Cellular membranes are fluid mosaics of phospholipids and proteins that allow selective permeability. The fluid mosaic model proposed by Singer and Nicolson describes membranes as a phospholipid bilayer with proteins embedded within. Membrane proteins carry out important functions like transport, signaling, and enzymatic activity through mechanisms like passive diffusion, facilitated diffusion, active transport, and cotransport.

1. Chapter 7
Membrane Structure and Function
In 1972, Singer and Nicolson proposed that the membrane is a
mosaic of proteins dispersed within a phospholipid bilayer, with only the
hydrophilic regions exposed to water. The fluid mosaic model states
that a membrane is a fluid structure with a “mosaic” of various proteins
embedded in it.
Function: The plasma
membrane exhibits selective
permeability, allowing some
substances to cross it more
easily than others

2. Cellular membranes are fluid
mosaics of lipids and proteins
• Membranes have two asymmetric leaflets
• Each leaflet has lateral fluidity
• Phospholipids are the most abundant lipid in the
plasma membrane (sphingolipids, glycolipids,
cholesterol also)—note: cholesterol only in animals &
some bacteria, not in plants
• Phospholipids are amphipathic molecules, containing
hydrophobic and hydrophilic regions
• The fluid mosaic model states that a membrane is a
fluid structure with a “mosaic” of various globular
proteins embedded in it—both integral and peripheral

3. Figure 7.2
Hydrophilic head
Hydrophobic tail
WATER
WATER
How many of these bilayers around nucleus?
Vacuole? Lysosome? ER lumen?

4. Figure 7.3
Glyco-
protein Carbohydrate Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Microfilaments
of cytoskeleton
Fibers of extra-
cellular matrix (ECM)
Cholesterol
Peripheral
proteins Integral
protein CYTOPLASMIC
SIDE OF
MEMBRANE
•Peripheral proteins usually on inner side of membrane & held their by ionic (with
charged lipid head) or hydrophobic (with a second hydrophobic protein) interaction
•Hydrophobic & hydrophilic regions of integral proteins
•Sugars usually on exterior leaflet; proteoglycans too

5. • Freeze-fracture studies of the plasma
membrane supported the fluid mosaic model
• Freeze-facture is a specialized preparation
technique that splits a membrane along the
middle of the phospholipid bilayer

6. LE 7-4
Knife
Cytoplasmic layerExtracellular layer
Cytoplasmic
layer
Plasma
membrane
Extracellular
layer
Proteins

7. Lateral movement
(~107
times per second)
Flip-flop
(~ once per month)
Movement of phospholipids
Rarely does a molecule flip-flop transversely across the membrane

8. LE 7-5b
ViscousFluid
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
carbon tails
•As temperatures cool, membranes switch from a fluid state to a
solid state
•The temperature at which a membrane solidifies depends on the
types of lipids
•Membranes rich in unsaturated fatty acids are more fluid than
those rich in saturated fatty acids
•Membranes must be fluid to work properly; they are usually
about as fluid as salad oil

9. 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

10. Figure 7.6
N-terminus
C-terminus
α helix
EXTRACELLULAR
SIDE
CYTOPLASMIC
SIDE
•Integral proteins that span the membrane are called transmembrane proteins
•The hydrophobic regions of an integral protein consist of one or more
stretches of nonpolar amino acids, often coiled into alpha helices

11. • Six major functions of membrane proteins
– Transport
– Enzymatic activity
– Signal transduction
– Cell-cell recognition
– Intercellular joining
– Attachment to the cytoskeleton and
extracellular matrix (ECM)

12. Figure 7.7
(a) Transport (b) Enzymatic
activity
(c) Signal
transduction
(d) Cell-cell
recognition
(e) Intercellular
joining
(f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)
Enzymes
ATP
Signaling
molecule
Receptor
Signal transduction
Glyco-
protein

