Without membranes, there would be no
cells, and thus no life
Why???
Cell Membrane functions
• Define cell boundaries (environment versus cytoplasm)
• Contain the cytoplasm
• Form a selectively permeable barrier that regulates what
enters and leaves the cell (ion pumps, channels)
• Allow “communication” (cell signaling) between the
external environment and the cytoplasm (integrins,
various receptors)
• Catalyze the production of intracellular signaling
molecules in response to extracellular signals. For
example, the enzyme adenylate cyclase produces a
small signaling molecule called cyclic AMP (cAMP)
Large and small substances move across cell
membranes in fundamentally different ways.
• Small moleculesA. Passive transport (simple and facilitated
diffusion)
B. Active transport
• Large molecules(endo and exocytosis)
A membrane’s molecular organization results in
selective permeability
• Membrane permeability is influenced by
size, chemical composition and
charge/polarity of the molecule trying to
cross the membrane
a. Membranes are more permeable to small molecules
than larger ones
b. Membranes are more permeable to hydrophobic
molecules
c. Membranes are most permeable to uncharged/nonpolar
molecules
Simple Diffusion
• Defined-the spontaneous net movement of
a substance from an area of its higher
concentration to an area of its lower
concentration until an equilibrium is
achieved
• Diffusion occurs because of the second
law of thermodynamics
LE 7-11a
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
Diffusion of one solute
Net diffusion
Equilibrium
LE 7-11b
Net diffusion
Net diffusion
Diffusion of two solutes
Net diffusion
Net diffusion
Equilibrium
Equilibrium
Osmosis
• Osmosis is a special case of diffusion
• It involves the diffusion of water across a
differentially permeable membrane
• Cell and tissues can gain or lose water by
osmosis depending on the type of
environment they exist in
Effect of solute on cell solutions
•
•
•
a.
b.
The solute concentration of the environment
determines whether a cell gains or loses water
The addition of solute lowers the concentration of
water (makes it less than 100%)
Three terms describe the tendency of one solution to
gain or lose water to another solution
Hypertonic (salty) solutions tend to gain water from
hypotonic solutions (less salty)
Isotonic solutions gain and lose water to one another
at the same rate.
LE 7-12
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
H2O
Selectively
permeable membrane: sugar molecules cannot pass
through pores, but
water molecules can
Osmosis
Same concentration
of sugar
LE 7-UN140
Environment
“Cell”
0.01 M sucrose
0.03 M sucrose
0.02 M glucose
0.01 M glucose
0.01 M fructose
Cell survival depends on balancing
water uptake and loss
• Plant and animal responses to being
placed in
• A. hypertonic solution
• B. hypotonic solutions
• C. Isotonic solutions
LE 7-13
Hypotonic solution
Isotonic solution
Hypertonic solution
Animal
cell
H2O
H2O
Turgid (normal)
H2O
H2O
Flaccid
H2O
Shriveled
Normal
Lysed
Plant
cell
H2O
H2O
H2O
Plasmolyzed
LE 7-14
Filling vacuole
Contracting vacuole
50 µm
50 µm
Traffic across membranes
•
•
•
•
•
•
•
•
•
•
A membrane’s molecular organization results in selective
permeability
Passive transport is diffusion across a membrane
Osmosis is the passive transport of water
Cell survival depends on balancing water uptake and loss
The solute concentration of the environment determines
whether a cell gains or loses water
Specific proteins facilitate the passive transport of selected
solutes (facilitated diffusion)
Active transport is the pumping of solutes against their gradients
Some ion pumps generate voltage across membranes
In cotransport, a membrane protein couples the transport of one
solute to another
Exocytosis and endocytosis transport large molecules
How do small molecules move
across cell membranes?
• Passive Transport is diffusion across a
membrane
A. Simple diffusion-membrane is permeable;
highlow concentration; no energy
required
B. Facilitated diffusion-diffusion-membrane
is impermeable (carrier molecule required)
highlow concentration; no energy
required
LE 7-17a
Passive transport
Diffusion
Facilitated diffusion
Facilitated Diffusion
• Specific proteins facilitate the passive
transport of selected solutes (facilitated
diffusion)
• Hydrophilic channels
• Rotating carriers (conformational changes)
LE 7-15a
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
LE 7-15b
Carrier protein
Solute
Active Transport
•
Active transport is the pumping of solutes
against their gradients
A. Membrane is impermeable (carrier required);
movement from low concentration to high
concentration; energy required
B. Sodium/potassium pump (neurons)
C. Plants often actively transport nutrients from soil
into the root cell (advantage of doing this?)
LE 7-17b
Active transport
ATP
Solution A (.2M glucose) is separated from solution B (.4 M
glucose) by a membrane which is impermeable to glucose .
Which solution is hypertonic?
1.
2.
3.
4.
A
B
Both A and B
Neither A nor B
58%
21%
12%
1
2
3
9%
4
Solution A (.2M glucose) is separated from solution B (.4 M
glucose) by a membrane which is impermeable to glucose .
Which solution will have a net gain of water?
1.
2.
3.
4.
A
B
Both A and B
Neither A nor B
47%
39%
11%
3%
1
2
3
4
In the Na+/K+ pump, the
ATPase enzyme is activated by
1.
2.
