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Fluid Mosaic: A description of membrane structure, depicting a cellular membrane as a mosaic of diverse protein molecules embedded in a fluid bilayers of phospholipid molecules.

Selective Permeability: A property of biological membranes that allow some substances to cross more easily than others and blocks the passage of other substances altogether.

Diffusion: The spontaneous movement of a substance down its concentration gradient from where it is more concentrated to where it is less concentrated.

Concentration Gradient: A region along which the density of a chemical substance increases or decreases. Cells often maintain concentration gradients of ions across their membranes. When a gradient exists, substances tend to move from where they are more concentrated to where th

Passive Transport: The diffusion of a substance across a biological membrane, with no expenditure of energy.

Osmosis: The diffusion of water across a selectively permeable membrane.

Tonicity: The ability of a solution surrounding a cell to cause that cell to gain or lose water.

Isotonic: Referring to a solution that, when surrounding a cell, has no effect on the passage of water into and out of the cell.

Hypotonic: Referring to a solution that, when surrounding a cell, will cause the cell to take up water.

Hypertonic: Referring to a solution that, when surrounding a cell, will cause the cell to lose water.

Osmoregulation: The homeostatic maintenance of solute concentrations and the balance of water gain and loss.

Facilitated Diffusion: The passage of a substance through a specific transport protein across a biological membrane down its concentration gradient.

Aquaporin: A transport protein in the plasma membrane of some plant or animal cells that facilitates the diffusion of water across the membrane (osmosis).

Active Transport: The movement of a substance across a biological membrane against its concentration gradient, aided by specific transport proteins and requiring an input of energy (often as ATP).

Exocytosis: The movement of materials out of the cytoplasm of a cell by the fusion of vesicles with the plasma membrane.

Endocytosis: Cellular uptake of molecules or particles via formation of new vesicles from the plasma membrane.

Phagocytosis: Cellular “eating”; a type of Endocytosis in which a cell engulfs macromolecules, other cells, or particles into its cytoplasm.

Pinocytosis: Cellular “drinking”; a type of Endocytosis in which the cell takes fluid and dissolved solutes into small membranous vesicles.

Receptor-Mediated Endocytosis: The movement of specific molecules into a cell by the inward budding of membranous vesicles, which contain proteins with receptor sites specific to the molecules being taken in.

Energy: The capacity to cause change, especially to perform work.

Kinetic Energy: The energy of motion; the energy of a mass of matter that is moving. Moving matter does work by imparting motion to other matter.

Heat: Thermal energy, the amount of energy associated with the movement of the atoms and molecules in a body of matter. Heat is energy in its most random form.

Potential Energy: The energy that matter possesses because of its location or arrangement. Water behind a dam possesses potential energy, and so do chemical bonds.

Chemical Energy: Energy available in molecules for release in a chemical reaction; a form of potential energy.

Thermodynamics: The study of energy transformation that occurs in a collection of matter.

First Law of Thermodynamics: The principle of conservation of energy. Energy can be transferred and transformed, but it cannot be created or destroyed.

Entropy: A measure of disorder. One form of disorder is heat, which is random molecular motion.

Second Law of Thermodynamics: The principle stating that every energy conversion reduces the order of the universe, increasing its entropy. Ordered forms of energy are at least partly converted to heat.

Cellular Respiration: The aerobic harvesting of energy from food molecules; the energy-releasing chemical breakdown of food molecules, such as glucose, and the storage of potential energy in a form that cells can use to perform work.

Exergonic Reaction: An energy-releasing chemical reaction in which the reactants contain more potential energy than the products. The reaction releases an amount of energy equal to the difference in potential energy between the reactants and the products.

Endergonic Reactions: An energy-requiring chemical reaction, which yields products with more potential energy than the reactants. The amount of energy stored in the products equals the difference between the potential energy in the reactants and that in the products.

Metabolism: The totality of an organism’s chemical reactions.

Metabolic Pathway: A series of chemical reactions that either builds a complex molecule or breaks down a complex molecule into simpler compounds.

Energy Coupling: In cellular metabolism, the use of energy released from an exergonic reaction to drive an endergonic reaction.

ATP: Adenosine triposphate, the main energy source for cells.

Phosphorylation: The transfer of a phosphate group, usually from ATP, to a molecule. Nearly all cellular work depends on ATP energizing other molecules by Phosphorylation.

Activation Energy: The amount of energy that reactants must absorb before a chemical reaction will start.

Enzyme: A macromolecule, usually a protein, that serves as a biological catalyst, changing the rate of a chemical reaction without being consumed by the reaction.

Substrate: (1) A specific substance (reactant) on which an enzyme acts. Each enzyme recognizes only the specific substrate or substrates of the reaction it catalyzes. (2) A surface in or on which an organism lives.

Active Site: The part of an enzyme molecule where a substrate molecule attaches (by means of weak chemical bonds); typically, a pocket or groove on the enzyme’s surface.

Induced Fit: The change in shape of the active site of an enzyme, caused by entry of the substrate, so that it binds more snugly to the substrate.

Cofactor: A nonprotein molecule or ion that is required for the proper functioning of an enzyme.

Coenzyme: An organic molecule serving as a cofactor. Most vitamins function as coenzymes in important metabolic reactions.

Competitive Inhibitor: A substance that reduces the activity of an enzyme by binding to the enzyme’s active site in place of the substrate.

Noncompetitive Inhibitor: A substance that reduces the activity of an enzyme without entering an active site. By binding elsewhere on the enzyme, a noncompetitive e inhibitor changes the shape of the enzyme so that the active sit no longer effectively catalyzes the conversion of s

Feedback Inhibitor: A method of metabolic control in which a product of a metabolic pathway acts as an inhibitor of an enzyme within that pathway.

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Definitions from Wiktionary under the GNU FDL.
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