lipids

1. Lipids, Membranes &
the First Cells
Chapter 6

2. Evolution of the Membrane
 The evolution of the plasma membrane was a
momentous event because it separated life
from non-life
 Before the cell RNA molecules clung to clay
particles, building copies as nucleotides
washed over them randomly

3. Lipid Membrane
 The formation of the membrane performed
three important tasks
 Separated the chemical composition of the inside
from the chemical composition of the outside
 Chemical reactions became more efficient as
reactants could collide more frequently
 Membrane could form a collective barrier

4. Lipid Formation
 Spark discharge
experiments succeeded in
production at least two
types of lipids
 How did these lipids behave
when they are immersed in
water
 Transmission electron
microscope
 Lipids spontaneously
formed enclosed
compartments filled with
water

5. What Characterizes
a Lipid
 Lipids are defined by a
physical property
 Solubility in water
 Not strictly characterized
by structure
 Therefore the structure of
lipids varies widely
 Hydrocarbons
 Hydrophobic

6. Lipids Found In Cells
 There are three types of lipids found in cells
 Lipids for energy storage
 Triacylglycerols
 Lipids for cell membranes
 Phospholipids
 Lipids for steroids
 Cholesterol, estrogen, & testosterone

7. Triglycerides
 Via condensation
reactions one glycerol
molecule is connected
to three fatty acids
 The glycerol and fatty
acid are linked by an
ester linkage

9. Steroids
 Family of lipids that has
a four ring structure
 Cholesterol is an
important membrane
component of plasma
membranes is some
organisms
 Others are important
hormone signals

10. Phospholipids
 Structure
 Glycerol linked to
phosphate group
 Glycerol linked to two
fatty acids
 For Archaea bacteria
 Glycerol linked to
phosphate group and two
isoprene units

11. Lipid Classification

12. What is a
Sphingolipid

13. Lipids & Disease
 Membrane lipids undergo constant metabolic
turnover
 Breakdown is performed by hydrolytic enzymes in
lysosomes
 Impaired degradation by a defect enzyme leads to
the accumulation of partial breakdown products
 Brain, liver & spleen
 Genetic basis for Niemann-Pick disease and Tay-
Sachs disease
 Metal retardation, paralysis, blindness, early death

14. Lipids in Membranes
 In order to spontaneously form a lipid bilayer
lipid must have
 Charges and polar bonds in the head region to
interact with water
 Long fatty acid tails to interact with each other
 Amphipathic

15. Lipid Bilayer
Formation & Energy
 Lipid structures form
spontaneously
 No energy input is required
 Energy Concepts
 Independent phospholipids are unstable in water high
potential energy
 Hydrophobic tails disrupt hydrogen bonds
 When tails interact with one another reach a lower potential
energy state
 Formation of these structures clearly decreases entropy
 But overall ∆H outweighs ∆S leading to a negative ∆G

16. What Will
Form
 Micelles
 Short tails
 Bilayers
 Long tails

17. Artificial Membranes
 Researchers produced
many types of vesicles
by using many different
types of phospholipids
 When phospholipids
are agitated by shaking
the layers break and re-
form small water
enclosed vesicles

18. Evolution of Membrane Models
 Today cell membranes are characterized by
what is known as a fluid mosaic model
 Over 100 years of research was performed
before this model
 Historical Perspectives

19. History &
Membranes

20. Cell
water soluble
lipid soluble
Charles Overton, 1890’s
 Question
 What is the composition of
the cell’s membrane?
 Experiment
 Added both water soluble
(hydrophilic) and lipid soluble
(hydrophobic) substances to
cells to determine if those
substances could enter cell
 Conclusion
 Cells have a lipid ‘coat’ on
their surface

21. Gorter and Grendel, 1925
Langmuir trough
 Question:
 What is the arrangement
of lipids in the plasma
membrane
 Experiment
 Measure surface area of
RBC
 Measure surface area of
lipid monolayer
 Compare areas
 Why use RBCs

22. Gorter and Grendel, Results
Results:
Surface area of monolayer = 2x the SA of RBCs, therefore lipid is oriented as a bilayer
Conclusions:

23. Gorter and Grendel,
Conclusion
 Lipids are two layers thick in
the membrane.
 bilayer
 Proposed that lipids were in
bilayer with polar groups
toward the aqueous
compartments and non-
polar fatty acid parts toward
the center of the bilayer
 Phospholipids can rotate,
diffuse and flip in the lipid
sea

24. protein
protein
Davson and Danielli, 1935
 A role for proteins
 Surface tension

25. Davson and Danielli
 1st model
 Evidence
 surface tension of oil droplets is high
 surface tension of cell membranes is low
 using starfish eggs
 Surface tension of oil droplets coated with protein
is low

26. Davson and Danielli
 2nd model
 Realized that there was a problem with the
Davson/Danielli 1st model
 Transport
 Model was revised to
allow for pores

27. Protein
Coat
Lipid
This was believed to confirm the Davson/Danielli model of the plasma membrane structure.
Robertson, 1959

28. Singer and Nicholson, 1972
 Disproved Robertson, Davson, & Danielli
 Lipid bilayer kept
 Protein coat lost
 This model had two important components
 Fluid
 all components are free to diffuse in the plane of the
membrane
 Mosaic
 heterogeneity in the membrane –
proteins and lipids interspersed
 AND, because of fluidity,
randomly distributed

29. Evidence for Fluid Mosaic
 Mixing of fluorescently tagged proteins on
hybrid cells
 Frye & Edidin, 1970
 Florescence recovery after photobleaching
 FRAP
 Mosaic
 Freeze fracture

30. Frye and Edidin, 1970
 Journal of cell science
 Used fluorescently labeled antibodies
Fluorophore

31. Membrane proteins
Mouse cell
Human cell
Hybrid cell
Mixed
proteins
after
1 hour
+
Frye and Edidin, 1970
 Explanations
 Proteins are free to diffuse in the membrane
 Newly synthesized membrane proteins are
inserted into the membrane
 Process is ATP dependent

32. Note: Experiment done at 37°C
Frye and Edidin (1970)

33. Frye and Edidin, 1970
 Is protein synthesis of new membrane
proteins responsible for intermixing
 Add protein synthesis inhibitor cyclohexamide
 Mixing still occurred
 Is intermixing an ATP dependent process
 Block ATP production with DNP, cyanide
 Mixing still occurred
 Conclusion
 Mixing is due to fluidity

34. (fusing)
virus
Mouse
cell
(a)
Incubation temperature (ºC)
Mosaics (%)
0
+
+
+
+
+
+
+
+
50
100
5 15 25 35
+
+
+
+
(c)
Frye and Edidin, 1970
 If intermixing is due to membrane fluidity,
then intermixing should be temperature
dependent

35. Fluorescence Recovery After
Photobleaching
 FRAF
 Measures lateral
diffusion of molecules
(lipids/proteins) in cell
membranes
 Method allows us to look
at populations of
molecules
 Information obtained
addresses whether
components are, in fact,
free to diffuse

36. 3
2
1
 Measure Recovery
 All labeled components
are free to diffuse
 Slow diffusion
 A fraction of the
population is not mobile
 Anchored proteins
 Conclusion
 Most but not all
components are free to
diffuse

37. Mosaic Evidence
 Scanning electron micrographs showed pits
and mounds studding the inner surfaces of
the bilayer

38. Proteins in the Bilayer
 Integral Membrane Proteins (IMPs)
 Cell surface receptors
 Adhesion molecules
 Transporters
 Ion channels
 Peripheral Membrane Proteins (PMPs)
 Associate non-covalently with the membrane
 Interact with IMPs of phospholidip head groups
 Can be inner or outer leaf of PM

39. IMPs Hydrophobic Regions
 Usually α helical
structure is found in the
transmembrane space
 There can be one α
helix or several
 These α helical
structures can also
form pores or tubes

40. IMPs Extracted Using
Detergents
 Detergents are small
amphipathic molecules
 They disrupt plasma
membranes and
hydrophobic regions of
the detergents binds to
hydrophobic region of
the protein
 Purification of protein
product will allow you to
test its function

41. Glycosylation of Proteins
 Never occurs on cytoplasmic proteins
 Uses specific enzymes of ER and golgi
 Two types
 N-linked = carbohydrates are attached to
asparigine (terminal amino group) synthesis of
CHO side chain begins in ER and is completed in
Golgi
 O-linked = carbohydrates are attached to serine
or threonine (hydroxyl groups) synthesis of CHO
side chain begins and ends in Golgi
 For review of the function see chapter 5 notes