3.
4.
5.
Release of K+
Binding of K+
Binding of Na+
Release of Na+
phosphorylation
29%
29%
19%
16%
6%
1
2
3
4
5
Figure 8.15 The sodium-potassium pump: a specific case of active transport
LE 7-16
EXTRACELLULAR [Na+] high
FLUID
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLASM
[Na+] low
[K+] high
Na+
Cytoplasmic Na+ bonds to
the sodium-potassium pump
P
ATP
P
ADP
Na+ binding stimulates
phosphorylation by ATP.
Phosphorylation causes
the protein to change its
conformation, expelling Na+
to the outside.
Loss of the phosphate
restores the protein’s
original conformation.
K+ is released and Na+
sites are receptive again;
the cycle repeats.
P
P
Extracellular K+ binds
to the protein, triggering
release of the phosphate
group.
LE 7-18
–
–
ATP
EXTRACELLULAR
FLUID
+
+
H+
H+
Proton pump
H+
–
+
H+
H+
–
+
CYTOPLASM
–
H+
+
Co-transport
•
•
In co-transport, a membrane protein
couples the transport of one solute to
another
In plants, transport of sucrose into cells
is coupled to the active transport of H+
ions out of the cell
LE 7-19
–
+
H+
ATP
H+
–
+
H+
Proton pump
H+
–
+
H+
–
+
H+
Sucrose-H+
cotransporter
Diffusion
of H+
H+
–
–
+
+
Sucrose
Movement of large molecules/cells
into and out of cells
•
Exocytosis and endocytosis transport
large molecules into and out of cells
• Exocytosis-out
• Endocytosis-in
a. Pinocytosis
b. Phagosytosis
c. Receptor-mediated endocytosis
LE 7-20b
PINOCYTOSIS
0.5 µm
Plasma
membrane
Pinocytosis
vesicles forming
(arrows) in a cell
lining a small
blood vessel
(TEM).
Vesicle
LE 7-20a
PHAGOCYTOSIS
EXTRACELLULAR
FLUID
CYTOPLASM
1 µm
Pseudopodium
Pseudopodium
of amoeba
“Food” or
other particle
Food
vacuole
Bacterium
Food vacuole
An amoeba engulfing a bacterium via
phagocytosis (TEM)
LE 7-20c
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Coated
pit
Ligand
A coated pit
and a coated
vesicle formed
during
receptormediated
endocytosis
(TEMs).
Coat
protein
Plasma
membrane
0.25 µm
Familial Hypercholesterolemia
• Symptoms/consequences
• Causes
• Genetics
Membrane Structure and Function
•
•
•
•
Membrane structure
Membrane models have evolved to fit
new data (science as a process)
A membrane is a fluid mosaic of lipids,
proteins and carbohydrates
There is a lot of experimental evidence
that favors the fluid mosaic model of
membrane structure.
History of Membrane Models
• Overton (1875) –Membranes contain lipids
(like dissolve like)
• Langmuir(1917)-Membranes have
amphipathic lipids (phospholipids)
• Gorter and Grendel(1925)-Phospholipid
bilayer
• Davson and Danielli (1935)-Phospholipids
and proteins (sandwich)
Figure 8.1 Artificial membranes (cross sections)
LE 7-2
WATER
Hydrophilic
head
Hydrophobic
tail
WATER
Figure 8.2 Two generations of membrane models
History of Membrane Models
(continued)
• Robertson (1950)-Electron micrographs
showing “trilaminate” structure
• Problems with current models
• Singer and Nicholson (1975)-Fluid mosaic
model
Figure 8.19 The three types of endocytosis in animal cells
Fluid Mosaic Model
• Consistent with all observations of
membrane properties to date
Figure 7-01
LE 7-5
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)
Movement of phospholipids
Viscous
Fluid
Unsaturated hydrocarbon
tails with kinks
Membrane fluidity
Saturated hydrocarbon tails
Cholesterol
Cholesterol within the animal cell membrane
In sucrose co-transport in plants, the active
transport of sucrose into plant cells is couple to
1. Facilitated
diffusion
2. ATP hydrolysis
3. A proton pump
4. 1 and 3
25%
1
25%
25%
2
3
25%
4
This model of membrane structure consisted of a
phospholipid bilayer sandwiched between 2 layers of
protein:
1. Gorter and
Grendle
2. Davson and
Danielli
3. Singer and
Nicholson
4. Overton
5. Robertson
20%
1
20%
20%
2
3
20%
4
20%
5
An increased synthesis of phospholipids containing
unsaturated fatty acids may be an adaptation by plants to:
1. Predators
2. Decreasing sunlight
3. Hypertonic
environments
4. Cooling temperatures
5. Warming
temperatures
20%
1
20%
20%
2
3
20%
4
20%
5
LE 7-4
Extracellular
layer
Proteins
Knife
Plasma
membrane
Extracellular layer
Cytoplasmic
layer
Cytoplasmic layer
LE 7-6
Membrane proteins
Mouse cell
Human cell
Hybrid cell
Mixed
proteins
after
1 hour
Figure 8.9 Some functions of membrane proteins
LE 7-8
EXTRACELLULAR
SIDE
N-terminus
C-terminus
a Helix
CYTOPLASMIC
SIDE