Transcript cell membrane

CELL MEMBRANE
Chapter 7
Cell Membrane
• Bilayer of phospholipids
• Phospholipid
– The 2 tails are hydrophobic fatty acids
– The head is a hydrophilic phosphate
group
– The tails and head are connected by
glycerol
• Cell membranes also contain proteins and
carbohydrates
Figure 8.1 Artificial membranes (cross sections)
Figure 8.4 The fluidity of membranes
• Held together by weak hydrophobic
interactions
• Lipids and proteins can drift laterally within
membrane
• Cholesterol regulates membrane fluidity
(makes less fluid when it is warmer, more
fluid when it is colder) and is only found in
animal cells
• Unsaturated fatty acids have “kinks” at the
double bonds causing them to not pack
together closely – makes membrane more
fluid
• Membrane proteins
–Integral proteins - generally
transmembrane with hydrophobic
regions
–Peripheral proteins - generally
attached to membrane’s surface
• Cell-to-cell recognition
–“Markers”- glycoproteins,
glycolipids, & oligosaccharides
(short polysaccharides)
Figure 8.9 Some functions of membrane proteins
Figure 8.7 The structure of a transmembrane protein
Figure 8.6 The detailed structure of an animal cell’s plasma membrane, in cross
section
TRAFFIC ACROSS MEMBRANE
• Selective Permeability - regulates
the type and rate of molecular traffic
into and out of cell
–Nonpolar (hydrophobic) molecules
• dissolve in membrane
• cross easily
• ex. CO2 and O2
–Polar (hydrophilic) molecules
• Small, uncharged pass through
easily (ex. water)
• Larger, uncharged will not easily
pass (ex. glucose)
• All ions, even small ones, have
difficulty penetrating hydrophobic
region (ex. Na+, H+)
• Passive Transport
–Requires no energy
–Diffusion - the net movement of a
substance down a concentration
gradient (from high to low
concentration)
• Concentration gradient - regular,
graded concentration change over a
distance
• Net movement - the overall
movement away from center of
concentration
• Results from random molecular
motion, although net movement may
be directional
• Increases entropy (increases
disorder)
• Decreases free energy (-ΔG) so it is
a spontaneous process
• Rate of diffusion depends on
permeability of membrane
• Water diffuses freely across most
membranes
• Net movement stops at equilibrium
Figure 8.10 The diffusion of solutes across membranes
–Osmosis -diffusion of water
• Water diffuses down its
concentration gradient
• Direction is determined by total
solute concentration, regardless
of type or diversity of solutes in
solutions
Figure 8.11 Osmosis
• Hypertonic solution- solution with a
greater solute concentration than
inside the cell
• Hypotonic solution- solution with a
lower solute concentration than
inside the cell
• Isotonic solution- solution with the
same solute concentration as inside
the cell
• Water moves from hypotonic to
hypertonic areas.
• Water potential (Ψ) is the
measure of the tendency for a
solution to take up water when
separated from pure water by a
selectively permeable membrane
• Water moves from high to low
water potential
• Water potential depends on
solute potential and pressure
potential (in cells)
–Ψ = Ψp + Ψs
»Ψp is pressure potential
»Ψs is solute potenial
–Water potential of pure water
is zero so Ψs of any solution
will always be negative
–Increasing solute, makes Ψs
more negative
–Increasing Ψs decreases
water potential.
–Pressure potential is influenced
by water movement into and out
of plant cells.
–Pressure potential is the
physical pressure exerted on
either side of a membrane.
–Increasing Ψp increases Ψ
–A positive pressure potential
means a plant cell is turgid and
a negative pressure means it is
flaccid.
