The Cell Membrane
0 His friends call him the plasma membrane
0 He is thin and flexible
0 He has two main functions:
0 Protection – protects the cell from the outside
environment
0 Regulation – controls what can enter and exit
the cell
0 He is selective: allows some things to pass
through more easily than others
0 He is selectively permeable: permeate is a
fancy way to say “pass through.”
What is the Fluid-Mosaic Model?
0 What’s a mosaic?
0 What does it mean to be fluid?
0 The cell membrane is NOT a rigid structure with
immovable components!
0 The cell membrane is fluid-like and flexible
0 Within the membrane, molecules can move around
What Is the Cell Membrane Made of?
The (Phospho-)Lipid Bilayer
0 LIPIDS: Phospholipids make up the majority of the cell
membrane
Hydrophilic heads (polar) are made of phosphates
(Phospho)
Hydrophobic tails (nonpolar) made out of fatty
acids (Lipid)
0 To protect the hydroPHOBIC
tails from water, they form a
bilayer which keeps the tails
inside and the water- loving
heads outside.
STEROLS
0 Cell membranes of eukaryotes contain sterols between the
tails of phospholipids
0 CHOLESTEROL – major membrane sterol in animals
0 Make the membrane more firm
0 Prevent the membrane from freezing at low temperatures
PROTEINS
0 Protein molecules - bring materials
into cell and receive signals from
outside cell
0 Integral proteins – embedded
within the bilayer
0 receptor proteins – detect
signals and transmit them
inside cell
0 transport proteins –passage
ways that allow certain
substances to pass
0 cell markers – carbohydrates
attached to help cells identify
or recognize other cells
0 Peripheral proteins – lie only on
one side of membrane (enzymes)
YouTube:
Membrane
Draw the Cell Membrane!
Your turn!
0 Use your notes and what you’ve learned so far to
complete the matching exercise in your notes
0 Protein (only) = B
0 Carbohydrate (only) = D
0 Lipid Bilayer = A
0 Phosphate Head = F
0 Fatty Acid Tail = G
0 Involved in Cell Recognition = D
0 Carbohydrate attached to a lipid = E
0 Helps move large material across the membrane = B
0 Carbohydrate attached to a protein = C
0 Outside cell = H
Solutions
0 Molecules dissolved in a liquid = SOLUTES
0 Liquid/fluid dissolving them = SOLVENT
0 This makes a SOLUTION
SALT is the solute and
0 In a salt solution, ______________
WATER is the solvent
_____________
0 In a sugar solution, sugar is the solute and water
is the solvent.
Dots = solute
Space = solvent
Concentration and Equilibrium
0 Solutions will spread out their dissolved molecules until
they are equal throughout.
0 EQUILIBRIUM = molecules are spread equally
0 CONCENTRATION = # of molecules in an area per unit volume.
0 CONCETRATION GRADIENT = the difference in the
concentration of molecules across a distance
High concentration: more solutes
per unit volume
Low concentration: less solutes 
per unit volume
What happens with a barrier?
(like a cell membrane)
0 If solutions on either side of the barrier have the same
concentration, they are at equilibrium.
0 At equilibrium, both the solvent and solute move back and
forth across the barrier: there is always movement.
Transport of Materials
Across the Cell Barrier
0 Materials move across the plasma
membrane in two ways:
0 Passive Transport
– movement across the membrane
without using energy
0 Active Transport
– movement across membrane
that requires energy
Types of Passive Transport:
1. Diffusion - simulation
0 Solutes move across a membrane from areas of high
concentration (crowded) to low concentration
0 Diffusion = random particle movements,
so does not use energy.
Imagine warm air
coming through an
open window…
Moving down the gradient
requires no energy
Because cell membranes are
selectively permeable….
0 ….concentration gradients can build up across a cell
(like a dam in a lake)
Potential Energy
What affects the rate of
diffusion?
