PRESENTATIONS
For 1/21/12
LECTURE CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
Chapter 7
Overview: Life at the Edge
The plasma membrane is the boundary that
separates the living cell from its surroundings
The plasma membrane exhibits selective
permeability, allowing some substances to
cross it more easily than others
The fluid mosaic model of membrane structure
Membrane Structure and
Function
States that a membrane is a fluid structure with a
mosaic of various proteins embedded in a
phospholipid bilayer
Lectures by
Erin Barley
Kathleen Fitzpatrick
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The Fluid Mosaic Model
Phospholipids are the most abundant lipid
in the plasma membrane
Phospholipids are amphipathic
molecules, containing hydrophobic and
hydrophilic region
The Fluidity of Membranes
Phospholipids can move laterally within the
bilayer but cannot easily flip sides
WATER
Hydrophilic
head
Hydrophobic
tail
Figure 7.2
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WATER
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The Fluidity of Membranes
Membrane proteins drift laterally
Movement of Proteins in the Bilayer
Proteins in the plasma membrane can drift laterally
within the bilayer
Unless anchored to the cytoskeleton
EXPERIMENT
Researchers labeled the plasma membrane proteins of a mouse
cell and a human cell with two different markers and fused the cells.
RESULTS
Membrane proteins
+
Mouse cell
Human cell
Hybrid cell
Figure 7.7
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Membrane Fluidity
Membranes must be fluid to work properly
usually about as fluid as salad oil
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
Mixed
proteins
after
1 hour
CONCLUSION
Membrane proteins move within the plane
of the plasma membrane.
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Cholesterol in the Membrane
The steroid cholesterol
Present in animal membranes
Acts as a "temperature buffer" of membrane fluidity
At warm temperatures (such as 37C), restrains
movement of phospholipids
At cool temperatures, it maintains fluidity by
preventing tight packing
Membranes rich in unsaturated fatty acids are
more fluid than those rich in saturated fatty acids
Cholesterol
(c) Cholesterol within the animal cell membrane
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Figure 7.8 b
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Figure 7.9
EXTRACELLULAR
SIDE
Peripheral proteins are bound to the surface
of the membrane
Integral proteins penetrate the hydrophobic
core
Integral proteins that span the membrane are called
transmembrane proteins
The hydrophobic regions of an integral protein
consist of one or more stretches of nonpolar amino
acids, often coiled into alpha helices
N-terminus
helix
C-terminus
CYTOPLASMIC
SIDE
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Peripheral Membrane Proteins
Six Major Functions of Membrane Proteins
Peripheral proteins
Are bound to the surface of the membrane
EXTRACELLULAR
SIDE OF
MEMBRANE
Fibers of
extracellular
matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
CYTOPLASMIC SIDE
OF MEMBRANE
Figure 7.5
Microfilaments
of cytoskeleton
Cholesterol
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Peripheral
protein
Integral
protein
Figure 7.10
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Figure 7.11
The Role of Membrane Carbohydrates in
Cell-Cell Recognition
HIV
Cells recognize each other by binding to surface
molecules
That usually carry attached carbohydrates
Membrane carbohydrates covalently bonded to
Lipids = glycolipids
Proteins = glycoproteins
Carbohydrates on the external side of the
plasma membrane vary among species,
individuals, and even cell types in an individual
Receptor
(CD4)
Co-receptor
(CCR5)
Receptor (CD4)
but no CCR5
Plasma
membrane
HIV can infect a cell that
has CCR5 on its surface,
as in most people.
HIV cannot infect a cell lacking
CCR5 on its surface, as in
resistant individuals.
Important for recognition of a cell
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Figure 7.12
Transmembrane
glycoproteins
Synthesis and Sidedness of
Membranes
Membranes have
distinct inside and
outside faces
Asymmetrical
distribution of proteins,
lipids, and associated
carbohydrates in the
plasma membrane
determined when the
membrane is built by the
ER and Golgi apparatus
Secretory
protein
Golgi
apparatus
Vesicle
ER
ER lumen
Glycolipid
Plasma membrane:
Cytoplasmic face
Extracellular face
Transmembrane
glycoprotein
Secreted
protein
Membrane
glycolipid
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Concept 7.2: Membrane structure results
in selective permeability
A cell must exchange materials with its
surroundings, a process controlled by the
plasma membrane
The Permeability of the Lipid Bilayer
Hydrophobic (nonpolar) molecules
can dissolve in the lipid bilayer and pass through the
membrane rapidly
CO2, O2, hydrocarbons
Think about why CO2 and O2 are hydrophobic
Polar molecules do not cross the membrane
easily
Sugars, carbohydrates, ions, water
These require transport proteins to enter or leave the
cell
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Transport Proteins
Transport proteins allow passage of hydrophilic
substances across the membrane
Some transport proteins, called channel proteins,
have a hydrophilic channel that certain molecules or
ions can use as a tunnel
Channel proteins called aquaporins facilitate the
passage of water
Other transport proteins, called carrier
proteins, bind to molecules and change shape
to shuttle them across the membrane
A transport protein is specific for the substance
it moves
From Wikipedia:
The 2003 Nobel Prize in Chemistry was awarded jointly to
Peter Agre for the discovery of aquaporins, and Roderick
MacKinnon for his work on the structure and mechanism of
potassium channels
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Figure 7.13
Molecules of dye
Concept 7.3: Passive transport is diffusion of
a substance across a membrane with no
energy investment
Diffusion is the tendency for molecules to spread
out evenly into the available space
Although each molecule moves randomly, diffusion
of a population of molecules may be directional
