test 1 notes - TEST#1 SUMMARIES The Functions of Life...

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TEST #1 SUMMARIES The Functions of Life . Maintaining boundaries and integrity of the organism - Movement Responsiveness, irritability, excitability - Intake of nutrients and digestion - Metabolism - Excretion - Growth and repair - Reproduction Most homeostatic mechanisms utilize a control system known as negative feedback . Negative feedback works like a thermostat. A sensor responds to a variable stimulus. When that variable is outside the normal range the sensor notifies a control center, which then responds directly or triggers a response by an effector . The response has the effect of reducing or negating the original stimulus by bringing the variable back within the normal range. A negative feedback mechanism is self regulating, it turns itself off. Very few processes in the body involve positive feedback and those that do use it to produce a quick response. Some external mechanism is necessary to turn off the process. Examples of positive feedback include: blood clotting, the labor contractions of childbirth, digestion in the stomach. A tissue is a group of cells working together to perform one or more specific functions. There are four tissue types in the human body: Epithelial tissue - found as the lining and covering of organs and body cavities, the secretory parts of organs and glands, the transport membranes of capillaries and alveolar sacs, and membranes which lubricate organs. Connective tissue - supports as bone, cartilage, tendons and ligaments, protects as the bony cavities and as protective immune cells in the blood, and stores nutrients. Nervous tissue - the tissue which carries information in the form of impulses throughout the body. Muscle tissue - contracts to perform movements such as skeletal muscle movements, propulsion in the GI tract, and pumping blood in the heart. Epithelial tissue is derived from all three germ layers: ectoderm, mesoderm, and endoderm . Connective tissue and muscle are mostly derived from the mesoderm. An exception is the connective glial cells, supportive cells of the nervous system which, together with nerve tissue and the skin, is derived from the ectoderm. Cell junctions : Epithelial cells, certain muscle cells, and others which lie side by side have specialized connections between them, called a junctional complex . In epithelium this connection is particularly significant because it allows a sheet of cells to create a barrier or semipermeable membrane, thus compartmentalizing a tissue and restricting passage of substances across the epithelium. Tight junctions (a.k.a. zona occludens ) are most important in restricting transport across an epithelial membrane, for instance in the wall of a capillary. Tight junctions are fusions of adjacent plasma membranes and vary somewhat in how "tight" they are.
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Depending on the extent and complexity of the fusions, some allow water and small molecules to pass through gaps between them (e.g. in the kidneys), and others allow very little to pass. In this way tight junctions allow these epithelial sheets to act as semipermeable membranes.
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