The document provides an outline and overview of key concepts about cell membranes: 1. Membranes are composed of a phospholipid bilayer with embedded proteins. The fluid mosaic model describes membranes as a fluid bilayer with proteins floating within. 2. Membrane components include phospholipids, transmembrane proteins, peripheral proteins, and glycoproteins/glycolipids. Electron microscopy techniques like TEM and freeze-fracture are used to study membrane structure. 3. Passive transport across membranes, like diffusion and facilitated diffusion, moves molecules down concentration gradients through channel or carrier proteins without energy expenditure.

1. Chapter 05
Lecture and
Animation Outline
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2. Membranes
Chapter 5
2

3. 3
Membrane Structure
• Phospholipids arranged in a bilayer
• Globular proteins inserted in the lipid
bilayer
• Fluid mosaic model – mosaic of proteins
floats in or on the fluid lipid bilayer like
boats on a pond

4. 4
PolarHydrophilicHeadsNonpolarHydrophobicTails
CH2
CH2
N+
(CH3)3
O
P
O
O–O
CH2H2C C
O O
O OC C
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH2
CH2
CH
CH2 CH
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH2
CH3 CH3
H
b. Space-filling modela. Formula c. Iconb. Space-filling modela. Formula c. Icon
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5. 5
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Glycoprotein
Extracellular
matrix protein Glycolipid
Cholesterol
Glycoprotein
Integral
proteins
Actin filaments
of cytoskeleton
Intermediate
filaments
of cytoskeleton
Peripheral
protein
Peripheral
protein
Glycolipid
Cholesterol
Glycoprotein
Integral
proteins
Actin filaments
of cytoskeleton
Intermediate
filaments
of cytoskeleton

6. 6
• Cellular membranes have 4 components
1. Phospholipid bilayer
• Flexible matrix, barrier to permeability
1. Transmembrane proteins
• Integral membrane proteins
1. Interior protein network
• Peripheral or Intracellular membrane proteins
1. Cell surface markers
• Glycoproteins and glycolipids

7. 7
• Both transmission electron microscope (TEM) and
scanning (SEM) used to study membranes
• One method to embed specimen in epoxy
– 1µm shavings
– TEM shows layers
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Plasma membrane of cell 1
Plasma membrane of cell 2
Cell 1
Cell 2
25 nm
© Dr. Donald Fawcett/ Photo Researchers, Inc.

8. • Freeze-fracture visualizes inside of
membrane
8
Knife
Cell
Medium
2. The cell often
fractures through the
interior, hydrophobic
area of the lipid
bilayer, splitting the
plasma membrane
into two layers.
3. The plasma membrane separates such
that proteins and other embedded
membrane structures remain within one
or the other layers of the membrane.
4. The exposed membrane is
coated with platinum, which
forms a replica of the
membrane. The underlying
membrane is dissolved away,
and the replica is then viewed
with electron microscopy .
Fractured
upper half
of lipid bilayer
Exposed lower
half of lipid bilayer
Exposed
lower half of
lipid bilayer
External surface
of plasma membrane
0.15 µm
1. A cell frozen in
medium is cracked
with a knife blade.
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© Dr. Donald Fawcett/Visuals Unlimited

9. 9
Phospholipids
• Structure consists of
– Glycerol – a 3-carbon polyalcohol
– 2 fatty acids attached to the glycerol
• Nonpolar and hydrophobic (“water-fearing”)
– Phosphate group attached to the glycerol
• Polar and hydrophilic (“water-loving”)
• Spontaneously forms a bilayer
– Fatty acids are on the inside
– Phosphate groups are on both surfaces

10. 10
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Plasma membrane of cell 1
Cell 1
Cell 2
Polar hydrophilic heads
Polar hydrophilic heads
Extracellular fluid
Intracellular fluid (cytosol)
Nonpolar
hydrophobic tails

11. • Bilayers are fluid
• Hydrogen bonding
of water holds the
2 layers together
• Individual
phospholipids and
unanchored
proteins can move
through the
membrane
11
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Mouse
cell
Fuse
cells
Intermixed
membrane proteins
Allow time for
mixing to occur
Human
cell

12. • Environmental influences on fluidity
– Saturated fatty acids make the membrane
less fluid than unsaturated fatty acids
• “Kinks” introduced by the double bonds keep them
from packing tightly
• Most membranes also contain sterols such as
cholesterol, which can either increase or decrease
membrane fluidity, depending on the temperature
– Warm temperatures make the membrane
more fluid than cold temperatures
• Cold tolerance in bacteria due to fatty acid
desaturases
12

