Cells, Homeostasis, and Disease
The cell is the smallest living component of a living organism. Organisms can be made up of a single cell, such as bacteria, or billions of cells, such as human beings. In large organisms, highly specialized cells that perform a common function are organized into tissue. Tissues, in turn, form organs, which are integrated into body systems.
Cells are complex organizations of specialized components, each component having its own specific function. The largest components of a normal cell are the cytoplasm, the nucleus, and the cell membrane. (See Cell components.)
The cytoplasm consists primarily of a fluid in which the tiny structures that perform the necessary functions to maintain the life of the cell are suspended. These tiny structures, called organelles, are the cell’s metabolic machinery. Each performs a specific function to maintain the life of the cell. Organelles include:
mitochondria — spherical or rod-shaped structures that are the sites of cellular respiration — the metabolic use of oxygen to produce energy, carbon dioxide, and water. (They produce most of the body’s adenosine triphosphate, which contains high-energy phosphate chemical bonds that fuel many cellular activities.)
ribosomes — the sites of protein synthesis
endoplasmic reticulum — an extensive network of two varieties of membrane-enclosed tubules: rough endoplasmic reticulum, which is covered with ribosomes; and smooth endoplasmic reticulum, which contains enzymes that synthesize lipids
Golgi apparatus — synthesizes carbohydrate molecules that combine with protein produced by the rough endoplasmic reticulum and lipids produced by the smooth endoplasmic reticulum to form such products as lipoproteins, glycoproteins, and enzymes
lysosomes — digest nutrients as well as foreign, obsolete, or damaged material in cells. (A membrane surrounding each lysosome separates its digestive enzymes from the rest of the cytoplasm. The enzymes digest nutrient matter brought into the cell by means of endocytosis, in which a portion of the cell membrane surrounds and engulfs matter to form a membrane-bound intracellular vesicle. The membrane of the lysosome fuses with the membrane of the vesicle surrounding the endocytosed material. The lysosomal enzymes then digest the engulfed material. Lysosomes digest the foreign matter ingested by white blood cells [WBCs] by a similar process, phagocytosis.)
peroxisomes — contain oxidases, enzymes that chemically reduce oxygen to hydrogen peroxide and hydrogen peroxide to water
cytoskeletal elements — a network of protein structures that maintain the cell’s shape and enable cell division and migration
centrosomes — contain centrioles, short cylinders adjacent to the nucleus that take part in cell division
microfilaments and microtubules — enable movement of intracellular vesicles (allowing axons to transport neurotransmitters) and formation of the mitotic spindle, the framework for cell division.
The cell’s control center is the nucleus, which plays a role in cell growth, metabolism, and reproduction. Within the nucleus, one or more nucleoli (dark-staining intranuclear structures) synthesize ribonucleic acid (RNA), a complex polynucleotide that controls protein synthesis. The nucleus also stores deoxyribonucleic acid (DNA), the double helix that carries genetic material and is responsible for cellular reproduction or division.
The semipermeable cell membrane forms the cell’s external boundary, separating it from other cells and from the external environment. The cell membrane consists of a double layer of phospholipids with protein molecules embedded in it. These protein molecules act as receptors, ion channels, or carriers for specific substances.
Each cell must replicate itself for life to continue. Cells replicate by division in one of two ways: mitosis (produces two daughter cells with the same DNA and chromosome content as the mother cell) or meiosis (produces four gametocytes, each containing half the number of chromosomes of the original cell). Most cells divide by mitosis; meiosis occurs only in reproductive cells. Some cells, such as nerve and muscle cells, typically lose their ability to reproduce after birth.
In the human body, most cells are specialized to perform one function. Respiration and reproduction occur in all cells. The specialized functions include:
movement — the result of coordinated action of nerve and muscle cells to change the position of a specific body part, contents within an organ, or the entire organism
conduction — the transmission of a stimulus, such as a nerve impulse, heat, or sound wave, from one body part to another
absorption — movement of substances through a cell membrane (for example, nutrients are absorbed and transported ultimately to be used as energy sources or as building blocks to form or repair structural and functional cellular components)
secretion — release of substances that act in another part of the body
excretion — release of waste products generated by normal metabolic processes.
Each of the following four types of tissue consists of several specialized cell types, which perform specific functions.
Epithelial cells line most of the internal and external surfaces of the body. Their functions include support, protection, absorption, excretion, and secretion.
Connective tissue cells are present in skin, bones and joints, artery walls, fascia, and body fat. Their major functions are protection, metabolism, support, temperature maintenance, and elasticity.
Nerve cells constitute the nervous system and are classified as neurons or neuroglial cells. Neurons perform these functions:
generating electrical impulses
conducting electrical impulses
influencing other neurons, muscle cells, and cells of glands by transmitting impulses.
Neuroglial cells support, nourish, and protect the neurons. The four types include:
oligodendroglia — produce myelin within the central nervous system (CNS)
astrocytes — provide essential nutrients to neurons and assist neurons in maintaining the proper bioelectrical potentials for impulse conduction and synaptic transmission
ependymal cells — involved in the production of cerebrospinal fluid
microglia — ingest and digest tissue debris when nervous tissue is damaged.
Muscle cells contract to produce movement or tension. The three types include:
skeletal (striated) muscle cells — extend along the entire length of skeletal muscles. These cells cause voluntary movement by contracting or relaxing together in a specific muscle. Contraction shortens the muscle; relaxation permits the muscle to return to its resting length.
smooth (nonstriated) muscle cells — present in the walls of hollow internal organs, blood vessels, and bronchioles. By involuntarily contracting and relaxing, these cells change the luminal diameter of the hollow structure and thereby move substances through the organ.
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