Mcat - The Cell (81)

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Prokaryotes

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A category of cell that includes bacteria, and cyanobacteria (blue-green algae).
- have outer membrane (cell wall)
- no membrane bound organelles
- no true nucleus

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Bacteria

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One of the two major classes of prokaryotic organism (the other being the Cyanobacteria). Bacteria are small (linear dimensions of around 1 m), noncompartmentalised, with circular DNA and ribosomes of 70S.

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Cyanobacteria

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Modern term for the blue green algae, prokaryotic cells that use chlorophyll on intracytoplasmic membranes for photosynthesis. The blue green colour is due to the presence of phycobiliproteins. Found as single cells, colonies or simple filaments. In Anabaena, in which the cells are arranged as a filament, heterocysts capable of nitrogen fixation occur at regular intervals. According to the endosymbiont theory Cyanobacteria are the progenitors of chloroplasts.

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Eukaryotes

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Organism whose cells have chromosomes with nucleosomal structure and separated from the cytoplasm by a two membrance nuclear envelope and compartmentalisation of a function in distinct cytoplasmic organelles.

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Nucleoid

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Region of cell in a bacterium that contains the DNA.

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Components of Bacteria

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- Cell Wall
- Cell Membrane
- Cytoplasm
- Ribosomes
- Flagella (sometimes)

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Eukaryotes

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Organism whose cells have chromosomes with nucleosomal structure and separated from the cytoplasm by a two membrance nuclear envelope and compartmentalisation of a function in distinct cytoplasmic organelles.
- protists, fungi, plants, animals
- cell wall in fungi & plants only
- has a nucleus
- membrane bound organelles

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Organelle

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Specific, usually subcellular, particles of membrane-bound organised living substances present in practically all eukaryotic cells, including mitochondria, the golgi complex, endoplasmic reticulum, lysosomes, centrioles and the cell centre, as well as the plastids of plant cells. Includes also the minute organs of protozoa concerned with such functions as locomotion and metabolism.

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Cytosol

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The fluid portion of a cells cytoplasm.

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Cytoplasm

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The protoplasm of a cell exclusive of that of the nucleus, it consists of a continuous aqueous solution (cytosol) and the organelles and inclusions suspended in it (phaneroplasm) and is the site of most of the chemical activities of the cell.

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Chromosome

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The self-replicating genetic structures of cells containing the cellular DNA that bears in its nucleotide sequence the linear array of genes.

The DNA of eukaryotes is subdivided into chromosomes, that consist of a number of chromosomes whose DNA is associated with various proteins. The chromosomes become more tightly packed at mitosis and become aligned on the metaphase plate. Each chromosome has a characteristic length and banding pattern.

In prokaryotes, chromosomal DNA is circular, and the entire genome is carried on one chromosome.

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Nucleus

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The major organelle of eukaryotic cells, in which the chromosomes are separated from the cytoplasm by the nuclear envelope.

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Centrioles

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They are self-replicating, short, fibrous, rod-shaped organelles of animal cells. Each centriole is a short cylinder containing nine pairs of peripheral microtubules, arranged orthogonally so as to form the wall of the cylinder.

Almost identical to basal body of cilium. The pericentriolar material, but not the centriole itself, is the major microtubule organising centre of the cell.

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Cytoskeleton

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That part of the cytoplasm that remains when organelles and internal membrane systems are removed. It consistes of microtubules, microfilaments, intermediate fibers and other accessory proteins.

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Microtubule

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Cytoplasmic tubule, 25nm outside diameter with a 5nm thick wall. Made of tubulin heterodimers packed in a three start helix (or of 13 protofilaments looked at another way) and associated with various other proteins (MAPs, dynein, kinesin). Microtubules of the ciliary axoneme are more permanent than cytoplasmic and spindle microtubules.

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Microfilament

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Cytoplasmic filament of F actin (5-7nm) which function in structure and movement in eukaryotic cells.

