Chapter 5
Membranes
I. The Structure of Membranes (5.1)
A. The fluid mosaic model shows proteins
embedded in a fluid lipid bilayer
1. Phospholipid
a. Hydrophilic head (glycerol and
phosphate group make it polar)
b. Hydrophobic tail (2 fatty acid chains
are nonpolar)
2. Globular proteins
a. proteins w/ nonpolar segments and
polar ends to fit in phospholipid bilayer.
3. Fluid mosaic model
a. random arrangement of proteins
“floating” in or on the fluid lipid bilayer.
http://www.susanahalpine.com/anim/Life/memb.htm
B. Cellular membranes consist of
four component groups
1. Phospholipid bilayer: 2 layers of
phospholipids with hydrophobic tails
pointed inward.
2. Transmembrane proteins (aka intergral
membrane proteins): allow substances to
pass through membrane.
3. Interior protein network: proteins in the
membrane that help support cell shape.
a. Peripheral membrane proteins: proteins
that are not part of the membrane
structure but help control cell movement.
4. Cell surface markers:
a. Glycoproteins and glycolipids act as
cell identity markers (IDs)
• Lipid rafts: sections of the cell membrane
that function as signal receptors and
involved in cell movement tightly packed
by cholesterol.
C. Electron microscopy has provided structural
evidence
Transmission electron microscopes (TEM)
and scanning electron microscopes (SEM)
used to study the plasma membrane.
II. Phospholipids: The Membrane’s
Foundation (5.2)
A. Phospholipids spontaneously form bilayers
1. Hydrophilic phosphate group facing out
2. Hydrophobic nonpolar fatty acids face in
3. Closely packed hydrophobic tails prevent
water soluble substances from passing
through.
B. The phospholipid bilayer is fluid
1. Hydrogen bonds help hold membrane
together.
2. Weak forces b/t phospholipids allow
movement of proteins and lipids.
C. Membrane fluidity can change
1. Saturated fats and cooler temps make
the membrane less fluid (solid) b/c lipids
are tightly packed.
2. Unsaturated fats and higher temps
make the membrane more fluid b/c lipids
are more loosely packed.
• http://telstar.ote.cmu.edu/Hughes/tutorial/c
ellmembranes/
III. Proteins: Multifunctional Components
(5.3)
A. Proteins and protein complexes
perform key functions
1. Transporters: move substances across
membrane in carriers or channels
2. Enzymes: carry out chemical reactions
3. Cell surface receptors: detect messages
4. Cell surface identity markers: ID
5. Cell-to-cell adhesion proteins: glue cells
together
6. Attachments to the cytoskeleton
B. Structural features of membrane proteins
1. The anchoring of proteins in the bilayer:
nonpolar sections of protein tied to
membrane. α-helix or -sheets
2. Transmembrane domains: area or areas of
protein anchored to nonpolar lipid bilayer.
α-helix
3. Pores: proteins
that form pipelike holes to
allow passage
for molecules
-sheets
IV. Passive Transport Across
Membranes
A. Passive transport: movement of
substances in and out of the cell w/OUT
using energy
B. Concentration gradient: difference in
concentration of a substance b/t the
inside and outside of a cell.
C. Transport can occur by simple diffusion
1. Diffusion: random movement of molecules
from an area of high concentration to an
area of lower concentration.
• http://www.stolaf.edu/people/giannini/flash
animat/transport/diffusion.swf
• http://www.wiley.com/college/pratt/047139
3878/student/animations/membrane_trans
port/index.html
• NOTE: Nonpolar molecules like O2 and
steroids have a much easier time moving
across the lipid bilayer membrane. Few
small polar molecules can cross very
slowly
• http://www.teachersdomain.org/resources/t
dc02/sci/life/cell/membraneweb/assets/tdc
02_int_membraneweb/tdc02_int_membra
neweb_swf.html
D. Proteins allow membrane diffusion to be
selective
1. Channel proteins: provide a passage for
polar molecules
2. Carrier proteins: bind to molecule and help
them cross membrane.
3. Selectively permeable: choose which
molecules can cross.
4. Diffusion of ions through channels
a. Ion channels: transport channel proteins
move ions (i.e. K+, Ca+2, Cl-) across
according to their concentration.
b. Gated channels: channels proteins that
open or close due to a stimulus.
c. Membrane potential: difference in voltage
across membrane causes channels to
open or close.
E. Carrier proteins and facilitated
diffusion
a. Facilitated diffusion: carrier proteins help
move molecules across membrane
according to the concentration difference.
b. Saturation: at high concentrations, all
carrier proteins will move molecules
across.
http://highered.mcgrawhill.com/sites/0072495855/student_view0/
chapter2/animation__how_facilitated_diffu
sion_works.html
c. Facilitated diffusion is:
- specific to certain molecules
- passive: high to low concentration
- saturates: only moves as fast as # of
carrier proteins available.
