1. Membrane Structure
and Function

2. Plasma MembranePlasma Membrane
Is the boundaryboundary that separates
the living cell from its nonliving
surroundings
Selectively PermeableSelectively Permeable (chooses
what may cross the membrane)
Fluid mosaicFluid mosaic of lipids and proteins
Lipid bilayerbilayer
Contains embeddedContains embedded proteinsproteins

4. PhospholipidsPhospholipids
Are the most abundantmost abundant lipid in the
plasma membrane
Are amphipathicamphipathic, containing both
hydrophilic (head) and hydrophobic
regions (tails)
HeadHead composed of phosphate group
attached to one carbon of glycerol
is hydrophilichydrophilic
Two fatty acid tailstails are hydrophobichydrophobic

5. Hydrophilic
head
HydrophobicHydrophobic
tailtail
WATER
WATER
Phospholipid Bilayer

6. Singer and NicolsonSinger and Nicolson
In 19721972, Singer and Nicolson, Proposed
that membrane proteinsmembrane proteins are dispersed
and individually inserted into theinserted into the
phospholipid bilayerphospholipid bilayer of the plasmaof the plasma
membranemembrane
Phospholipid
bilayer
Hydrophilic region
of protein
Hydrophobic region of protein

7. Fluid Mosaic ModelFluid Mosaic Model
A membrane is a fluid structurefluid structure
with a “mosaic” of various proteins
embedded in it when viewed from
the top
PhospholipidsPhospholipids can move laterallylaterally a
small amount and can “flex” their
tails
Membrane proteinsMembrane proteins also move side
to side or lateralllaterally making the
membrane fluid

8. Freeze-fractureFreeze-fracture studies of the plasma
membrane support the fluid mosaic model
of membrane structure
A cell is frozen and fractured with a knife. The fracture planefracture plane
often follows the hydrophobic interior of a membraneoften follows the hydrophobic interior of a membrane, splitting
the phospholipid bilayer into two separated layerstwo separated layers. The
membrane proteins go wholly with one of the layersmembrane proteins go wholly with one of the layers.

9. The Fluidity of Membranes
Phospholipids in the plasma membrane
Can move within the bilayer two ways
Lateral movement
(~107
times per second)
Flip-flop
(~ once per month)

10. The type of hydrocarbon tailstype of hydrocarbon tails in
phospholipids Affects the fluidity of
the plasma membrane
Fluid Viscous
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
Carbon tails
The Fluidity of Membranes

11. The Fluidity of Membranes
The steroid cholesterolsteroid cholesterol Has different
effects on membrane fluidity at
different temperatures
7.5
Cholesterol

12. Membrane Proteins and Their Functions
A membrane is a collage of differentcollage of different
proteins embeddedproteins embedded in the fluid matrix
of the lipid bilayer
Fibers of
extracellular
matrix (ECM)

13. Types of Membrane Proteins
Integral proteinsIntegral proteins
Penetrate the hydrophobic core of the lipid
bilayer
Are often transmembrane proteinstransmembrane proteins,
completely spanning the membrane
EXTRACELLULAR
SIDE

14. Types of Membrane Proteins
Peripheral proteinsPeripheral proteins
Are appendages loosely bound to the
surface of the membrane

15. Six Major Functions of Membrane
Proteins
Figure 7.9
Transport. (left) A protein that spans the membrane
may provide a hydrophilic channel across the
membrane that is selective for a particular solute.
(right) Other transport proteins shuttle a substance
from one side to the other by changing shape. Some
of these proteins hydrolyze ATP as an energy source
to actively pump substances across the membrane.
Enzymatic activity. A protein built into the membrane
may be an enzyme with its active site exposed to
substances in the adjacent solution. In some cases,
several enzymes in a membrane are organized as
a team that carries out sequential steps of a
metabolic pathway.
Signal transduction. A membrane protein may have
a binding site with a specific shape that fits the shape
of a chemical messenger, such as a hormone. The
external messenger (signal) may cause a
conformational change in the protein (receptor) that
relays the message to the inside of the cell.
(a)
(b)
(c)
ATP
Enzymes
Signal
Receptor