42. Transport Proteins
 Facilitated Diffusion
 Requires no ATP moves with a concentration gradient
 Channels
 Ion channels
 Ions move according to an electrochemical gradient
 Usually specific to one type of ion
 Aquaporins
 Transporters
 Glucose transporter
 Changes shape
 Active Transport
 Requires ATP moves against concentration gradient
 Ion pumps

43. Gramicidin – Ion Channel
 Ions carry charge
movement of ions
produces and
electric current
 Can carry H+, K+ and Na+
Ions travel down this pore

44. Aquaporins
 Very specific/selective channel
 Only water goes through
 Water is able to cross the plasma membrane
10X faster than without aquaporins

45. Gated Channels
 Aquaporins and ion channels are gated
 Open or close in response to signal
 Voltage gated channels
 Open in response to depolarization of membrane
 Ligand gated channels
 Open in response to a chemical signal
 Remember no energy is needed for transport
 Powered by diffusion along an
electrochemical gradient

46. Carrier Proteins
 Lipid bilayer is not permeable to glucose, yet
glucose is a main source of cellular energy
 Researchers used RBCs to extract and purify
a glucose transporter

47. GLUT – 1
 After isolating and analyzing many proteins
from RBCs ghosts researchers found one
protein that increased membrane
permeability to glucose
 This protein was added to liposomes (artificial
membranes) and it transported glucose at the
same rate as a living cell

48. Active Transport Pumps
 Requires energy – ATP
 ATP ADP + P
 The phosphate group is transiently and covalently
attached to a protein in a process known as
protein phosphorylation
 Phosphorylation of proteins often leads to a
change in protein shape or protein conformation
 Leads to a decrease in entropy
 Movement is against a concentration gradient
 Important for establishing electrochemical
gradients

51. Cell Membrane

52. Selective Permeability
 Artificial membrane systems
proved to be invaluable in
determining the permeability of
membranes
 Allowed researches to change
one parameter at a time and
asses the effect
 How rapid is diffusion
 What happens when a different
type of phospholipid is used
 Does permeability change with
cholesterol or other molecules

53. Membranes are Highly
Selective
 Small nonpolar molecules
move across quickly
 CO2, O2, N2, hydrophobic
molecules
 Small polar molecules have
intermediate permeability
 H2O, glycerol, urea
 Large uncharged polar
 Glucose
 Ions
 Na+, K+, Cl-
Permeability cm/sec

54. Does Lipid Composition Affect
Permeability
 Two properties affect
permeability
 Number of double bonds
 Saturated vs. unsaturated
 Packing
 Double bonds produce
spaces between tails
 Atoms are in one plane
locked in place
 Spaces reduce strength of
hydrophobic interactions
 Tail length

55. Fluidity and Double Bond
Character
 Degree of hydrophobic interactions increases
with saturated fats
 Fluidity increases with double bonds

56. Fluidity and Double Bond
Character

57. Cholesterol and Permeability
 Cholesterol is a large bulky ring structure
 Cholesterol should increase the density of
hydrophobic interactions

58. Cholesterol and Permeability

59. Diffusion
 Diffusion is a process which occurs
spontaneously due to an increase in entropy
 Diffusion occurs from an area of high
concentration to an area of low concentration
 Diffusion across a plasma membrane

60. Osmosis
 Osmosis is the diffusion of
water from higher
concentration to lower
concentration
 Only occurs when solutions
are separated by a
membrane that is
permeable to some
molecules and not others
 Movement is spontaneous
 Driven by an increase in
entropy when the solute
becomes more dilute
Entropy decreased 5X on one
Side up increased 10X on the other
The system had a net gain of entropy

61. Osmosis
 If water is more concentrated on one side of
a membrane then there will be a net
movement of water

62. Osmosis & Diffusion
 Osmosis and diffusion reduce differences in
chemical composition between the inside and
outside of membrane bound vesicles
 Therefore, it is unlikely that interiors differed
radically from the external environment
 Lipid bilayers become capable of creating a
specialized internal environment due to
specialized protein transporters