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Chapter 8 Section 2
Key Ideas
 What
determines the direction in which
passive transport occurs?
 Why is osmosis important?
 How do substances move against their
concentration gradients?
What’s the difference
between active and passive
 In
active transport, the cell is required to
use energy to move a substance.
 In passive transport, the cell does not use
Passive Transport
 When
a space is filled evenly, a state
called equilibrium is reached.
 The amount of a particular substance in a
given volume is called the concentration
of the substance.
 When one area has a higher
concentration than another area does, a
concentration gradient exists.
Substances move from an area of higher
concentration to an area of lower
This movement down the concentration
gradient is called diffusion.
Some substances enter and leave the cell by
diffusion across the cell membrane. In
passive transport, substances cross the cell
membrane down their concentration
Simple Diffusion
Occurs when small, nonpolar molecules can
pass directly through the lipid bilayer.
Oxygen diffuses into the cell through the lipid
bilayer. The concentration of oxygen is higher
outside the cell than it is inside. Oxygen
moves DOWN the concentration gradient.
Carbon dioxide is often higher inside the cell
than it is outside. So carbon dioxide diffuses
out of the cell.
Facilitated Diffusion
 Many
ions and polar molecules do not
diffuse easily through the nonpolar lipid
 During facilitated diffusion, transport
proteins help these substances diffuse
through the cell membrane.
 Two types of transport proteins are
channel proteins and carrier proteins.
Channel Proteins
Ions, sugars, and amino acids can diffuse
through the cell membrane through channel
They serve are tunnels through the lipid
Each channel allows for the diffusion of
specific substances that have the right size
and charge. Example: only sodium ions can
pass through the sodium ion channel.
Carrier Proteins
Carrier proteins transport substances that fit
within their binding site.
A carrier protein binds to a specific substance
on one side of the cell membrane.
This binding causes the protein to change
As the protein’s shape changes, the
substance is moved across the membrane
and is released on the other side.
Water can diffuse across a selectively
permeable membrane in a process called
It’s a type of passive transport.
Allows cells to maintain water balance as
their environment changes.
Polar water molecules do not diffuse directly
through the bilayer. The cell membrane
contains channel proteins that only water
molecules can pass through. It is a form of
facilitated diffusion.
Predicting Water Movement
Water moves out: if the solution is hypertonic,
water moves out of the cell. The cell loses
water and shrinks.
Water moves in: if the solution is hypotonic,
water moves into the cell. The cell gains
water and expands in size.
No net change in water movement: if the
solution is isotonic, water diffuses into and out
of the cell at equal rates. The cell stays the
same size.
Effects of Osmosis
If left unchecked, the swelling caused by a
hypotonic solution could cause a cell to burst.
Rigid cell walls of plants and fungi prevent the
cells from expanding too much. Many plants
are healthiest in a hypotonic state.
Some unicellular eukaryotes have contractile
vacuoles, which collect excess water inside
the cell land force the water out of the cell.
 Animal
cells have neither cell walls nor
contractile vacuoles.
 But most animal cells can avoid swelling
caused by osmosis by actively removing
solutes from the cytoplasm.
Active Transport
 Sometimes
cells must transport substances
against their concentration gradients. This
is called active transport because the
cells must use energy to move these
 The energy needed for active transport is
supplied by ATP.
 Many
active transport processes use
carrier proteins to move substances. The
carrier proteins require energy to “pump”
substances against their concentration
Sodium-potassium pump
 http://highered.mcgraw-
 Page 182 diagram
Endocytosis & Exocytosis
 Many
substances are too large to be
transported by carrier proteins. Instead,
they move across the cell membrane in