POL

1. Cell Membranes
5.1 – 5.4

2. Chapter 5 Cell Membranes and Signaling
Key Concepts
5.1 Biological Membranes Have a Common Structure and Are
Fluid
5.2 Passive Transport across Membranes Requires No Input
of Energy
5.3 Active Transport Moves Solutes against Their
Concentration Gradients
5.4 Bulk Transport: Large Molecules Cross Membranes via
Vesicles

3. Chapter 5 Opening Question
1. Predict, based on its molecular structure, whether or not
caffeine can move through the cell membrane. Justify your
prediction. Remember, only small, non polar molecules can
pass directly through the phospholipid bilayer.

4. Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
A membrane’s structure and
functions are determined
by its constituents: lipids,
proteins, and
carbohydrates.
The general design of
membranes is known as
the fluid mosaic model.
Phospholipids form a
continuous bilayer which is
like a “lake” in which a
variety of proteins “float.”
Biological membranes are
primarily made of what?

5. Figure 5.1 Membrane Structure

6. Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
The lipid molecules are usually
phospholipids with two
regions:
• Hydrophilic regions—
electrically charged “heads”
associate with water
molecules
• Hydrophobic regions—
nonpolar fatty acid “tails”
that do not dissolve in water
• This two-sided nature makes a
phospholipid amphipathic. Why can’t water soluble molecules
move through the phospholipid
bilayer?

7. Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
A bilayer is formed when the fatty acid “tails” associate
with each other and the polar “heads” face the
aqueous environment.
How are biological
membranes arranged?

8. Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
Membranes may differ in lipid composition; there
are many types of phospholipids.
Phospholipids may differ in:
• Fatty acid chain length
• Degree of saturation
• Kinds of polar groups present

9. Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
Cholesterol is an important
component of animal cell
membranes.
Hydroxyl groups interact with
the polar heads of
phospholipids.
Cholesterol is important in
modulating membrane
fluidity; other steroids
function as hormones.
What does cholesterol do
for a membrane?

10. In-Text Art, Chapter 5, p. 84 (2)

11. Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
The fatty acids make the membrane somewhat fluid. This
allows some molecules to move laterally within the
membrane
Membrane fluidity is influenced by:
• Lipid composition—short, unsaturated chains
increase fluidity
 Some organism, like those that hibernate, can
change the amount of unsaturated chains to
increase or decrease fluidity – what should they do
in winter?
• Temperature—fluidity decreases in colder conditions

12. Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
All biological membranes contain proteins; the ratio of
proteins to phospholipids varies.
Peripheral membrane proteins lack hydrophobic
groups and are not embedded in the bilayer.
Integral membrane proteins are at least partly
embedded in the phospholipid bilayer.
What kind of
membrane protein
is easiest to
remove in an
experiment?

13. In-Text Art, Chapter 5, p. 85
What kind of general kind of amino acid side chains would
be found on the outside of this protein?

14. Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
Anchored membrane proteins have hydrophobic lipid
components that anchor them in the bilayer.
Proteins are asymmetrically distributed on the inner
and outer membrane surfaces.
Transmembrane proteins extend through the
bilayer; they may have domains with different
functions on the inner and outer sides of the
membrane.

15. Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
Some membrane proteins can move within the
phosopholipid bilayer; others are restricted.
• Cell fusion experiments illustrate this migration.
Proteins inside the cell can restrict movement of
membrane proteins, as can attachments to the
cytoskeleton.

16. Figure 5.2 Rapid Diffusion of Membrane Proteins (Part 1)

17. Figure 5.2 Rapid Diffusion of Membrane Proteins (Part 2)
Why do the
membrane
proteins distribute
around the cell?

18. Figure 5.2 Rapid Diffusion of Membrane Proteins (Part 3)
1. Construct a graph using the data above.
2. Use the graph to predict what percentage of cells would
have mixed at 18C and at 30C.
0

19. Apply The Concept: Page 84
The membrane lipids of a cell can be labeled with a fluorescent tag so
the entire surface of the cell will glow evenly under ultraviolet light. If
a strong laser light is then shone on a tiny region of the cell, that
region gets bleached (the strong light destroys the fluorescent tag)
and there is a “hole” in the cell surface fluorescence (though not an
actual hole in the cell’s membrane). After the laser is turned off, the
hole gradually fills in with fluorescent lipids that diffuse in from other
parts of the membrane. The time it takes for the “hole” to disappear
is a measure of membrane fluidity. The table shows some data for
cells with altered membrane compositions. Explain the effect of
each alteration.

20. Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
Diverse carbohydrates are located on
the outer cell membrane and play a
role in communication.
• Glycolipid—carbohydrate
covalently bonded to a lipid –
recognition site
• Glycoprotein—one or more
oligosaccharides covalently bonded
to a protein
• Proteoglycan—protein with more
and longer carbohydrates bonded
to it
What do
glycolipids do?

21. Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
Cells can adhere to one another through
interactions between cell surface carbohydrates
and proteins (glycoproteins).

22. Concept 5.1 Biological Membranes Have a Common Structure
and Are Fluid
Membranes are constantly forming, transforming
into other types, fusing, and breaking down.
Though membranes appear similar, there are
major chemical differences among the
membranes of even a single cell.

