The document discusses the structure and composition of plasma membranes, describing the phospholipid bilayer and various integral and peripheral proteins that make up the fluid mosaic model. It explains the functions of cell membranes, including transport, signaling, and cell-cell interactions, and how different proteins are involved in processes like transport, reception, enzymatic activity, and adhesion. The fluidity and dynamics of the membrane are influenced by the phospholipids and cholesterol that make up the bilayer.

1. Plasma Membrane

2. Three Views of a Cell Membrane
A. A cross section of a plasma membrane (human red blood cell).
B. 2-D view of a cell membrane.
C. 3-D view of a cell membrane.

3. Membrane Functions
3. transport
4. Signal detection
5. Cell-cell
communication
2. Localize
specific
functions
1. boundaries
6. Cell-cell
adhesion

4. Biological Membrane Composition
Lipids(Phospholipids,
Sphingolipids And
cholesterol)
Proteins (integral,
Peripheral, Lipid
anchored)
Carbohydrates
(peripheral Sugar
moeities)

5. Biologocal Membrane composition
Figure 5.1
glycocalyx
proteins
integral
protein
peripheral
protein
cytoskeleton
cholesterol
Phospholipid bilayer:
a double layer of
phospholipid
molecules whose
hydrophilic “heads”
face outward, and
whose hydrophobic
“tails” point inward,
toward each other.
Glycocalyx: sugar
chains that attach to
proteins and
phospholipids, serving
as protein binding sites
and as cell lubrication
and adhesion
molecules.
Cholesterol molecules
that act as a patching
substance and that
help the cell maintain
an optimal level of
fluidity.
Proteins, which are
integral, meaning
bound to the
hydrophobic interior of
the membrane, or
peripheral, meaning not
bound in this way.
phospholipids
cell
interior
cell
exterior

6. The Phospholipid Bilayer
• The fatty acid chains are hydrophobic,
meaning they avoid water, while the
phosphate group is hydrophilic, meaning it
readily bonds with water.

7. The Phospholipid Bilayer
polar
head
nonpolar
tails
watery
extracellular
fluid
watery
cytosol
hydrophilic
hydrophobic
hydrophobic molecules
pass through freely
hydrophilic molecules
do not pass
through freely
hydrophilic
(a) (b)
Phospholipid molecule Phospholipid bilayer
–
Hydrophilic heads exposed
hydrophobic tails protected

8. Packing arrangements of lipid molecules in an aqueous environment.

9. Micelles
Micelles are closed lipid monolayers;
• a fatty acid core and polar surface
or
• a polar core with fatty acids on the surface
10
micelle formed by phospholipids in
aqueous solution.
inverse micelle formed by
phospholipids in organic solvent.

10. Liposomes
11
' LIPO ' meaning fat
' Soma ' meaning body
concentric bilayered vesicles in which an
aqueous volume is entirely enclosed by a
membraneous lipid bilayer.

11. FIGURE 2-20 Cross-sectional views of the three structures formed by
phospholipids in aqueous solutions. The white spheres depict the hydrophilic heads
of the phospholipids, and the squiggly black lines (in the yellow regions) represent the
hydrophobic tails. Shown are a spherical micelle with a hydrophobic interior composed
entirely of fatty acyl chains; a spherical liposome, which has two phospholipid layers
and an aqueous center; and a two-molecule-thick sheet of phospholipids, or bilayer,
the basic structural unit of biomembranes.

12. Membrane Structure
• Not all membranes are identical
– Membranes with different functions differ in their
chemical composition and structure
• Fluid Mosaic Model best describes our current
understanding of membrane structure

13. Fluid mosaic model
• 1972 - Singer and Nicolson
It views the membrane as a fluid phospholipid bilayer
that has a mosaic of proteins either fixed within it or
capable of moving laterally across it.

14. Mosaic quality of the membrane
• Membranes have their own unique collection of proteins
• Proteins form different patterns
• Combination of proteins makes membrane unique
• So, the membrane is said to be mosaic

15. Fluidity
• Fluidity is defined as “easy of flow”.
• Membranes are in motion with fast drifting
lipids and slower drifting proteins

16. Factors affecting membrane fluidity
Membrane
fluidity
temperature
Lipid
composition
unsaturated
FA chains
Cholesterol

17. Presence of unsaturated FA chains
Lipids bilayer can be converted
to rigid crystalline (or gel) state
at characteristic freezing point.
Phase transition can be
avoided
 if the hydrocarbon chains
are short/ have double bonds.
Lower
fluidity
Higher
fluidity

18. Presence of unsaturated FA chains
Reduces the tendency of the hydrocarbon
tails to interact with one another.
Cis-double bonds produce kinks in the chains
that make them more difficult to pack together.
 so that the membrane remains fluid at lower
temperatures.

19. Cholesterol affects fluidity
At body temperature it
lessens fluidity
By restraining the
movement of
phospholipids
 At colder temperatures
it adds fluidity
 By not allowing
phospholipids to pack
close together.

20. Lipid Mobility
 Lipids are free to move
laterally in a lipid bilayer
Flip-flop movement is
very energetically
unfavorable
 catalyzed by flippases.

21. Membrane Proteins
Integral proteins
Peripheral proteins
Lipid anchored proteins

22. Integral proteins
The hydrophobic region
23
Outside of cell
Hydrophilic region

23. Peripheral proteins
 loosely bound to the surface of membrane.
24
Bound to bilayer surface by ionic interactions

24. Lipid Anchored Proteins

25. Proteins of the Plasma Membrane Provide 6
Membrane Functions:
1) Transport Proteins
2) Receptor Proteins
3) Enzymatic Proteins
4) Cell Recognition Proteins
5) Attachment Proteins
6) Intercellular Junction
Proteins

26. 1) Transport Proteins
Carrier Proteins – bind to a
substance and carry it
across membrane,
change shape in process
Channel Proteins – channel
for lipid insoluble
molecules and ions to
pass freely through

27. 2) Receptor Proteins
– Bind to chemical
messengers (Ex.
hormones) which sends
a message into the cell
causing cellular reaction.

28. 3) Enzymatic Proteins
– Carry out enzymatic
reactions right at the
membrane when a
substrate binds to the
active site.

29. 4) Cell Recognition Proteins
– Glycoproteins (and
glycolipids) on
extracellular surface
serve as ID tags
(which species, type
of cell, individual).

30. 5) Attachment Proteins
- Attach to cytoskeleton
(to maintain cell shape
and stabilize proteins)
and/or the extracellular
matrix (integrins connect
to both).

31. 6) Intercellular Junction Proteins
– Bind cells together

32. Carbohydrates diversify membranes
• Membrane carbohydrates
are only found on the
outside (external) face of
membranes
• Attach to lipids or protein
(glycolipid/ glycoprotein)
• Enable cells to
distinguish/ recognize
one another

33. Membrane trafficking
• Materials must move in
and out of the cell
through the plasma
membrane.
• Some materials move
between the
phospholipids.
• Some materials move
through the proteins.

34. Membrane trafficking
simple
diffusion
facilitated
diffusion
active
transport
ATP