16. Cell-cell recognition. Some glyco-proteins serve as
identification tags that are specifically recognized
by other cells.
Intercellular joining. Membrane proteins of adjacent cells
may hook together in various kinds of junctions, such as
gap junctions or tight junctions
Attachment to the cytoskeleton and extracellular matrix
(ECM). Microfilaments or other elements of the
cytoskeleton may be bonded to membrane proteins,
a function that helps maintain cell shape and stabilizes
the location of certain membrane proteins. Proteins that
adhere to the ECM can coordinate extracellular and
intracellular changes
(d)
(e)
(f)
Glyco-
protein
Six Major Functions of Membrane Proteins

17. The Role of Membrane Carbohydrates
in Cell-Cell Recognition
Cell-cell recognitionCell-cell recognition
IIs a cell’s ability to distinguish one
type of neighboring cell from another
Membrane carbohydratesMembrane carbohydrates
Interact with the surface molecules
of other cells, facilitating cell-cell
recognition

18. Synthesis and Sidedness of Membranes
Membranes have distinct inside anddistinct inside and
outside facesoutside faces
This affects the movementaffects the movement of
proteins synthesizedproteins synthesized in the
endomembrane systemendomembrane system (Golgi and(Golgi and
ER)ER)

19. Synthesis and Sidedness of Membranes
Membrane proteins and lipidsMembrane proteins and lipids are made in
the ERER andand Golgi apparatusGolgi apparatus
ER

20. Membrane Permeability
Membrane structurestructure results in
selective permeabilityselective permeability
A cell must exchange materialscell must exchange materials
with its surroundingswith its surroundings, a process
controlled by the plasma
membrane

21. Permeability of the Lipid Bilayer
Hydrophobic moleculesHydrophobic molecules
Are lipid solublelipid soluble and can pass
through the membrane rapidlyrapidly
Polar moleculesPolar molecules
Do NOT cross the membrane
rapidlyrapidly

22. Transport Proteins
Transport proteinsTransport proteins
Allow passage of hydrophilichydrophilic
substancessubstances across the
membrane

23. Passive Transport
Passive transportPassive transport is diffusion of a
substance across a membrane
with no energyno energy investment
COCO22, H, H22O, and OO, and O22 easily diffuse
across plasma membranes
Diffusion of water is known as
OsmosisOsmosis

24. Simple Diffusion
DiffusionDiffusion
Is the tendency for molecules of any substance to
spread out evenlyspread out evenly into the available space
Move from high to low concentrationhigh to low concentration
DownDown the concentration gradient

25. Effects of Osmosis on Water Balance
OsmosisOsmosis
Is the movement of water
across a semipermeable
membrane
Is affectedaffected by theby the
concentration gradient ofconcentration gradient of
dissolved substancesdissolved substances called thecalled the
solution’ssolution’s tonicitytonicity

26. Water Balance of Cells Without Walls
Tonicity
Is the ability of a solution to
cause a cell to gain or lose
water
Has a great impact on cellsimpact on cells
without wallswithout walls

27. Three States of Tonicity

28. Isotonic Solutions
If a solution is isotonicisotonic
The concentration of solutesconcentration of solutes is the
samesame as it is inside the cell
There will be NO NETNO NET movement of
WATER

29. Hypertonic Solution
If a solution is hypertonichypertonic
The concentration of solutesconcentration of solutes is greatergreater
than it is inside the cell
The cell will lose waterlose water (PLASMOLYSIS)(PLASMOLYSIS)

30. Hypotonic Solutions
If a solution is hypotonichypotonic
The concentration of solutesconcentration of solutes is lesless
than it is inside the cell
The cell will gain watergain water

31. Water Balance in Cells Without
Walls
Animal cell. An animal cell fares best in an isotonicisotonic
environment unless it has special adaptations to offset
the osmotic uptake or loss of water.

