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.
More Parts of the Cell Membrane
0 Also embedded in the
lipid bilayer are proteins
and carbohydrate chains
0 Protein molecules bring
materials into the cell and
receive signals from
outside the cell
0 Carbohydrate molecules
(attached to proteins or
lipids) have antenna to
help cells identify or
recognize other cells
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.
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
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…
Types of Passive Transport:
2. Osmosis
0 A special name for diffusion of water!
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.
Types of Passive Transport:
3. Facilitated Diffusion
0 Large molecules or those with a charge need the
help of a protein to pass across a cell membrane
0 Proteins form a channel
and molecules move
through the “doorway.”
Each channel is specific
to a particular type of
molecule
Doesn’t require energy
=> passive transport
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