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Ribosome

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A small particulate organelle found in prokaryotes and eukaryotes and also within mitochondria and chloroplasts, but differing in size and composition. Made of two subunits, each being an RNA protein complex. Ribosomes are responsible for the translation of mRNA which may occur in the cytoplasm (see polyribosomes) or on rough endoplasmic reticulum.

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Endoplasmic Reticulum (ER)

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Membrane system that ramifies through the cytoplasm. The membranes of the endoplasmic reticulum are separated by 50-200 nm and the cisternal space thus enclosed constitutes a separate compartment. It is involved in the transport of materials throughout the cell.

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Smooth Endoplasmic Reticulum (SER)

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An internal membrane structure of the eukaryotic cell. Biochemically similar to the rough endoplasmic reticulum, but lacks the ribosome binding function.

Tends to be tubular rather than sheet like, may be separate from the rough endoplasmic reticulum or may be an extension of it. Abundant in cells concerned with lipid metabolism and proliferates in hepatocytes when animals are challenged with lipophilic drugs.

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Rough Endoplasmic Reticulum (RER)

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Membrane organelle of eukaryotes that forms sheets and tubules.

Contains the receptor for the signal receptor particle and binds ribosomes engaged in translating mRNA for secreted proteins and the majority of transmembrane proteins. Also a site of membrane lipid synthesis. The membrane is very similar to the nuclear outer membrane. The lumen contains a number of proteins that possess the C terminal signal KDEL.

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Golgi Apparatus

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Intracellular stack of membrane bounded vesicles in which glycosylation and packaging of secreted proteins takes place. Part of the GERL complex.

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GERL Complex

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An acronym which stands for the Golgi apparatus, the Endoplasmic Reticulum, and the Lysosomes. It refers to a system of different organelles and vesicles within a eukaryotic cell which together make, secrete, and degrade proteins.

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Vesicles

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A closed membrane shell, derived from membranes either by a physiological process (budding) or mechanically by sonication.

Vesicles of dimensions in excess of 50nm are believed to be important in intracellular transport processes.

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Lysosome

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Membrane-bound vesicles that contain hydrolytic enzymes involved in intracellular digestion. Lysosomes fuse with endocytotic vacuoles, thereby breaking down the material ingested by the cell. They can also break down old cell components to release their molecular building blocks into the cytosol for reuse.

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Autolysis

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Spontaneous lysis (rupture) of cells or organelles produced by the release of internal hydrolyic enzymes. Normally associated with the release of lysosomal enzymes.

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Cell Membrane

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The structure enveloping a cell composed of a phospholipid bilayer embedded with proteins. One or both ends of the proteins may extend beyond the layer and act as receptors or cell adhesion molecules.

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Phospholipid Bilayer

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The phospholipid bilayer describes the composition of the cell membrane. It has both a hydrophilic (outer) phosphoric acid and a hydrophobic (inner) fatty acid region. This allows the layer to regulate the passage of material in and out of the cell. Small, non-polar molecules generally pass freely through the membrane but large molecules and charged ions do not and require carrier proteins to cross the membrane.

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Receptor

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A molecular structure within a cell or on the surface characterised by selective binding of a specific substance and a specific physiologic effect that accompanies the binding, for example, cell surface receptors for peptide hormones, neurotransmitters, antigens, complement fragments and immunoglobulins and cytoplasmic receptors for steroid hormones.

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kUnit I FC 1.1: The role of hemoglobin in oxygen transport to tissues.

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• Hemoglobin is packaged in RBCs that circulate throughout the body via the bloodstream.
• In the lungs, hemoglobin picks up oxygen molecules that bind loosely to the iron in the centre of each heme group.
• In the tissues, oxygen detaches from hemoglobin to move down its concentration gradient and into tissue cells.
• Each molecule of hemoglobin can bind four oxygen molecules.
• Each RBC contains approximately 250 x 106 hemoglobin molecules.

