Topic 5. The Working Cell

Bio 166

fluid mosaic

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

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.

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 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 they are less concentrated.

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 to gain or lose water.

isotonic

Referring to a solution that, when surrounding a cell, has no effect on the passage of water into or 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 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; involves glycolysis, the citric acid cycle, and oxidative phosphorylation (the electron transport chain and chemiosmosis).

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 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.

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 triphosphate, the main energy source for cells.

phosphorylation

The production of ATP by chemiosmosis during the light reactions of photosynthesis.

activation energy

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

enzymes

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

A specific substance (reactant) on which an enzyme acts. Each enzyme recognizes only the specific substrate or substrates of the reaction it catalyzes.

active site

The part of an enzyme molecule where a substrate molecule attaches (by mean of weak chemical bonds); typically, apocket 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.

cofactors

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. A competitive inhibitor's structure mimics that of the enzyme's 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 inhibitor changes the shape of the enzyme so that the active site no longer effectively catalyzes the conversion of substrate to product.

feedback inhibition

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