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Chapter 7
Membrane Structure and Function
Figure 7.1 The plasma membrane
Figure 7.2
Phospholipid bilayer
(cross section)
WATER
Hydrophilic
head
Hydrophobic
tail
WATER
Figure 7.3 The fluid mosaic
model for membranes
Hydrophillic region
of protein
Phospholipid
bilayer
Video
Hydrophobic region of protein
Figure 7.8 The structure of a
transmembrane protein
EXTRACELLULAR
SIDE
N-terminus
C-terminus
a Helix
CYTOPLASMIC
SIDE
Figure 7.4 Research Method
Freeze-Fracture
APPLICATION
A cell membrane can be split into its two layers, revealing the ultrastructure of the
membrane’s interior.
TECHNIQUE
A cell is frozen and fractured with a knife. The fracture plane often follows the
hydrophobic interior of a membrane, splitting the phospholipid bilayer into two
separated layers. The membrane proteins go wholly with one of the layers.
Extracellular
layer
Knife
Proteins
Cytoplasmic
Plasma
membrane layer
RESUTS
These SEMs show membrane proteins (the “bumps”) in the two layers,
demonstrating that proteins are embedded in the phospholipid bilayer.
Extracellular layer
Cytoplasmic layer
• A specialized
preparation technique,
freeze-fracture, splits
a membrane along the
middle of the
phospholid bilayer
prior to electron
microscopy.
• This shows protein
particles interspersed
with a smooth matrix,
supporting the fluid
mosaic model.
Fig. 8.3
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
1. Membranes are fluid
• Membrane molecules are held in place by
relatively weak hydrophobic interactions.
• Most of the lipids and some proteins can drift
laterally in the plane of the membrane, but rarely
flip-flop from one layer to the other.
Fig. 8.4a
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 7.5 The
fluidity of
membranes
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
• The steroid cholesterol is wedged between
phospholipid molecules in the plasma
membrane of animals cells.
• At warm temperatures, it restrains the
movement of phospholipids and reduces
fluidity.
• At cool temperatures, it maintains fluidity by
preventing tight packing.
Fig. 8.4c
Figure 7.6 Inquiry Do membrane
proteins move?
EXPERIMENT
Researchers labeled the plasma membrane proteins of a mouse
cell and a human cell with two different markers and fused the cells. Using a microscope,
they observed the markers on the hybrid cell.
RESULTS
Membrane proteins
+
Mouse cell
Human cell
Hybrid cell
Mixed
proteins
after
1 hour
CONCLUSION
The mixing of the mouse and human membrane proteins
indicates that at least some membrane proteins move sideways within the plane
of the plasma membrane.
Glycocalyx
Figure 7.7 The detailed structure of
an animal cell’s plasma membrane,
in cross section
Fibers of
extracellular
matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
Microfilaments
of cytoskeleton
Cholesterol
Peripheral
protein
EXTRACELLULAR
SIDE OF
MEMBRANE
Integral CYTOPLASMIC SIDE
protein OF MEMBRANE
• Membranes have distinctive inside and outside
faces.
– The two layers may differ
in lipid composition, and
proteins in the membrane
have a clear direction.
– The outer surface also has
carbohydrates.
– This asymmetrical
orientation begins during
synthesis of new membrane
in the endoplasmic
reticulum.
Fig. 8.8
• The carbohydrates attached to some of the
proteins and lipids of the cell membrane are
added as the membrane is refined in the Golgi
apparatus; the new membrane then forms
transport vesicles that travel to the cell surface.
On which side of the vesicle membrane are the
carbohydrates?
– Interior surface of the vesicle membrane
– Exterior (cytoplasmic) surface of the vesicle
membrane
• The proteins in the plasma membrane may
provide a variety of major cell functions.
Fig. 8.9
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
video
• In the absence of other forces, a substance will
diffuse from where it is more concentrated to
where it is less concentrated, down its
concentration gradient.
– This spontaneous process decreases free energy and
increases entropy by creating a randomized mixture.
• Each substance diffuses down its own
concentration gradient, independent of the
concentration gradients of other substances.
Fig. 8.10b
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 7.11 The diffusion of
solutes across a membrane
(a) Diffusion of one solute. The membrane Molecules of dye
Membrane (cross section)
has pores large enough for molecules
of dye to pass through. Random
movement of dye molecules will cause
some to pass through the pores; this
will happen more often on the side
WATER
with more molecules. The dye diffuses
from where it is more concentrated
to where it is less concentrated
(called diffusing down a concentration
gradient). This leads to a dynamic
Net diffusion
Net diffusion
equilibrium: The solute molecules
continue to cross the membrane,
but at equal rates in both directions.
