2. 5.1 The Nature of the Plasma Membrane

3. The Nature of the Plasma Membrane
The plasma membrane is a thin, fluid entity that manages to be very flexible and yet is stable enough to stay together despite being continually remade due to the constant movement of materials in and out of it.

4. The Plasma Membrane
In animal cells, the plasma membrane has four principal components:
1. A phospholipid bilayer.
2. Molecules of cholesterol interspersed within the bilayer.

5. The Plasma Membrane
3. Proteins that are embedded in or that lie on the bilayer.
4. Short carbohydrate chains on the cell surface, collectively called the glycocalyx, that function in cell adhesion and as binding sites on proteins.

6. The Plasma Membrane glycocalyx phospholipids cholesterol proteins
cell exterior
cytoskeleton
integral
protein
cell interior
peripheral
protein
Phospholipid
bilayer: a double
layer of
phospholipid
molecules whose
hydrophilic “heads”
face outward, and
whose hydrophobic
“tails” point inward,
toward each other.
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.
Glycocalyx: sugar
chains that attach
to proteins and
phospholipids,
serving as protein
binding sites and
as cell lubrication
and adhesion
molecules.
Figure 5.1

7. The Phospholipid Bilayer
Phospholipids are molecules composed of two fatty acid chains linked to a charged phosphate group.

8. 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.

9. The Phospholipid Bilayer
(a) Phospholipid molecule
(b) Phospholipid bilayer
watery
extracellular
fluid
polar
head P P -
hydrophilic
hydrophobic
nonpolar
tails
hydrophilic
watery
cytosol
Hydrophobic molecules
pass through freely.
Hydrophilic molecules
do not pass
through freely.
Figure 5.2

10. The Phospholipid Bilayer
Such phospholipids arrange themselves into bilayers—two layers of phospholipids in which the fatty acid “tails” of each layer point inward (avoiding water), while the phosphate “heads” point outward (bonding with it).

11. The Phospholipid Bilayer
Phospholipids take on this configuration in the plasma membrane because a watery environment lies on either side of the membrane.

12. The Phospholipid Bilayer
In animal cells, the cholesterol molecules that are interspersed between phospholipid molecules in the plasma membrane perform two functions:
They act as a patching material that helps keep some small molecules from moving through the membrane.
They keep the membrane at an optimal level of fluidity.

13. The Phospholipid Bilayer
Some plasma membrane proteins are integral, meaning they are bound to the hydrophobic interior of the phospholipid bilayer.
Others are peripheral, meaning they lie on either side of the membrane but are not bound to its hydrophobic interior.

14. Membrane Protein Functions
In animal cells, the cholesterol molecules that are interspersed between phospholipid molecules in the plasma membrane perform two functions:
structural support
cell identification, by serving as external recognition proteins that interact with immune system cells

15. Membrane Protein Functions
communication, by serving as external receptors for signaling molecules
transport, by providing channels for the movement of compounds into and out of the cell

16. The Plasma Membrane (a) Structural support (b) Recognition
(c) Communication
(d) Transport
cell exterior
cell interior
Membrane proteins
can provide structural
support, often when
attached to parts of
the cell’s scaffolding
or “cytoskeleton.”
Protein fragments
held within
recognition proteins
can serve to identify
the cell as “normal” or
“infected” to immune
system cells.
Receptor proteins,
protruding out from
the plasma membrane,
can be the point of
contact for signals
sent to the cell via
traveling molecules,
such as hormones.
Proteins can serve
as channels
through which
materials can pass
in and out of
the cell.
Figure 5.3

17. The Plasma Membrane
The plasma membrane today is described by a conceptualization called the fluid-mosaic model.
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.

18. 5.2 Diffusion, Gradients, and Osmosis

19. Diffusion, Gradients, and Osmosis
Diffusion is the movement of molecules or ions from a region of their higher concentration to a region of their lower concentration.

20. Diffusion, Gradients, and Osmosis
A concentration gradient defines the difference between the highest and lowest concentrations of a solute within a given medium.
Through diffusion, compounds naturally move from higher to lower concentrations, meaning down their concentration gradients.

21. Diffusion, Gradients, and Osmosis
(a) Dye is dropped in.
(b) Diffusion begins.
(c) Dye is evenly distributed.
water
molecules
dye
molecules
Figure 5.4

22. Diffusion, Gradients, and Osmosis
Energy must be expended to move compounds against their concentration gradients, meaning from a lower to a higher concentration.

23. Diffusion, Gradients, and Osmosis
A semipermeable membrane is one that allows some compounds to pass through freely while blocking the passage of others.

24. Diffusion, Gradients, and Osmosis
Osmosis is the net movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration.

25. Diffusion, Gradients, and Osmosis
Because the plasma membrane is a semipermeable membrane, osmosis operates in connection with it.
Osmosis is a major force in living things; it is responsible for much of the movement of fluids into and out of cells.

26. salt
solute
(a) An aqueous solution
divided by a semipermeable
membrane has a solute
—in this case, salt—
poured into its right chamber.
solvent
semipermeable membrane
(b) As a result, though
water continues to flow in
both directions through the
membrane, there is a net
movement of water toward
the side with the greater
concentration of solutes in it.
osmosis
(c) Why does this occur?
Water molecules that are
bonded to the sodium (Na+)
and chloride (Cl–) ions that
make up salt are not free to
pass through the membrane
to the left chamber of the
container.
pure water
water bound to
salt ions
Figure 5.5

27. Osmotic Imbalances
Osmotic imbalances can cause cells either to dry out from losing too much water or, in the case of animal cells, to break from taking too much water in.
Plant cells generally do not have this problem because their cell walls limit their uptake of water.