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Pinocytosis

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Uptake of fluid filled vesicles into cells (endocytosis). Macro pinocytosis and micro pinocytosis are distinct processes, the latter being energy independent and involving the formation of receptor ligand clusters on the outside of the plasma membrane and clathrin on the cytoplasmic face.

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Nuclear Membrane

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An intracellular structure, consisting of two concentric membranes, enclosing the nucleoplasm and separating it from the cytoplasm. The areas where these two membranes fuse are called nuclear pore complexes.

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Envelope

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An intracellular structure, consisting of two concentric membranes, enclosing the nucleoplasm and separating it from the cytoplasm. The areas where these two membranes fuse are called nuclear pore complexes.

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Nuclear Pores

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Openings in the nuclear envelope, diameter about 10 nm, through which molecules such as nuclear proteins synthesised in the cytoplasm) and mRNA must pass. Pores are generated by a large protein assembly.

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Histones

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Proteins associated with DNA in chromosomes.

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Nucleolus

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A small dense body (sub organelle) within the nucleus of eukaryotic cells, visible by phase contrast and interference microscopy in live cells throughout interphase. Contains RNA and protein and is the site of synthesis of ribosomal RNA. The nucleolus surrounds a region of one or more chromosomes (the nucleolar organiser) in which are repeated copies of the DNA coding for ribosomal RNA.

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Ribosomes

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Small cellular components composed of specialised ribosomal RNA and ribonucleic acid (RNA).

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Secretory Vesicles

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Membrane bounded vesicle derived from the Golgi apparatus and containing material that is to be released from the cell. The contents may be densely packed, often in an inactive precursor form (zymogen).

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Exocytosis

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Release of material from the cell by fusion of a membrane bounded vesicle with the plasma membrane.

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Vesicle

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A closed membrane shell, derived from membranes either by a physiological process (budding) or mechanically by sonication. Vesicles of dimensions in excess of 50nm are believed to be important in intracellular transport processes.

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Vacuole

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Membrane bounded vesicle of eukaryotic cells. Secretory, endocytotic and phagocytotic vesicles can be termed vacuoles. Botanists tend to confine the term to the large vesicles found in plant cells that provide both storage and space filling functions.

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Lysosome

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Membrane bounded cytoplasmic organelle containing a variety of hydrolytic enzymes that can be released into a phagosome or to the exterior. Release of lysosomal enzymes in a dead cell leads to autolysis (and is the reason for hanging game, to tenderise the muscle), but it is misleading to refer to lysosomes as suicide bags, since this is certainly not their normal function. Part of the GERL complex or trans Golgi network.

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Autolysis

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Spontaneous lysis (rupture) of cells or organelles produced by the release of internal hydrolyic enzymes. Normally associated with the release of lysosomal enzymes.

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Microbodies

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Membrane-bound cytoplasmic particles containing enzymes and other substances.

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Peroxisome

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Organelle containing peroxidase and catalase, sometimes as a large crystal. A site of oxygen utilisation, but not of ATP synthesis. In plants, associated with chloroplasts in photorespiration and considered to be part of a larger group of organelles, the microbodies.

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Glyoxysome

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An organelle found only in plants that stores the enzymes responsible for the conversion of lipids to carbohydrates.

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Mitochondria

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A small intracellular organelle which is responsible for energy production and cellular respiration. They are semiautonomous, which means they carry their own DNA and ribosomes, which enable them to produce some of their own proteins and self replicate by binary fission. They are believed to have been early prokaryotic cells that evolved a symbiotic relationship with the ancestors of eukaryotic cells.

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Intermembrane Space

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Region between the two membranes of mitochondria and chloroplasts. On the endosymbiont hypothesis, this space would represent the original phagosome.

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Cristae

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The folded membranes inside a mitochondrion.

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Mitochondrial Matrix

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The substance occupying the space enclosed by the inner membrane of a mitochondrion; it contains enzymes, filaments of DNA, ribosomes, granules, and inclusions of protein crystals, glycogen, and lipid.