Equilibrium
(b) Diffusion of two solutes. Solutions of
two different dyes are separated by a
membrane that is permeable to both.
Each dye diffuses down its own concentration gradient. There will be a net
diffusion of the purple dye toward the
left, even though the total solute
concentration was initially greater on
the left side.
Net Osmosis
diffusion
Osmosis video
Net diffusion
Net diffusion
Net diffusion
Equilibrium
Equilibrium
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
Same concentration
of sugar
Figure 7.12
Osmosis
Selectively
permeable membrane: sugar molecules cannot pass
through pores, but
water molecules can
Water molecules
cluster around
sugar molecules
More free water
molecules (higher
concentration)
Fewer free water
molecules (lower
concentration)
Osmosis

Water moves from an area of higher
free water concentration to an area
of lower free water concentration
• Unbound water molecules will move from the
hypotonic solution where they are abundant to
the hypertonic solution where they are rarer.
• This diffusion of water across a selectively
permeable membrane is a special case of passive
transport called osmosis.
• Osmosis continues
until the solutions
are isotonic.
Fig. 8.11
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
HYPOTONIC
Solute potential= Osmotic potential =0
HYPERTONIC
Solute potential is less than zero
Water pot.= Solute + Pres. Pot. =0
Pressure for water to diffuse in
•
A solution of 1 M glucose is separated by a
selectively permeable membrane from a solution of
0.2 M fructose and 0.7 M sucrose. The membrane is
not permeable to the sugar molecules. Which of the
following statements is correct?
–
–
–
–
–
Side A is hypotonic relative to
side B.
The net movement of water
will be from side B to side A.
The net movement of water
will be from side A to side B.
Side B is hypertonic relative to
side A.
There will be no net movement
of water.
An artificial cell consisting of an aqueous solution
enclosed in a selectively permeable membrane has just
been immersed in a beaker containing a different
solution. The membrane is permeable to water and to the
simple sugars glucose and fructose but completely
impermeable to the disaccharide sucrose.
• Which solute(s) will exhibit a net diffusion
into the cell?
–
–
–
sucrose
glucose
fructose
• Which solute(s) will exhibit a net diffusion
out of the cell?
–
–
–
sucrose
glucose
fructose
• Which solution is hypertonic to the other?
– the cell contents
– the environment
• In which direction will there be a net
osmotic movement of water?
– out of the cell
– into the cell
– neither
• After the cell is placed in the beaker, which of
the following changes will occur?
– The artificial cell will become more flaccid.
– The artificial cell will become more turgid.
– The entropy of the system (cell plus surrounding
solution) will decrease.
– The overall free energy stored in the system will
increase.
– The membrane potential will decrease
Figure 7.13 The water balance of living cells
Hypotonic solution
(a) Animal cell. An
animal cell fares best
in an isotonic environment unless it has
special adaptations to
offset the osmotic
uptake or loss of
water.
H2O
Isotonic solution
(b) Plant cell. Plant cells
are turgid (firm) and
generally healthiest in
a hypotonic environment, where the
uptake of water is
eventually balanced
by the elastic wall
pushing back on the
cell.
Osmosis Animation
H2O
H2O
Normal
Lysed
Turgid (normal)
Turgid Leaf
H2O
Shriveled
H2O
H2O
H2O
Hypertonic solution
Flaccid
H2O
Plasmolyzed
Plasmolysis video
Pearson Diffusion & Osmosis
Lab
• Print out Answers to Lab quiz tonight.
Figure 7.14 The contractile vacuole of Paramecium:
an evolutionary adaptation for osmoregulation
Filling vacuole
50 µm
Contractile Vacuole
(a) A contractile vacuole fills with fluid that enters from a system of canals
radiating throughout the cytoplasm.
50 µm
Contracting vacuole
(b) When full, the vacuole and canals contract, expelling fluid from the cell.
•
You observe plant cells under a microscope that have just been
placed in an unknown solution. First the cells plasmolyze; after
a few minutes, the plasmolysis reverses and the cells appear
normal. What would you conclude about the unknown solute?
1. It is hypertonic to the plant cells, and its solute can not cross the
pant cell membranes.
2. It is hypotonic to the plant cells, and its solute can not cross the pant
cell membranes.
3. It is isotonic to the plant cells, but its solute can cross the plant cell
membranes.
4. It is hypertonic to the plant cells, but its solute can cross the plant
cell membranes.
5. It is hypotonic to the plant cells, but its solute can cross the plant
cell membranes.
• For a cell living in an isotonic environment (for
example, many marine invertebrates) osmosis is
not a problem.
– Similarly, the cells of most land animals are bathed
in an extracellular fluid that is isotonic to the cells.
