The document summarizes key aspects of cellular membranes. It discusses how plasma membranes form a boundary around cells and compartments and are composed of a phospholipid bilayer with embedded and peripheral proteins. The membrane allows selective transport of materials, acts as a scaffold for enzymes, and transmits signals. The fluid mosaic model describes membranes as a fluid bilayer with embedded protein mosaic. Membranes are dynamic structures that allow movement of proteins and transport of materials while maintaining cell integrity.

1. Cellular Membranes

2. Introduction
• The plasma
membrane is the
outer boundary of
the cell.
• Plasma membranes
of all cell types
exhibit characteristic
trilaminar
appearance in
electron micrographs

3. An Overview of Membrane
Functions

4. 3.1 An Overview of Membrane
Functions (1)
• Compartmentalization – Membranes form
continuous sheets that enclose intracellular
compartments.
• Scaffold for biochemical activities –
Membranes provide a framework that organizes
enzymes for effective interaction.
• Selectively permeable barrier – Membranes
allow regulated exchange of substances
between compartments.

5. 5
Cell Membrane
The cell
membrane is
flexible and
allows a
unicellular
organism to
move

6. 6
Homeostasis
• Balanced internal condition of cells
• Also called equilibrium
• Maintained by plasma membrane
controlling what enters & leaves
the cell

7. 7
Functions of Plasma
Membrane
 Protective barrier
 Regulate transport in & out of cell
(selectively permeable)
 Allow cell recognition
 Provide anchoring sites for filaments
of cytoskeleton

8. 8
Functions of Plasma
Membrane
 Provide a binding site for enzymes
 Interlocking surfaces bind cells
together (junctions)
Contains the cytoplasm (fluid in cell)

9. 9
Structure of the Cell
Membrane

10. 10
Phospholipids
Cholesterol
Proteins
(peripheral and integral)
Carbohydrates (glucose)
Membrane Components

11. 11
Phospholipids
Make up the cell
membrane
Contains 2 fatty
acid chains that
are nonpolar
Head is polar &
contains a –PO4
group &
glycerol

12. 3.1 An Overview of Membrane
Functions (2)
• Transporting solutes – Membrane proteins
facilitate the movement of substances between
compartments.
• Responding to external signals – Membrane
receptors transduce signals from outside the cell
in response to specific ligands.
• Intracellular interaction – Membranes mediate
recognition and interaction between adjacent
cells.
• Energy transduction – Membranes transduce
photosynthetic energy, convert chemical energy
to ATP, and store energy.

13. 8.2 A Brief History of Studies on
Plasma Membrane Structure (1)
• Membranes were
found to be mostly
composed of lipids
because their
dissolving power
matched that of oil.

14. A Brief History of Studies on
Plasma Membrane Structure (2)
• The lipid bilayer:
-accounted for the
2:1 ratio of lipid to
cell surface area
-the most
energetically
favored orientation
for the polar head
groups is facing the
aqueous
compartments
outside of the
bilayer

15. A Brief History of Studies on
Plasma Membrane Structure (3)
• The nature and importance of the lipid
bilayer:
– Lipid composition can influence the activity of
membrane proteins and determine the
physical state of the membrane.
– The cohesion of bilayers to form a continuous
sheet makes cells deformable and facilitates
splitting and fusion of membranes.

16. A Brief History of Studies on
Plasma Membrane Structure (4)
• Protein-lined pores in the membrane account for the
movement of polar solutes and ions across cell
boundaries.

17. A Brief History of Studies on
Plasma Membrane Structure (4)
•The fluid-mosaic model
Core lipid bilayer exists in a fluid state, capable of movement
Membrane proteins form a mosaic of particles penetrating the lipids

18. 8.3 The Chemical Composition of
Membranes (1)
• Membrane composition
– The lipid and protein components are bound
together by non-covalent bonds.
– Membranes also contain carbohydrates.
– Protein/lipid ratios vary among membrane
types

19. The Chemical Composition of
Membranes (2)
• Membrane lipids
– Are amphipathic
– Three main types of membrane lipids:
• Phosphoglycerides are diacylglycerides with
small functional head groups linked to the glycerol
backbone by phosphate ester bonds.
• Sphingolipids are ceramides formed by the
attachment of sphingosine to fatty acids.
• Cholesterol is a smaller and less amphipathic lipid
that is only found in animals.

