The Cell Membrane
Overview
• Cell membrane separates living cell from nonliving
surroundings
– thin barrier = 8nm thick
• Controls traffic in & out of the cell
– selectively permeable (or semi-permeable)
– allows some substances to cross more easily than
others
• hydrophobic vs hydrophilic
• Made of phospholipids, proteins & carbohydrates
Phospholipids
• Fatty acid tails
– hydrophobic
Phosphate
• Phosphate group head
– hydrophilic
• Arranged as a bilayer
Fatty acid
Phospholipid bilayer
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
polar
hydrophilic
heads
Are they saturated or unsaturated fat molecules?
More than lipids…
• In 1972, S.J. Singer
& G. Nicolson
proposed that
membrane proteins
are inserted into
the phospholipid
bilayer.
• They called it the
Fluid Mosaic Model.
Membrane is a collage of proteins & other molecules
embedded in the fluid matrix of the lipid bilayer
Glycoprotein
Extracellular fluid
Glycolipid
Phospholipids
Cholesterol
Peripheral
protein
Transmembrane
proteins
Cytoplasm
Filaments of
cytoskeleton
Membrane Proteins
• Proteins determine membrane’s specific functions
– cell membrane & organelle membranes each have
unique collections of proteins
• Membrane proteins:
– peripheral proteins
• loosely attached to the surface of the membrane
– integral proteins
• stick into the lipid bilayer
• usually go all the way across the whole membrane
Many Functions of Membrane Proteins
Outside
Plasma
membrane
Inside
Transporter
Enzyme
activity
Cell surface
receptor
Cell surface
identity marker
Cell adhesion
Attachment to the
cytoskeleton
Membrane carbohydrates
• Play a key role in cell-cell recognition
– ability of a cell to distinguish one cell from
another
– important in organ &
tissue development
– basis for rejection of
foreign cells by
immune system
Check Your Understanding…
1. What does semi-permeable mean?
2. Which biomolecule makes up membranes?
3. What do proteins do in membranes?
4. What do carbohydrates do on membranes?
5. Why do we use the “fluid mosaic model” to describe membranes?
Movement across the
Cell Membrane
Diffusion
• 2nd Law of Thermodynamics
– universe tends towards disorder (entropy)
 Diffusion

movement from high  low concentration
Diffusion
• Move from HIGH to LOW concentration
– “passive transport”
– no energy needed
diffusion
Diffusion across cell membrane
• Cell membrane is the boundary between
inside & outside…
– separates cell from its environment
Why do molecules need to cross the membrane?
OUT
IN
food
carbohydrates
sugars, proteins
amino acids
lipids
salts, O2, H2O
OUT
IN
cell needs materials in & products or waste out
waste
ammonia
salts
CO2
H2O
products
Diffusion through the phospholipid bilayer
• Which molecules can get through directly?
– fats & other lipids
 What molecules can NOT
lipid
inside cell
get through directly?
salt
NH3

polar molecules
 H2O

ions
 salts, ammonia

outside cell
sugar
aa
H2O
large molecules
 starches, proteins
Channels through cell membrane
• Membrane becomes semi-permeable with
protein channels
– specific channels allow specific material across
cell membrane
inside cell
NH3
H2O
salt
aa
sugar
outside cell
Facilitated Diffusion
• Diffusion through protein channels
– channels move specific molecules across
cell membrane
facilitated = with help
– no energy needed
open channel = fast transport
high
low
“The Bouncer”
Active Transport
• Cells may need to move molecules against the concentration
gradient (from low concentration to high concentration)
– shape change transports the molecule from
one side of membrane to other
– protein “pump”
– “costs” energy = ATP
ATP
Getting through the cell membrane
• Passive Transport
– Simple diffusion
• diffusion of nonpolar, hydrophobic molecules
– lipids
– high  low concentration gradient
– Facilitated transport
• diffusion of polar, hydrophilic molecules
• through a protein channel
– high  low concentration gradient
• Active transport
– diffusion against concentration gradient
• low  high
– uses a protein pump
– requires ATP
ATP
Transport summary
simple
diffusion
facilitated
diffusion
active
transport
ATP
How about large molecules?
• Moving large molecules into & out of cell
– through vesicles & vacuoles
• endocytosis
• exocytosis
Endocytosis
phagocytosis
pinocytosis
receptor-mediated
endocytosis
fuse with lysosome
for digestion
non-specific
process
triggered by
molecular signal
Diffusion Review
• Which molecules can move through membranes
without help?
• What is the difference between passive and
active transport?
• If molecules can move across the membrane,
why do our cells still need vesicles?
The Special Case of Water
Movement of water across
the cell membrane
2007-2008
Osmosis is…
when water moves across a membrane to
balance concentrations.
Concentration of water
• Direction of osmosis is determined by
comparing total solute concentrations
– Hypertonic - more solute, less water
– Hypotonic - less solute, more water
– Isotonic - equal solute, equal water
water
hypotonic
hypertonic
net movement of water
Managing water balance
• Cell survival depends on balancing water
uptake & loss
freshwater
balanced
saltwater
Check for Understanding
• At the bottom of your notes, explain what
would happen if you took a fish from the
ocean and put it in a freshwater lake.
• Use the words:
– concentration
– osmosis
– membrane
– semi-permeable