Transcript video slide

Chapter 7
Membrane Structure and
Function
{
7.1 Cellular membranes are fluid
mosaics of lipids and proteins
Overview: Life at the Edge

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The plasma membrane is a selectively permeable boundary,
allowing some substances to cross it more easily than others
Phospholipids have hydrophobic and hydrophilic regions
The fluid mosaic model states that a membrane is a fluid structure
with a “mosaic” of various proteins embedded in it

Main components:
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Carbohydrates
Integral proteins
Phospholipids
Peripheral proteins
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-2
Figure 7.2 Phospholipid bilayer
(cross section)
Hydrophilic
head
WATER
Hydrophobic
tail
WATER
Fig. 7-3
In 1972, J. Singer and G. Nicolson proposed that
the membrane is a mosaic of proteins dispersed
within the bilayer, with only the hydrophilic
regions exposed to water
Phospholipid
bilayer
Hydrophobic regions
of protein
Hydrophilic
regions of protein
Fig. 7-4
The fluid mosaic model is supported by freeze-fracture
studies, (a specialized preparation technique that splits
a membrane along the middle of the phospholipid
bilayer)
TECHNIQUE
RESULTS
Extracellular
layer
Knife
Plasma membrane
Proteins
Inside of extracellular layer
Cytoplasmic layer
Inside of cytoplasmic layer
Fig. 7-5
• Phospholipids in
the plasma
membrane can
move within the
bilayer
• Most of the
lipids, and some
proteins, drift
laterally
• Rarely does a
molecule flipflop transversely
across the
membrane
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)
(a) Movement of phospholipids
Fluid
Unsaturated hydrocarbon
tails with kinks
Viscous
Saturated hydrocarbon tails
(b) Membrane fluidity
Cholesterol
(c) Cholesterol within the animal cell membrane
As temperatures cool, membranes switch
from a fluid state to a solid state
 The temperature at which a membrane
solidifies depends on the types of lipids
 Membranes rich in unsaturated fatty
acids are more fluid that those rich in
saturated fatty acids
 Membranes must be fluid to work
properly; they are usually about as fluid as
salad oil

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The steroid cholesterol has different effects on
membrane fluidity at different temperatures
 At warm temperatures (such as 37°C), cholesterol
restrains movement of phospholipids
 At cool temperatures, it maintains fluidity by
preventing tight packing

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Membrane Proteins and Their Functions
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A membrane is a collage of different proteins embedded in the fluid
matrix of the lipid bilayer
Proteins determine most of the membrane’s specific functions
Peripheral proteins-bound to the surface of the membrane
Integral proteins-completely embedded in membrane

Transmembrane proteins: span membrane
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-7
Figure 7.7 The detailed structure of an animal cell’s plasma membrane, in a cutaway view
Fibers of
extracellular
matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
http://www.youtube.com/
watch?v=Rl5EmUQdkuI
Fig. 7-8
The hydrophobic regions of an integral protein consist of one or more stretches of
nonpolar amino acids, often coiled into alpha helices
N-terminus
C-terminus
 Helix
EXTRACELLULAR
SIDE
CYTOPLASMIC
SIDE
Fig. 7-9
Signaling molecule
Enzymes
Six major
functions of
membrane
proteins
ATP
(a) Transport
Receptor
Signal transduction
(b) Enzymatic activity
(c) Signal transduction
(e) Intercellular joining
(f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)
Glycoprotein
(d) Cell-cell recognition
The Role of Membrane Carbohydrates in Cell-Cell
Recognition
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Cells recognize each other by binding to surface molecules, often
carbohydrates, on the plasma membrane
Membrane carbohydrates may be covalently bonded to lipids
(forming glycolipids) or more commonly to proteins (forming
glycoproteins)
Cell surface carbohydrates vary from species to species

Why blood transfusions must be type-specific
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Chapter 7
Membrane Structure and
Function
{
Concept 7.2: Membrane structure
results in selective permeability
Membranes are selectively permeable
A cell must exchange materials with its
surroundings, a process controlled by the plasma
membrane
 Plasma membranes are selectively permeable,
regulating the cell’s molecular traffic

