This document defines and describes several key terms related to membrane transport in cells, including: active transport, which uses ATP to move substances against a concentration gradient; diffusion, the passive movement of particles down a concentration gradient; endocytosis and exocytosis, the processes by which cells take in and release materials via membrane vesicles; facilitated diffusion, which uses membrane proteins to assist diffusion; and osmosis, the passive movement of water across a membrane in response to differences in solute concentration. Concentration gradients, semi-permeable membranes, vesicles, and transport pumps are also discussed in the context of membrane transport.

1. Active transport
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movement of substances
across membranes using
energy in the form of ATP.
ATP
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(adenosine triphosphate)
energy molecule.
Concentration
gradient
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a gradient resulting from an
unequal distribution of ions
across the cell membrane.

2. Diffusion
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passive movement of
particles from a region of
higher concentration to a
region of lower
concentration.
Endocytosis
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the process in which the cell
takes in materials from the
outside by infolding of the
membrane to form a vesicle.
Exocytosis
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the process in which the cell
releases materials to the
outside by discharging them
as membrane-bounded
vesicles that pass through
the cell membrane.

3. Facilitated diffusion
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diffusion through a
membrane that requires
proteins.
Hypertonic
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a more concentrated
solution relative to another
fluid.
Hypotonic
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a less concentrated solution
relative to another fluid.

4. Osmoregulation
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control of the water balance
of a living organism.
Osmosis
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passive movement of water
molecules from a region of
lower solute concentration to
a region of higher solute
concentration.
Secretion
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when material is released
from a cell.

5. Semi-permeable
membrane
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membrane that allows some
substances to diffuse through
but not others. Transport pumps
proteins in the plasma
membrane that use ATP to
move substances across the
membrane. Vesicles a bubble-
like membranous structure that
stores and transports cellular
products.