Cell Membrane
The cell membrane or plasma membrane
completely surrounds the cell and serves as
a barrier between the cell and its
environment.
 Allows entrance of needed substances
(glucose, amino acids, lipids, etc.) and
prevents entrance of other substances.
 Allows waste products (CO2) to exit the cell

The cell membrane allows some molecules
to enter the cell but prevents entrance of
other molecules.
 The cell membrane retains certain
molecules inside the cell but allows other
molecules to exit.
 Therefore, the cell membrane is said to be
selectively permeable to molecules.
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Lipid molecule = 1 glycerol + 3 fatty acids

Phospholipid molecule = 1 glycerol + 1 phosphate
group + 2 fatty acids
The head region of the phospholipid is also called
the polar region and is hydrophilic (water loving)
The tail region of the phospholipid is also called
the nonpolar region and is hydrophobic (water
hating)
The cell membrane is composed of a phospholipid
bilayer.
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The cell membrane is composed of a
phospholipid bilayer.
The heads of the bilayer can interact with
water because they are polar.
 The tails of the bilayer cannot interact with
water because they are nonpolar.
 Therefore, water soluble molecules cannot
move through the bilayer easily.

Transporting across the cell
membrane
There are two types of cellular transport that occurs
across the cell membrane.
- Passive Transport
- Active Transport
Passive Transport
- does not require energy
- moves from an area of high
concentration to an area of low
concentration ( a downhill flow)
Passive Transport (con’t)
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Passive transport includes
- diffusion
- facilitated diffusion
- osmosis
Diffusion- occurs when molecules move
randomly away from each other in a liquid or
gas.
Example: sugar/water
salt/water
smoke/air
Passive Diffusion
moves from an area of high concentration to an
area of low concentration ( a downhill flow)
Diffusion (con’t)

Diffusion is the continual
moving about and
bumping of particles into
each other. This continues
until all the particles of a
substance are equally
distributed throughout the
container. However, the
bumping never stops!
Facilitated Diffusion
Passive Transport
Osmosis

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Osmosis is the
diffusion of WATER
across a semipermeable membrane
osmosis occurs in
response to the
concentration of
solutes dissolved in
water.
Water moves low
solutes to high solutes
A simple rule to remember is:
Salt is a solute, when it is
concentrated inside or outside
the cell, it will draw the water
in its direction. This is also
why you get thirsty after eating
something salty.
Osmosis in a Isotonic solution

If the concentration of a
solute (salt) is equal of on
both sides, the water will
move back in forth but it
won't have any result on
the overall amount of
water on either side.
"ISO" means the same
Animation of Osmosis (Cell in an
Isotonic Environment)
Inside of Cell
Cell will remain same
Outside of Cell
Equal number of solutes inside and outside of cell
Osmosis in a Hypertonic solution

The word "HYPER"
means more, in this case
there are more solute
(salt) molecules outside
the cell, which causes
the water to be sucked
in that direction.In plant
cells, the central vacuole
loses water and the cells
shrink, causing wilting.
Osmosis Animation (Cell in
Hypertonic Environment)
Inside of Cell
Cell will
shrivel
Outside of Cell
Higher percent of solutes outside of cell
Osmosis in a Hypotonic Solution

The word "HYPO" means
less, in this case there are
less solute (salt) molecules
outside the cell, since salt
sucks, water will move
into the cell. The cell will
gain water and grow
larger. In plant cells, the
vacuoles will fill and the
plant becomes stiff and
rigid, the cell wall keeps
the plant from bursting
Osmosis Animation (Cell in
Hypotonic Environment)
Inside of Cell
Cell will swell
Outside of Cell
Lower percent of solutes outside of cell
Osmosis in action
Facilitated Diffusion
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a process whereby a large
substance passes through a
membrane with a aid of a
facilitator.
The facilitator is an
integral membrane protein
that spans the width of the
membrane.
The force that drives the
molecule from one side of
the membrane to the other
is the force of diffusion.
Active Transport
 Energy
is needed in active transport
 Active transport is the movement from
an area of low concentration to an area
of high concentration (uphill flow)
 Active transport use carrier proteins that
act as pumps.
Active Transport
Active transport is the movement from an area of
low concentration to an area of high concentration
(uphill flow)
Active Transport (con’t)
 Active
transport include: Bulk Transport
Bulk Transport is for solutes to large to
cross a cell membrane. Examples:
endocytosis
exocytosis
Endocytosis
 During endocytosis the
cell membrane Folds into
a pouch that encloses the
particles. The pouch
pinches off INSIDE the
Cell to form VESICLE
(membrane-wrapped
bubbles). The VESICLE
can then fuse with other
Organelles
(LYSOSOMES) or
Release its contents into
the Cytoplasm.
Endocytosis con’t(Pinocytosis
& Phagocytosis)
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Pinocytosis and
Phagocytosis are two
types of endocytosis.
Pinocytosis is
sometimes referred to
as “cell drinking”
Solutes or Fluids
outside the Cell
Membrane can be
brought into the
Cytoplasm by
pinocytosis.
Pinocytosis takes in liquids
Pinocytosis
Pinocytosis is the ingestion of dissolved materials by endocytosis.
The cytoplasmic membrane folds in and pinches off placing small
droplets of fluid in a pinocytic vesicle. The liquid contents of the
vesicle is then slowly transferred to the cytosol.
Phagocytosis
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Phagocytosis is like
pinocytosis except the
cell engulfs a food
particle or other cells
instead of a drop of
liquid.
"CELL EATING"
Phagocytosis takes in solid particles
Phagocytosis
Phagocytosis is the ingestion of solid particles by
endocytosis. The cytoplasmic membrane folds in and
pinches off placing the particle in a phagocytic vacuole. The
phagocytic vacuole then fuses with lysosomes and the
material is degraded.
Exocytosis

During exocytosis waste
and cell products leave
the cell. Products made
in the cell are packaged
in golgi vesicles, which
then fuse with the cell
membrane and secrete
material out of the cell.
Exocytosis
During exocytosis, a cell releases waste products or
specific secretion products by the fusion of a
vesicle with the cytoplasmic membrane.
How active transport occurs