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
-
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
Figure 12.1
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
Peripheral proteins
Integral proteins
Peripheral
protein
Examples of Integral membrane proteins
Figure 12.9: bacteriorhodopsin
Yellow tubes represent a-helices.
light harvesting protein
Figure 12.10-porin, a pore or channel
across 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 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 12.19
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 12.16 Primary active transport in animals:
Na+/K+ ATPase Pump
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!
- An ATP driven antiport transport
Figure 12.20. The ATPase pump drives a secondary active
transport (symport) of glucose into cell.
Na+ ion gradient
antiport
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 12.22. The K+ ion channel
K+ ions desolvated
K+ ions still solvated
by water
K+ ion flow
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 channel
than 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 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 12.25. 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