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Cells Of The Human Body

.. lum of skeletal muscle stores calcium ions that function in muscle contraction. GOLGI APPARATUS The Golgi apparatus is composed of flattened membranous sacs, containing cisternae, that are stacked on each other like dinner plates. The Golgi apparatus modifies, packages, and distributes proteins and lipids manufactured by the rough and smooth endoplasmic reticula. Proteins produced at the ribosomes of the rough endoplasmic reticulum are surrounded by a vesicle, or little sac, that forms from the membrane of the endoplasmic reticulum. The vesicle moves to the Golgi apparatus, fuses with the membrane of the Golgi apparatus, and releases the protein into the cisterna of the Golgi apparatus. The Golgi apparatus concentrates and, in some cases, chemically modifies the proteins by synthesizing and attaching carbohydrate molecules to the proteins to form glycoproteins or attaching lipids to proteins to form lipoproteins.

The proteins are then packaged into vesicles that pinch off from the margins of the Golgi apparatus and are distributed to various locations. Some vesicles carry proteins to the plasma membrane where the proteins are secreted from the cell by exocytosis; other vesicles contain proteins that become part of the plasma membrane; and still other vesicles contain enzymes that are used within the cell. The Golgi apparatuses are most numerous and most highly developed in cells that secrete large amounts of protein or glycoproteins, such as cells in the salivary glands and the pancreas. SECRETORY VESICLES The membrane-bound secretory vesicles that pinch off from the Golgi apparatus move to the surface of the cell, their membranes fuse with the plasma membrane, and the contents of the vesicle are released to the exterior by exocytosis. The membranes of the vesicles are then incorporated into the plasma membrane.

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Secretory vesicles accumulate in many cells, but their contents frequently are not released to the exterior until a signal is received by the cell. For example, secretory vesicles that contain the hormone insulin do not release it until the concentration of glucose in the blood increases and acts as a signal for the secretion of insulin from the cells. LYSOSOMES Lysosomes are membrane-bound vesicles that pinch off from the Golgi apparatus. They contain a variety of hydrolytic enzymes that work as intracellular digestive systems. Vesicles taken into the cell fuse with the lysosomes to form one vesicle and to expose the phagocytized materials to hydrolytic enzymes. Various enzymes within lysosomes digest nucleic acids, proteins, polysaccharides, and lipids.

Certain white blood cells have large numbers of lysosomes that contain enzymes to digest phagocytized bacteria. Lysosomes also digest organelles of the cell that are no longer functional in a process called autophagia. Also, when tissues are damaged cells release their enzymes, which digest both damaged and healthy cells. In other cells the lysosomes move to the plasma membrane, and the enzymes are secreted by exocytosis. For example, the normal process of bone remodeling involves the breakdown of bone tissue by specialized bone cells.

Enzymes responsible for that degradation are released into the extracellular fluid from lysosomes produced by those cells. PEROXISOMES Peroxisomes are membrane-bound vesicles that are smaller than lysosomes. Peroxisomes contain enzymes that break down fatty acids and amino acids. Hydrogen peroxide which breaks down hydrogen peroxide to water and oxygen. Cells that are active in detoxification, such as liver and kidney cells, have many peroxisomes.

MITOCHONDRIA Mitochondria usually are small, rod-shaped structures. In living cells, time lapse photomicrography shows that mitochondria constantly change shape from spherical to rod-shaped or even to long, threadlike structures. Mitochondria are the major sites of ATP production, which is the major energy source for most endergonic chemical reactions within the cell. Each mitochondrion has an inner and outer membrane separated by an intermembranous space. The outer membrane has a smooth contour, but the inner membrane has numerous infoldings called cristae that project like shelves into the interior of the mitochondria.

A complex series of mitochondrial enzymes forms two major enzyme systems that are responsible for oxidative metabolism and most ATP synthesis. The enzymes of the citric acid (or Krebs) cycle are found in the matrix, which is the substance located in the space formed by the inner membrane. The enzymes of the electron transport chain are embedded within the inner membrane. Cells with a greater energy requirement have more mitochondria with more cristae than cells with lower energy requirements. Within the cytoplasm of a given cell, the mitochondria are more numerous in areas in which ATP is used. Increases in the number of mitochondria result from the division of preexisting mitochondria.