Concentration of the solution
Temperature of the solution
Pressure also speeds up particle
motion
Types of Passive Transport:
2. Osmosis
0 the process by which water molecules diffuse across
a cell membrane from an area of higher concentration
to an area of lower concentration
0 Water molecules (fast and small) pass through the cell’s
selectively permeable membrane
0 Solute molecules are too large to pass -- only the water
diffuses until equilibrium is reached.
0Three conditions that control
the direction of osmosis- In
each of the following conditions
we are comparing the solute
concentration outside the cell
to inside the cell.
HYPOTONICITY
0 the concentration of solute molecules outside the cell
is lower than the concentration in the cytosol.
0 This will cause water to flow into the cell until
equilibrium is reached.
HYPERTONICITY
0 the concentration of solute molecules outside the
cell is higher than the concentration in the cytosol.
0 This will cause water to flow out of the cell until
equilibrium
is reached.
ISOTONICITY
0 the concentration of solutes outside the and inside the cell are
equal so water will flow in and out of the cell at equal rates.
How cells deal with osmosis:
0Many cells, especially unicellular fresh water
organisms, must deal with living in extreme
hypotonic environments.
0 -water continually rushes into the cell forcing it to
constantly rid itself of excess water. (Paramecium are
unicellular freshwater organisms with this problem.)
0 contractile vacuoles -organelles that remove excess
water by collecting it and then contracting, pumping the
water out of the cell. This action requires energy.
0 Multi-cellular organisms respond to hypotonic
environments by pumping solutes out. This helps control
the flow of water into the cell.
0Plants in a hypotonic environment can stand upright.
0 water fills the cell pressing the membrane up against
the cell wall which is strong enough to resist breaking.
0 The pressure water molecules exert against the cell wall
is called turgor pressure.
0 In hypertonic environments water leaves the cell
through osmosis, the cell membrane then shrinks away
from the cell wall causing the plant to wilt. This process
is called plasmolysis.
0 Animal cells like red blood cells have no cell
walls to stop them from expanding and when
in a hypotonic environment will swell until
they burst. The bursting of cells is called
cytolysis.
0 VIDEO
Types of Passive Transport:
3. Facilitated Diffusion
0 Large molecules or those with a charge (not
soluble in lipids) need the help of a protein to pass
across a cell membrane
0 Proteins form a channel
and molecules move
through the “doorway.”
From high to low
concentration.
Each channel is specific
to a particular type of
molecule
Doesn’t require energy
=> passive transport
Each carrier protein is specific for a
certain molecule
0 As soon as the molecule binds to its carrier protein, the carrier
protein changes shape
0 The change in shape shields the molecule from the
hydrophobic interior of the lipid bi-layer
0 The molecule can then be delivered either into or out of the cell
0 Example- glucose is too large to diffuse across the membrane but
essential for the cell
0 VIDEO -
Diffusion Through Ion Channels0 allow for the passage of ions through the cell
membrane
0Each ion channel is specific for one kind of ion
0Some ion channels are always open
0Some ion channels have gates that open and close as
needed
0 They open and close in response to 3 different stimuli
0 Stretching of the cell membrane
0 Electrical signals
0 Chemicals in their environment
Active Transport
0 Movement AGAINST the concentration gradient
(because it moves solutes from
low to high concentration—where it’s already
crowded)
0 Difference in solution concentrations = concentration gradient
0 Three types of active
transport…
Active Transport
1. Pump
0 Pump – a protein PUSHES
molecules across the membrane
Ex: the Sodium and
Potassium (Na/K) Pump.
YouTube:
Na/K Pump
Active Transport
2. Endocytosis
0 Endocytosis (endo=in): a pocket (vacuole) forms
around a large molecule outside the cell and buds inward
to release the material inside the cell.
Active Transport
3. Exocytosis
0 Exocytosis (exo=out): a vacuole inside the cell fuses
with the cell membrane and forces the material outside the
cell.
Animation