Hydrophobic (nonpolar) molecules, such as
hydrocarbons, pass through the membrane rapidly
 Polar molecules, (sugars), do not cross the membrane
easily

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Transport Proteins

Transport proteins allow passage of
hydrophilic substances across the membrane

Channel proteins are transport proteins that act
like tunnels


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aquaporins facilitate the passage of water
 3 billion water molecules/aquaporin/second!!!!
Carrier proteins, bind to molecules and change
shape to shuttle them across the membrane
A transport protein is specific for the
substance it moves
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Chapter 7
Membrane Structure and
Function
{
Concept 7.3: Passive transport is diffusion
of a substance across a membrane with no
energy investment
Diffusion
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Diffusion is the tendency for molecules to spread out evenly into
the available space
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At dynamic equilibrium, as many molecules cross one way as cross
in the other direction
Substances diffuse down their concentration gradient

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molecules may exhibit a net movement in one direction
Does not require energy/work
The diffusion of a substance across a biological membrane is
passive transport because it requires no energy from the cell to
make it happen
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-11
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
Net diffusion
Equilibrium
(a) Diffusion of one solute
Net diffusion
Net diffusion
(b) Diffusion of two solutes
Net diffusion
Net diffusion
Equilibrium
Equilibrium
Effects of Osmosis on Water Balance
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Osmosis is the diffusion of water across a selectively permeable
membrane
Water diffuses across a membrane from the region of lower solute
concentration to the region of higher solute concentration
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
http://www.youtube.com/
watch?v=VY0mZUDvbH4
Fig. 7-12
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
H2O
Selectively
permeable
membrane
Osmosis is the diffusion of
water across a selectively
permeable membrane
Water diffuses across a
membrane from the
region of lower solute
concentration to the
region of higher solute
concentration
Osmosis
Same concentration
of sugar
Water Balance of Cells Without Walls
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Isotonic solution: Solute concentration is the same as that inside the
cell; no net water movement across the plasma membrane

Hypertonic solution: Solute concentration is greater than that inside
the cell; cell loses water
Hypotonic solution: Solute concentration is less than that inside the
cell; cell gains water

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-13
Hypotonic solution
H2O
Isotonic solution
H2O
H2O
Hypertonic solution
H2O
(a) Animal
cell
Lysed
H2O
Normal
H2O
Shriveled
H2O
H2O
(b) Plant
cell
Turgid (normal)
Flaccid
Plasmolyzed

