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BIOL 200 (Section 921)
Lecture # 6-7; June 26/27, 2006
UNIT 5: MEMBRANES
• Reading:
• ECB (2nd ed.) Chap 2, pp 70-74 (to review
carbohydrate and lipid chemistry); Chap 11, pp
365 –386; Chap 12 pp. 389-410 (essential) 411421 (optional), and related questions [119a,b,c,d,e,f, 11-10 to 11-14, 11-17, 11-18; 129abce; 12-11; 12-12; 12.13; 12-15; 12-18]
MEMBRANES - LIPIDS, LIPID BILAYERS:
OBJECTIVES
1. Explain the condensation reactions that occur to
assemble lipids and to form glycolipids
2. Explain the connection between the fluid mosaic
model and the evidence supporting it.
3. Recognize that models of membranes have to be
revised constantly to account for new experimental
data.
4. Understand the properties and general synthesis of
phospholipids, glycolipids, cholesterol, and various
glycosides in membrane structure
5. Understand the properties of integral and peripheral
proteins in membrane structure
MEMBRANE PROTEINS - OBJECTIVES
1. Explain the difference between peripheral and
integral membrane proteins
2. Explain the forces that anchor proteins of each
of these classes to membranes.
3. Explain how proteins can form 'aqueous pores'
for transport of water, ions and other charged
molecules.
4. Explain how membranes differ from bimolecular
phospholipid leaflets.
5. Explain the evidence for fluid mobility of
proteins within the plane of the membrane
MEMBRANE FUNCTION -TRANSPORTES,
CHANNELS AND MEMBRANE POTENTIAL
LEARNING OBJECTIVES
1.
2.
3.
4.
5.
6.
7.
Each membrane has specific functions which are
reflected in the functioning systems located in it.
To understand fundamental transport processes across
membranes and the role of membrane proteins.
To understand the linkages between electrical forces,
ATP hydrolysis and specific ion transport pumps.
To understand ion selectivity, gated channels and
membrane potential. Nernst equation
To understand how a membrane potential is generated
and propagated.
To understand the link between ion channels and nerve
impulses.
Understand the control of directed secretion and its
relation to nerve impulse transmission
Functions of membranes [Becker et al. The World of the Cell Fig. 7-2]
Biological membranes [Fig. 11-4]
Fluid Mosaic Model of Biological Membranes [Becker Fig. 7-5]
Key features of Fluid Mosaic Model
Membrane lipids:
• arranged in a bilayer
• fluid (free to move in plane of bilayer)
• asymmetrically arranged (different lipid components on
one face of the bilayer than the other)
Membrane proteins:
• globular units with hydrophobic domains embedded in the
hydrophobic core of the membrane,
• "fluid", in the lipid bilayer, unless anchored by interactions
with other proteins.
[Becker et al. The World of the Cell]
LIPID CHEMISTRY
Read pp. 73-74 for a review of chemical
structures and terminology of different lipid
molecules
Triacylglycerols form oil droplets
Oil seed
embyro
Fat molecules are hydrophobic, whereas
Phospholipids are amphipathic [Fig. 11-10]
11_10_Fat_phospholip.jpg
Membrane lipids have both polar and
non-polar regions (amphipathic)
[Fig. 11-7]
11_07_amphipathic.jpg
Phosphatidylcholine is the most common
phospholipid in cell membranes [Fig. 11-6]
11_06_Phosphatidylch.jpg
Sphingolipids: serine backbone, 2 acyl tails
(Often glycolipids)
[Becker et al. The World of the Cell]
Fig. 11-8: hydrophilic,
water forms H-bonds
with atom carrying
uneven charge
distribution
Fig. 11-9: hydrophobic,
water doesn’t interact
with solute; forms cagelike structures
Amphipathic phospholipids form a bilayer in water.
11_11_bilayer.in.H20.jpg
What interactions stabilize the bilayer structure?
Three characteristics of
membrane lipids
1. Fluidity
2. Asymmetry
3. Permeability
Membrane lipid fluidity
• The ease with which the lipid molecules move in
the plane of the bilayer
• Depends on temperature and the phospholipid
composition [higher the temperature, lipids with
longer tails and fewer double bonds are
synthesized in temp.- adapting bacteria/yeast]
• Depends on the length and unsaturation of fatty
acids
– Shorter hydrocarbon tails = increased fluidity
– Increased unsaturation (# double bonds) = increased
fluidity
Movement of phospholipid molecules within Membranes [Becker Fig. 7-5]
The effect of chain length and the number of double bonds
on the melting point of fatty acids [Becker et al. Fig. 7-13]
What is the role of cholesterol in animal cell membrane fluidity?
11_16_Cholesterol.jpg
Membrane flexibility (fluidity) can be studied by
either phospholipid vesicles or a synthetic
phospholipid bilayers
FRAP – fluorescence recovery after photobleaching
11_36_Photobleaching_technique
s.jpg
Membrane Fluidity:
Experimental evidence
Below are results from two FRAP
experiments. What can you conclude about
the membrane in cell 2 compared to cell 1?
What characteristics would you predict that
cell 2’s membrane lipids would have?
Cell 1
Cell 2
Lipid asymmetry – The two halves of the bilayer
are different. Membranes have a distinct lipid profile
in inside and outside layers of bilayer [Fig. 11-17]
extracellular
space
sphingomyelin
glycolipid
PC
cholesterol
PS
PI
PE
cytosol
How is lipid asymmetry created?
• All membrane lipids are made in the SER.
• Enzymes in the SER join fatty acids and glycerol and
phosphate and head groups to make phospholipids
• The phospholipid is inserted into one of the monolayers
• Enzymes called FLIPPASES flip some of these lipids into
the other bilayer, so that the whole membrane will grow.