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Semiautonomous

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Of systems or processes that are not wholly independent of other systems or processes

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Binary fission

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Division of a cell into two daughter cells after DNA replication and nuclear division (mitosis). A form of asexual reproduction.

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Cell Wall

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In plants the primary wall is pectin rich, the secondary wall mostly composed of cellulose. In bacteria, cell wall structure is complex: the walls of gram-positive and gram-negative bacteria are distinctly different. Removal of the wall leaves a protoplast or spheroplast.

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Centriole

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They are self-replicating, short, fibrous, rod-shaped organelles of animal cells. Each centriole is a short cylinder containing nine pairs of peripheral microtubules, arranged orthogonally so as to form the wall of the cylinder. Almost identical to basal body of cilium. The pericentriolar material, but not the centriole itself, is the major microtubule organising centre of the cell. Centrioles divide prior to mitosis and the daughter centrioles and their associated pericentriolar material come to lie at the poles of the spindle.

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Centrosome

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The microtubule organising centre which, in animal cells, surrounds the centriole and which will divide to organise the two poles of the mitotic spindle. By directing the assembly of a cells skeleton, this organelle controls division, motility and shape.

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Cytoskeleton

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That part of the cytoplasm that remains when organelles and internal membrane systems are removed.

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Tubulin

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Abundant cytoplasmic protein (55 kD), found mainly in two forms, _ and _. A tubulin heterodimer (one _, one _), constitutes the protomer for microtubule assembly. Multiple copies of tubulin genes are present (and are expressed) in most eukaryotic cells studied so far. The different tubulin isoforms seem, however, to be functionally equivalent. _ _ tubulin is localised in centrosome and is involved in nucleation of microtubule assembly during the cell cycle. Highly conserved from yeast to mammals.

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Cilia

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Microtubular, hairlike structures that some single-celled organisms use for locomotion through liquids. They also cover the cells of certain tissues, such as the epithelium lining the lungs, and help those cells sweep away fluids or particles. Cilia are smaller than flagella.

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Flagella

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In bacteria, a whiplike motility appendage present on the surface of some species. Flagella are composed of a protein called flagellin. Bacteria can have a single flagellum, a tuft at one pole, or multiple flagella covering the entire surface. In eukaryotes, flagella are threadlike protoplasmic extensions used to propel flagellates and sperm. Flagella have the same basic structure as cilia but are longer in proportion to the cell bearing them and present in much smaller numbers.

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Actin

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A filamentous proteins (42 kD) involved in muscle contraction in both smooth and striated muscle and also serves as an important structural molecule for the cytoskeleton of many eukaryotic cells. It is the main constituent of the thin filaments of muscle fibres. The filaments (known also as filamentous or f-actin) can be dissociated into their globular subunits; each subunit is composed of a single polypeptide 375 amino acids long. This is known as globular or g-actin. In conjunction with myosin, actin is responsible for the contraction and relaxation of muscle.

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Myosin

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A family of motor ATPases that interact with F actin filaments.

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Simple Diffusion

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The process of becoming diffused or widely spread, the spontaneous movement of molecules or other particles in solution, owing to their random thermal motion, to reach a uniform concentration throughout the solvent, a process requiring no addition of energy to the system.

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Osmosis

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The movement of solvent through a membrane impermeable to solute, in order to balance the chemical potential due to the concentration differences on each side of the membrane. Frequently mis used in the popular press.

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Hypertonic

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Of a fluid, sufficiently concentrated to cause osmotic shrinkage of cells immersed in it. Note that a mildly hyperosmotic solution is not necessarily hypertonic for viable cells, that are capable of regulating their volumes by active transport.

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Hypotonic

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Having a lesser osmotic pressure than a reference solution, which is ordinarily assumed to be blood plasma or interstitial fluid; more specifically, refers to a fluid in which cells would swell.

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Lyse

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To break up, to disintegrate, to effect lysis

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Isotonic

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A biological term denoting a solution in which body cells can be bathed without a net flow of water across the semipermeable cell membrane. Also, denoting a solution having the same tonicity as some other solution with which it is compared, such as physiologic salt solution and the blood serum.