13. 13
Membrane Proteins
• Various functions:
1. Transporters
2. Enzymes
3. Cell-surface receptors
4. Cell-surface identity markers
5. Cell-to-cell adhesion proteins
6. Attachments to the cytoskeleton

14. 14
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Enzyme Cell surface receptor
Cell surface identity marker Cell-to-cell adhesion Attachment to the cytoskeleton
Outside
cell
Inside
cell
Transporter

15. 15
Structure relates to function
• Diverse functions arise from the diverse
structures of membrane proteins
• Have common structural features related
to their role as membrane proteins
• Peripheral proteins
– Anchoring molecules attach membrane
protein to surface

16. • Anchoring molecules are modified lipids with
1. Nonpolar regions that insert into the internal portion
of the lipid bilayer
2. Chemical bonding domains that link directly to
proteins
16
Protein anchored to
phospholipid
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17. 17
• Integral membrane proteins
– Span the lipid bilayer (transmembrane
proteins)
• Nonpolar regions of the protein are embedded in
the interior of the bilayer
• Polar regions of the protein protrude from both
sides of the bilayer
– Transmembrane domain
• Spans the lipid bilayer
• Hydrophobic amino acids arranged in α helices
Membrane Proteins

18. • Proteins need only a single transmembrane
domain to be anchored in the membrane, but
they often have more than one such domain
18
a. b.
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Single transmembrane
domain
Many transmembrane domains

19. 19
• Pores
– Extensive nonpolar regions within a
transmembrane protein can create a pore
through the membrane
– Cylinder of β sheets in the protein secondary
structure called a β-barrel
• Interior is polar and allows water and small polar
molecules to pass through the membrane

20. 20
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β-pleated sheets

21. 21
Passive Transport
• Passive transport is movement of
molecules through the membrane in which
– No energy is required
– Molecules move in response to a
concentration gradient
• Diffusion is movement of molecules from
high concentration to low concentration
– Will continue until the concentration is the
same in all regions

22. 22
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a. b. c. d.

23. 23
• Major barrier to crossing a biological
membrane is the hydrophobic interior that
repels polar molecules but not nonpolar
molecules
– Nonpolar molecules will move until the
concentration is equal on both sides
– Limited permeability to small polar molecules
– Very limited permeability to larger polar
molecules and ions

24. • Facilitated diffusion
– Molecules that cannot cross membrane easily
may move through proteins
– Move from higher to lower concentration
– Channel proteins
• Hydrophilic channel when open
– Carrier proteins
• Bind specifically to molecules they assist
• Membrane is selectively permeable
24

25. 25
Channel proteins
• Ion channels
– Allow the passage of ions
– Gated channels – open or close in response
to stimulus (chemical or electrical)
– 3 conditions determine direction
• Relative concentration on either side of membrane
• Voltage differences across membrane
• Gated channels – channel open or closed

26. 26
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Extracellular fluid Extracellular fluid
Cytoplasm
a.
Cytoplasm

27. 27
Carrier proteins
• Can help transport both ions and other
solutes, such as some sugars and amino
acids
• Requires a concentration difference
across the membrane
• Must bind to the molecule they transport
– Saturation – rate of transport limited by
number of transporters

28. 28
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b.
Extracellular fluid
Cytoplasm

29. 29
Osmosis
• Cytoplasm of the cell is an aqueous
solution
– Water is solvent
– Dissolved substances are solutes
• Osmosis – net diffusion of water across a
membrane toward a higher solute
concentration

30. 30
Urea molecule
Semipermeable
membrane
Water
molecules
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31. 31
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32. 32
Osmotic concentration
• When 2 solutions have different osmotic
concentrations
– Hypertonic solution has a higher solute concentration
– Hypotonic solution has a lower solute concentration
• When two solutions have the same osmotic
concentration, the solutions are isotonic
• Aquaporins facilitate osmosis

33. Osmotic pressure
• Force needed to stop osmotic flow
• Cell in a hypotonic solution gains water causing
cell to swell – creates pressure
• If membrane strong enough, cell reaches
counterbalance of osmotic pressure driving
water in with hydrostatic pressure driving water
out
– Cell wall of prokaryotes, fungi, plants, protists
• If membrane is not strong, may burst
– Animal cells must be in isotonic environments
33

