Maintaining a Balance
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The plasma membrane is a SELECTIVELY
PERMEABLE membrane that allows nutrients
and wastes to enter and exit the cell
◦ Homeostasis: maintaining balance in the cell

Selective Permeability – allows some
molecules to pass through and keeps others
out
◦ E.g. Screen on a window

Water can freely enter and exit the cell. Other
particles such as Na+ and Ca+ ions are only
allowed in at certain times (selective
permeability)
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The plasma membrane is composed of a
PHOSPHOLIPID BILAYER (double layer)
A phospholipid molecule is a lipid which is
composed of a glycerol, a phosphate, and
two fatty acids
The two fatty acid tails of the phospholipid
are NONPOLAR (not attracted to water)
whereas the head of the phospholipid
molecule is POLAR (attracted to water)
Polar Head
(glycerol + phosphate)
Nonpolar Tail
(fatty acids)

The two layers of phospholipid molecules
make a sandwich with the fatty acid tails
forming the interior of the membrane and the
phospholipid heads facing the watery
environments found inside and outside the
cell

Cholesterol is mixed into the plasma membrane
and helps keep the fatty acid tails
from sticking together
Transport proteins span across
the plasma membrane and move
larger molecules through the
membrane
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Water can pass freely through the plasma
membrane by DIFFUSION
Diffusion – the movement of particles from an
area of higher concentration to an area of
lower concentration
◦ Like gravity  High to Low
◦ E.g. Perfume spreads from point of origin to evenly
disperse around the room
Gas spreads out by diffusion!
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In the cell, WATER always moves to reach an
equal concentration of both sides of the
membrane
The diffusion of water across a semipermeable membrane is called OSMOSIS
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If two solutions with different solute
concentrations are separated by a selectively
permeable membrane (i.e. the plasma
membrane), the water molecules will move
from the solution with HIGHER WATER (lower
solute) CONCENTRATION to the solution with
LOWER WATER (higher solute)
CONCENTRATION.
The solute doesn’t move; only the WATER
moves
90% Water
70% Water
80% Water
80% Water
Water moves from high concentration of water
to lower concentration of water
15% Sugar
(85% H2O)
H2O
H2O
30% Sugar
(70% H2O)
Cell
H2O
Environment
H2O
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Facilitated Diffusion – transport of materials
across a membrane with the aid of transport
proteins (no energy)
Proteins act like a gate or channel that opens
and closes to let materials in and out
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Isotonic Solution – the concentration of water is
EQUAL on both sides of the membrane. Water
moves in and out of the cell equally
Hypotonic Solution – the solute concentration is
higher inside the cell than outside the cell. Water
will move into the cell by osmosis and the cell
will SWELL in size
Hypertonic Solution – the solute concentration is
lower inside the cell than outside the cell. Water
will move out of the cell by osmosis and cell will
SHRINK in size

CONCENTRATION GRADIENT - When there is
an unequal concentration of solutes on
different sides of a membrane
WITH concentration gradient
AGAINST concentration gradient

PASSIVE TRANSPORT - Particles naturally
move WITH the concentration gradient
◦ High  Low

NO ENERGY is needed
◦ Diffusion = passive
◦ Osmosis = passive
◦ Facilitated Diffusion = passive
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Active Transport – moves materials AGAINST
the concentration gradient
◦ Low  High
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REQUIRES ENERGY!
Active transport uses TRANSPORT PROTEINS
+ ENERGY to move materials from low
concentration to high concentration
ENERGY!
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Endocytosis – the whole cell engulfs
something
Exocytosis – the whole cell ejects something