Transcript Membrane Structure and Function Chapter 8

Membrane Structure and
Function
Chapter 5
Membrane Structure
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A boundary separating the cell from its environment
About 8 nm thick
Controls chemical traffic in and out of the cell
Selectively permeable
Unique structure determines function and solubility
characteristics
Artificial Membranes
Two Generations of Membrane
Models
1935
Always same thickness
Coated with proteins
1972
Proteins span the membrane
Not all the same
The Fluidity
of Membranes
Membranes rarely flip-flop –
too unstable
Frequent lateral movement
Double bonds in unsaturated
lipid tails cause kinks which
contribute to fluidity
Cholesterol reduces fluidity at
high temps.
Increases fluidity at low temps.
Membrane Proteins
• Membrane proteins contribute to the
mosaic quality of the structure.
• Different proteins convey different
properties to each membrane.
• Integral proteins - within the membrane.
• Peripheral proteins - attached to membrane
surface
• Proteins attach to cytoskeleton or to extracellular
fibers to help give animal cells a stronger
framework
Evidence of the Drifting of
Membrane Proteins
Membrane Carbohydrates
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Found only on the outside of the membrane.
Function in cell to cell recognition.
Sorting embryonic cells into tissues.
Immune defense.
Usually oligosaccharides (15 or less sugar units)
Glycolipids or Glycoproteins
Cell Plasma Membrane
Structure of a Transmembrane
Protein
Helical secondary
Structure of the
non-polar portion
Bacteriorhodopsin
has 7 transmembrane
Domains and is a
specialized
transport protein.
Functions of
Membrane
Proteins
Sidedness of
the Plasma
Membrane
Membranes have
distinct cytoplasmic
and extracellular sides.
Permeability of the Lipid Bilayer
• Nonpolar (hydrophobic) molecules
– hydrocarbons, oxygen, carbon dioxide
– cross with ease
– the smaller the faster
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Polar (hydrophilic) molecules
- small molecules, water, ethanol, pass easily
- larger, glucose, will not pass through the lipid
membranes
- all ions have difficulty passing
Selective Permeability
Transport Proteins
Provide hydrophilic
tunnel for ions.
They are specific
for the substances
they transport
Diffusion of
Salts Across
Membranes
Each dye diffuses down
its own concentration
gradient
Passive Transport
• Diffusion across a membrane
• Down a concentration gradient (High to
Low)
• Net directional movement
• Spontaneous, no energy required,
decreases free energy
Diffusion Animation 8.2
Diffusion is the net movement down a
concentration gradient until a dynamic
equilibrium is reached (there is still
movement, no NET movement).
Osmosis The Passive Transport of Water
The Selectively Permeable
Membrane
Towards Equilibrium
The Water Balance of Living Cells
Evolutionary Adaptations for
Osmoregulation in Paramecium
Filling vacuole
Fresh water organisms
are hypertonic to their
environment. They
take up water by
osmosis. A system is
needed to remove
excess water
Contracting vacuole
Osmoregulation in Saltwater Fish
While seawater is isotonic to many organisms, it is
hypertonic to some, they will loose water by osmosis.
Fish conserve water and pump out salts
Water Potential Y
• Measures the tendency of water to leave
one place in favor of another.
• Water will always move from an area of
high water potential to an area of low
water potential.
• Water potential is affected by two physical
factors
1.Addition of solute, always lowers the water
potential
2.Pressure, an increase in pressure will
increase the water potential
Y = Yp + Ys
• Pressure potential, Yp, is usually positive in
living cells, negative in dead xylem.
• Water potential, Y, is zero for pure water.
• Cell sap has a negative solute potential, Ys, as
it always contains solute.
• Water potential, Y, can be negative, zero or
positive, depending on the Ys and Yp
values.
Facilitated Diffusion
Facilitated Diffusion
• Diffusion of solutes with the help of
transport proteins.
• Passive - no energy required
• These solutes need a protein to facilitate
their diffusion because they are too polar
to pass through the lipid bilayer.
Active Transport
• Pumps molecules across the membrane
against their concentration gradients.
• Requires energy, in the form of ATP
• Used to help maintain ionic gradients
across membranes.
• These ionic gradients represent potential
energy.
Electrochemical Gradient
• Two forces drive the movement of ions
into a cell, the ion’s conc. gradient and the
electrical force – together called the
electrochemical gradient.
Nerve Cells
Upon activation, Na+ enters the
cell by diffusion down its
electrochemical gradient.
In order to be ready for the next
signal, Na+ must be actively
pumped out against its gradient.
The Sodium Potassium Pump
Animal electrogenic pump.
Passive and Active Transport
Some Ion Pumps Generate Voltage
Across Membranes
• All cells have voltages across their plasma membranes
• The cytoplasm of a cell is negative compared to the
extracellular fluids.
• This voltage is called a membrane potential, ranges from
-50 to -200 mV
• Membrane potential can act like a battery.
A Plant Electrogenic Pump
Energy can be stored
by creating a voltage
across membranes.
Using ATP, H+ is
pumped out of the
cell to create a
gradient.
Proton pumps are
the main electrogenic
pumps of plants,
fungi and bacteria
Cotransport
A membrane protein couples
the transport of one product
to another.
The ATP-driven pump stores
energy by concentrating a
substance on one side of
the membrane.
H+ falls back down its conc.
gradient taking sucrose with it.
Sucrose accumulation in a
plant cell.
Exocytosis
Secretion of macromolecules by the
fusion of vesicles with the plasma
membrane
Endocytosis
Cholesterol Enters Cells by
Receptor Mediated Endocytosis
• In the blood, cholesterol is bound to lipid and
protein complexes called LDLs
• The LDLs bind to LDL receptors on cell
membranes initiating endocytosis.
• In the disease, familial hypercholesterolemia,
the LDL receptors are defective, cholesterol
cannot enter the cell and accumulates in the
blood causing atherosclerosis.
Aquaporin – facilitates water
diffusion (osmosis)
Tetrameric protein,
four identical
subunits
Each subunit forms
a water channel
Osmotic Potential