The smallest unit of life. All organisms are composed of cells, tiny compartments limited by a MEMBRANE, in which is contained the full complement of genetic material of the organism. Unicellular organisms such as bacteria and protozoa are composed of one cell which contains all non-nutritional components needed for survival. Multicellular organisms, which include most plants and animals, have cells which are often arranged in tissues or organs, and have specialized functions necessary for survival and reproduction.
Cells can be subdivided into PROKARYOTES, which have no NUCLEUS, and EUKARYOTES, in which a nucleus is present, in which the DNA (deoxyribonucleic acid) is kept separate from other cellular material (see EUKARYOTE). Prokaryotes are bacteria: these have a simple structure, characterized by the presence of a rigid cell wall surrounding the cell membrane. Bacteria have a simple GENOME, and require only basic nutrients to survive. They reproduce by binary fission, in which the cell pinches in two, to form two genetically identical daughter cells. In contrast to the relatively simple structure of prokaryotes, eukaryotic cells are more complex. The DNA is packaged into a number of strands (see CHROMOSOMES), which are confined to the nucleus, providing a greater degree of control over the expression of specific genes. In addition to a nucleus, complex eukaryotes also possess a variety of small specialized structures or ORGANELLES necessary for cell survival. Continuous with the cell nucleus is the ROUGH ENDOPLASMIC RETICULUM, a series of highly convoluted membranous folds to which RIBOSOMES are attached on the cytosolic surface. Ribosomes are catalytic units composed of RIBONUCLEIC ACID (RNA) and proteins, which mediate the translation of messenger RNA into proteins during gene expression.
SMOOTH ENDOPLASMIC RETICULUM lacks ribosomes, and is involved in the synthesis of LIPIDS, important for the formation of cell membranes. Proteins synthesized at ribosomes on the rough endoplasmic reticulum enter the endoplasmic reticulum, en route to a series of membrane stacks called the GOLGI APPARATUS, where they may be modified before transport to their site of action. Some proteins are synthesized by free ribosomes in the cell CYTOSOL. Proteins made at these sites are released directly into the cell cytosol and act at intracellular sites. Other organelles are important for cell metabolism. Membrane-bound sacs called LYSOSOMES contain enzymes used in intracellular digestion. Similarly, PEROXISOMES contain oxidative enzymes which produce hydrogen peroxide for some cellular functions. Most of the energy used to fuel metabolic processes in eukaryotic cells is produced by the MITOCHONDRIA, which use oxygen obtained during respiration in the production of ADENOSINE TRIPHOSPHATE (ATP). In plant cells, organelles called chloroplasts harness energy from light to produce ATP and to form carbohydrates from carbon dioxide.
While prokaryotic cell sizes and shapes are limited by their cell wall, eukaryotic cell shape is maintained by a CYTOSKELETON, formed from networks of protein FILAMENTS and MICROTUBULES. These structures also allow for intracellular trafficking and motility, and lead to the enormous variety in shape that exists in many cell types in higher organisms. Different shapes underlie the highly specialized functions which cells in higher eukaryotes are required to perform for the survival of the organism. For example, a NEURON is a cell in which the distinguishing feature is the presence of an AXON and DENDRITES, fine elongated processes supported by the cytoskeleton of filaments and microtubules. These features allow neurons to conduct electrical charge, and engage in fast intracellular signalling.
Cells therefore represent complex units of life. While neurons and other eukaryotic cells contain similar basic features, specialization has led to highly defined functions of cells within tissues, which collectively permit the organism to sustain life.
FIONA M.INGLIS
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