When muscles enlarge as a result of exercise, the number of mitochondria within the muscle cells increases to provide the additional ATP required for muscle contraction. The information for making some mitochondrial proteins is stored in DNA contained within the mitochondria themselves, and those proteins are synthesized on ribosomes within the mitochondria. The structure of many other mitochondrial proteins is determined by nuclear DNA, however, and these proteins are synthesized on ribosomes within the cytoplasm and then transported into the mitochondria. Both the mitochondrial DNA and mitochondrial ribosomes are very different from those within the nucleus and cytoplasm of the cell. In addition, unlike nuclear DNA, mitochondrial DNA does not have associated proteins. CENTRIOLES AND SPINDLE FIBERS The centrosome is a special area of the cytoplasm close to the nucleus that contains two centrioles. Each centriole is a small, cylindrical organelle about 0.3 – 0.5 um in length and 0.15 um in diameter, and the two centrioles are usually found perpendicular to each other within the centrosome. The wall of the centriole is made up of nine evenly spaced, side by side units, or triplets. Each unit consists of three microtubules located side by side and joined together. The centrosome is the center of microtubule formation. Microtubules appear to have some control over the distribution of actin and intermediate filaments. Through its control of microtubule formation, the centrosome is closely involved in determining cell shape and movement.

The microtubules extending from the centrosomes are constantly growing and shrinking. Before there is cell division, the two centrioles double in number, the centrosome divides into two, and one centrosome, containing two centrioles, moves to each end of the cell. Spindle fibers extend out in all directions from the centrosome. These microtubules grow and shrink even more rapidly than those of nondividing cells. If a spindle fiber comes in contact with a kinetochore, the fiber attaches itself to the kinetochore and stops growing or shrinking. Eventually spindle fibers from each centromere attach to the kinetochores of all the chromosomes. Then the chromosomes are pulled apart and moved by the microtubules toward the two centrosomes during cell division. CILIA AND FLAGELLA Cilia are appendages that come from the surface of cells and are capable of movement. They are usually found on only one surface of a given cell and vary in number from one to thousands per cell.

Cilia are cylindrical in shape, about 10 um in length and 0.2 um in diameter, and the shaft of each cilium is covered by the plasma membrane. Two centrally located microtubules and nine peripheral pairs of fused microtubules extend from the base to the tip of each cilium. Movement of the microtubules past each other, a process that requires energy from ATP, is responsible for movement of the cilia. A basal body is located in the cytoplasm at the base of the cilium. There are many cilia on surface cells that line the respiratory tract and the female reproductive tract.

In these regions cilia move with a power stroke in one direction and a recovery stroke in the other direction. Their motion moves materials over the surface of the cells. Flagella have a similar structure like cilia but are longer, and there is usually only one per cell. Whereas, cilia moves small particles across the cell surface, flagella moves the cell. MICROVILLI Microvilli are cylindrically shaped extensions of the plasma membrane about 0.5-1 um in length and 90 nm in diameter. Many microvilli are on each cell increasing the cell surface area. Microvilli are only one tenth to one twentieth the size of cilia.

