Page 72 - Demo 1

PASSIVE TRANSPORT

Passive transport happens when substances cross the plasma
membrane without any input of energy from the cell. There are different types
of passive transport; however, in all cases, substances move down a
concentraon gradient. No energy is needed because molecules move from an
area where they have a higher concentraon to an area where they have a lower
concentraon. Concentration refers to the number of parcles of a substance per
unit of volume such that the more the parcles of a substance in a given volume,
the higher the concentraon. A substance always moves from an area where it
is more concentrated to an area where it is less concentrated. There are
several different types of passive transport, including diffusion, facilitated
diffusion, and osmosis.

Diffusion

Most molecules are in constant moon, and how a molecule moves is a
totally random event. If two kinds of molecules are combined, they soon mix
and the random movement of molecules always tends to generate a uniform
mixture.

During diffusion, substances move down their concentraon gradient
unl they are distributed equally to achieve equilibrium. This process does not
require a cell membrane because molecules are in moon. In cells, substances
simply move from the side of the cell membrane where it is more concentrated
to the side where it is less concentrated. Substances that can squeeze between
the lipid molecules in the plasma membrane by simple diffusion are generally
very small, hydrophobic molecules, such as molecules of oxygen and carbon
dioxide.

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