• Effects of osmosis
–In hypertonic solution: animal
cells shrivel, plant cell are
plasmolyzed (cell membrane
pulls away from cell wall)
–In a hypotonic solution: animal
cells are lysed (pop), plant cells
are turgid (firm)
–In isotonic: animal cells normal,
plant cells are flaccid (limp)
Figure 8.12 The water balance of living cells
• Osmoregulation - controlling
water balance
–Contractile vacuoles pump out
water (ex. Paramecium)
–Pumping out salts (ex. Bony fish)
–Facilitated Diffusion - diffusion
across membrane with the help of
transport proteins
• Passive because solutes move
down their concentration gradient
and no energy is required
• Facilitated Diffusion Animation
Figure 8.13 The contractile vacuole of Paramecium: an evolutionary adaptation for
osmoregulation
Figure 8.14 Two models for facilitated diffusion
• Transport proteins
–Specific for the solutes that they
transport
–Conformational change in protein allows
solute to be transferred to other side
–Gated proteins - channel opens in
response to electrical or chemical signal
–Ex. Aquaporins – channel proteins that
transport water via facilitated diffusion
»Problems with aquaporins associated
with glaucoma, cataracts, and kidney
diseases
• Active Transport - an energy
requiring process during which a
transport protein pumps a molecule
across the membrane, against its
concentration gradient (from low to
high concentration)
–Requires +ΔG
–Helps cell maintain steep ionic
gradients across cell membrane
–Uses ATP (energy)
–
Sodium Potassium Pump animation
Figure 8.15 The sodium-potassium pump: a specific case of active transport
–Ex. Sodium-potassium pump
• Transport protein has binding sites
for Na+ on interior side and sites for
K+ on exterior side
• Na+ binds to protein and stimulates
ATP to phosphorylize the protein
thereby changing its shape
• This changed shape allows Na+ to
be expelled outside of cell and
allows K+ to bind on outside of
protein
• K+ binding triggers release of
phosphate from protein
• Loss of phosphate restores
proteins original shape and expels
K+ into cell
• Na+ K+ pump translocates 3 Na+
ions out of cell for every 2 K+ ions
pumped into cell.
• Ion pumps generate voltage across
membrane
–Because anions and cations are
distributed unequally across cell
membranes, all cells have voltages
across their membranes (batteries)
–Membrane potential - voltage
across membrane
• ranges from -50 to -200 mv (the
inside of the cell is more negative
than the outside)
–That negative inside favors the
passive transport of cations into the
cell and anions out of cell.
–Electrochemical gradient - diffusion
gradient resulting from combined
effects of membrane potential and
concentration gradient
• Ions may not always diffuse down
their concentration gradient, but
always diffuse down their
electrochemical gradient
–Electrogenic pump - a transport
protein that generates voltage
across membrane
• Ex. Sodium potassium pump (in
animals): 3 Na+ move out and only
2 K+ move in (net charge of +1 on
outside of cell)
• Ex. Proton pump (in bacteria,
fungi, and plants): actively
transports H+ outside of cell
Figure 8.17 An electrogenic pump
Ion pump animation
• Cotransport - a process where a
single ATP-powered pump actively
transports one solute and indirectly
drives the transport of other solutes
against their concentration gradient
•
Cotransport animation
Figure 8.18 Cotransport
–One example in plants:
• An ATP driven proton pump sends
H+ outside of the cell
• Then H+ diffuses back into cell via
a specific transport protein
• As H+ diffuses, sucrose can ride
the proton’s “coattails” and move
into the cell (against its own
concentration gradient).
– One example in humans:
• If someone has severe diarrhea or
is badly dehydrated from running…
–Give person solution high in
glucose and salt
–Solutes transported to blood
causes water to move into blood
from colon (rehydration)
–Cotransport involves Na+ and
glucose so both needed
Figure 8.19 The three types of endocytosis in animal cells
• Exocytosis - process of exporting
macromolecules (ex. proteins and
polysaccharides) from a cell by fusion
of vesicles with a cell membrane
• Endocytosis - process of importing
macromolecules (ex. proteins and
polysaccharides) into a cell by forming
vesicles derived from the cell
membrane
•
•
Endo animation
More…
–Phagocytosis - endocytosis of
solid particles
–Pinocytosis - endocytosis of fluid
droplets
–Receptor-mediated endocytosis a ligand binds to a receptor site in a
coated pit and causes a vesicle to
form and ingest material. This is
more discriminating than
pinocytosis.