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Chapter 7 Outline
Membrane Structure and Function
Guiding Questions
1. Describe the structure of the cell membrane.
2. By what methods are substances transported across membranes?
Fluid Mosaic Model
 The membrane is a fluid structure with various proteins embedded in or attached to a double layer
(bilayer) of phospholipids
o Fluid = capable of flowing and easily changing shape
o Mosaic = containing various components of different type
 Phospholipids are amphipathic – one side is polar, the other side is non polar.
 Describe the structure of a phospholipid.
 Describe the structure of the lipid bilayer.
Selective Permeability
Fluidity
 Affected by temperature
 Can be controlled
o Fatty acid saturation
o Cholesterol concentration
Membrane Proteins
 Types
o Integral/Transmembrane
o Peripheral
 Structure/Orientation
o Established by Endoplasmic Reticulum
o Determined by type of amino acids at various points on the protein.
o Very important for function of protein in the membrane
 Function
o Transport
o Enzymatic Activity
o Signal Transduction
o Intercellular Joining
o Cell-Cell Recognition
o Attachment to Cytoskeleton and Extracellular Matrix
Movement across a plasma membrane
 Passive Transport: does not require external energy, moves down a concentration gradient.
o Diffusion
 Movement of particles from high concentration to low concentration due to random
motion.
 Moves toward dynamic equilibrium
o Osmosis
 Passive transport of water from. . .
 Low solute concentration to high solute concentration.
 High water potential to low water potential
 Hypotonic solution to hypertonic solution
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o Facilitated Diffusion
 Movement of a substance from high concentration to low concentration (down the
concentration gradient) through a membrane protein.
 Does not require energy
Active Transport: does require energy, moves a substance up its concentration gradient from low
concentration to high concentration.
o Examples
 Sodium-Potassium Pump
 Proton Pump
Cotransport
o Couple down hill diffusion of one substance to up hill transport of another.
o Incorporates both active and passive transport.
Exocytosis
o A substance is released to the exterior of a cell
Endocytosis: A substance is taken in to a cell
o Phagocytosis
o Pinocytosis
o Receptor mediated endocytosis
Chapter 48
What is the role of transport and channel proteins in propagating signals along a neuron, producing an action
potential, and forming/maintaining resting potential?
Osmosis and Water Potential
Osmosis: Diffusion of water across a selectively permeable membrane
1. From a region of low solute concentration to a region of high solute concentration.
2. From high water potential to low water potential.
1. From a region of low solute concentration to a region of high solute concentration.
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Hypertonic solution: Having a larger solute concentration than another solution.
Hypotonic solution: Having a smaller solute concentration than another solution.
Isotonic solution: Having the same solute concentration as another solution.
 If solutes are not allowed to pass through a membrane, water will move through the membrane from a
hypotonic solution to a hypertonic solution.
 Always moving toward equilibrium—having equal solute concentrations on both sides of a membrane.
2. From high water potential to low water potential.
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The physical property predicting the direction in which water will flow, governed by solute
concentration and applied pressure.
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Water Potential = Pressure Potential + Solute Potential
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Water moves from high water potential to low water potential
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Effect of solute on water potential
o Zero solute: Ψs = 0
o Adding solute makes Ψs negative
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Effect of pressure on water potential
o Atmospheric pressure: Ψp = 0
o Increase pressure: Ψp larger/more positive
o Decrease pressure: Ψp smaller/more negative
(Ψ = Ψp + Ψs)