Microvilli does not move, and they are supported with actin filaments, not microtubules. They are found in the intestine, kidney, and other areas in which absorption is an important function. In some locations of the body, microvilli are highly modified to work as sensory receptors. SUMMARY OF CELL PARTS CELL PARTS STRUCTURE FUNCTION Plasma Membrane Lipid bilayer composed of phospholipids and Outer boundary of cells that controls entry cholesterol with proteins that extend across and exit of substances; receptor or are buried in either surface of the lipid molecules function in intercellular bilayer communication; marker molecules enable cells to recognize one another Nucleus Nuclear envelope Double membrane around the nucleus; the Separates nucleus from cytoplasm and outer membrane is continuous with the controls movement of materials into and endoplasmic reticulum; nuclear pores go out of the nucleus through the nuclear envelope Chromatin Thin strands of DNA, histones, and DNA controls protein synthesis and the other proteins; condenses to form chemical reactions of the cell; DNA is chromosomes during cell division the genetic or hereditary material Nucleolus One to four dense bodies making up of Large and small ribosomal subunits are made ribosomal RNA and proteins here Cytoplasm: Cytosol Fluid Part Water with dissolved ions and molecules; Contains enzymes that start colloid with suspended proteins decomposition and synthesis reactions; ATP is produced in glycolysis reactions Cytoskeleton Microtubules Hollow tubes composed of the protein Support the cytoplasm and form centrioles, tubulin; 25 nm in diameter spindle fibers, cilia, and flagella; responsible for cell movements Actin filaments Small fibrils of the protein actin; 8 nm in Support the cytoplasm and form centrioles, diameter microvilli, responsible for cell movement SUMMARY OF CELL PARTS CELL PARTS STRUCTURE FUNCTION Intermediate filaments Protein fibers; 10 nm in diameter Support the cytoplasm Cytoplasmic inclusions Groups of molecules made or taken in Function depends on the molecules; energy by the cell; may be surrounded storage, oxygen transport, skin color, by a membrane and others Cytoplasm: Organelles Ribosome Ribosomal RNA and proteins form large and Site of protein synthesis small subunits; attached to endoplasmic reticulum or free Rough endoplasmic reticulum Membranous tubules and flattened sacs with Protein synthesis and transport to Golgi attached ribosomes apparatus Smooth endoplasmic reticulum Membranous tubules and flattened sacs with Makes lipids and carbohydrates; attached ribosomes makes harmful chemical; stores calcium Golgi apparatus Flattened membrane sacs stacked on each other Modification, packaging, and distribution of proteins and lipids for secretion or internal use Secretory vesicle Membrane-bound sac pinched off Golgi Carries proteins and lipids to cell surface apparatus for secretion Lysosome Membrane-bound vesicle pinched off Golgi Contains digestive enzymes apparatus Peroxisome Membrane-bound vesicle One site of lipid and amino acid breakdown and breaks down hydrogen peroxide Mitochondria Round, rod-shaped, or threadlike Major site of ATP production when oxygen structures; surrounded by double membrane; is available inner membrane forms cristae SUMMARY OF CELL PARTS CELL PARTS STRUCTURE FUNCTION Centrioles Pair of cylindrical organelles in the centrosome Centers for microtubule formation; consisting of triplets of parallel microtubules determine cell polarity during cell division; form the basal bodies of cilia and flagella Spindle fibers Microtubules extending from the centrosome to Assist in the separation of chromosomes chromosomes and other parts of the cell during cell division Cilia Extensions of the plasma membrane containing Move materials over the surface of cells doublets of parallel microtubules Flagellum Extensions of the plasma membrane containing In humans, responsible for movement of doublets of parallel microtubules spermatozoa Microvilli Extension of the plasma membrane containing Increase surface area of the plasma microfilaments membrane for absorption and secretion; modified to form sensory receptors CELL METABOLISM Cell metabolism is all the decomposition and synthesis reactions in the cell. The breakdown of food molecules such as carbohydrates, lipids, and proteins releases energy that is used to synthesize ATP. Each ATP molecule has a portion of the energy stored from the chemical bonds of the food molecules.

The ATP molecules are small energy packets that are used to drive other chemical reactions or processes such as active transport. ATP production takes place in cytosol and in mitochondria through lots of chemical reactions. Food molecules transfer energy to ATP. If a cell was to receive all the energy from food molecules, it would literally burn up. To show ATP production from food molecules: the breakdown of sugar glucose.

For example: sugar from a candy bar. Once glucose is put into a cell, lots of reactions takes place inside the cytosol. These chemical reactions, glycolysis, change the glucose to pyruvic acid. Pyruvic acid can go into different biochemical pathways, if oxygen is available. Aerobic respiration happens when oxygen is available. Pyruvic acid molecules enter mitochondria through chemical reactions called citric acid cycle and the electron transport chain, which are then changed to carbon dioxide and water. Energy stored in each glucose molecule can produce 36-38 ATP molecules through aerobic respiration.

Anaerobic respiration happens without oxygen and includes the change of pyruvic acid to lactic acid. There is a production of two ATP molecules for each glucose molecule used. Anaerobic respiration doesn’t produce as much ATP as aerobic respiration. But it does allow cells to work for a short time when oxygen is too Medicine and Health Care.


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