Unformatted text preview: Click the Exit Button: Water transport has a special status in physiology because of its inﬂuence on cell volume and
because its movement has its own unique terminology. Under normal conditions most cells in the body neither swell not
shrink because there is no net water movement across the membrane. Forces hat might cause water to cross membranes
are absent However, in some locations in the body the movement of water is a normal occurrence. In glands, for example,
water must be transported across specialized epithelial cells. As well, when you drink water it moves across epithelial
cells lining the intestine to be absorbed into the bloodstream. Water transport is always waive, is unaffected by membrane mtentials, Won ' t. The ﬂow of water across a membrane down its concentration gradient is called osmosis. &—" On the screen you see a condition that drives osmosis. At body temperature the concentration of pure water is 55.5 molar.
The Wan in intracellular ﬂuid is 3W3 molar). Thus, W
concentration is roughly 55.2 molar (55.5 e 0.3 molar). In this case there is a 0.3 molar smaller water concentration inside
the cell than outside. Water molecules will move passively into the cell down their concentration gradient, causing the cell
to swell and eventually burst. Our-ost- 'Spcclucmotvnbrtnmpoﬂ Go to the next screen: The direction of passive water
ﬂow into or out of a cell depends on the direction of the water m b' concentration gradient across the plasma membrane. In the
‘3’ MM" case shown on the screen water moves out of the cell because
i? W" WM total solute concentration is lower inside the cell than it is
7—} m” m W” ”W” W .. ~ » .— outside, which makes the water concentration higher inside 4) the cell. ““1 ”it ‘ """""" Osmosis refers solute con All H: n . ., museum I: e concentration of water in body ﬂuid
is only about 0.2% lower than pure water. A difference in the
total solute concentration across a membrane is always a accompanied by a difference in water concentration; it is ' ’ ssible to have one without the other. The total solute
pa’m'cleconcennamm' Inﬁgn is gown as igos§l§§ °""°"' ' W m °"""" ”“9“ Go to the next screen: Here are some important
terms you need to know. Go to the next screen: We oﬁen use the term Gaunt-dry Totucomnmon dwmmtwm osmotic pressure in place of osmolarity. When a
solution’s osmolarity is based on its total solute
concentration, its tom is determined by how it affgts 333“ ”W ‘ “an“ ”th'm” ”9"" cell volume: which depends not only on the solute Ice-ounce Twaduuommvnmmohdry l . . .
\ “a o “" " " concentratlon but also on the solute permeablllty of cell
a? ”Law "WWW“ ‘ ”hm" ““5"” °""°“'"’ " W" “WW" membranes. A solution is isotonic when it does not alter cell volume; the cell neither shrinks nor swells. A
solution that causes cells to shrink is hypertonic. A 5.0a M solution that causes cells to swell is hypotonic. New} c — c Rug 5 call t-u burs
08m - M can at “W - ' . . .
a." "I.“ a... Chck the See Example Button. The distinction between
hmmmmm osmolarity and tomcity is best illustrawd when a cell is placed in a 300 mOsm solution of urea, a substance that permeates
most cell membranes readily. The osmolarity of this solution
equals the initial osmolarity inside the cell (300m0sm), making
it iso-osmotic with the intracellular ﬂuid. Under these
condition, water does not move into or out of the cell because
the water concentration is the same on both sides of the cell
membrane. Over time urea moves into the cell due to the initial
presence of an inwardly directed concentration gradient.
Intracellular solutes, in contrast, are retained inside the cell "ML! mmwwmdhm
mum-mum m... K- 0 Page7ofll ...
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- Spring '07