dynamic At equilibrium, as many molecules cross
the membrane in one direction as in the other
Animation: Membrane Selectivity
Animation: Diffusion
Membrane (cross section)
WATER
Net diffusion
Net diffusion
Equilibrium
(a) Diffusion of one solute
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
(b) Diffusion of two solutes
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Effects of Osmosis on Water Balance
Substances diffuse down their concentration
gradient
The diffusion of a substance across a biological
membrane is passive transport because no
energy is expended by the cell to make it
happen
Osmosis is the diffusion of water across a
selectively permeable membrane
A membrane that lets water across but not some
solutes
Water diffuses across a membrane from the
region of lower solute concentration to the
region of higher solute concentration until the
solute concentration is equal on both sides
Because water molecules are clustered around the
dissolved solute molecules, and are not free
Free water equilibrates
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Figure 7.14
Lower
concentration
of solute (sugar)
Higher
concentration
of solute
Same concentration
of solute
Sugar
molecule
H 2O
Selectively
permeable
membrane
Water Balance of Cells Without Walls
Tonicity is the ability of a surrounding solution to
cause a cell to gain or lose water
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
Osmosis
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Figure 7.15
Water Balance of Cells with Walls
Cell walls help maintain water balance
A plant cell in a hypotonic solution swells until
the wall opposes uptake
Hypotonic
solution
Isotonic
solution
Hypertonic
solution
(a) Animal cell
H 2O
H 2O
H 2O
H 2O
the cell is now turgid (firm)
If a plant cell and its surroundings are isotonic,
there is no net movement of water into the cell;
the cell becomes flaccid (limp)
Lysed
(b) Plant cell
H2O Cell wall
Normal
H 2O
Shriveled
H 2O
H 2O
the plant will wilt
Turgid (normal)
Flaccid
Plasmolyzed
Osmosis
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Facilitated Diffusion: Passive Transport
Aided by Proteins
Carrier proteins undergo a subtle change in
shape that translocates the solute-binding site
across the membrane
In facilitated diffusion, channel proteins speed the
passive movement of molecules across the plasma
membrane
provide corridors that allow a specific molecule or ion to cross
the membrane down their diffusion gradients
Solute
Carrier protein
(b) A carrier protein alternates between two conformations, moving a
solute across the membrane as the shape of the protein changes.
The protein can transport the solute in either direction, with the net
Figure 7.17 B
movement being down the concentration gradient of the solute.
Figure 7.17 A
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Figure 7.18-6
Concept 7.4: Active transport uses energy
to move solutes against their gradients
EXTRACELLULAR [Na+] high
FLUID
[K+] low
allows cells to maintain concentrations of solutes that differ than the
concentrations in their surroundings
N a+
N a+
Active transport moves substances against their
concentration gradients
N a+
N a+
N a+
N a+
N a+
N a+
CYTOPLASM Na+
1
[Na+]
low
[K+] high
P
ADP
2
ATP
P
3
Active transport requires energy
in the form of ATP
Or the concentration gradient of other molecules
K+
Active transport is performed by specific proteins
embedded in the membranes
The sodium-potassium pump is one type of active
transport system
Animation: Active Transport
K+
6
K+
K+
K+
K+
5
4
P
Pi
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Figure 7.19
Passive transport
Active transport
How Ion Pumps Maintain Membrane
Potential
Membrane potential
Is the voltage difference across a membrane
Voltage is created by differences in the
distribution of positive and negative ions across
a membrane
And is contributed by the combined concentration of
and electrical gradients of ions across a membrane
Diffusion
Facilitated diffusion
ATP
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An electrogenic pump
Two combined forces, collectively called the
electrochemical gradient, drive the diffusion
of ions across a membrane
A chemical force (the ion s concentration
gradient)
An electrical force (the effect of the membrane
potential on the ion s movement)
is a transport protein that generates voltage across a
membrane
The sodium-potassium pump is the major
electrogenic pump of animal cells
The main electrogenic pump of plants, fungi, and
bacteria is a proton pump
ATP
-
+
Proton pump
+
EXTRACELLULAR
FLUID
H+
H+
H+
CYTOPLASM
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-
H+
H+
+
+
H+
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Figure 7.20
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Cotransport: Coupled Transport by a
Membrane Protein
Cotransport
active transport of a solute driven by a
concentration gradient of another solute
Concept 7.5: Bulk transport across the
plasma membrane occurs by exocytosis and
endocytosis
Large molecules, such as polysaccharides and
proteins, cross the membrane in bulk
via vesicles
Bulk transport requires energy
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Exocytosis
Endocytosis
In exocytosis, transport vesicles migrate to the
membrane, fuse with it, and release their
contents
Many secretory cells use exocytosis to export
their products
In endocytosis, the cell takes in macromolecules
by forming vesicles from the plasma membrane
Endocytosis is a reversal of exocytosis, involving
different proteins
There are three types of endocytosis
Phagocytosis ( cellular eating )
We wont talk about:
Pinocytosis ( cellular drinking )
Receptor-mediated endocytosis
Animation: Exocytosis
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Figure 7.22a
Phagocytosis
Pseudopodium
of amoeba
Bacterium
Food vacuole
Pseudopodium
1 m
In phagocytosis a cell engulfs a particle in a
vacuole
The vacuole fuses with a lysosome to digest
the particle
EXTRACELLULAR
FLUID
Solutes
Food
or other
particle
An amoeba engulfing a bacterium
via phagocytosis (TEM).
Animation: Phagocytosis
Food
vacuole
CYTOPLASM
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