6. Facilitated diffusion in red blood cells
C. Osmosis is the movement of
water across membranes
1. Solvent: medium, usually water, in which
solute is dissolved.
2. Solute: substance dissolved in solvent
***Solutes and solvents move by diffusion
(high concentration to low concentration)
3. Osmosis: movement of water to
area of higher solute concentration.
a. Osmotic pressure: the concentration of
all solutes in a solution.
http://highered.mcgrawhill.com/sites/0072495855/student_view0
/chapter2/animation__how_osmosis_wor
ks.html
b. Hypertonic: higher solute concentration
c. Hypotonic: lower solute concentration
d. Isotonic: when two solutions have the
same osmotic (solute) concentration.
4. Aquaporins: water channels
• Movement of water is
assisted by special
water channels.
• People with
Nephrogenic Diabetes
Insipidus have
nonfunctional
aquaporins and have
trouble retaining water.
5. Osmotic pressure
a. Hydrostatic pressure: pressure of
cytoplasm pushing on cell membrane
due to osmosis.
a. Osmotic pressure: force to stop flow
of water, depends on solute
concentration inside and outside of
cell.
6. Maintaining osmotic balance
a. Extrusion: contractile vacuole pumps out
water that constantly enters due to
osmosis.
b. Isosmotic Regulation: Increasing the
internal solute concentration to match the
environment.
• http://trc.ucda
vis.edu/biosci
10v/bis10v/we
ek7/parameci
um.mov
c. Turgor/ Turgor pressure: plant cells are
usually hypertonic to their environment to
increase hydrostatic pressure. This
increases the pressure of the cell
membrane against the cell wall
maintaining cell shape.
• http://www.teachersdomain.org/resources/t
dc02/sci/life/cell/membraneweb/assets/tdc
02_int_membraneweb/tdc02_int_membra
neweb_swf.html
V. Active Transport
Across Membranes
(5.5)
A. Active transport :
movement of
substances UP
CONCENTRATION
GRADIENT that
requires ENERGY from
ATP.
B. Active transport uses energy to move
materials against a concentration gradient
1. Uniporters: carrier proteins that move
only 1 type of molecule.
2. Symporters: carrier proteins that move 2
molecules in the same direction.
3. Antiporters: carrier proteins that move 2
molecules in opposite directions.
3 types of carrier proteins using active transport
C. The sodium-potassium pump
runs directly on ATP
1. Sodium-potassium pump: carrier protein
that moves 3 sodium ions (Na+) out of the
cell for every 2 potassium ions (K+) it
moves in. It uses ATP to move these ions
from areas of low concentration to areas of
high concentration.
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_the_
sodium_potassium_pump_works.html
STEPS of Na+/K+ Pump
1. 3 Na+ bind to protein changing its
conformation (shape).
2. Protein separates ATP to ADP and binds
to phosphate group to becoming
phosphorylated
3. Phosphorylated protein moves 3 Na+ to
outside fluid
4. Protein has new conformation and binds
to K+ from outside
5. Binding to K+ causes a conformation in
protein to release phosphate group
6. The protein is now free of phosphate
group and changes back to original
conformation with a high affinity for 3 Na+
http://www.brookscole.com/chemistry_d/templates/student_resources/s
hared_resources/animations/ion_pump/ionpump.html
D. Coupled transport uses ATP
indirectly
1. Coupled transport: the energy released to
move a molecule down its concentration
gradient is used to move a different
molecule against its concentration
gradient.
EXAMPLE:
• Sodium-potassium pump uses ATP to
move sodium ions outside of cell where
there is a higher concentration.
• A symporter moves sodium ions into the
cell and simultaneously moves glucose
into the cell where there is a higher
concentration.
2. Countertransport
An antiporter moves
one molecule, such as
Na+, is moved down its
concentration gradient.
The energy is used to
move another molecule,
such as H+, in the
opposite direction
against its
concentration gradient.
Coupled transport and
Countertransport
• http://highered.mcgrawhill.com/olc/dl/120068/bio04.swf
VI. Bulk Transport by Endocytosis
and Exocytosis
A. Bulk transport: process moving LARGE,
POLAR molecules across membrane
B. Bulk material enters the cell in vesicles
1. Endocytosis: plasma membrane takes in
food particles and liquids.
2. Phagocytosis (eat): cell takes in an
organism or large organic molecules
(WBC take in bacteria)
3. Pinocytosis (drink): cell takes in liquid
(mammalian egg cell take in nutrients)
3. Receptor-mediated endocytosis: molecules
bind to receptors on the membrane. Clathrin
proteins help to trap these molecules and form
a vesicle. Molecules like LDLs enter through
this process.
C. Material can leave the cell by exocytosis
1. Exocytosis: movement of large materials
out of the cell via vesicles. (moves cell wall
materials, excess water, enzymes,
hormones, neurotransmitters and waste).
• http://highered.mcgrawhill.com/olc/dl/120068/bio02.swf