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Aka semipermeable
Means that the cell membrane allows some,
but not all, molecules to cross.
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Small nonpolar molecules- don’t want to
bond to anything so they slip through the
membrane (CO2 and O2)
Water- so small it slips through between the
heads of phospholipids and the tails kick it
out on the other side.
Anything bigger or polar need special
assistance.

Receptors- proteins that detects a signal
molecule and performs an action in response.
◦ Look for specific molecule then let the cell know the
molecule has arrived.

Ligand- molecule the receptor binds to
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1. Intracellular Receptors- within cell
 Detects the important, small nonpolar molecules that
cross the membrane
 Example: Hormones- bind to receptor, receptor takes it
to nucleus to turn on genes in DNA to make specific
proteins.
 http://highered.mcgrawhill.com/sites/0072507470/student_view0/chapter17/an
imation__intracellular_receptor_model.html

2. Membrane
Receptor- used for
molecules that
cannot cross the
membrane
 Once the molecule
binds to receptor, the
receptor changes shape
to announce its
presence to the cell so
cell can make
appropriate response
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The selective permeable membrane allows
some molecules to enter and exit cell without
making the cell work
Passive Transport- movement across cell
membrane without cell using energy.
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Movement of
molecules from a
region of HIGHER
concentration to a
region of LOWER
concentration
Goal is to reach
EQUILIBRIUMmolecules are all
evenly spaced

Note: molecules continue to move
around but still stay spread out

Concentration gradient- difference in concentration
from one location to another
HIGHER CONCENTRATION
LOWER CONCENTRATION
* Most molecules diffuse DOWN concentration gradient

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Moving from an area of high water
concentration to an area of low water
concentration
Solutes =
water concentration
Solution outside the cell is compared to the
inside of the cell

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The solution is isotonic to the cell if it has the
same concentration of solutes as the cell
Equal amounts of water are entering and
leaving cell= cell size stays the same

Has more solutes than the cell
Low water,
MORE SOLUTES
Water leaves to
the lower water
concentration
**water flows out of cell. Cell can shrivel up and
die from lack of water

More water outside of cell= water moves
inside the cell
More water,
LOW SOLUTES
Water enters
cell. Cell has
less water
concentration
**water is entering the cell, making it BIGGER
What type of solution is each of these cells in?
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Diffusion of molecules
across a membrane
using TRANSPORT
PROTEINS (channels
and carrier proteins)
Still PASSIVE transport
because no energy is
required. Molecules
still moving DOWN,
DOWN, DOWN, DOWN,
DOWN the
concentration
gradient 

Channel proteins- transport polar molecules
◦ gated to regulate flow

Carrier proteins- transport ions and larger
molecules
◦ Have receptors that bind to specific molecule.
◦ When molecule is bound, the protein changes shape
in order to release molecule to other side.
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Drives molecules across membrane AGAINST
(UP) the concentration gradient
Moves molecules from lower to higher
concentration
Proteins for active diffusion called Pumps
Uniport- One molecule one way
Symport- 2+ different molecules,
same direction
Antiport- 2+ different molecules,
different directions

Vesicles transport large substances without
crossing through membrane

ENDOCYTOSIS- taking large molecules INTO
A CELL by engulfing them in a membrane.
(like swallowing it)

EXOCYTOSIS◦ Opposite of endocytosis
◦ Releasing substances OUTof cell by the vesicle
fusing with the membrane and releasing its
contents.