28. Solute Concentration
Cells will gain or lose water relative to their surroundings in accordance with what the solute concentration is inside the cell as opposed to outside it.

29. Solute Concentration
A cell will lose water to a surrounding solution that is hypertonic—a solution that has a greater concentration of solutes in it than does the cell’s cytoplasm.
A cell will gain water when the surrounding solution is hypotonic to the cytoplasmic fluid.

30. (a) Hypertonic surroundings
(b) Isotonic surroundings
(c) Hypotonic surroundings
H2O
Animal cell:
plasma membrane
H2O
H2O
Plant cell:
H2O
plasma membrane
cell wall
H2O
H2O
wilted
turgid
Net movement of
water out of cell
Balanced water
movement
Net movement of
water into cell
Figure 5.6

31. Solute Concentration
Water flow is balanced between the cell and its surroundings when the surrounding fluid and the cytoplasmic fluid are isotonic to each other—when they have the same concentration of solutes.

32. Plasma Membranes and Diffusion
Animation 5.1: Plasma Membranes and Diffusion

33. 5.3 Moving Smaller Substances In and Out

34. Moving Smaller Substances In and Out
Some compounds are able to cross the plasma membrane strictly through diffusion; others require diffusion and special protein channels; still others require protein channels and the expenditure of cellular energy.

35. ATP Passive transport Active transport simple diffusion
facilitated diffusion
ATP
Materials move down
their concentration
gradient through the
phospholipid bilayer.
The passage of materials
is aided both by a
concentration gradient
and by a transport
protein.
Molecules again move
through a transport
protein, but now energy
must be expended to
move them against their
concentration gradient.
Figure 5.7

36. Transport Through the Plasma Membrane
Active transport is any movement of molecules or ions across a cell membrane that requires the expenditure of energy.
Passive transport is any movement of molecules or ions across a cell membrane that does not require the expenditure of energy.

37. Types of Passive Transport
There are two forms of passive transport: simple diffusion and facilitated diffusion.
For either form of transport to bring about a net movement of materials into or out of a cell, a concentration gradient must exist.

38. Types of Passive Transport
A concentration gradient is all that is required for simple diffusion to operate.
Facilitated diffusion, however, requires both a concentration gradient and a protein channel.

39. Facilitated Diffusion
In facilitated diffusion, transport proteins function as channels for larger hydrophilic substances—substances that, because of their size and electrical charge, cannot diffuse through the hydrophobic portion of the plasma membrane.

40. Facilitated Diffusion
glucose
cell exterior
plasma
membrane
cell interior
1. The transport
protein has a
binding site for
glucose that
is open to the
outside of the cell.
2. Glucose binds
to the binding
site.
3. This binding
causes the protein
to change shape,
exposing glucose
to the inside of
the cell.
4. Glucose passes
into the cell and
the protein
returns to its
original shape.
Figure 5.8

41. Active Transport
Cells cannot rely solely on passive transport to move substances across the plasma membrane.
A cell may need to maintain a greater concentration of a given substance on one side of its membrane.
Yet, passive transport equalizes concentrations of substances on both sides of the plasma membrane.

42. Active Transport To deal with such needs, cells use active transport.
Chemical pumps move compounds across the plasma membrane against their concentration gradients.

43. Active Transport
One example of such transport is the pumping of glucose into cells that line the small intestines.

44. 5.4 Moving Larger Substances In and Out

45. Moving Larger Substances In and Out
Larger materials are brought into the cell through endocytosis and moved out through exocytosis.

46. Exocytosis and Endocytosis
Both mechanisms employ vesicles, the membrane-lined enclosures that alternately bud off from membranes or fuse with them.

47. Exocytosis
In exocytosis, a transport vesicle moves from the interior of the cell to the plasma membrane and fuses with it, at which point the contents of the vesicle are released to the environment outside the cell.

48. Exocytosis (a) Exocytosis (b) Micrograph of exocytosis
extracellular fluid
protein
plasma membrane
transport vesicle
cytosol
Figure 5.10

49. Endocytosis
There are two principal forms of endocytosis: pinocytosis and phagocytosis.

50. Endocytosis
Pinocytosis is the movement of moderate-sized molecules into a cell by means of the creation of transport vesicles produced through an infolding or “invagination” of a portion of the plasma membrane.

51. Endocytosis
Phagocytosis is when certain cells use pseudopodia or “false feet” to surround and engulf whole cells, fragments of them, or other large organic materials.

52. Formation of a pinocytosis vesicle.1 2
captured
molecules
receptors
coated
pit
vesicle 3 4
In this form of pinocytosis, called clathrin-mediated endocytosis, cell-surface
receptors bind to individual molecules of the substance to be taken into the cell and
then move laterally across the plasma membrane to a pit, coated on its underside
with the protein clathrin, that will become a vesicle that moves into the cell.
Formation of a pinocytosis vesicle.
(b) Phagocytosis
bacterium
(or food particles)
pseudopodium
vesicle
In phagocytosis, food particles—or perhaps whole organisms—are taken in by means
of “false feet” or pseudopodia that surround the material. Pseudopodia then fuse
together, forming a vesicle that moves into the cell’s interior with its catch enclosed.
A human immune system cell called a
macrophage (colored blue) uses
phagocytosis to ingest an invading yeast
cell.
Figure 5.11

53. Endocytosis
In pinocytosis, materials are brought into the cell inside vesicles that bud off from the plasma membrane.