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Chapter 7 Membrane Structure
and 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 Composition
phospholipids Proteins Cholesterol Question:
How are the materials arranged?
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Phospholipid Bilayer
Phospholipids Hydrophilic
heads Hydrophobic
tails
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Membrane Models
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Fluid Mosaic Model 1972
New model to fit the new evidence with membranes.
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Fluid Mosaic Model
The way the phospholipids and proteins behave in a membrane.
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“Fluid”
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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“Mosaic”
Proteins: float in a sea of phospholipids.
Proteins form a collage or mosaic pattern that shifts over time.
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Protein Function in Membranes
Transport. Enzymatic activity. Receptor sites for signals. Cell adhesion. Cell-cell recognition. Attachment to the cytoskeleton.
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Types of Membrane Proteins
Integral - inserted into the phospholipid bilayer.
Peripheral - are attached to the membrane surface.
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I or P?(mark these on the list above)
Transport –Channels and pumps (Integral)
Enzymes- I or P Receptors- P or sometimes I Cell adhesion- P or I Cell to cell recognition- P
(glycoproteins) Attachment to cytoskeleton- I
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Question?
How do the integral proteins stick to the membrane?
By the solubility of their amino acids.
Non-polar will be in the hydrophobic area
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Hydrophilic Amino Acids
Hydrophobic Amino Acids
Hydrophilic Amino Acids
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Membranes are Bifacial
Inside layer is different than outer layer
The proteins have specific orientations.
Carbohydrates are found only on the outer surface.
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Carbohydrates
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Membrane Carbohydrates
Branched oligosaccharides form glycophospholipids and glycoproteins on external surface.
Function – cell to cell recognition
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Cholesterol
Keeps membranes the right fluidity level Not too flimsy Not too rigid
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End of Part 1 Create a Paper Model of a
membrane
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Question
How do materials get across a cell's membrane?
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Problems
phospholipid bilayer is hydrophobic. Hydrophilic materials don't cross easily.
Large molecules don't cross easily. Too big to get through the membrane.
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Mechanisms
1. Passive Transport
2. Active Transport
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Passive Transport
Movement across membranes that does NOT require cellular energy.
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Types of Passive Transport
1. Diffusion
2. Osmosis
3. Facilitated Diffusion
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Diffusion
The net movement of atoms, ions or molecules down a concentration gradient.
Movement is from: High Low
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Equilibrium
When the concentration is equal on both sides.
There is no net movement of materials.
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Factors that Affect Diffusion
1. Concentration
2. Temperature
3. Pressure
4. Particle size
5. Mixing
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Osmosis
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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Tonicity
The concentration of water relative to a cell. 1. Isotonic (same)
2. Hypotonic (below)
3. Hypertonic (above)
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Isotonic
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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Hypotonic
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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Hypertonic
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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Facilitated Diffusion
RequiresTransport protein that helps materials through the cell membrane.
Doesn't require energy (ATP).
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Aquaporins
Newly found channels for osmosis.
GFP labeled Aquaporins
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Active Transport
Movement across membranes that DOES require cellular energy.
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Types of Active Transport
1. Carrier-Mediated
2. Endocytosis
3. Exocytosis
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Carrier-Mediated Transport
General term for the active transport of materials into cells AGAINST the concentration gradient.
Movement is: low high
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Examples
1. Na+- K+ pump
2. Electrogenic or H+ pumps
3. Cotransport
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Na+- K+ pump
Moves Na+ ions out of cells while moving K+ ions in.
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Electrogenic or H+ pumps
Also called Proton pumps. Create voltages across
membranes for other cell processes.
Used by plants, fungi and bacteria.
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Cotransport
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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Exocytosis
Moves bulk material out of cells.
Example - secretion of enzymes.
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Endocytosis
Moves bulk materials into cells.
Several types known.
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Types
1. Pinocytosis - liquids
2. Phagocytosis – solids
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Carbohydrates
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Forming vesicles
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Summary
Know membrane structure. Be able to discuss the various
methods by which cells move materials through membranes.
Be able to solve problems in osmosis.