20. Chemical structure of membrane
lipids

21. Chemical structure of membrane
lipids

22. The Chemical Composition of
Membranes (3)
• The Nature and
Importance of the Lipid
Bilayer
– Membrane lipid
composition is
characteristic of specific
membranes.
– Lipids give membranes the
ability to fuse, form
networks, and separate
charge.
– Lipid bilayers assemble
spontaneously in aqueous
solutions as in liposomes.

23. The Chemical Composition of
Membranes (3)

24. The dynamic properties of plasma
membranes

25. 25
FLUID- because individual phospholipids and
proteins can move around freely within the layer,
like it’s a liquid.
MOSAIC- because of the pattern produced by the
scattered protein molecules when the membrane
is viewed from above.
FLUID MOSAIC MODEL

26. 26
Polar heads are hydrophilic “water loving”
Nonpolar tails are hydrophobic “water fearing”
Cell Membrane
Makes membrane “Selective” in what crosses

27. 27

28. 30
Small molecules and larger hydrophobic
molecules move through easily.
e.g. O2, CO2, H2O
Semipermeable Membrane

29. 31
Ions, hydrophilic molecules larger than
water, and large molecules such as proteins
do not move through the membrane on their
own.
Semipermeable Membrane

30. The Chemical Composition of
Membranes (4)
• The Asymmetry of
Membrane Lipids
– Inner and outer membrane
leaflets have different lipid
compositions.
– Provides different physico-
chemical properties
appropriate for different
interactions
– Membrane lipids move
easily within a leaflet but
only rarely “flip-flop”

31. The Chemical Composition of
Membranes (4)
• Membrane Carbohydrates
– Membranes contain carbohydrates covalently
linked to lipids and proteins on the
extracellular surface of the bilayer.
– Glycoproteins have short, branched
carbohydrates for interactions with other cells
and structures outside the cell.
– Glycolipids have larger carbohydrate chains
that may be cell-to-cell recognition sites.

32. Membrane Carbohydrates

33. 8.4 The Structure and
Functions of Membrane
Proteins (1)
• Membrane proteins
attach to the bilayer
asymmetrically, giving
the membrane a
distinct “sidedness”
• Membrane proteins
can be grouped into
three distinct classes

34. The Structure and Functions of
Membrane Proteins (2)
• Integral Membrane Proteins
– Are amphiphatic, with hydrophic domains
anchoring them in the bilayer and hydrophilic
regions forming functional domains outside of
the bilayer.
– Channel proteins have hydrophilic cores that
form aqueous channels in the membrane-
spanning region.

35. The Structure and Functions of
Membrane Proteins (3)
• Distribution of Integral Proteins: Freeze-
Fracture Analysis
– Freeze-fracture technique divides the
phospholipid leaflets of the membrane.
– Integral membrane proteins appear as bumps
and pits using the electron microscope.
– The heterogeneity of protein distribution is
shown.

36. Freeze-fracture

37. The Structure and Functions of
Membrane Proteins (4)
• Studying the
Structure and
Properties of Integral
Membrane Proteins
– Determining
membrane
sidedness: The
orientation of integral
proteins can be
determined using
nonpenetrating agents
that label the proteins.

38. The Structure and Function of
Membrane Proteins (7)
• Peripheral proteins are attached to the
membrane by weak bonds and are easily
solubilized.
• Lipid-anchored membrane proteins are
distinguished both by the types of lipid anchor
and their orientaiton.
– Glycophosphatidylinositol (GPI)-linked proteins
found on the outer leaflet can be released by inositol-
specific phospholipases.
– Some inner-leaflet proteins are anchored to
membrane lipids by long hydrocarbon chains.