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Facilitated Diffusion

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A process by which substances are conveyed across cell membranes faster than would be possible by diffusion alone. This is generally achieved by proteins that provide a hydrophilic environment for polar molecules throughout their passage through the plasma membrane, acting as either shuttles or pores.

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Active Transport

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Transport of ions, nutrients or other molecules into a cell against a concentration gradient, this requires the expenditure of energy through ATP hydrolysis.

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Endocytosis

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Uptake of material into a cell by the formation of a membrane bound vesicle.

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Phagocytosis

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Endocytosis of particulate material, such as microorganisms or cell fragments. The material is taken into the cell in membrane bound vesicles (phagosomes) that originate as pinched off invaginations of the plasma membrane. Phagosomes fuse with lysosomes, forming phagolysosomes in which the engulfed material is killed and digested.

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Exocytosis

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Release of material from the cell by fusion of a membrane bounded vesicle with the plasma membrane.

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Tissue

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A group of similar cells united to perform a specific function.

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Epithelial Tissue

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The covering of internal and external surfaces of the body, including the lining of vessels and other small cavities. It consists of cells joined by small amounts of cementing substances. Epithelium is classified into types on the basis of the number of layers deep and the shape of the superficial cells.

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Connective Tissue

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Rather general term for mesodermally derived tissue that may be more or less specialised. Cartilage and bone are specialised connective tissue, as is blood, but the term is probably better reserved for the less specialised tissue that is rich in extracellular matrix (collagen, proteoglycan etc.) and that surrounds other more highly ordered tissues and organs.

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Nervous Tissue

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A highly differentiated tissue composed of nerve cells, nerve fibres, dendrites, and a supporting tissue

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Muscle Tissue

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Tissue specialised for contraction. See twitch muscle, catch muscle: Cardiac muscle (heart muscle) is a striated but involuntary muscle responsible for the pumping activity of the vertebrate heart. The individual muscle cells are joined through a junctional complex known as the intercalated disc and are not fused together into multinucleate structures as they are in skeletal muscle. Skeletal muscle is a rather non-specific term usually applied to the striated muscle of vertebrates that is under voluntary control. The muscle fibres are syncytial and contain myofibrils, tandem arrays of sarcomeres. Smooth muscle is muscle tissue in vertebrates made up from long tapering cells that may be anything from 20-500m long. Smooth muscle is generally involuntary and differs from striated muscle in the much higher actin/myosin ratio, the absence of conspicuous sarcomeres and the ability to contract to a much smaller fraction of its resting length. Smooth muscle cells are found particularly in blood vessel walls, surrounding the intestine (especially the gizzard in birds) and in the uterus. The contractile system and its control resemble those of motile tissue cells (for example fibroblasts, leucocytes) and antibodies against smooth muscle myosin will cross react with myosin from tissue cells, whereas antibodies against skeletal muscle myosin will not.

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Virus

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Viruses are obligate intracellular parasites of living but noncellular nature, consisting of DNA or RNA and a protein coat. They range in diameter from 20-300nm.

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Capsid

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A protein coat that covers the nucleoprotein core or nucleic acid of a virion. Commonly shows icosahedral symmetry and may itself be enclosed in an envelope (as in the Togaviridae). The capsid is built up of subunits (some integer multiple of 60, the number required to give strict icosahedral symmetry) that self assemble in a pattern typical of a particular virus. The subunits are often packed, in smaller capsids, into 5 or 6 membered rings (pentamers or hexamers) that constitute the morphological unit (capsomere). The packing of subunits is not perfectly symmetrical in most cases and some units may have strained interactions and are said to have quasi equivalence of bonding to adjacent units.

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Obligate

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Without an alternative system or pathway

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Virion

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A single virus particle, complete with coat.

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Bacteriophage

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Viruses that have a specific affinity for and infect bacteria.