1. Chapter 7 Membrane Structure and Function
A sandwich model
, by Hugh Davson and James Danielli

2. In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins
Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions
In 1972, J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water

3. Amphipathic molecules
Fig. 7-2
Amphipathic molecules
WATER
Hydrophilic
head
Hydrophobic
tail
WATER

4. Phospholipid bilayer Hydrophobic regions of protein Hydrophilic
Fig. 7-3
Phospholipid
bilayer
Hydrophobic regions
of protein
Hydrophilic
regions of protein

5. Freeze fracture TECHNIQUE RESULTS Extracellular layer Proteins
Fig. 7-4
TECHNIQUE
RESULTS
Extracellular
layer
Proteins
Inside of extracellular layer
Knife
Plasma membrane
Cytoplasmic layer
Inside of cytoplasmic layer

6. The fluidity of membrane
Fig. 7-5
The fluidity of membrane
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)
(a) Movement of phospholipids
Fluid
Viscous
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
carbon tails
(b) Membrane fluidity
Cholesterol
(c) Cholesterol within the animal cell membrane

7. Cholesterol
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

8. 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.

9. A 2008 study conducted in Finland showed that sterols can accumulate in heart valves, suggesting that dietary sterols might increase the risk of aortic valve stenosis.[9]
In 2009, Cardiologist Dr. William Davis noting evidence that plant sterols are implicated in increased cardiovascular events, diseased aortic valves, and carotid atherosclerotic plaque.

10. Membrane proteins Mixed proteins after 1 hour Mouse cell Human cell
Fig. 7-6
RESULTS
Membrane proteins
Mixed proteins
after 1 hour
Mouse cell
Human cell
Hybrid cell

11. Fibers of extracellular matrix (ECM) Glyco- Carbohydrate protein
Fig. 7-7
Fibers of
extracellular
matrix (ECM)
Glyco-
protein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE

12. EXTRACELLULAR N-terminus SIDE C-terminus CYTOPLASMIC SIDE  Helix
Fig. 7-8
EXTRACELLULAR
SIDE
N-terminus
C-terminus
CYTOPLASMIC
SIDE
 Helix

13. Six major functions of membrane proteins:
Fig. 7-9
Signaling molecule
Enzymes
Receptor
ATP
Signal transduction
(a) Transport
(b) Enzymatic activity
(c) Signal transduction
Glyco-
protein
(d) Cell-cell recognition
(e) Intercellular joining
(f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)
Six major functions of membrane proteins:

14. Synthesis and sideness of membranes

15. ER 1 Transmembrane glycoproteins Secretory protein Glycolipid Golgi 2
Fig. 7-10 ER 1
Transmembrane
glycoproteins
Secretory
protein
Glycolipid
Golgi
apparatus 2
Vesicle 3
Plasma membrane:
Cytoplasmic face 4
Extracellular face
Transmembrane
glycoprotein
Secreted
protein
Membrane glycolipid

16. Concept 7.2:
Membrane structure results in
selective permeability

17. Membrane (cross section)
Fig. 7-11
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
Net diffusion
Equilibrium
(a) Diffusion of one solute
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
(b) Diffusion of two solutes

18. Osmosis is the diffusion of water
Fig. 7-12
Lower
concentration
of solute (sugar)
Higher concentration
of sugar
Same concentration
of sugar
H2O
Selectively
permeable
membrane
Osmosis is the diffusion of water
across a selectively permeable membrane
Osmosis

19. Fig. 7-13
Tonicity is the ability of a solution to cause a cell to gain or lose water
Hypotonic solution
Isotonic solution
Hypertonic solution
H2O
H2O
H2O
H2O
(a) Animal
cell
Lysed
Normal
Shriveled
H2O
H2O
H2O
H2O
(b) Plant
cell
Turgid (normal)
Flaccid
Plasmolyzed

20. 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.

22. (a) A contractile vacuole fills with fluid that enters from
Fig Osmoregulation
50 µm
Filling vacuole
(a) A contractile vacuole fills with fluid that enters from
a system of canals radiating throughout the cytoplasm.
Contracting vacuole
(b) When full, the vacuole and canals contract, expelling
fluid from the cell.

23. (b) A carrier protein (Shape change)
Fig. 7-15
EXTRACELLULAR FLUID
Channel protein
Solute
CYTOPLASM
(a) A channel protein
Solute
Carrier protein
(b) A carrier protein (Shape change)

24. Some diseases are caused by malfunctions in specific transport systems, for example the kidney disease cystinuria胱胺酸尿症(#)
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?

25. Fig. 7-16-7 Active transport
EXTRACELLULAR
FLUID
[Na+] high
Na+
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
Na+
[Na+] low
ATP
Na+ P P
CYTOPLASM
[K+] high
ADP 1 2 3
(Phosphorylation) K+ K+ K+ K+ K+ P K+ P
(dephosphorylation) 6 5 4

26. Facilitated diffusion
Fig. 7-17
Passive transport
Active transport
ATP
Diffusion
Facilitated diffusion

27. 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

28. Two combined forces A chemical force An electrical force
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)

29. sodium-potassium pump :
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

30. – EXTRACELLULAR FLUID + ATP – + H+ H+ Proton pump H+ – + H+ H+ H+ – +
Fig. 7-18 –
EXTRACELLULAR
FLUID +
ATP – + H+ H+
Proton pump H+ – + H+ H+ H+ – +
CYTOPLASM H+ – +

31. 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

32. – + H+ ATP H+ – + H+ H+ – + H+ H+ – + H+ H+ – + – + Diffusion of H+
Fig. 7-19 – + H+
ATP H+ – +
Proton pump H+ H+ – + H+ H+ – + H+
Diffusion
of H+
Sucrose-H+
cotransporter H+ –
Sucrose + – +
Sucrose

33. 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

34. Fig. 7-20 PHAGOCYTOSIS EXTRACELLULAR FLUID CYTOPLASM 1 µm Pseudopodium
of amoeba
“Food”or
other particle
Bacterium
Food
vacuole
Food vacuole
An amoeba engulfing a bacterium
via phagocytosis (TEM)
PINOCYTOSIS
0.5 µm
Plasma
membrane
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM)
Vesicle
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Coated
pit
Ligand
A coated pit
and a coated
vesicle formed
during
receptor-
mediated
endocytosis
(TEMs)
Coat
protein
Plasma
membrane
0.25 µm

35. You should now be able to:
Define the following terms: amphipathic molecules, aquaporins.
Explain how membrane fluidity is influenced by temperature and membrane composition
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

36. Explain how transport proteins facilitate diffusion
Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps
Explain how large molecules are transported across a cell membrane