The document discusses the evolution of cell membranes from early RNA molecules clinging to clay particles to the modern fluid mosaic model. Key events include the formation of lipid bilayers that separated internal and external chemistry, allowing more efficient reactions. Experiments showed lipids spontaneously forming enclosed compartments and lipid bilayers with integral membrane proteins that gave membranes a mosaic-like structure. The fluid mosaic model proposes membranes are fluid with lipids and proteins able to diffuse freely within the plane of the bilayer. Transport proteins like channels and carriers allow selective permeability while pumps use ATP to transport molecules against gradients.

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