CHAPTER 12 Membrane Structure and Function

Biological Membranes are composed of Lipid Bilayers and Proteins -Biological membranes define the external boundaries of cells and separate compartments within cells. -A biological membrane consists of two layers of lipid molecules AND -proteins embedded in or associated with the lipid bilayer Figure Lipid bilayers are the structural basis for biological membranes -Noncovalent interactions among lipid molecules make membranes flexible and self-sealing. -Polar head groups contact aqueous medium -Nonpolar tails point toward the interior of membrane

Membrane Fluidity This lipid will result in a more fluid membrane at lower temps Cholesterol helps disrupt hydrophobic interaction of fatty acid chains. More fluidity

Figure 12.8 Integral and peripheral membrane proteins Integral proteins Peripheral proteins Peripheral protein

Examples of Integral membrane proteins Figure 12.9: bacteriorhodopsin Yellow tubes represent  -helices. light harvesting protein Figure porin, a pore or channel across the membrane. Note the  -sheet 2 o structure

Examples of peripheral membrane proteins Figure 12.11: prostaglandin H 2 synthase-1

Lipid Bilayers and Membranes are Dynamic Structures Figure Billion times slower ~2  m is 1 sec -Some anchored proteins can diffuse laterally just as rapidly as a phospholipid. -Membrane fluidity is maintained at lower temperatures by adjusting the ratio of saturated and unsaturated fatty acyl groups

Membrane Transport - Membranes are selectively permeable barriers that restrict the free passage of most molecules How then do molecules and ions traverse the membrane bilayer? Through three types of integral membrane proteins: 1. Channels and pores 2. Passive transporters 3. Active transporters

Passive Transport -Passive transport is called facilitated diffusion because it does NOT require an energy source -Transport would otherwise be very slow in absence of protein -Protein binds solutes and transports them down a concentration gradient Types of passive transport systems -Uniport – transporter carries only a single type of solute -Some transporters carry out co-transport of two solutes: -Symport – same direction -Antiport – opposite directions

Figure Types of passive transport Uniport -Passive transport is called facilitated diffusion because it does NOT require an energy source -Transport would otherwise be very slow in absence of protein -Protein binds solutes and transports them down a concentration gradient

Active Transport - Active transport is similar to passive transport BUT requires energy to move a solute up its concentration gradient -Active transport of charged molecules or ions may result in a charge gradient across the membrane. Transport against a membrane potential (or voltage). Types of active transport -Primary active transport is powered by a direct source of energy as ATP, light or electron transport -Secondary active transport is driven by an ion concentration gradient.

Figure Primary active transport in animals: Na + /K + ATPase Pump - An ATP driven antiport transport Exterior: [K + ] = 5mM [Na + ] = 145 mM Cytosol: [K + ] = 140mM [Na + ] = 5-15 mM > 30% of ATP generated is used to maintain this gradient and that’s when resting!

Figure The ATPase pump drives a secondary active transport (symport) of glucose into cell. antiport Na + ion gradient

Pores and Channels (pores are used for bacteria and channels for animals) - Pores and channels are trans-membrane proteins with a central passage for diffusion of ions and small molecules -Passage can be in either direction and is very fast relative to pumps. -Solutes of appropriate size, charge, molecular structure (geometry) can diffuse down a concentration gradient -Process requires no energy -Are selective for specific solutes based on conditions to traverse the membrane

Figure The K + ion channel K + ion flow K + ions still solvated by water K + ions desolvated

Figure The K + ion channel Note the peptide carbonyl groups interacting with the K + ions in the 3 Å region.

Figure The K + ion channel More energy is released (exothermic) in resolvation in channel than the cost of desolvation of water (endothermic). This is why the channel is selective for K +

Figure The K + ion channel Na + ions are smaller than K + ions Why don’t Na + ions traverse the K + channel?? More energy is needed (endothermic) in desolvation of water in channel than released during resolvation within the channel.

Figure The K + ion channel Hydrated K+ ions Repulsion of adjacent K + ion is channel pushes the ions through the channel.

Endocytosis and Exocytosis How do cells import/export molecules too large for transport via pores, channels or transport proteins? Endocytosis – macromolecules are engulfed by plasma membrane and brought into the cell inside a lipid vesicle Exocytosis – materials to be excreted from the cell are enclosed in vesicles that fuse with the plasma membrane and release the vesicle contents into the extracellular space

Assignment Read Chapter 11 Read Chapter 12 Read Clinical Insight Page 201