13. 1. Which of the following best describes the structure of a
biological membrane?
a. two layers of phospholipids with proteins embedded
between the two layers
b. a mixture of covalently linked phospholipids and proteins
that determines which solutes can cross the membrane
and which cannot
c. two layers of phospholipids with proteins either spanning
the layers or on the surface of the layers
d. a fluid structure in which phospholipids and proteins move
freely between sides of the membrane
e. two layers of phospholipids (with opposite orientations of
the phospholipids in each layer) with each layer covered
on the outside with proteins

14. 1. Which of the following best describes the structure of a
biological membrane?
a. two layers of phospholipids with proteins embedded
between the two layers
b. a mixture of covalently linked phospholipids and proteins
that determines which solutes can cross the membrane
and which cannot
c. two layers of phospholipids with proteins either spanning
the layers or on the surface of the layers
d. a fluid structure in which phospholipids and proteins move
freely between sides of the membrane
e. two layers of phospholipids (with opposite orientations of
the phospholipids in each layer) with each layer covered
on the outside with proteins

15. Sidedness of
Membranes
•Membranes have distinct
inside and outside faces
•The asymmetrical
distribution of proteins,
lipids and associated
carbohydrates in the
plasma membrane is
determined when the
membrane is built by the
ER and Golgi apparatus
Transmembrane
glycoproteins Secretory
protein
Golgi
apparatus
Vesicle
Attached
carbohydrate
ER
lumen
Glycolipid
Transmembrane
glycoprotein
Plasma membrane:
Cytoplasmic face
Extracellular face
Membrane
glycolipid
Secreted
protein

16. The Permeability of the Lipid
Bilayer
• Hydrophobic (nonpolar) molecules, such as
hydrocarbons, can dissolve in the lipid bilayer
and pass through the membrane rapidly
• Polar molecules, such as sugars, do not
cross the membrane easily
View Membrane Transport Video

17. Transport Proteins
• Transport proteins allow passage of hydrophilic
substances across the membrane
• Some transport proteins, called channel proteins, have
a hydrophilic channel that certain molecules or ions
can use as a tunnel
• Channel proteins called aquaporins facilitate the
passage of water
• Other transport proteins, called carrier proteins, bind
to molecules and change shape to shuttle them
across the membrane
• A transport protein is specific for the substance it
moves

18. Passive transport is diffusion of a
substance across a membrane with
no energy investment
• Diffusion is the tendency for molecules to
spread out evenly into the available space
• Although each molecule moves randomly,
diffusion of a population of molecules may
exhibit a net movement in one direction
• At dynamic equilibrium, as many molecules
cross one way as cross in the other direction

19. Figure 7.10
Molecules of dye Membrane (cross section)
WATER
(a) Diffusion of one solute
(b) Diffusion of two solutes
Net diffusion Net diffusion
Net diffusionNet diffusion
Net diffusion Net diffusion
Equilibrium
Equilibrium
Equilibrium

20. Effects of Osmosis on Water
Balance
• Osmosis is the diffusion of water across a
selectively permeable membrane
• The direction of osmosis is determined by a
difference in total solute concentration (but
pressure, gravity, matrix can influence)
• Water diffuses across a membrane from the
region of lower solute concentration (higher
water potential) to the region of higher solute
concentration (lower water potential)

21. Figure 7.11
Lower concentration
of solute (sugar)
Higher concentration
of solute
More similar
concentrations of solute
Sugar
molecule
H2O
Selectively
permeable
membrane
Osmosis
Selectively permeable
membrane: sugar
molecules cannot pass
through pores, but
water molecules can

22. Water Balance of Cells Without
Walls
• Tonicity is the ability of a solution to cause a
cell to gain or lose water
• Isotonic solution: solute concentration is the
same as that inside the cell; no net water
movement across the plasma membrane
• Hypertonic solution: solute concentration is
greater than that inside the cell; cell loses
water
• Hypotonic solution: solute concentration is less
than that inside the cell; cell gains water

