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Campbell Biology Concepts and Connections The Working Cell

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).

adenosine triphosphate (ATP)

Main energy source for cells.


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

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; involves glycolysis, the citric acid cycle, and oxidative phosphorylation (the electron transport chain and chemiosmosis).

chemical energy

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


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


A nonprotein molecule or ion that is required for the proper functioning of an enzyme. See also coenzyme.

competitive inhibitor

A substance that reduces the activity of an enzyme by binding to the enzyme's active site in place of the substrate. A competitive inhibitor's structure mimics that of the enzyme's substrate.

concentration gradient

An increase or decrease in the density of a chemical substance in an area. 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 they are less concentrated.


The spontaneous tendency of a substance to move down its concentration gradient from where it is more concentrated to where it is less concentrated.

endergonic reaction

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.


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


The capacity to perform work, or to rearrange matter.

energy coupling

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


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


A protein (or RNA molecule) that serves as a biological catalyst, changing the rate of a chemical reaction without itself being changed into a different molecule in the process.

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.


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

facilitated diffusion

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

feedback inhibition

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

first law of thermodynamics

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

fluid mosaic

A description of membrane structure, depicting a cellular membrane as a mosaic of diverse protein molecules embedded in a fluid bilayer made of phospholipid molecules.


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.

hypertonic solution

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

hypotonic solution

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

induced fit

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

isotonic solution

A solution having the same solute concentration as another solution, thus having no effect on passage of water in or out of the cell.

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.


The totality of an organism's chemical reactions.

noncompetitive inhibitor

A substance that impedes the activity of an enzyme without entering an active site. By binding elsewhere on the enzyme, a noncompetitive inhibitor changes the shape of the enzyme so that the active site no longer functions.


Method by which organisms regulate solute concentrations and balance the gain and loss of water.


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.


Cellular "eating"; a type of endocytosis whereby a cell engulfs macromolecules, other cells, or particles into its cytoplasm.


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


Cellular "drinking"; a type of endocytosis in which the cell takes fluid and dissolved solutes into small membranous vesicles.

potential energy

The energy that matter possesses because of its location or arrangement. Water behind a dam and chemical bonds possess potential energy.

receptor-mediated endocytosis

The movement of specific molecules into a cell by the inward budding of membranous vesicles. The vesicles contain proteins with receptor sites specific to the molecules being taken in.

second law of thermodynamics

The principle whereby every energy conversion reduces the order of the universe, increasing its entropy. Ordered forms of energy are at least partly converted to heat.

selective permeability

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


(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.


The study of energy transformation that occurs in a collection of matter. See first law of thermodynamics; second law of thermodynamics.


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

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