32. Water Balance of Cells with Walls
Cell WallsCell Walls
Help maintain water balance
Turgor pressureTurgor pressure
Is the pressure of water inside a plant cell
pushing outward against the cell membrane
If a plant cell is turgidturgid
It is in a hypotonichypotonic environment
It is very firm,firm, a healthy state in mosta healthy state in most
plantsplants
If a plant cell is flaccidflaccid
It is in an isotonic or hypertonicisotonic or hypertonic environment

33. Water Balance in Cells with Walls
Plant cell. Plant cells are turgid (firmturgid (firm) and generally
healthiest in a hypotonic environmenthypotonic environment, where the uptake of
water is eventually balanced by the elastic wall pushing back
on the cell.

34. How Will Water Move Across
Semi-Permeable Membrane?
Solution A has 100 molecules of
glucose per ml
Solution B has 100 molecules of
fructose per ml
How will the water molecules move?How will the water molecules move?
There will be no net movement of waterno net movement of water since the
concentration of solute in each solution is equal

35. How Will Water Move Across
Semi-Permeable Membrane?
Solution A has 100 molecules of
glucose per ml
Solution B has 75 molecules of
fructose per ml
How will the water molecules move?How will the water molecules move?
There will be a net movement of water from
Solution B to Solution A until both solutions have
equal concentrations of solute

36. How Will Water Move Across
Semi-Permeable Membrane?
Solution A has 100 molecules of
glucose per ml
Solution B has 100 molecules of
NaCl per ml
How will the water moleculesHow will the water molecules
move?move?
Each molecule of NaCl will dissociate to form a Na+ ion and a
Cl- ion, making the final concentration of solutes 200 molecules
per mil. Therefore, there will be a net movement of water from
Solution A to Solution B until both solutions have equal
concentrations of solute

37. Facilitated Diffusion
Facilitated diffusionFacilitated diffusion
Is a type of PassivePassive Transport Aided
by ProteinsProteins
In facilitated diffusion
Transport proteinsTransport proteins speed the
movement of molecules across the
plasma membrane

38. Facilitated Diffusion & Proteins
Channel proteinsChannel proteins
Provide corridors that allow a specific
molecule or ion to cross the membrane
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
A channel protein (purple) has a channel through which
water molecules or a specific solute can pass.

39. Facilitated Diffusion & Proteins
Carrier proteinsCarrier proteins
Undergo a subtle change in shape that
translocates the solute-binding site
across the membrane
A carrier proteincarrier protein alternates between two conformationsalternates between two conformations, moving a solute
across the membrane as the shape of the protein changes. The protein
can transport the solute in either directioncan transport the solute in either direction, with the net movement being
down the concentration gradientdown the concentration gradient of the solute.

40. Active Transport
Active transport
Uses energyUses energy to move solutes
againstagainst their concentration
gradients
Requires energy, usually in
the form of ATPATP

41. The sodium-potassium pumpsodium-potassium pump
Is one type of active transport system
Active Transport
P
P i
EXTRACELLULAR
FLUID
Na+ binding stimulates
phosphorylation by ATP.
2
Na+
Cytoplasmic Na+
binds to
the sodium-potassium pump.
1
K+
is released and Na+
sites are receptive again;
the cycle repeats.
3
Phosphorylation causes the
protein to change its
conformation, expelling Na+
to
the outside.
4
Extracellular K+
binds to the
protein, triggering release of the
Phosphate group.
6
Loss of the phosphate
restores the protein’s
original conformation.
5
CYTOPLASM
[Na+
] low
[K+
] high
Na+
Na+
Na+
Na+
Na+
P ATP
Na+
Na+
Na+
P
ADP
K+
K+
K+
K+ K+
K+
[Na+
] high
[K+
] low