34. 34
HumanRedBloodCellsPlantCells
Shriveled cells Normal cells
Flaccid cell Normal turgid cell
Hypertonic
Solution
Isotonic
Solution
Hypotonic
Solution
Cells swell and
eventually burst
Cell body shrinks
from cell wall
© David M.Phillips/Visuals Unlimited
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
5 µm 5 µm 5 µm

35. 35
Maintaining osmotic balance
• Some cells use extrusion in which water is
ejected through contractile vacuoles
• Isosmotic regulation involves keeping cells
isotonic with their environment
– Marine organisms adjust internal
concentration to match sea water
– Terrestrial animals circulate isotonic fluid
• Plant cells use turgor pressure to push the
cell membrane against the cell wall and
keep the cell rigid

36. 36
Active Transport
• Requires energy – ATP is used directly or
indirectly to fuel active transport
• Moves substances from low to high
concentration
• Requires the use of highly selective carrier
proteins

37. 37
• Carrier proteins used in active transport
include
– Uniporters – move one molecule at a time
– Symporters – move two molecules in the
same direction
– Antiporters – move two molecules in opposite
directions
– Terms can also be used to describe facilitated
diffusion carriers

38. 38
Sodium–potassium (Na+
–K+
) pump
• Direct use of ATP for active transport
• Uses an antiporter to move 3 Na+
out of
the cell and 2 K+
into the cell
– Against their concentration gradient
• ATP energy is used to change the
conformation of the carrier protein
• Affinity of the carrier protein for either Na+
or K+
changes so the ions can be carried
across the membrane

39. 39
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Na+
K+
6. Dephosphorylation of protein triggers
change back to original conformation,
with low affinity for K+
. K+
diffuses into
the cell, and the cycle repeats.
1. Carrier in membrane binds
intracellular sodium.
2. ATP phosphorylates protein with
bound sodium.
5. Binding of potassium causes
dephosphorylation of protein.
4. This conformation has higher affinity
for K+
. Extracellular potassium binds
to exposed sites.
3. Phosphorylation causes
conformational change in protein,
reducing its affinity for Na+
. The Na+
then diffuses out.
P
P
P
P
P
ATP
+
ADP
Extracellular
Intracellular

40. 40
Coupled transport
• Uses ATP indirectly
• Uses the energy released when a
molecule moves by diffusion to supply
energy to active transport of a different
molecule
• Symporter is used
• Glucose–Na+
symporter captures the
energy from Na+
diffusion to move glucose
against a concentration gradient

41. 41
Na+
/ K+
pump
Na+
Glucose
K+
Outside
of cell
ATP
ADP + Pi
Inside
of cell
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Coupled
transport
protein

42. Bulk Transport
• Endocytosis
– Movement of substances into the cell
– Phagocytosis – cell takes in particulate matter
– Pinocytosis – cell takes in only fluid
– Receptor-mediated endocytosis – specific
molecules are taken in after they bind to a receptor
• Exocytosis
– Movement of substances out of cell
– Requires energy
42

43. 43
Cytoplasm
Cytoplasm
0.1 µm
1 µm
Solute
Bacterial
cells
Plasma
membrane
a. Phagocytosis
Plasma
membrane
b. Pinocytosis
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a: © CDC/Dr. Edwin P. Ewing, Jr. b: © BCC Microimaging, Inc.Reproduced with permission

44. • In the human genetic disease familial hypercholesterolemia, the
LDL receptors lack tails, so they are never fastened in the clathrin-
coated pits and as a result, do not trigger vesicle formation. The
cholesterol stays in the bloodstream of affected individuals,
accumulating as plaques inside arteries and leading to heart
attacks. 44
Receptor protein
Coated pit
Clathrin
Coated vesicle
90 nm
Target molecule
c. Receptor-mediated endocytosis
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(both): © Reproduced with permission from M.M. Perry and A.B. Gilbert, “Yolk transport in the ovarian follicle of the hen (Gallus domesticus): lipoprotein-like
particles at the periphery of the oocyte in the rapid growth phase,” Journal of Cell Science, 39:257-72, October 1979. © The Company of Biologists

45. • Exocytosis
– Discharge of materials out of the cell
– Used in plants to export cell wall material
– Used in animals to secrete hormones,
neurotransmitters, digestive enzymes
45
a. b.
Plasma membrane
Secretory vesicle
Secretory product
Cytoplasm
70 nm
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b: © Dr. Brigit Satir