Osmoregulation, the control of water balance, is a necessary
adaptation for life in such environments
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Facilitated Diffusion: Passive
Transport Aided by Proteins
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In facilitated diffusion, transport proteins speed the passive
movement of molecules (such as ions and hydrophilic substantces)
across the plasma membrane
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Channel proteins provide corridors and include:
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Aquaporins, for facilitated diffusion of water
Ion channels that open or close in response to a stimulus (gated
channels)
Facilitated diffusion is still passive because the solute moves down
its concentration gradient
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-15
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
(a) A channel protein
Carrier protein
(b) A carrier protein
Solute
The Need for Energy in Active Transport
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Active transport moves substances against their concentration
gradient (from the side where they are less concentrated to the side
where they are more concentrated).
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requires energy, (ATP)
Active transport is performed by specific proteins embedded in the
membranes
Active transport allows cells to maintain concentration gradients
that differ from their surroundings
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-16-1
The sodium-potassium pump is one type of active
transport system
EXTRACELLULAR
FLUID
[Na+] high
[K+] low
Na+
Na+
CYTOPLASM
Na+
[Na+] low
[K+] high
1 Cytoplasmic Na+ binds to
the sodium-potassium pump.
Fig. 7-16-2
The sodium-potassium pump is one type of active
transport system
Na+
Na+
Na+
P
ADP
ATP
2 Na+ binding stimulates
phosphorylation by ATP.
Fig. 7-16-3
The sodium-potassium pump is one type of active
transport system
Na+
Na+
Na+
P
3 Phosphorylation causes
the protein to change its
shape. Na+ is expelled to
the outside.
Fig. 7-16-4
The sodium-potassium pump is one type of active
transport system
P
P
4 K+ binds on the
extracellular side and
triggers release of the
phosphate group.
Fig. 7-16-5
The sodium-potassium pump is one type of active
transport system
5 Loss of the phosphate
restores the protein’s original
shape.
Fig. 7-16-6
The sodium-potassium pump is one type of active
transport system
K+ is released, and the
cycle repeats.
Fig. 7-16-7
EXTRACELLULAR
FLUID
Na+
[Na+] high
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLASM
1
Na+
[Na+] low
[K+] high
P
ADP
2
ATP
P
3
P
P
6
5
4
http://www.youtube.com/
watch?v=P-imDC1txWw
Fig. 7-17
Passive transport
Active transport
ATP
Diffusion
Facilitated diffusion
How Ion Pumps Maintain Membrane Potential
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Membrane potential is the voltage difference across a membrane
Voltage is created by differences in the distribution of positive and
negative ions
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The inside of the cell is negatively (-) charged
A positively charged ion (like Na+) is attracted to the negative charges
inside the cell
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Two combined forces, collectively called the electrochemical
gradient, drive the diffusion of ions across a membrane:
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A chemical force (the ion’s concentration gradient)
An electrical force (the effect of the membrane potential on the ion’s
movement)
An electrogenic pump is a transport protein that generates voltage
across a membrane

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In animal cells: Na-K pump
In plants/fungi/bacteria cells: proton pump
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Figure 7.18 An electrogenic pump
–
ATP
EXTRACELLULAR
FLUID
+
–
+
H+
H+
Proton pump
H+
–
+
H+
H+
–
+
CYTOPLASM
–
H+
+
Fig. 7-19
–
+
H+
ATP
–
H+
+
H+
Proton pump
H+
–
H+
+
–
H+
+
H+ Diffusion
of H+
Sucrose-H+
cotransporter
H+
Sucrose
–
Cotransport occurs when
– active
transport of a solute indirectly drives
transport of another solute
+
+
Sucrose
Chapter 7
Membrane Structure and
Function
{
Concept 7.5: Bulk transport across
the plasma membrane occurs by
exocytosis and endocytosis
Exocytosis & Endocytosis
Large molecules, such as polysaccharides and proteins,
cross the membrane in bulk via vesicles
 Bulk transport requires energy
 In exocytosis, transport vesicles migrate to the membrane,
fuse with it, and release their contents
 transport vesicles migrate to the membrane, fuse with it,
and release their contents, (used by secretory cells)
 In endocytosis, the cell takes in macromolecules by
forming vesicles from the plasma membrane (3 types)
 Phagocytosis (“cellular eating”)
 Pinocytosis (“cellular drinking”)
 Receptor-mediated endocytosis

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-20
In phagocytosis a
cell engulfs a
particle in a vacuole
then fuses with a
lysosome to digest
the particle
PHAGOCYTOSIS
1 µm
CYTOPLASM
EXTRACELLULAR
FLUID
Pseudopodium
Pseudopodium
of amoeba
“Food”or
other particle
Bacterium
Food
vacuole
Food vacuole
An amoeba engulfing a bacterium
via phagocytosis (TEM)
PINOCYTOSIS
0.5 µm
Plasma
membrane
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM)
Vesicle
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
In receptor-mediated
endocytosis, binding of
ligands (molecule that
binds to receptor) to
receptors triggers
vesicle formation
Coated
vesicle
Coated
pit
Ligand
A coated pit
and a coated
vesicle formed
during
receptormediated
endocytosis
(TEMs)
Coat
protein
Plasma
membrane
0.25 µm
In pinocytosis,
molecules are taken
up when
extracellular fluid is
“gulped” into tiny
vesicles
http://www.youtube.com/
watch?v=W6rnhiMxtKU
http://www.youtube.com/
watch?v=U9pvm_4-bHg