23. Passive Transport

24. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
Diagram how sugar molecules dissolve in coffee.
Predict how decreasing the temperature of the coffee
would affect the sugar molecules.
View this simulation.

25. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
Selective permeability: biological membranes allow
some substances, but not others, to pass
Two processes of transport across membranes:
1. Passive transport does not require metabolic
energy.
• A substance moves down its concentration
gradient.
2. Active transport does require input of metabolic
energy.
• A substance moves against its concentration
gradient.

26. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
Passive transport can occur by:
• Simple diffusion through the phospholipid bilayer
• Facilitated diffusion through channel proteins or aided by
carrier proteins
• Osmosis is a kind of facilitated diffusion
• Passive transport is random, requires a concentration
gradient (a difference in concentration across an area), and
will continue until the concentrations are equal – this is the
driving for for all passive transport.
• Passive transport does not require cellular energy (ATP).

27. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
Diffusion is the process of random movement toward
equilibrium; a net movement from regions of greater
concentration to regions of lesser concentration.

28. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
Speed of diffusion depends on three factors:
• Diameter of the molecules—smaller
molecules diffuse faster.
• Temperature of the solution—higher
temperatures lead to faster diffusion.
• Concentration gradient—the greater the
concentration gradient, the faster a substance
will diffuse.

29. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
Cell cytoplasm is an aqueous solution, as is
the surrounding environment.
Diffusion of each solute depends only on its
own concentration.
A higher concentration inside the cell causes
the solute to diffuse out; higher concentration
outside causes the solute to diffuse in.

30. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
Some molecules cross the phospholipid bilayer
by simple diffusion:
• O2, CO2, and small, nonpolar, lipid-soluble
molecules.
Polar (hydrophilic) molecules do not pass
through—they are not soluble in the
hydrophobic interior of the membrane.
• Amino acids, sugars, ions, water

31. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
Osmosis is the diffusion of water across
membranes through special channel proteins.
It depends on the concentration of water
molecules on either side of the membrane—
water moves down its concentration gradient.
The higher the total solute concentration, the lower
the concentration of water molecules.
-Osmosis is a special kind of what?
-What is the difference between osmosis and
diffusion?

32. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
Osmotic pressure: pressure that must be applied to a
solution to prevent flow of water across a membrane
by osmosis
Π = cRT
c = total solute concentration
R = the gas constant
T = absolute temperature

33. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
The higher concentration of a substance on one
side of a membrane represents stored energy
(potential energy).
If a membrane allows water, but not solutes, to
pass through, the net movement of water
molecules will be toward the solution with the
higher solute concentration and the lower
concentration of water molecules.
-Which way does water move?

34. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
When comparing two solutions
separated by a membrane:
• A hypertonic solution has a
higher solute concentration.
Cells shrivel in hypertonic
solutions.
• Isotonic solutions have equal
solute concentrations.
• A hypotonic solution has a
lower solute concentration. Cells
swell in hypotonic solutions.
Why is saline
used in IVs, and
not pure water?

35. Figure 5.3 Osmosis Can Modify the Shapes of Cells

36. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
Concentration of solutes in the environment
determines the direction of osmosis in all animal
cells.
In other organisms, cell walls limit the volume of
water that can be taken up.
Turgor pressure is the internal pressure against
the cell wall—as it builds up, it prevents more
water from entering.
What helps plant cells maintain turgor pressure?

37. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
Facilitated diffusion:
Transport proteins help hydrophilic substances cross
the cell membrane.
Channel proteins are integral membrane proteins that
form channels across the membrane through which
some substances can pass.
Substances can also bind to carrier proteins to speed
up diffusion.
Both processes operate in either direction.

38. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
Channel Proteins:
Ion channels: channel proteins that allow specific ions
to pass through
Most are gated channels—they open when a stimulus
causes the protein to change shape.
• Ligand-gated—the stimulus is a ligand, a chemical
signal.
• Voltage-gated—the stimulus is a change in
electrical charge difference across the membrane.

39. Figure 5.4 A Ligand-Gated Channel Protein Opens in Response to a Stimulus

40. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
Water crosses membranes at a faster rate than simple
diffusion.
It may “hitchhike” with ions such as Na+
as they pass
through ion channels.
Aquaporins are channels that allow large amounts of
water to move along its concentration gradient.
What do aquaporins do?

41. Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 1)

42. Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 2)
1. Explain the look of the oocytes above.
2. Identify and justify the claim that the cells with
aquaporins have an increased permeability to water.

43. Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 3)

44. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
Carrier proteins in the membrane facilitate diffusion
by binding substances.
Glucose transporters are carrier proteins in mammalian
cells.
Glucose molecules bind to the carrier protein and
cause the protein to change shape—it releases
glucose on the other side of the membrane.

45. Figure 5.6 A Carrier Protein Facilitates Diffusion (Part 1)

46. Concept 5.2 Passive Transport across Membranes Requires No
Input of Energy
Glucose is quickly broken
down in the cell, so there is
always a strong
concentration gradient that
favors glucose uptake.
But the system can become
saturated—when all of the
carrier molecules are
bound, the rate of diffusion
reaches a maximum.

