CHAPTER 12Membrane 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 12.1

- 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 morefluid membrane at lower temps

Cholesterol helpsdisrupt hydrophobicinteraction of fatty acidchains.

More fluidity

Figure 12.8 Integral and peripheral membrane proteins

Integral proteins

Peripheral proteins

Peripheral protein

Examples of Integral membrane proteins

Figure 12.9: bacteriorhodopsinYellow tubes represent a-helices.light harvesting protein

Figure 12.10-porin, a pore or channelacross the membrane. Note the b-sheet 2o structure

Examples of peripheral membrane proteins

Figure 12.11: prostaglandin H2 synthase-1

Lipid Bilayers and Membranes are Dynamic Structures

Figure 12.15

Billion times slower

~2 mm is1 sec

- Some anchored proteins can diffuse laterally just as rapidly as a phospholipid.

- Membrane fluidity is maintained at lower temperaturesby adjusting the ratio of saturated and unsaturated fattyacyl groups

Membrane Transport

- Membranes are selectively permeable barriers that restrictthe 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 concentrationgradient

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 12.19Types of passive transport

Uniport

- Passive transport is called facilitated diffusion because it doesNOT 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 againsta membrane potential (or voltage).

Types of active transport

- Primary active transport is powered by a direct source of energyas ATP, light or electron transport

- Secondary active transport is driven by an ion concentration gradient.

Figure 12.16 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 maintainthis gradient and that’s when resting!

Figure 12.20. The ATPase pump drives a secondary activetransport (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 centralpassage 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 12.22. The K+ ion channel

K+ ion flow

K+ ions still solvatedby water

K+ ions desolvated

Figure 12.23. The K+ ion channel

Note the peptide carbonyl groups interacting with the K+

ions in the 3 Å region.

Figure 12.24. The K+ ion channel

More energy is released (exothermic) in resolvation in channelthan the cost of desolvation of water (endothermic).

This is why the channel is selective for K+

Figure 12.24. The K+ ion channel

Na+ ions aresmaller thanK+ ions

Why don’t Na+ ions traversethe K+ channel??

More energy is needed (endothermic) in desolvation of water in channel than released during resolvation within the channel.

Figure 12.25. The K+ ion channel

Hydrated K+ ions

Repulsion of adjacent K+ ion is channel pushes theions 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 membraneand release the vesicle contents into the extracellular space

Assignment

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