23. Figure 7.12
Hypotonic Isotonic Hypertonic
H2O
Lysed Normal Shriveled
Plasma
membrane
Cell
wall
Turgid (normal) Flaccid Plasmolyzed
(a)Animalcell(b)Plantcell
Plasma
membrane
H2OH2O
H2OH2O
H2OH2O
H2O

24. Water Balance of Cells with
Walls
• Cell walls help maintain water balance
• A plant cell in a hypotonic solution swells until the
wall opposes uptake; the cell is now turgid (firm)
• If a plant cell and its surroundings are isotonic,
there is no net movement of water into the cell; the
cell becomes flaccid (limp), and the plant may wilt
• In a hypertonic environment, plant cells lose water;
eventually, the membrane pulls away from the
wall, a usually lethal effect called plasmolysis
Video: PlasmolysisVideo: Plasmolysis

25. LE 7-14
Filling vacuole
50 µm
50 µm
Contracting vacuole
The protist Paramecium, which is hypertonic to its pond water environment, has a
contractile vacuole that acts as a pump

26. 2. Which of the following statements about osmosis is
correct?
a. If a cell is placed in an isotonic solution, more water will
enter the cell than leaves the cell.
b. Osmotic movement of water into a cell would likely occur
if the cell accumulates water from its environment.
c. The presence of aquaporins (proteins that form water
channels in the membrane) should speed up the process
of osmosis.
d. If a solution outside the cell is hypertonic compared to the
cytoplasm, water will move into the cell by osmosis.
e. Osmosis is the diffusion of water from a region of lower water
concentration to a region of higher water concentration.

27. 2. Which of the following statements about osmosis is
correct?
a. If a cell is placed in an isotonic solution, more water will
enter the cell than leaves the cell.
b. Osmotic movement of water into a cell would likely occur
if the cell accumulates water from its environment.
c. The presence of aquaporins (proteins that form water
channels in the membrane) should speed up the process
of osmosis.
d. If a solution outside the cell is hypertonic compared to the
cytoplasm, water will move into the cell by osmosis.
e. Osmosis is the diffusion of water from a region of lower water
concentration to a region of higher water concentration.

28. Facilitated Diffusion: Passive
Transport Aided by Proteins
• In facilitated diffusion, transport proteins speed
movement of molecules across the plasma
membrane
• Channel proteins provide corridors that allow a
specific molecule or ion to cross the membrane
• Carrier proteins undergo a subtle change in
shape that translocates the solute-binding site
across the membrane

29. Figure 7.14
(a) A channel
protein
(b) A carrier protein
Carrier protein
Channel protein Solute
Solute
EXTRACELLULAR
FLUID
CYTOPLASM

30. Active transport uses energy to
move solutes against their gradients
• Facilitated diffusion is still passive because
the solute moves down its concentration
gradient
• Some transport proteins, however, can move
solutes against their concentration gradients

31. The Need for Energy in Active
Transport
• Active transport moves substances against
their concentration gradient
• Active transport requires energy, usually in the
form of ATP
• Active transport is performed by specific
proteins embedded in the membranes
• Active transport allows cells to maintain
concentration gradients that differ from their
surroundings
• The sodium-potassium pump is one type of
active transport system

32. Figure 7.15
EXTRACELLULAR
FLUID
CYTOPLASM
1 2
5
6
4
3
[Na+
] low
[K+
] high
[Na+
] high
[K+
] low
Na+
K+
K+
K+
K+
K+
K+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
ATP
ADP
P
P
P
i
P
Cytoplasmic Na+
bonds to
the sodium-potassium pump
Na+
binding stimulates
phosphorylation by ATP.
Phosphorylation causes
the protein to change its
conformation, expelling Na+
to the outside.
Extracellular K+
binds
to the protein, triggering
release of the phosphate
group.
Loss of the phosphate
restores the protein’s
original conformation.
K+
is released and Na+
sites are receptive again;
the cycle repeats.

