In biology, a cell is an elementary living system capable of independent existence and self-replication and development. Contained and protected by a lipid membrane, it is the basic structure of life metabolic processes of all animals and plants. There are two types of cells: prokaryotic and eukaryotic. A prokaryotic cell is a primitive unicellular (one-celled) living entity; it lacks a nucleus and has no organelles, with its genetic material drifting freely in the cytoplasm. Eukaryotic cells, on the other hand, cluster together to make up multicellular (many-celled) organisms, forming organs and tissues; a eukaryotic cell has a nucleus, which contains its genetic material (DNA) inside, and organelles that drift in the cytoplasm; organelles are cellular substructures that perform important functions, doing maintenance and providing the cell with energy for it to replicate. Bacteria are prokaryotic cells, while animals and plants consist of billions of eukaryotic cells. The only eukaryotic cell that lives independently as a one-celled entity is a protozoan, such as an amoeba.
The structure and functions of each cell show certain characteristics common to all cells, reflecting a common origin from primary organic complexes. The specific features of various cells are the result of specialization in the process of evolution. All cells similarly regulate metabolism, duplicate and use hereditary material, and acquire and utilize energy. At the same time, unicellular eukaryotic organisms, such as protozoans, differ sharply in terms of size, shape, and behavior. Prokaryotic cells, such as bacteria, also differ in shape and size (round, spiral, rod-like). No less diverse are the cells of multicellular organisms. For example, in man, lymphoid cells are small (about 10 microns in diameter) and rounded and participate in immunological reactions, whereas some neurons (nerve cells) have processes (axons) more than a meter long and perform the body’s basic regulatory functions.
Characteristics
Every eukaryotic cell has three principal parts: 1) the external lipid membrane, which envelops and protects the cell; 2) the nucleus, which contains the genetic material; and 3) the cytoplasm. Within each of these, it is possible to distinguish certain substructures, which differ in shape, size, internal structure, chemical properties, and function. These substructures are called organelles, which are vital to the cell and in fact are found in all cells. Others are the products of cell activity and represent temporary formations. Mitochondria, Golgi apparatus, endoplasmic reticulum (ER), and lysosomes are important organelles that reside in the cytoplasm. The segregation of various biochemical functions takes place by means of these specialized structures, facilitating the accomplishment within the same cell of diverse processes, including the synthesis and decomposition of many substances.
Genetic information on the structure of the proteins characteristic of an organism of a particular species is stored in DNA (deoxyribonucleic acid), the principal component of nuclear organelles called chromosomes. Another, most important property of DNA is its capacity for self-replication, which ensures both the stability of the hereditary information and its continuity— that is, its transmission to succeeding generations. Meanwhile, Ribonucleic acids, the immediate participants in protein synthesis, are synthesized on limited sections of the DNA molecule (embracing several genes), which act as templates. The transcription of the DNA code is accomplished by the synthesis of messenger RNA (m-RNA). Protein synthesis is, in effect, a reading of information from the RNA template.
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| Above, a eukaryotic cell and its organelles. |
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| A rod-shaped bacterium, which is a prokaryotic cell. |