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Chapter 7
Membrane Structureand Function
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Plasma Membrane
The membrane at the
boundary of every cell.Functions as a selective
barrier for the passage of
materials in and out of cells.
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Membrane
CompositionLipids
ProteinsQuestion:
How are the materialsarranged?
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Membrane Models
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Fluid Mosaic Model
1972New model to fit the new
evidence with membranes.Example of
Science as a Process.
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Fluid Mosaic Model
Refers to the way the lipids
and proteins behave in amembrane.
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Fluid
Refers to the lipid bilayer.
Molecules are not bondedtogether, so are free to shift.
Must remain "fluid" formembranes to function.
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Ways to keep the
membrane fluidLipid changes or shifts:
Cold hardening of plants(shift to saturated fatty acids).
Hibernating animals
(Cholesterol increase).
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Mosaic
Proteins: float in a sea of
lipids.Proteins form a collage or
mosaic pattern that shifts
over time.
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Types of Membrane
ProteinsIntegral - inserted into the
lipid bilayer.Peripheral - not embedded in
the lipid bilayer, but are
attached to the membranesurface.
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Hydrophilic
Amino Acids
Hydrophobic
Amino AcidsHydrophilic
Amino Acids
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Question?
How do the integral proteins
stick to the membrane?By the solubility of their
amino acids.
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Protein Function in
MembranesTransport.
Enzymatic activity.
Receptor sites for signals.
Cell adhesion.
Cell-cell recognition.
Attachment to the cytoskeleton.
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Carbohydrates
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Membrane
CarbohydratesBranched oligosaccharides
form glycolipids andglycoproteins on externalsurface.
Function - recognition of"self" vs "other.
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Question
How do materials get acrossa cell's membrane?
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Problems
Lipid bilayer is hydrophobic.Hydrophilic materials don'tcross easily.
Large molecules don't cross
easily. Too big to getthrough the membrane.
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Passive Transport
Movement acrossmembranes that does NOTrequire cellular energy.
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Types of Passive
Transport1. Diffusion
2. Osmosis3. Facilitated Diffusion
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Equilibrium
When the concentration isequal on both sides.
There is no net movement ofmaterials.
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Factors that Effect
Diffusion1. Concentration
2. Temperature3. Pressure
4. Particle size
5. Mixing
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Osmosis
Diffusion of water.
Water moving from an area ofhigh concentration to an areaof low concentration.
No cell energy is used.
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Tonicity
The concentration of waterrelative to a cell.
1. Isotonic (same)
2. Hypotonic (below)
3. Hypertonic (above)
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Isotonic
Isosmotic solution.
Cell and water are equal insolute concentration.
No net movement of water in
or out of the cell.
No change in cell size.
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Hypotonic
Hypoosmotic solution
Cell's water is lower than theoutside water (more solutes).
Water moves into the cell.
Cell swells, may burst or thecell is turgid.
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Hypertonic
Hyperosmotic solution
Cell's water is higher than theoutside water (less solutes)
Water moves out of the cell.
Cell shrinks or plasmolysisoccurs.
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Facilitated Diffusion
Transport protein that helpsmaterials through the cellmembrane.
Doesn't require energy (ATP).
Works on a downhillconcentration gradient.
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Active Transport
Movement acrossmembranes that DOESrequire cellular energy.
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Carrier-Mediated
TransportGeneral term for the active
transport of materials intocells AGAINST theconcentration gradient.
Movement is: low high
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Examples
1. Na+- K+ pump
2. Electrogenic or H+
pumps3. Cotransport
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Na+- K+ pump
Moves Na+ ions out of cellswhile moving K+ ions in.
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Electrogenic or
H+
pumpsAlso called Proton pumps.
Create voltages acrossmembranes for other cellprocesses.
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Endocytosis
Moves bulk materials intocells.
Several types known.
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Types
1. Pinocytosis - liquids
2. Phagocytosis - solids3. Receptor Mediated - uses
receptors to "catch" specific
kinds of molecules.
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Forming vesicles
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Summary
Know membrane structure.
Be able to discuss thevarious methods by whichcells move materials through
membranes.Be able to solve problems in
osmosis.
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