The Plasma Membrane
Honors Anatomy& Physiology
Plasma Membrane
boundary between inside & outside of
cell
 flexible structure
 dynamic role in cellular activities

Plasma Membrane
Fluid Mosaic Model

resembles an ever-moving sea of
fluid lipids that has large proteins
bobbing along throughout the lipids
Cell Membrane: Lipid-Bilayer

Fluid Mosaic Model
◦ phospholipids
Membrane Lipids
Phospholipids (~70% of membrane)
Cholesterol (20%)
1.
2.
◦
changes fluidity of membrane
Glycolipids (5%)
3.
◦
◦
sugar molecule attached to a lipid
outer membrane only
Lipid Rafts
4.
◦
control protein-protein interactions in
membrane
Membrane Proteins

Integral Proteins
◦
◦
◦
◦
◦

go all the way thru the membrane
channel proteins
carrier proteins
receptor proteins
enzymes
Peripheral Proteins
◦ on inside or outside of membrane
◦ +/- attached to integral proteins
Plasma Membrane
Glycocalyx
“sugar-coating” on cell surface
 important in cell-cell recognition

Cell Junctions
3 factors binding cells together:
1. glycoproteins
◦
2.
3.
sticky
membrane contours of 2 cells fit
together
cell junctions form
Cell Junctions: Tight Jcts

integral proteins
in 2 adjacent
cells fuse
together
◦ impermeable jct
◦ prevent
molecules
moving thru ECF
between cells
Cell Junctions: Desmosomes

anchoring jcts
◦ holds cells together in thickening called a
plaque
Cell Junctions: Gap Jcts
allows 2 adjacent cells to pass ions,
small molecules
 important in cardiac muscle

◦ allows synchronized contractions
Cell Junctions
Interstitial Fluid
ECF
 from blood
 water, a.a. sugars, fatty acids,
vitamins, hormones, enzymes,
neurotransmitters

◦ cells must take in what it requires
Membrane Transport
plasma membrane is selectively
permeable
 2 ways substances can pass:

1. Passive Transport
2. Active Transport
Passive Transport
1.
2.
3.
Diffusion
Facilitated Diffusion
Osmosis
Diffusion

molecules or ions move from hi  lo
concentrations
◦ due to KE
◦ Factors that speed up diffusion:
1. concentration gradient
2. temperature
3. size of particles
Diffusion

lipid bilayer nonpolar so small
nonpolar molecules allowed to pass
◦ oxygen molecules
◦ carbon dioxide
◦ small, uncharged polar molecules
 water
 glycerol
Facilitated Diffusion

polar substances move across
membrane down concentration gradient
using a protein
1. carrier-mediated


integral proteins carry specific molecules
ligand attaches to protein  which changes
shape  molecule enters cell
2. channel-mediated



selective to specific ion or water (aquaporins)
leakage channels always open
gated channels controlled by electrical or
chemical signals
Carrier-Mediated
Facilitated Diffusion
Leakage Channel
Gated Channel
Osmosis
diffusion of water thru selectively
permeable membrane
1. simple diffusion thru membrane
 small polar molecule that “wiggles”
thru nonpolar bilayer when
membrane lipids randomly move
 Aquaporins (leakage channels)
unsaturated fatty acid tails &
cholesterol leave tiny spaces

Isotonic Solutions

same concentration of nonpenetrating
solutes as found inside cells
◦ 0.9% saline
◦ 5% glucose
◦ body fluids
Hypertonic Solutions
higher concentration of
nonpenetrating solutes than inside
cells
 cells crenate (shrink)
 used for extreme edema (excess water
in extracellular spaces)

Hypotonic Solutions
more dilute than inside cells
 cells take in water  burst = cytolysis

◦ (hemolysis if RBC)
◦ used in extremely dehydrated patients
Active Transport
requires proteins that combine
specifically and reversibly
w/transported substance
 solutes move against their
concentration gradient

◦ so cell must expend nrg
Active Transport Processes

Pumps
◦ Primary Active Transport
◦ Secondary Active Transport

Vesicular Transport
◦ Endocytosis
 Phagocytosis
 Pinocytosis
◦ Receptor-Mediated Endocytosis
◦ Exocytosis
Primary Active Transport

hydrolysis of ATP provides nrg by:
1. transferring its 3rd phosphate group to
the protein pump
2. pump changes configuration (shape)
3. causing ligand to move across
membrane
4. are specific (no pump in particular cell
 no transport)
Na+K+Pump
[K+] inside cell 10x > outside cell
 [Na+] outside cell 10x > inside cell
 concentration gradient necessary for
all cells to maintain normal fluid vol.

◦ leakage channels in membrane allow both
to diffuse slowly but continuously
◦ diffuse according to electrochemical
gradients

antiporter: moves 2 substances in
opposite directions
http://brookscole.cengage.com/chemistry_d/te
mplates/student_resources/shared_resources/a
nimations/ion_pump/ionpump.html
http://www.siskiyous.edu/class/bio12b/SodPotas
sXchngPmp.swf
2◦ Active Transport
1◦ pump indirectly drives 2◦ pump
moving other solutes
 nrg stored in the electrochemical
gradient created from 1◦ pump used
to drive 2◦ pump

◦ Na+ moves back into cell (leakage
channels) as symporter
◦ sugars
Vesicular Transport

move fluids made of large particles &
macromolecules
◦ Endocytosis
◦ Exocytosis
◦ Transcytosis
 substances move across cell organelle 
organelle
Phagocytosis: Cell Eating
Pinocytosis: Cell Drinking
Endocytosis:
Receptor-Mediated
Exocytosis
ejects substances out of cell
 stimulated by:

◦ hormone binding to receptor
◦ change in membrane voltage

release of:
◦
◦
◦
◦
hormones
neurotransmitters
mucus
cell waste
Resting Membrane Potential
consequence of pumps, especially
Na+/K+ pump, a difference in charge
exists across membrane = voltage
 in resting state all plasma membranes
have resting membrane potential of
-50 to -100mV
(-) sign indicates inside of cell (-)
compared to outside
 so we say all cells are polarized

Resting Membrane Potential

exists only at the membrane
◦ overall inside and outside neutral
Cell-Environment
Interactions
always involves plasma membrane
 glycocalyx is key
