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AP Biology
Chapter 7: Membrane Structure & Function
AP Biology
Phospholipids
Fatty acid
Phosphate
An amphipathic molecule Phosphate head
hydrophilic
Fatty acid tails hydrophobic
Arranged as a bilayer “repelled by water”
“attracted to water”
AP Biology
Cell membrane defines cell Cell membrane separates living cell from
aqueous environment Selectively Permeable
allows some substances to cross more easily than others Hydrophilic vs. hydrophobic substances
Fluid Mosaic of lipids & proteins Contains embedded proteins
AP Biology
Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer
Extracellular fluid
Cholesterol
Cytoplasm
Glycolipid
Transmembraneproteins
Filaments ofcytoskeleton
Peripheralprotein
Glycoprotein
Phospholipids
1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model
AP Biology
Freeze Fracture Freeze fracture studies supports the
fluid mosaic model of the membrane structure
A cell is frozen and fractured with a knife Fracture plan follows the hydrophobic
interior of the membrane The membrane proteins go wholly with one of
the layers
AP Biology
Fluid Mosaic Model A membrane is a fluid structure with a
“mosaic” of various proteins embedded in it
Phospholipids & proteins can move Laterally & horizontally
making the membrane fluid
Lateral movement(~107 times per second)
Flip-flop(~ once per month)
AP Biology
Membrane Fluidity The type of hydrocarbon tails in the
phospholipids affects the fluidity of the plasma membrane. Saturated vs. unsaturated
Fluid Viscous
Unsaturated hydrocarbontails with kinks
Saturated hydro-Carbon tails
AP Biology
Membrane Fluidity The steroid cholesterol affects
membrane fluidity at different temperatures in animal cells Known as the “temperature buffer” Hinders solidification
Cholesterol
AP Biology
Membrane Proteins Proteins determine membrane’s specific functions
Classes of membrane proteins: peripheral proteins
loosely bound to surface of membrane
attachment to cytoskeleton for support
cell surface identity marker (antigens) integral proteins
penetrate lipid bilayer, usually across whole membrane transmembrane protein transport proteins
channels, pumps
AP Biology
Membrane Permeability A cell must exchange molecules with
its surroundings Hydrophobic molecules
Are lipid soluble and can pass through rapidly
Hydrophilic molecules Do not cross the membrane rapidly
Impedes movement of polar molecules/ions
What substances have difficulty passing through?
Polar molecules Water, glucose
Ions (charged) salts, ammonia
What substances can NOT get through directly?
large molecules starches, proteins
AP Biology
Integral Proteins Create Channels
polarhydrophilic
heads
nonpolarhydrophobic
tails
polarhydrophilic
heads
Specific protein channels transport hydrophilic molecules
H2Osugar
lipids
salt
AP Biology
Proteins domains anchor molecule
Within membrane nonpolar amino acids
hydrophobic anchors protein
into membrane
On outer surfaces of membrane in fluid polar amino acids
hydrophilic extend into
extracellular fluid & into cytosol
Polar areasof protein
Nonpolar areas of protein
AP Biology
Many Functions of Membrane Proteins
Outside
Plasmamembrane
InsideTransporter Cell surface
receptorEnzymeactivity
Cell surface identity marker
Attachment to thecytoskeleton
Cell adhesion
“Antigen”
“Channel”
AP Biology
Membrane carbohydrates Play a key role in cell-cell recognition
ability of a cell to distinguish one cell from another antigens
important in organ & tissue development
blood types basis for rejection of
foreign cells by immune system
Glycoprotein
Glycolipid
AP Biology
Synthesis and Sidedness of Membranes Membranes have
distinct inside and outside faces Membrane proteins
and lipids are made in the ER Proteins have a
directional orientation in the membrane
AP Biology
PM
Inside Cell
Outside Cell
RULE: What is on the inside of the vesicle, ends up on the outside of the
plasma membrane
AP Biology
PM
Inside Cell
Outside Cell
- Membrane fusion proteins will fuse the membrane of the transport vesicle
to the plasma membrane…
AP Biology
PM
Inside Cell
Outside Cell
- Notice that the proteins inside the vesicle are now outside the cell. And
the proteins stuck in the vesicle membrane will now be in the plasma
membrane…
AP Biology
PM
Inside Cell
Outside Cell
RULE: What is on the inside of the vesicle, ends up on the outside of the
plasma membrane
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Movement Across the Cell Membrane
INfoodcarbohydratessugars, proteinsamino acidslipidssalts, O2, H2O
OUTwasteammoniasaltsCO2
H2O products
cell needs materials in & products or waste out
IN
OUT
AP Biology
Passive Transport The diffusion of a substance across a
membrane with no energy investment Diffusion is the tendency of molecules
of any substance to spread out evenly into the available space Down the concentration gradient
movement from HIGH LOW concentration movement from HIGH LOW concentration
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1. Simple Diffusion Move from HIGH to LOW concentration
“passive transport” No energy needed Nonpolar, hydrophobic molecules Lipids, O2, CO2
Can easily diffuse Water is small enough and can slowly diffuse
OSMOSIS
movement of O2
HIGHLOW
AP Biology
2. Osmosis Osmosis is the diffusion
of water across a semipermeable membrane Tonicity
Is the ability of a cell to gain or lose water
Depends on the amount of solutes in the solution
Diffusion of water from
HIGH concentration of H2O LOW concentration of H2O
Diffusion of water from
HIGH concentration of H2O LOW concentration of H2O
AP Biology
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
hypotonic hypertonicwater
net movement of waterHIGH concentration of H2O LOW concentration of H2O HIGH concentration of H2O LOW concentration of H2O
AP Biology
Managing water balance Hypotonic
a cell in fresh water or pure water high concentration of water around cell
problem: cell gains water, swells & can burst
example: Paramecium ex: water continually enters Paramecium cell
solution: contractile vacuole pumps water out of cell Uses ATP
plant cells Turgor pressure
the pressure of water inside a plant cell pushing against the cell membrane freshwater
1
Plant cells are healthiest
Animal cells burst
AP Biology
Managing water balance Hypertonic
a cell in salt water low concentration of water
around cell problem: cell loses water &
can die example: shellfish solution: take up water or
pump out salt plant cells
plasmolysis = wilt can recover
saltwater
2
Plasma membrane shrinks = wilt
I’m shrinking,I’m shrinking!