They only transfer specific lipids.
• Glycolipids get their sugar groups in the Golgi only on the
non-cytosolic monolayer of the membrane. It then travels
as a vesicle and fuses with the plasma membrane, where it
maintains the orientation of glycolipids in the noncytosolic monolayer [see Fig. 11-15].
• It makes the membrane asymmetrical.
ATP
Phospholipids moved between
leaflets by translocator proteins
ER
New
phospholipids
added to
cytoplasmic
leaflet
Phospholipid
translocator
Membrane
protein
Both sides
enlarged
Different organelles have
different lipid compositions
lipid
PC
PE
Cholesterol
Sphingomyelin
Glycolipids
ER
40
17
6
5
Trace
PM
24
7
17
19
7
Relative permeability of
lipid bilayer [Fig. 12-2]
•Cell membrane acts as
barriers
•Rate of crossing the
membrane varies with the size
and the solubility of the
molecule
•Small, hydrophobic molecules
cross the membrane most
rapidly
•Other moleules require special
transport proteins
•Synthetic bilayers have been
used to study permeability
Functions of membrane proteins [Fig. 11-20]
e.g. Na+ pump
Structural
linkers e.g
integrin
receptors
Membrane proteins associate with the lipid
bilayer in several different ways [Fig. 11-21]
11_21_proteins.associ.jpg
Fig. 11-23: a-helix spans the bilayer (why?)
H-bonded
polypeptide
chain inside,
Hydrophobic
R-groups
outside.
Five transmembrane a helices form a water channel across the
lipid bilayer. Where are the hydrophillic and hydrobobic aa sidechains?
11_24_hydrophl.pore.jpg
Bacteriorhodopsin acts as a proton pump. Two polar aa side chains are
Involved in proton transfer process.
11_28_Bacteriorhodop.jpg
Fig. 11-27: solubilizing membrane
proteins with detergent like TritonX-100
Fig. 11-27: solubilizing membrane proteins -the results
Water-soluble complex
of membrane protein
and detergent
Water soluble complex
of lipid + detergent
Panel 4-5: protein electrophoresis
[Becker et al. The World of the Cell]
Spectrin-based cell cortex of human red blood cells provides
mechanical strength and shape to the cell [Fig. 11-32]
11_31_spectrin_network.jpg
Genetic abnormalities in spectrin result in abnormal,
spherical and fragile red blood cells causing anemia
Glycocalyx functions in cell-cell recognition, protection, lubrication and adhesion
Glycocalyx:
sugar coat
Fig. 11-32
Can plasma membrane proteins move?
CELL FUSION EXPT.
Figure 11-34 (p.383)
- Surface proteins of cultured cells are labeled with antibodies
coupled to fluorescent dyes (red and green).
- The "red" and "green" cells are then mixed and can fuse.
- In time, labeled proteins from each cell mix showing
membrane fluidity
What restricts lateral mobility of plasma membrane proteins?
Extracellular
matrix
Cell cortex
Intercellular protein-protein interactions
Diffusion barriers
Tight Junctions: Gut epithelium cells showing
apical and basolateral domains [Fig. 11-39]
Fig. 21-22: tight junctions seal epithelia
Inject dye into either apical face or basolateral face-can’t cross tight junction
Epithelium is
sheet of cells
Tight junctions wrap around the apical face of each cell
Membrane transport
[Becker et al.]
ECB 2nd Ed. Table 12-1
Membrane permeability
12_02_diffusion_rate.jpg
Carrier has conformational change,
carries solute [Fig. 12-3]
Carrier protein
Channel protein
channel opens, selectively
allows ions in.
Passive vs. active transport [Fig. 12-4]
Passive transport
Active transport
Two components of an electrochemical gradient =
Concentration gradient + membrane potential
12_08_electroch_gradient .jpg
Three ways of active transport of molecules
via the cell membranes [Fig. 12-9]
The Na+/K+ pump uses ATP hydrolysis to pump
Na+ out and K+ in to the cell both against their
electrochemical gradient [Fig. 12-10]
Mechanism of the Na+/K+ pump [Fig. 12-12]
12_12_Na_K_cyclic.jpg
Carrier protein-mediated transport [Fig. 12-13]
12_13_Carrier_proteins.jpg
Transport of glucose across the gut lining is mediated by
two types of glucose carriers
12_15_glucose_gut.jpg
Fig. 12-5: Each organelle has its own subset of
unique channels and carriers
ERCa
Ca2+
transporters
Osmosis=movement of water
molecules across a semi-permeable membrane
in response to a concentration gradient
hypotonic
Fig. 12-15:
‘ghost’
Cells use different ways to prevent
osmotic swelling [Fig. 12-16]
Similarities and differences in carrier-mediated
transport in animal and plants [Fig. 12-18]
Animal cell
Plant cell
A K+ channel protein
12_19_selectivity_filter.jpg
The quick closing of the leaves of Venus Flytrap in
response to touch is due to the mechanical stimulation of
ion channels changing the turgor pressure [Fig. 12-21]
12_21_Venus _flytrap.jpg
The patch-clamp technique
12_22_patch_clamp_record .jpg
Measurement of ion channel activity as current by the
Patch-clamp technique [Fig. 12-23]
12_23_current _measured .jpg
Gated ion channels [Fig. 12-24]
12_24_Gated _ion_chan .jpg
Fig. 12-26: Touch-induced opening of voltage-gated
Ion channels generate (1) an electric impulse followed
by a rapid water loss by hinge cells, causing (3) folding
of Mimosa leaflets.
Fig. 12-27: The unequal distribution of ions across thebBilayer
produces the membrane potential (electrical potential difference
across the membrane).
Potassium ion channels are important in generating the
membrane potential across the plasma membrane [Fig. 12-28]