• Organisms without rigid walls have osmotic
problems in either a hypertonic or hypotonic
environment and must have adaptations for
osmoregulation to maintain their internal
environment.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
video
video
EXTRACELLULAR
FLUID
Figure 7.15
Two types of
transport
proteins that
carry out
facilitated
diffusion
Channel protein
Solute
CYTOPLASM
(a) A channel protein (purple) has a channel through which
water molecules or a specific solute can pass.
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
movement being down the concentration gradient of the solute.
Figure 7.17 Review: passive and active transport compared
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.
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.
Active transport
animation
Creates
membrane
potential in
nerve cells
Figure 7.18 An electrogenic
pump
EXTRACELLULAR
FLUID
–
+
–
ATP
+
H+
H+
Proton pump
H+
–
H+
+
H+
–
+
CYTOPLASM
H+
–
+
Figure 7.19 Cotransport: active transport
driven by a concentration gradient
–
+
H+
ATP
–
H+
+
H+
Proton pump
H+
–
+
H+
–
+
H+ Diffusion
of H+
Sucrose-H+
cotransporter
H+
–
+
–
+
Sucrose
Cotransport
•
An experiment is designed to study the mechanism of sucrose uptake
by plant cells. Cells are immersed in a sucrose solution, and the pH
of the solution is monitored with a pH meter. Samples of the cells are
taken at intervals, and the sucrose in the sampled cells is measured.
The measurements show that sucrose uptake by the cells correlates
with a rise in the pH of the surrounding solution. The magnitude of
the pH change is proportional to the starting concentration of sucrose
in the extracellular solution. A metabolic poison known to block the
ability of cells to regenerate ATP is found to inhibit the pH changes
in the extracellular solution. Based on this information which of the
following statements would you predict is correct? *
1. Sucrose uptake is the result of simple diffusion
2. Hydrogen ion movement is the result of facilitated diffusion.
3. Sucrose moving through the membrane forces hydrogen
ions in to the cell
4. Sucrose and Hydrogen ions are transported in opposite
directions across the membrane
5. Sucrose transport is the result of a hydrogen ion
cotransporter.
Do Lab simulation
• Go to this site:
• http://midpac.edu/~biology/Intro%20Biolog
y/PH%20Biology%20Lab%20Simulations/
biomembrane1/intro.html
• Do the lab simulation on Membrane
Structure and Transport, then take the
practice test – print out the test to turn in.
Figure 7.20 Exploring Endocytosis in
Animal Cells video
In phagocytosis, a cell
engulfs a particle by
wrapping pseudopodia
around it and packaging
it within a membraneenclosed sac large
enough to be classified
as a vacuole. The
particle is digested after
the vacuole fuses with a
lysosome containing
hydrolytic enzymes.
PHAGOCYTOSIS
EXTRACELLULAR
FLUID
1 µm
CYTOPLASM
Pseudopodium
Pseudopodium
of amoeba
“Food” or
other particle
Bacterium
Food
vacuole
Food vacuole
An amoeba engulfing a bacterium via
phagocytosis (TEM).
In pinocytosis, the cell
“gulps” droplets of
extracellular 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.
PINOCYTOSIS
0.5 µm
Plasma
membrane
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM).
Vesicle
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, but 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.
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Ligand
Coated
pit
A coated pit
and a coated
vesicle
formed
during
receptormediated
endocytosis
(TEMs).
Coat
protein
Plasma
membrane
0.25 µm
• In pinocytosis, “cellular drinking”, a cell creates a
vesicle around a droplet of extracellular fluid or small
particles.
– This is a non-specific process.
Fig. 8.19b
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Receptor-mediated endocytosis is very specific in
what substances are being transported.
• This process is triggered when extracellular substances
bind to special receptors, ligands, on the membrane
surface, especially near coated pits.
• This triggers the formation of a vesicle
Fig. 8.19c
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Receptor-mediated endocytosis enables a cell to
acquire bulk quantities of specific materials that
may be in low concentrations in the
environment.
– Human cells use this process to absorb cholesterol.
– Cholesterol travels in the blood in low-density
lipoproteins (LDL), complexes of protein and lipid.
– These lipoproteins bind to LDL receptors and enter
the cell by endocytosis.
– In familial hypercholesterolemia, an inherited
disease, the LDL receptors are defective, leading to
an accumulation of LDL and cholesterol in the
blood.
– This contributes to early atherosclerosis.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Do Lab simulation
• Go to this site:
• http://midpac.edu/~biology/Intro%20Biolog
y/PH%20Biology%20Lab%20Simulations/
biomembrane2/intro.html
• Do the lab simulation on Membrane
Transport and Communication, then take
the practice test – print out the test to turn
in.
Chapter 7
Membrane Structure
and Function