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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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Phospholipids
Proteins
Question:
◦ How are the materials arranged?
 Phospholipids
 Hydrophilic
heads
 Hydrophobic
tails
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phospholipid bilayer.
Proteins coat the surfaces.
Sometimes called the “sandwich” model.
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Biochemical work.
TEM pictures show the membrane as a double
line.
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Not all membranes in a cell were the same.
How could the proteins stay in place?
Result - the model was questioned and
tested by scientific process.
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New model to fit the new evidence with
membranes.
Example of “Science as a Process”.
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Refers to the way the phospholipids and
proteins behave in a membrane.
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Refers to the phospholipid bilayer.
Molecules are not bonded together, so are
free to shift.
Must remain "fluid" for membranes to
function.
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phospholipid changes or shifts:
Cold hardening of plants (shift to unsaturated
fatty acids).
Hibernating animals (Cholesterol increase).
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Proteins: float in a sea of phospholipids.
Proteins form a collage or mosaic pattern that
shifts over time.
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TEM pictures of fractured membranes.
Cell fusion studies.
Tagging of membrane proteins by antibodies.
 Transport.
 Enzymatic
activity.
 Receptor sites for signals.
 Cell adhesion.
 Cell-cell recognition.
 Attachment to the cytoskeleton.
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Integral - inserted into the phospholipid
bilayer.
Peripheral - not embedded in the
phospholipid bilayer, but are attached to the
membrane surface.
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How do the integral proteins stick to the
membrane?
By the solubility of their amino acids.
Hydrophilic
Amino Acids
Hydrophobic
Amino Acids
Hydrophilic
Amino Acids
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The phospholipid composition of the two
layers is different.
The proteins have specific orientations.
Carbohydrates are found only on the outer
surface.
Carbohydrates
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Branched oligosaccharides form
glycophospholipids and glycoproteins on
external surface.
Function - recognition of "self" vs "other”.
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How do materials get across a cell's
membrane?
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phospholipid bilayer is hydrophobic.
Hydrophilic materials don't cross easily.
Large molecules don't cross easily. Too big to
get through the membrane.
1. Passive Transport
2. Active Transport
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Movement across membranes that does NOT
require cellular energy.
1. Diffusion
2. Osmosis
3. Facilitated Diffusion
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The net movement of atoms, ions or
molecules down a concentration gradient.
Movement is from: High
Low
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When the concentration is equal on both
sides.
There is no net movement of materials.
1.
2.
3.
4.
5.
Concentration
Temperature
Pressure
Particle size
Mixing
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Diffusion of water.
Water moving from an area of its high
concentration to an area of its low
concentration.
No cell energy is used.
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The concentration of water relative to a cell.
1. Isotonic (same)
2. Hypotonic (below)
3. Hypertonic (above)
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Isosmotic solution.
Cell and water are equal in solute
concentration.
No net movement of water in or out of the
cell.
No change in cell size.
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Hypoosmotic solution
Cell's water is lower than the outside water
(more solutes).
Water moves into the cell.
Cell swells, may burst or the cell is turgid.
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Hyperosmotic solution
Cell's water is higher than the outside water
(less solutes)
Water moves out of the cell.
Cell shrinks or plasmolysis occurs.
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Common to AP exams
Usually depend on being able to define
“high” and “low” water or solute
concentrations.
U-tube problems
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Transport protein that helps materials
through the cell membrane.
Doesn't require energy (ATP).
Works on a downhill concentration gradient.
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Newly found channels for osmosis.
GFP labeled
Aquaporins
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Movement across membranes that DOES
require cellular energy.
1. Carrier-Mediated
2. Endocytosis
3. Exocytosis
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General term for the active transport of
materials into cells AGAINST the
concentration gradient.
Movement is: low
high
1. Na+/ K+ pump
2. Electrogenic or H+ pumps
3. Cotransport
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Moves Na+ ions out of cells while moving K+
ions in.
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Also called Proton pumps.
Create voltages across membranes for other
cell processes.
Used by plants, fungi and bacteria.
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Movement of H+ that allows other materials
to be transported into the cell as the H+
diffuses back across the cell membrane.
Example - Sucrose transport
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Moves bulk material out of cells.
Example - secretion of enzymes.
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Moves bulk materials into cells.
Several types known.
1. Pinocytosis - liquids
2. Phagocytosis - solids
3. Receptor Mediated - uses receptors to
"catch" specific kinds of molecules.
Carbohydrates
Forming
vesicles
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Know membrane structure.
Be able to discuss the various methods by
which cells move materials through
membranes.
Be able to solve problems in osmosis/water
potential.