47. Active Transport

48. Concept 5.3 Active Transport Moves Solutes against Their
Concentration Gradients
Cells maintain an internal environment with a different
composition than the outside environment.
This requires work—energy from ATP is needed to
move substances against their concentration
gradients (active transport).
Specific carrier proteins move substances in only one
direction, either into or out of the cell.

49. Table 5.1
-Compared to passive
transport, in which direction
does active transport move
molecules?

50. Table 5.1
-Compared to passive transport, in which direction does
active transport move molecules?

51. Concept 5.3 Active Transport Moves Solutes against Their
Concentration Gradients
1. Justify the claim that energy is needed to move a
substance from an area of low concentration to an
area of high concentration.
2. Describe how much energy is required to allow
BBs to fall through a strainer versus how much
energy is required to take the BBs from the outside
and put them back in the strainer before they fall out.
(think Gibbs free energy – spontaneous reactions).
Think about how much energy is required to walk up a
steep hill versus to walk down the hill.

52. Concept 5.3 Active Transport Moves Solutes against Their
Concentration Gradients
Two types of active transport:
Primary active transport involves direct hydrolysis of
ATP for energy.
Secondary active transport uses the energy from an
ion concentration gradient or an electrical gradient.
The gradients are established by primary active
transport.
-Where does secondary
active transport get the
energy needed to work?

53. Concept 5.3 Active Transport Moves Solutes against Their
Concentration Gradients
The sodium–potassium (Na+
–K+
) pump is an
integral membrane protein that pumps Na+
out of
a cell and K+
in.
One molecule of ATP moves two K+
and three Na+
ions.

54. Figure 5.7 Primary Active Transport: The Sodium–Potassium Pump
How many sodium ions are pumped out of a cell?

55. Concept 5.3 Active Transport Moves Solutes against Their
Concentration Gradients
Secondary active transport uses energy that is
“regained” by letting ions move across the
membrane with their concentration gradients.
• Example: after the Na+
–K+
pump establishes a
concentration gradient of
Na+
, then passive diffusion of Na+
back into
the cell can provide energy for glucose
transport.
One protein usually moves both the ion and the
transported molecule across the membrane.
Review page 92 – What molecules move by secondary
active transport?

56. Bulk Transport

57. Concept 5.4 Large Molecules Cross Membranes via Vesicles
Endocytosis and Exocytosis
Macromolecules are too large or too charged to pass
through biological membranes, so instead they cross
within vesicles.
To take up or to secrete macromolecules, cells must
use endocytosis and exocytosis.

58. Concept 5.4 Large Molecules Cross Membranes via Vesicles
Exocytosis moves materials out of the cell in vesicles.
The vesicle membrane fuses with the cell membrane
and the contents are released into the environment.
Exocytosis is important in the secretion of substances
made by cells such as digestive enzymes and
neurotransmitters.

59. Concept 5.4 Large Molecules Cross Membranes via Vesicles
Endocytosis brings macromolecules and particles into
eukaryotic cells.
The cell membrane invaginates (folds) around the
particle and forms a vesicle.
The vesicle then separates from the membrane.

60. Figure 5.8 Endocytosis and Exocytosis

61. Concept 5.4 Large Molecules Cross Membranes via Vesicles
Endocytosis depends on receptors—proteins that
bind to specific molecules (ligands).
The receptors are integral membrane proteins on the
cell membrane.
The resulting vesicle includes both the receptor and its
ligand, plus other substances present near the site of
invagination.

62. Concept 5.4 Large Molecules Cross Membranes via Vesicles
Phagocytosis (“cellular eating”): a specialized
cell engulfs a large particle or another cell
• A food vesicle (phagosome) forms and
usually fuses with a lysosome, where the
contents are digested.
Pinocytosis (“cellular drinking”): vesicles are
smaller and bring in fluids and dissolved
substances

63. Concept 5.4 Large Molecules Cross Membranes via Vesicles
Receptor endocytosis (receptor-mediated
endocytosis) brings specific large molecules into
a cell via specific receptors.
This allows cells to control internal processes by
controlling location and abundance of each type
of receptor on the cell membrane.
It also plays a role in cell signaling.
What kind of lipid is imported primarily by receptor-
mediated endocytosis? (see page 94)

64. Concept 5.4 Large Molecules Cross Membranes via Vesicles
The receptors are located in membrane regions called
coated pits.
The cytoplasmic surface of a pit is coated by another
protein (often clathrin).
When receptors bind to their ligands, the coated pit
invaginates and forms a coated vesicle.
Clathrin stabilizes the vesicle.

65. Figure 5.9 Receptor Endocytosis

66. Concept 5.4 Large Molecules Cross Membranes via Vesicles
Once inside, the vesicle loses its clathrin coat and
fuses with a membrane-enclosed compartment called
an endosome.
Receptors may be recycled to the cell membrane or
degraded in a lysosome. This is an important
mechanism for controlling the abundance of each kind
of receptor on the cell surface.