42. Comparison of Passive & Active Transport
Passive transport. Substances diffuse spontaneously
down their concentration gradients, crossing a
membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport
proteins in the membrane.
Active transport. Some transport proteins
act as pumps, moving substances across a
membrane against their concentration
gradients. Energy for this work is usually
supplied by ATP.
Diffusion. Hydrophobic
molecules and (at a slow
rate) very small uncharged
polar molecules can diffuse through the lipid
bilayer.
Facilitated diffusion. Many hydrophilic
substances diffuse through membranes with
the assistance of transport proteins,
either channel or carrier proteins.
ATP

43. Maintenance of Membrane Potential by Ion
Pumps
Membrane potentialMembrane potential
Is the voltage difference across a
membrane
An electrochemical gradientelectrochemical gradient
Is caused by the concentration
electrical gradient of ions across a
membrane
An electrogenic pumpelectrogenic pump
Is a transport protein that generates
the voltage across a membrane

44. Proton Pump
EXTRACELLULAR
FLUID
+
H+
H+
H+
H+
H+
H+
Proton pump
ATP
CYTOPLASM
+
+
+
+
–
–
–
–
–
+

45. Cotransport
CotransportCotransport
Occurs when active transportactive transport of a
specific solute indirectly drives the
active transport of another solute
Involves transport by a membranemembrane
proteinprotein
Driven by a concentration gradientconcentration gradient

46. Example of Cotransport
Cotransport: active transport driven bydriven by
a concentration gradienta concentration gradient

47. Bulk Transport
Bulk transport across the plasma
membrane occurs by exocytosisexocytosis
and endocytosisand endocytosis
Large proteins
Cross the membrane by
different mechanisms

48. Exocytosis & Endocytosis
In exocytosisexocytosis
Transport vesiclesTransport vesicles migrate to
the plasma membrane, fuse
with it, and release their
contents
In endocytosisendocytosis
The cell takes in macromolecules
by forming new vesicles fromforming new vesicles from
the plasma membranethe plasma membrane

49. Endocytosis

50. Exocytosis

51. In phagocytosisphagocytosis, a cell
engulfs a particle byengulfs a particle by
Wrapping pseudopodiaWrapping pseudopodia
around it and packaging
it within a membrane-
enclosed sac large
enough to be classified
as a vacuolevacuole. The
particle is digested after
the vacuole fuses with a
lysosome containing
hydrolytic enzymes.
Three Types of Endocytosis
PHAGOCYTOSIS
In pinocytosispinocytosis, the cell
““gulps” droplets ofgulps” droplets of
extracellular fluidextracellular fluid into tiny
vesicles. It is not the fluid
itself that is needed by the
cell, but the molecules
dissolved in the droplet.
Because any and all
included solutes are taken
into the cell, pinocytosis
is nonspecific in the
substances it transports.

52. 0.25 µm
RECEPTOR-MEDIATED ENDOCYTOSIS
Receptor
Ligand
Coat protein
Coated
pit
Coated
vesicle
A coated pit
and a coated
vesicle
formed
during
receptor-
mediated
endocytosis
(TEMs).
Plasma
membrane
Coat
protein
Receptor-mediated endocytosis enables the
cell to acquire bulk quantities of specific
substances, even though those substances
may not be very concentrated in the
extracellular fluid. Embedded in the
membrane are proteins with
specific receptor sites exposed to
the extracellular fluid. The receptor
proteins are usually already clustered
in regions of the membrane called coated
pits, which are lined on their cytoplasmic
side by a fuzzy layer of coat proteins.
Extracellular substances (ligands) bind
to these receptors. When binding occurs,
the coated pit forms a vesicle containing the
ligand molecules. Notice that there are
relatively more bound molecules (purple)
inside the vesicle, other molecules
(green) are also present. After this ingested
material is liberated from the vesicle, the
receptors are recycled to the plasma
membrane by the same vesicle.