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Passive Transport in the Cell
Osmosis and Diffusion
Homeostasis
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The goal of all cells at all times is to stay in balance. This
is referred to as HOMEOSTASIS. Cells and organisms
will do whatever it takes to keep the inside in a state of
homeostasis regardless of any changes that are happening
outside.
Brownian Motion
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All molecules in all substances are vibrating or moving at
all times.
In a solid, the movements are very small.
In a liquid, such as the fluids both surrounding and inside a
cell, these movements are bigger.
As the molecules move around, they pass through the cell
membrane.
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They always move from a region where there is more to
a region where there is less.
This is called the concentration gradient.
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animat
ion__how_diffusion_works.html
Diffusion
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This is the movement of molecules such as Oxygen, Sugar,
and Carbon Dioxide from one region to another.
Because these are relatively small molecules, they can
pass through the cell membrane until they are equal on
the inside and the outside. This is referred to as dynamic
equilibrium.
Diffusion works best for small cells, and this is part of the
reason why all cells are microscopic.
Osmosis
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Osmosis specifically refers to the movement of water
molecules.
When water concentration is equal on the inside and
outside of the cell, we refer to this as isotonic.
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When water moves into the cell, we refer to it as
hypotonic.
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Animal cells that have a lot of water move in will burst.
Plant cells that have a lot of water move in will press
against the cell wall and become very firm.
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When water moves out of a cell, it is referred to as
hypertonic.
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Animal cells that have water move out will shrink or
shrivel. Plant cells will do the same thing, but within the
cell wall.
 Note: Diffusion, osmosis, and
facilitated
diffusion (in which a carrier protein helps
larger or charged molecules cross the cell
membrane, p. 56) are all PASSIVE, meaning
that the cell does not have to use any
energy to move materials in or out.
 This
is very beneficial to the cell, but
movements like these must always go in
the direction of the concentration
gradient – from high concentration to
low concentration. What if the cell needs
to move materials AGAINST the
concentration gradient?