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General Features of Cells

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Cell, smallest unit of an organism that can function independently. All living organisms are made of cells, and it is generally held that nothing less than a cell can truly be said to be alive. Some microscopic organisms, such as bacteria and protozoa, are single cells whereas animals and plants are composed of many millions of cells assembled into tissues and organs. Although viruses and cell-free extracts are able to perform many individual functions of a living cell, they lack the capacity shown by cells of independent survival, growth, and replication and are therefore not considered to be living. Biologists study cells to learn how cells are made from molecules and how cells co-operate to make an organism as complex as a human being. Before we can comprehend how a healthy human body functions, how it develops and ages, and what goes wrong with it in disease, we need to understand the cells of which it is made. General Features of Cells Cells exist in many different sizes and shapes. Some of the smallest bacterial cells are short cylindrical objects less than one micron, or �m (each �m being a millionth of a metre), in length. At the other extreme, nerve cells have complex shapes including many long thin extensions, and may reach lengths of several metres (those in the neck of a giraffe provide a dramatic example). Most plant cells are typically 20 to 30 �m long, polygonal, and defined by rigid cell walls. Most cells in animal tissues are compact in shape, 10 to 20 �m in diameter, with a deformable and often richly folded surface. ...read more.


It is especially important in animal cells, which lack a rigid cell wall, as the cytoskeleton maintains the structure and shape of the cell. The cytoskeleton provides a framework for the organization of the cell and an anchorage for organelles and enzymes. It is also responsible for the many movements that the cell can produce. In many cells, the cytoskeleton is not a permanent structure but continually dismantled and reassembled. It is formed from three principal types of protein filaments: microtubules, actin filaments, and intermediate filaments. These are linked to each other and to other structures in the cell by a variety of accessory proteins. Cell movements in eukaryotic cells almost always depend on actin filaments or microtubules. Many cells possess flexible "hairs" on their surface, called cilia or flagella, which contain a core bundle of microtubules capable of regular energy-driven bending movements. Sperm cells swim by means of flagella, for example, and cells lining the intestine or other ducts in the vertebrate body carry fields of cilia on their surfaces that sweep fluids and particles in a specific direction. Large bundles of actin filaments are found in muscle cells where, together with the protein called myosin, they produce forceful contractions. The movements associated with cell division in animals or plants depend on actin filaments and microtubules, which carry chromosomes and other components of the cell into the two segregating daughter cells. Many other internal movements are needed by plant and animal cells in order to develop a particular shape or to maintain their complicated internal structure. Mitochondria and Chloroplasts Mitochondria are among the most conspicuous organelles in the cytoplasm, and are present in nearly all eukaryotic cells. ...read more.


Intercellular Junctions To form a multicellular organism, cells must not only differentiate into specialized types, but also be bound together into tissues and organs. Eukaryotes have evolved a number of different ways to satisfy this need. In higher plants, the cells not only remain connected by cytoplasmic bridges (called plasmodesmata), they are also imprisoned in a rigid honeycomb of chambers walled with cellulose that the cells themselves have secreted (cell walls). The cells of most animals are bound together by a relatively loose network of large extracellular organic molecules (called the extracellular matrix) and by adhesion between plasma membranes. Often attachments between the cells hold them together to form a multicellular sheet, or epithelium. Epithelial sheets frequently form the outer boundary of a tissue or organ, providing a surface barrier that regulates the entry and exit of material. Cell Signalling During the development of an embryo, each different type of cell becomes programmed to respond in a particular way, and there is consequently a need for messages or signals to pass between different cells. The cell also has to work in harmony with its surroundings, which in a multicellular organism means co-operating with its neighbours. The importance of such "social controls" on cell division becomes apparent when the controls fail, resulting in cancer, which usually kills the organism. Cells co-ordinate their many activities through a system of signalling reactions that serves a role similar to the electrical system of a motor car or the nervous system of a small animal. Molecules, often produced by other cells, act on cell-surface receptors which initiate cascades of biochemical reactions in the cytoplasm of the target cell. Changes in the concentration of the chemical signals (specific ions and molecules) regulates the activity of proteins and the expression of genes in the target cell.1 ...read more.

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