chapter 7 membrane structure and function, by hugh davson and james danielli a sandwich model in...
TRANSCRIPT
Chapter 7 Membrane Structure and Function
, by Hugh Davson and James DanielliA sandwich
model
In 1972, J. Singer and G. Nicolson
the placement of membrane proteins
Fig. 7-4
TECHNIQUE
Extracellularlayer
KnifeProteins Inside of extracellular layer
RESULTS
Inside of cytoplasmic layer
Cytoplasmic layerPlasma membrane
Freeze fracture
Fig. 7-5
Lateral movement(~107 times per second)
Flip-flop(~ once per month)
(a) Movement of phospholipids
(b) Membrane fluidity
Fluid Viscous
Unsaturated hydrocarbontails with kinks
Saturated hydro-carbon tails
(c) Cholesterol within the animal cell membrane
Cholesterol
The fluidity of membrane
• The steroid cholesterol has different effects on membrane fluidity at different temperatures
• At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids
• At cool temperatures, it maintains fluidity by preventing tight packing
Cholesterol
Phytosterol• The FDA has approved the following claim for phytosterols: "Foods containing at least 0.4 gram per serving of plant sterols, eaten twice a day with meals for a daily total intake of at least 0.8 gram, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease."*
• The mechanism behind phytosterols and the lowering of cholesterol occurs as follows: the incorporation of cholesterol into micelles in the gastrointestinal tract is inhibited, decreasing the overall amount of cholesterol absorbed (see cholesterol absorption inhibitor). This may in turn help to control body total cholesterol levels, as well as modify HDL, LDL and TAG levels. Many margarines, butters, breakfast cereals and spreads are now enriched with phytosterols and marketed towards people wishing to lower their cholesterol levels.
•dietary sterols might increase the risk of aortic valve stenosis.[9]
Fig. 7-7
Fibers ofextracellularmatrix (ECM)
Glyco-protein
Microfilamentsof cytoskeleton
Cholesterol
Peripheralproteins
Integralprotein
CYTOPLASMIC SIDEOF MEMBRANE
GlycolipidEXTRACELLULARSIDE OFMEMBRANE
Carbohydrate
Fig. 7-9
(a) Transport
ATP
(b) Enzymatic activity
Enzymes
(c) Signal transduction
Signal transduction
Signaling molecule
Receptor
(d) Cell-cell recognition
Glyco-protein
(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)
•Six major functions of membrane proteins:
Fig. 7-10ER
1
Transmembraneglycoproteins
Secretoryprotein
Glycolipid
2Golgiapparatus
Vesicle
3
4
Secretedprotein
Transmembraneglycoprotein
Plasma membrane:
Cytoplasmic face
Extracellular face
Membrane glycolipid
Synthesis and
sideness of membranes
Fig. 7-11Molecules of dye Membrane (cross section)
WATER
Net diffusion Net diffusion Equilibrium
(a) Diffusion of one solute
Net diffusion
Net diffusion
Net diffusion
Net diffusion
Equilibrium
Equilibrium
(b) Diffusion of two solutes
Lowerconcentrationof solute (sugar)
Fig. 7-12
H2O
Higher concentrationof sugar
Selectivelypermeablemembrane
Same concentrationof sugar
Osmosis
Osmosis is the diffusion of water
across a selectively permeable membrane
Hypotonic solution
(a) Animal cell
(b) Plant
cell
H2O
Lysed
H2O
Turgid (normal)
H2O
H2O
H2O
H2O
Normal
Isotonic solution
Flaccid
H2O
H2O
Shriveled
Plasmolyzed
Hypertonic solution
Fig. 7-13
Tonicity is the ability of a solution to cause a cell to gain or lose water
Aquaporin
• Aquaporins are integral membrane proteins • Aquaporins are "the plumbing system for cells
• Agre said he discovered aquaporins "by serendipity." His lab had an N.I.H. grant to study the Rh blood group antigen. They isolated the Rh molecule but a second molecule, 28 kilodaltons in size (and therefore called 28K) kept appearing.
• http://en.wikipedia.org/wiki/File:Aquaporin_Z.png
Fig. 7-15
EXTRACELLULAR FLUID
Channel protein
(a) A channel protein
Solute CYTOPLASM
Solute Carrier protein
(b) A carrier protein (Shape change)
cystinuria胱胺酸尿症 (#) caused by malfunctions in specific
transport systems • Cystinuria is characterized by the inadequate reabsorption of cystine during the filtering process in the kidneys, thus resulting in an excessive concentration of this amino acid.
• Cystine may precipitate out of the urine, if the urine is neutral or acidic, and form crystals or stones in the kidneys, ureters, or bladder.
• Cystine?
2
Fig. 7-16-7 Active transportEXTRACELLULAR
FLUID [Na+] high [K+] low
[Na+] low
[K+] high
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLASM ATP
ADP P
Na+ Na+
Na+
P 3
K+
K+ 6
K+
K+
5 4
K+
K+
P P
1(Phosphoryla
tion)
(dephosphorylation)
How Ion Pumps Maintain Membrane Potential
• Membrane potential is the voltage difference across a membrane
• Voltage is created by differences in the distribution of positive and negative ions
collectively called the electrochemical gradient, drive the diffusion of ions across a membrane:
–A chemical force •(the ion’s concentration gradient)
–An electrical force• (the effect of the membrane potential on the ion’s movement)
Two combined forces
• An electrogenic pump is a transport protein that generates voltage across a membrane•sodium-potassium pump :
•The sodium-potassium pump is the major electrogenic pump of animal cells
• proton pump •The main electrogenic pump of plants, fungi, and bacteria is a proton pump
Cotransport: Coupled Transport by a Membrane Protein
• Cotransport occurs when active transport of a solute indirectly drives transport of another solute
• Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell
Fig. 7-19
Proton pump
–
–
–
–
–
–
+
+
+
+
+
+
ATP
H+
H+
H+
H+
H+
H+
H+
H+
Diffusionof H+
Sucrose-H+
cotransporter
Sucrose
Sucrose
Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and
endocytosis• In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents– Many secretory cells use exocytosis to export their products
• In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane
– There are three types of endocytosis:– Phagocytosis (“cellular eating”)– Pinocytosis (“cellular drinking”)– Receptor-mediated endocytosis
Fig. 7-20 PHAGOCYTOSIS EXTRACELLULARFLUID
CYTOPLASM Pseudopodium
“Food”orother particle Food
vacuole
PINOCYTOSIS
1 µm
Pseudopodiumof amoeba
Bacterium
Food vacuole
An amoeba engulfing a bacteriumvia phagocytosis (TEM)
Plasmamembrane
Vesicle
0.5 µm
Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM)
RECEPTOR-MEDIATED ENDOCYTOSIS
Receptor Coat protein Coat
edvesicle
Coatedpit
Ligand
Coatprotein
Plasmamembrane
A coated pit and a coated vesicle formedDuring receptor-mediated endocytosis(TEMs)
0.25 µm
You should now be able to:1. Define the following terms:
amphipathic molecules, aquaporins.2. Explain how membrane fluidity is
influenced by temperature and membrane composition
3. Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions