cells differ from animal cells in that the membrane is surrounded by a cell wall. They also have larger vacuoles (fluid-filled pouches), and contain chloroplasts that convert light energy to chemical energy for the synthesis of glucose.

Prokaryotic cells In prokaryotic cells, found in bacteria and cyanobacteria, the DNA forms a simple loop and there is no nucleus. The prokaryotic cell also lacks organelles such as mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, and centrioles, which perform specialized tasks in eukaryotic cells.

Cell composition Primarily, cells are composed of the elements that make up the majority of organic compounds - namely, oxygen, hydrogen, carbon, and nitrogen - and the complete contents of a cell are called the protoplasm. The composition of the protoplasm varies, but the products of its breakdown when the cell dies are mostly proteins. It also contains carbohydrates, fats, and the lipoids, lecithin and cholesterin, besides inorganic salts such as the phosphates and chlorides of potash, soda, and lime. Water, which provides a favourable environment for biochemical reactions, makes up 60 to 65% of each cell. Its distribution through the cell membrane is controlled by osmosis. Most cells contain vacuoles, which are spaces within cells surrounded by membrane and containing a solution. Plant cells have large vacuoles containing a solution of sugars and other substances called cell sap; animal cells have smaller vacuoles which may contain either food or water. Some cells have flexible, hairlike attachments called cilia and flagellum; these are found on protozoa as well as in human oviducts and respiratory tracts and are used for locomotion, capturing food, and removing particles of foreign material.

Cell function The cell wall in most animal cells is not a substantial membrane, and the shape of the cell is maintained by surface tension or chemical action. All cells are dynamic at some stage of their life cycle, in the sense that they use energy to perform a variety of cell functions: movement, growth, maintenance and repair of cell structure, reproduction of the cell, and manufacture of specialized cell products such as enzymes and hormones. These functions are also the result of interactions of organic molecules. The endoplasmic reticulum (ER), which is found in all cells, is a system of membranes running throughout the cytoplasm and is the site for the production of fats and proteins. Connected to this is the Golgi apparatus, a collection of vesicles and folded membranes, which stores and later transports the proteins manufactured by the ER. Mitochondria are also found in the majority of cells. These are sometimes called the ‘powerhouses’ of the cell as they contain the enzymes involved in the cell's metabolic activities that enable the release of energy from food by combining it with oxygen. Chloroplasts are only found in plant cells. They contain chlorophyll which absorbs sunlight and converts this to energy through the process of photosynthesis.

Cell reproduction New cells are produced from existing cells. They reproduce by division (mitosis), with each cell dividing to produce two new cells, both of which contain a copy of the genetic programme. Simple cell division, or asexual reproduction, normally results in the production of two identical daughter cells, each containing a set of chromosomes identical with those of the parent cell. In sexual reproduction, the DNA of two different organisms of the same species combines to produce a cell with a new combination of genes. When this occurs between single-celled organisms, it is called conjugation. In multicellular organisms, sexual reproduction requires the production of male and female germ cells (sperm and eggs) by a process called meiosis. During this process a cell divides twice, but its chromosomes are duplicated only once. Thus, four germ cells are produced, each containing half the normal number of chromosomes. In the male organism the germ cells develop into sperm; in the female they develop into eggs. A sperm and an egg then unite (fertilization) to form a new cell, called a zygote, which has a complete set of chromosomes, and which has received half its genetic information from each parent, thus producing a new individual. Cell differentiation is the process by which a daughter cell becomes different from its parent in appearance or function, or both, even though both parent and daughter cell contain identical genetic information.

Cell death Like all living things, cells die. In humans, external cells, such as skin cells, flake off, while the dead cells from internal organs are passed out of the body with waste products. The lifespan of cells varies - for example, white blood cells live for about 13 days, red blood cells live about 120 days, and liver cells live about 18 months. Nerve cells can live for as long as 100 years. See also apoptosis.

Cell theory English scientist Robert Hooke first used the word ‘cell’ in his Micrographia (1665) to describe the structure of plant tissue. As curator of instruments at the Royal Society of London, he had followed the development and improvement of the microscope and in a drawing of the microscopic structure of cork, he showed walls surrounding empty spaces and referred to these structures as ‘cells’. He described similar structures in the tissue of other trees and plants and noted that in some tissues the cells were filled with a liquid while in others they were empty. He concluded from this that the function of the cells was to transport substances through the plant. Soon afterwards, the Dutch scientist Anton van Leeuwenhoek, who developed the light microscope, was able to describe these cells in greater detail. In 1839, with the help of improved microscopes, two German scientists, Matthias Schleiden and Theodor Schwann, formulated the current theory which states that cells are the basic units of construction of all living things. This concept was taken further by German pathologist, Rudolf Virchow, who advanced the idea that new cells are formed by the division of existing cells, and that this gives rise to growth and reproduction in plants and animals. Towards the end of the 19th century techniques were developed for staining cell parts which enabled scientists to detect tiny cell structures. However, it was not until the introduction of more advanced optical techniques and equipment in the mid-20th century, such as the electron microscope, that these minute structures could be examined in detail.

Cell studies The study of cells (cytology) is not restricted to describing just the structures of cells (morphology). One of the main concepts underlying modern cytology is that the function of each structure can be understood as a series of biochemical reactions. The understanding of these functions has been greatly advanced by the development of cell fractionation techniques, using an ultracentrifuge to separate specific intracellular structures from the remainder of the cell. Another technique is tissue culture, by which specific kinds of cells can be isolated and grown for study. In 1976 the barrier between the plant and animal kingdoms was broken by the achievement in the laboratory (in the UK, USA, and Hungary) of the fusion of a plant and animal cell to form a hybrid.