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Biology - Chapter 7 - Membrane Structure and Function
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Introduction
o Membranes define the boundaries of the cell and its various internal compartments
▪ Transport proteins : regulate movement of substances into and out of the cell and its
organelles
▪ Receptors : detect external signals
▪ Selective Permeability : allowing some substances to cross more easily than others
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Grendel : extracted lipids from red blood cells
▪ Hugh Davson and James Danielli : sandwich model in which phospholipid bilayer lies
between two layers of globular proteins
▪ S
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Singer and G
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2 Selective Permeability
-Hydrophobic (nonpolar) molecules can dissolve in the lipid bilayer and pass through the membrane
rapidly
-Polar molecules do not cross the membrane easily
• Transport Proteins
o Transport proteins allow the passage of hydrophilic substances across the membrane
▪ Some transport proteins, called channel proteins, have a hydrophilic channel that
certain molecules can use
• Channel proteins called aquaporins facilitate the passage of water
▪ Carrier proteins bind to molecules and change shape to shuttle them across the
membrane
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4 - Active Transport
• Active transport mechanisms can be divided into:
o Directive active transport - the accumulation of solute molecules or ions on one side of the
membrane is coupled directly to an exergonic chemical reaction
o Indirect active transport - requires energy but depends on the simultaneous transport of
two solutes, with the favorable movement of one solute down its gradient driving the
unfavorable movement of the other solute up its gradient
o The sodium potassium pump is one type of active transport system (used by neurons to
send an action potential through the axon, received by another cell)
o The resulting electrochemical potentials for potassium and sodium ions are essential as the
driving force for coupled transport as well as for the transmission of nerve impulses
• Membrane potential is the voltage difference across a membrane
• Voltage is created by differences in the distribution of positive and negative ions across a
membrane
• Two combined forces, collectively called electrochemical gradient, drive the diffusion of ions'
across a membrane
o A chemical force (the ions' concentration gradient)
o An electrical force (the effect of the membrane potential on the ions' movement)
• An electrogenic pump is a transport protein that generates voltage across a membrane
• The sodium potassium pump is the major electrogenic pump of amical cells
• The main electrogenic pump of plants, fungi, and bacteria is a proton pump
• Electrogenic pumps help store energy that can be used for cellular work
• Cotransport : Coupled Transport by a membrane protein
• Cotransport occurs when active transport of a solute indirectly drives transport of other solutes
• Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active
transport of nutrients into the cell
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0832 L bars/mole K
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