Download 7.3: Passive transport is diffusion of a substance across a membrane

7.3: Passive transport is diffusion of a substance across a membrane with no
energy investment

diffusion: movement of molecules of any substance so that they spread out evenly into
the available space
In the absence of other forces, a substance will diffuse from where it is more concentrated to
where it is less concentrated.

 Basically, a substance moves down its concentration gradient.
Passive transport: diffusion of a substance across a biological membrane is called passive
transport because a cell does not energy to make this happen.
Effects of Osmosis on Water Balance
Water diffuses across the membrane from the region of lower solute concentration to that of
higher solute concentration until the solute concentrations on both sides of the membrane are
equal.

Osmosis: diffusion of water across a selectively permeable membrane
Water Balance of Cells Without Walls




tonicity: ability of a solution to cause a cell to gain or lose water
isotonic: when a cell’s environment has no effect on the passage of water into and out of
the cell because it’s the same as inside of the cell; no net movement
hypertonic: a solution that, when surrounding a cell, will cause a cell to lose water; the
cell will most likely shrivel up and die
 high solute, low solvent
hypotonic: a solution that causes water to enter the cell faster than it leaves; the cell will
most likely swell and burst
 low solute, high solvent
A cell without rigid walls cannot tolerate excessive intake or excessive loss of water

osmoregulation: control of water balance
Water Balance of Cells with Walls
The cell’s wall helps maintain the cell’s water balance.



turgid: a cell can become very firm because the cell wall doesn’t allow the cell to expand
too much when water goes in
flaccid: a cell can become very limp if the cell and its environment are isotonic and no
water enters the cell
plasmolysis: in a hypertonic environment, the cell shrivels and causes the plant to dies
Facilitated Diffusion: Passive Transport Aided by Proteins

facilitated diffusion: the spontaneous passage of molecules or ions across a biological
membrane with the assistance of specific transmembrane transfer proteins
 most transport proteins are very specific, allowing only certain substances in
Two types of transport proteins:
1. channel proteins: provide corridors that allow specific molecules or ions to cross the
membrane
ex: aquaporins (water channels) and ion channels (open or close in response to a stimulus)
2. carrier proteins: alternates between two shapes, moving a solute across a membrane during
the shape change