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Managing water balance Isotonic
animal cell immersed in mild salt solution
no difference in concentration of water between cell & environment problem: none
no net movement of water flows across membrane equally, in
both directions
cell in equilibrium volume of cell is stable
example: blood cells in blood plasma slightly salty IV solution in hospital
balanced
3
I couldbe better…
That’sperfect!
AP Biology
Cell (compared to beaker) hypertonic or hypotonic
Beaker (compared to cell) hypertonic or hypotonic
Which way does the water flow? in or out of cell
.05 M
.03 M
Do you understand Osmosis…
AP Biology
How Will Water Move Across Semi-Permeable
Membrane?Solution A has 100 molecules of glucose per ml
Solution B has 100 molecules of fructose per ml
How will the water molecules How will the water molecules move?move?
There will be no net movement of waterno net movement of water since the concentration of solute in each
solution is equal
AP Biology
How Will Water Move Across Semi-Permeable Membrane?
Solution A has 100 molecules of glucose per ml
Solution B has 75 molecules of fructose per ml
How will the water molecules How will the water molecules move?move?
There will be a net movement of water from Solution B to Solution A until both solutions have equal concentrations of
solute
AP Biology
How Will Water Move Across Semi-Permeable Membrane?
Solution A has 100 molecules of glucose per ml
Solution B has 100 molecules of NaCl per ml
How will the water molecules How will the water molecules move?move?
Each molecule of NaCl will dissociate to form a Na+ ion and a Cl- ion, making the final concentration of solutes 200 molecules per mil. Therefore, there will be a net movement
of water from Solution A to Solution B until both solutions have equal concentrations of solute
AP Biology
3. Facilitated Diffusion Diffusion through protein channels
channels move specific polar molecules or ions across cell membrane A type of passive transport No energy needed High to low concentration
Channel proteins Water or specific solute
Carrier proteins Undergoes a subtle shape change Ex: Glucose carrier protein open channel = fast transport
facilitated = with help
HIGH
LOWHIGH
LOW
AP Biology
Aquaporins Water moves rapidly into & out of cells
evidence that there were water channels protein channels allowing flow of water
across cell membrane
1991 | 2003
Peter AgreJohn Hopkins
Roderick MacKinnonRockefeller
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Active Transport
conformational change
Cells may need to move molecules against concentration gradient conformational shape change transports solute from one side of
membrane to other protein “pump” “costs” energy = ATP
Phosphorylation Examples
Sodium Potassium Pump Proton Pump
ATP
LOW
HIGH
AP Biology
Ion Pumps Ion pumps maintain membrane potential
In membranes of the chloroplast, mitochondria, lysosome and nerve cells Membrane potential
Voltage difference across a membrane
Electrochemical gradient Caused by the ion concentration difference across
a membrane
Electrogenic pump Transport protein that generates the voltage
across a membrane
AP Biology
Sodium Potassium Pump
Na+ binding stimulatesphosphorylation by ATP.
2
Na+ Cytoplasmic Na+ binds tothe sodium-potassium pump.
1
K+ is released and Na+
sites are receptive again; the cycle repeats.
3
Phosphorylation causes the protein to change its
conformation, expelling Na+ to the outside.
4
Extracellular K+ binds to the protein, triggering release of the
Phosphate group.
6 Loss of the phosphaterestores the protein’s original conformation.
5
CYTOPLASM
[Na+] low[K+] high
Na+
Na+
Na+
Na+
Na+
P ATP
Na+
Na+
Na+
P
ADP
K+
K+
K+
K+ K+
K+
[Na+] high[K+] low
Found in nerve cells
AP Biology
Proton Pumps Creates
electrochemical gradient across a membrane
Cotransport Active transport of one
solute indirectly drives the active transport of another solute Ex: Sucrose
cotransporter
EXTRACELLULARFLUID
+
H+
H+
H+
H+
H+
H+ Proton pump
ATP
CYTOPLASM
+
+
+
+–
–
–
–
–
+
AP Biology
How about large molecules? Moving large molecules into & out of cell
through vesicles & vacuoles Exocytosis
Transport vesicles migrate to the plasma membrane, fuse with it, release their contents
Endocytosis The cell takes in macromolecules by
forming a vesicle or vacuole from the plasma membrane
exocytosis
AP Biology
Endocytosis
Phagocytosis“cell eating”
Pinocytosis“cell drinking”
receptor-mediated endocytosis
“ligand-receptor”
Vacuole fuses with lysosome for digestion
non-specificprocess, fluid “gulped” into a vesicle
triggered byspecific molecular signal
AP Biology
Summary Passive transport. Substances diffuse spontaneously
down their concentration gradients, crossing a membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport proteins in the membrane.
Active transport. Some transport proteins act as pumps, moving substances across a
membrane against their concentration gradients. Energy for this work is usually
supplied by ATP.
Diffusion. Hydrophobicmolecules and (at a slow
rate) very small uncharged polar molecules can diffuse through the lipid
bilayer.
Facilitated diffusion. Many hydrophilic substances diffuse through membranes with
the assistance of transport proteins,either channel or carrier proteins.
ATP
