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Ch 5: The Working Cell

Quiz yourself by thinking what should be in each of the black spaces below before clicking on it to display the answer.

activation energy   The amount of energy that reactants must absorb before a chemical reaction will start.  
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.  
active transport   The movement of a substance across a biological membrane against its concentration gradient, aided by specific transport proteins and requiring input of energy (often as ATP).  
ADP   A molecule composed of adenosine and two phosphate groups. The molecule ATP is made by combining a molecule of ADP with a third phosphate in an energy-consuming reaction.  
ATP   A molecule composed of adenosine and three phosphate groups; the main energy source for cells.  
calorie   The amount of energy that raises the temperature of 1 g of water by 1°C.  
chemical energy   Energy stored in the chemical bonds of molecules; a form of potential energy.  
concentration gradient   An increase or decrease in the density of a chemical substance within a given region. Cells often maintain concentration gradients of hydrogen ions across their membranes. When a gradient exists, the ions or other chemical substances involved tend to move from where they are more concentrated to where they are less concentrated.  
conservation of energy   The principle that energy can neither be created nor destroyed.  
diffusion   The spontaneous movement of particles of any kind down a concentration gradient; that is, movement of particles from where they are more concentrated to where they are less concentrated.  
endocytosis   The movement of materials into the cytoplasm of a cell via vesicles or vacuoles.  
Energy   The capacity to perform work, or to move matter in a direction it would not move if left alone.  
Entropy   A measure of disorder, or randomness. One form of disorder is heat, which is random molecular motion.  
enzyme inhibitors   A chemical that interferes with an enzyme's activity by changing the enzyme's shape, either by plugging up the active site or binding to another site on the enzyme.  
enzymes   A protein that serves as a biological catalyst, changing the rate of a chemical reaction without itself being changed in the process.  
exocytosis   The movement of materials out of the cytoplasm of a cell via membranous vesicles or vacuoles.  
facilitated diffusion   The passage of a substance across a biological membrane down its concentration gradient, aided by specific transport proteins.  
feedback regulation   A method of metabolic control in which the end product of a metabolic pathway acts as an inhibitor of an enzyme within that pathway.  
hypertonic   In comparing two solutions, referring to the one with the greater concentration of solutes.  
hypotonic   In comparing two solutions, referring to the one with the lower concentration of solutes.  
induced fit   The interaction between a substrate molecule and the active site of an enzyme, which changes shape slightly to embrace the substrate and catalyze the reaction.  
isotonic   Having the same solute concentration as another solution.  
kinetic energy   Energy of motion. Moving matter performs work by transferring its motion to other matter, such as leg muscles pushing bicycle pedals.  
metabolism   The total of all the chemical reactions in an organism.  
osmoregulation   The control of the gain or loss of water and dissolved solutes in an organism.  
osmosis   The diffusion of water across a selectively permeable membrane.  
passive transport   The diffusion of a substance across a biological membrane without any input of energy.  
phagocytosis   Cellular "eating"; a type of endocytosis whereby a cell engulfs large molecules, 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.  
plasmolysis   A phenomenon that occurs in plant cells in a hypertonic environment. The cell loses water and shrivels, and its plasma membrane pulls away from the cell wall, usually killing the cell.  
Potential energy   Stored energy; the energy that an object has due to its location and/or arrangement. Water behind a dam and chemical bonds both possess potential energy.  
receptor-mediated endocytosis   The movement of specific molecules into a cell by the inward budding of vesicles. The vesicles contain proteins with receptor sites specific to the molecules being taken in.  
signal transduction pathway   A series of molecular changes that converts a signal on a target cell's surface to a specific response inside the cell.  
substrate   (1) A specific substance (reactant) on which an enzyme acts. Each enzyme recognizes only the specific substrate of the reaction it catalyzes. (2) A surface in or on which an organism lives.  
transport protein   A membrane protein that helps move substances across a cell membrane.  


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