39. 8.5 Membrane Lipids and
Membrane Fluidity (1)
• Membrane lipids exist in gel or liquid-
crystal phases depending on temperature,
lipid composition and saturation in the
presence of cholesterol.
– Liquid-crystal membranes predominate
– Unsaturated fatty acids lower the temperature
at which the liquid-crystal/gel phase transition
occurs (transition temperature).

40. The structure of the lipid bilayer
depends on the temperature

41. Membrane Lipids and Membrane
Fluidity (2)
• The Importance of Membrane Fluidity
– The fluidity of membranes is a compromise
between structural rigidity and complete
fluidity.
– Membrane fluidity makes it possible for
proteins to move in the membrane and for
membranes to assemble and grow.

42. Membrane Lipids and Membrane
Fluidity (3)
• Maintaining Membrane Fluidity
– Organisms (other than birds and mammals) maintain
membrane fluidity as temperature changes by altering
the composition of membrane lipids.
– Remodeling lipid bilayers involves saturation or
desaturation of acyl chains and replacement of acyl
chains by phospholipases or acyltransferases.
– The importance of these mechanisms has been
verified using mutants unable to carry out certain
desaturation reactions in response to cold.

43. Membrane Lipids and Membrane
Fluidity (4)
• Lipid rafts
– Outer leaflet of plasma membrane contains
specialized regions
– Cholesterol and sphingolipids tend to pack
together to form highly ordered microdomains
forming lipids rafts that float within the more
fluid and disordered environment.
– Provide a favorable environment for cell-
surface receptors and GPI-anchored proteins.

44. Lipids rafts

45. 8.6 The Dynamic Nature of the
Plasma Membrane (1)
• The Diffusion of Membrane Proteins after Cell Fusion
– Cell fusion be induced by certain viruses, or with polyethylene
glycol.
– Labeled proteins have shown that membrane proteins can move
between fused cell.

46. 8.6 The Dynamic Nature of the
Plasma Membrane (1)

47. The Dynamic Nature of the Plasma
Membrane (2)
• Restrictions on Protein
and Lipid Mobility
– Proteins can be
labeled and tracked by
fluorescence
recovery after
photobleaching
(FRAP) and single
particle tracking
(SPT).
– Proteins can be
immobile, mobile in a
directed manner, or
exhibit random
movement.

48. Patterns of movement of integral
membrane proteins

49. The Dynamic Nature of the Plasma
Membrane (3)
• Control of Membrane Protein Mobility
– Protein movements are slower than predicted by
protein size and membrane viscosity.
– Protein movements are limited by interactions with the
cytoskeleton, other proteins, and extracellular
materials.
– Techniques that can drag tagged proteins within the
membrane, indicate that some proteins have barriers
to lateral diffusion.
– Genetically modified proteins missing either
intracellular or extracellular domains are less
restricted.

50. The Dynamic Nature of the Plasma
Membrane (4)
• Membrane Lipid Mobility
– Phospholipid diffusion is restricted within the
bilayer.
– Phospholipids are confined for very brief
periods to certain areas and then hop from
one confined area to another.
– Fences restricting motion are constructed of
rows of integral membrane proteins bound to
the membrane skeleton by their cytoplasmic
domains.

51. The Dynamic Nature of the Plasma
Membrane (5)
• Membrane Domains
and Cell Polarity
– Differences in protein
distribution are
evident in cells of
organized tissues.
– In epithelia, the
proteins of the apical
membrane are
distinct from those of
the lateral and basal
membranes

52. The plasma membrane of the
human erythrocyte

53. Properties of the Plasma Membrane
https://www.youtube.com/watch?v=QQtqPG8cCpE
https://www.bioexplorer.net/plasma-membrane-functions.html/
Structure and Composition of Cell Membrane | Biology
https://www.youtube.com/watch?v=dDiogdOiC24
Membrane fluidity
https://www.youtube.com/watch?v=tvMwO02jO3o
Freeze Fracture Electron Microscopy of Cell Membranes