33. Figure 7.16
ATP
Passive transport Active transport
Diffusion Facilitated diffusion

34. Maintenance of Membrane
Potential by Ion Pumps
• Membrane potential is the voltage difference
across a membrane (differences in the
distribution of positive and negative ions across a
membrane)
• Two combined forces, 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)

35. H+
ATP
CYTOPLASM
EXTRACELLULAR
FLUID
Proton pump
H+
H+
H+
H+
H+
+
+
+
+
+
–
–
–
–
–
•An electrogenic pump is a transport protein that generates the
voltage across a membrane
•The main electrogenic pump of plants, fungi, and bacteria is a
proton pump

36. 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

37. Figure 7.18
Sucrose
ATP
H+
H+
H+
H+
H+
Proton pump
−
−
−
−
+
+
+
+
Sucrose-H+
cotransporter
H+
H+
H+
H+
Sucrose
Diffusion of H+

38. 3. Which of the following amino acids would most
likely be present in the outer side (facing the lipid
tails) of a transmembrane domain of an integral
membrane protein?
a. a charged amino acid like lysine
b. a polar amino acid like serine
c. a special amino acid like glycine or
proline
d. a hydrophobic amino acid like valine
e. any of the above, with no preference

39. 3. Which of the following amino acids would most
likely be present in the outer side of a
transmembrane domain of an integral membrane
protein?
a. a charged amino acid like lysine
b. a polar amino acid like serine
c. a special amino acid like glycine or
proline
d. a hydrophobic amino acid like valine
e. any of the above, with no preference

40. 4. Assume that each of the following items experiences a
similar magnitude of energy difference driving their diffusion
across a pure lipid bilayer. If ranked in order from fastest to
slowest, which of the following items would likely be second in
terms of how much of it crosses the bilayer in a given time?
a. molecular oxygen
b. sucrose
c. insulin
d. glucose
e. water

41. 4. Assume that each of the following items experiences a
similar magnitude of energy difference driving their diffusion
across a pure lipid bilayer. If ranked in order from fastest to
slowest, which of the following items would likely be second in
terms of how much of it crosses the bilayer in a given time?
a. molecular oxygen (first because a gas)
b. Sucrose (needs active transport)
c. Insulin (ligand for receptor signaling)
d. Glucose (needs active transport)
e. Water (second from channels & size)

42. 5. Consider various transport systems in a hypothetical cell
(see figure). Which one of these systems would both be a
passive system and not alter the membrane potential through
its operation?

43. 5. Consider various transport systems in a hypothetical cell
(see figure). Which one of these systems would both be a
passive system and not alter the membrane potential through
its operation?
A
B
C
D
E

44. Bulk Transport: Exocytosis
• In exocytosis, transport vesicles migrate to the
membrane, fuse with it, and release their contents
• Many secretory cells use exocytosis to export their
products
Bulk Transport: Endocytosis
• In endocytosis, the cell takes in macromolecules by
forming vesicles from the plasma membrane
• Endocytosis is a reversal of exocytosis, involving
different proteins
• Small molecules and water enter or leave the cell through the
lipid bilayer or by transport proteins
• Large molecules, such as polysaccharides and proteins, cross
the membrane via vesicles (bulk transport)

45. Figure 7.19
Phagocytosis Pinocytosis
Receptor-Mediated
Endocytosis
Solutes
Pseudopodium
“Food”
or
other
particle
Food
vacuole
CYTOPLASM
EXTRACELLULAR
FLUID
Plasma
membrane
Coated
pit
Coated
vesicle
Coat
protein
Receptor

46. LE 7-20c
Receptor
RECEPTOR-MEDIATED ENDOCYTOSIS
Ligand
Coated
pit
Coated
vesicle
Coat protein
Coat
protein
Plasma
membrane
0.25 µm
A coated pit
and a coated
vesicle formed
during
receptor-
mediated
endocytosis
(TEMs).
•Three types of endocytosis:
–Phagocytosis (“cellular
eating”): Cell engulfs
particle in a vacuole
–Pinocytosis (“cellular
drinking”): Cell creates
vesicle around fluid
–Receptor-mediated
endocytosis: Binding of
ligands to receptors
triggers vesicle formation