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metabolic equilibrium actively maintained by several complex biological mechanisms that operate via the autonomic nervous system to offset disrupting changes; process by which organisms maintain a stable internal environment


the strength of a solution; the amount of solute dissolved in a solvent


the process by which molecules move from an area of higher concentration to an area of lower concentration


diffusion of molecules through a semipermeable membrane from a place of higher concentration to a place of lower concentration until the concentration on both sides is equal

concentration gradient

a difference in the concentration of a substance across a distance


equality of distribution; state where there is no net movement of molecules

passive transport

the movement of substances across a cell membrane without the use of energy by the cell

active transport

energy-requiring process that moves material across a cell membrane against a concentration gradient


process by which a cell takes material into the cell by infolding of the cell membrane


the process by which a substance is released from the cell through a vesicle that transports the substance to the cell surface and then fuses with the membrane to release the substance


the dissolved substance in a solution


a liquid substance capable of dissolving other substances

positive feedback loop

Situation in which a change in a certain direction provides information that causes a system to change further in the same direction.

negative feedback loop

A feedback loop that causes a system to change in the opposite direction from which it is moving

semipermeable membrane

a membrane (as a cell membrane) that allows some molecule to pass through but not others

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