Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Chapter 7
Membrane Structure and
Function
PowerPoint lectures are originally from Campbell / Reece Media Manager
and Instructor Resources for BIOLOGY, 7th & 8th Edition by N. A.
Campbell & J. B. Reece Copyright 2005 & 2008 Pearson Education,
Inc. Publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: Life at the Edge
• The plasma membrane is the boundary that
separates the living cell from its nonliving
surroundings
• The plasma membrane exhibits selective
permeability, allowing some substances to
cross it more easily than others
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins
• Phospholipids are the
most abundant lipid in
the plasma membrane
• Phospholipids are
amphipathic
molecules, containing
hydrophobic and
hydrophilic regions
Hydrophilic
head
Hydrophobic
tails
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Membrane Model
• Membranes have been chemically
analyzed and found to be made of
proteins and lipids
• In 1972, Singer and Nicolson proposed
that the membrane is a mosaic of
proteins dispersed and individually
inserted into the phospholipid bilayer
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• The fluid mosaic model states that a membrane
is a fluid structure with a “mosaic” of various
amphipatic proteins embedded in it
Hydrophilic region
of protein
Hydrophobic region of protein
Phospholipid
bilayer
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The Fluidity of Membranes
• Phospholipids in the plasma
membrane can move within the bilayer
• Most of the lipids, and some proteins,
drift laterally
• Rarely does a molecule flip-flop
transversely across the membrane
LE 7-5a
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)
Movement of phospholipids
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The Fluidity of Cell Membranes is affected by:
1- Temperature.
2- Fatty acids’ composition.
• 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
LE 7-5b
Viscous Fluid
Unsaturated hydrocarbon
tails with kinks
Membrane fluidity
Saturated hydro-
carbon tails
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• 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
• Cholesterol acts as a “temperature buffer for
• Membrane fluidity
Cholesterol and Membranes’ Fluidity
LE 7-5c
Cholesterol
Cholesterol within the animal cell membrane
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• Some proteins in the plasma
membrane can drift within the bilayer
• Proteins are much larger than lipids
and move more slowly
• To investigate whether membrane
proteins move, researchers fused a
mouse cell and a human cell
Membrane Proteins
LE 7-6
Membrane proteins
Mixed
proteins
after
1 hour Hybrid cell
Human cell
Mouse cell
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Membrane Proteins and Their Functions
• 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.
• According to their location, membrane proteins are
classified into:
– Peripheral proteins are bound to the surface of
the membrane
– Integral proteins penetrate the hydrophobic core
and often span the membrane
LE 7-7
Fibers of extracellular matrix (ECM)
Glycoprotein
Carbohydrate
Microfilaments
of cytoskeleton
Cholesterol
Integral
protein
Peripheral
proteins
CYTOPLASMIC SIDE
OF MEMBRANE
EXTRACELLULAR
SIDE OF
MEMBRANE
Glycolipid
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• 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
Membrane Proteins
LE 7-8
EXTRACELLULAR
SIDE N-terminus
C-terminus CYTOPLASMIC
SIDE a Helix
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• Six major functions of membrane proteins:
1) Transport proteins -form small openings for molecules to difuse through
2) Enzymatic Proteins - carry out metabolic reactions
3) Signal transduction proteins - molecular triggers that set off cell responses (such as release of hormones or opening of channel proteins)
4) Cell to cell Recognition Proteins - ID tags, to identify cells to the body's immune system
5) Intercellular joining- link adjacent cells together
6) Attachment to the cytoskeleton and extracellular matrix (ECM)
Functions of Membrane Proteins
LE 7-9a
Enzymes Signal
Receptor ATP
Transport Enzymatic activity Signal transduction
LE 7-9b
Glyco- protein
Cell-cell recognition Intercellular joining Attachment to the
cytoskeleton and extra-
cellular matrix (ECM)
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Transport
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Enzymatic Activity
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Receptors for Signal Transduction
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Cell-Cell Recognition
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The Role of Membrane Carbohydrates in Cell-Cell Recognition
• 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)
• Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual
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Synthesis and Sidedness of Membranes
• Membranes have distinct inside and outside
faces
• The asymmetrical distribution of proteins,
lipids, and associated carbohydrates in the
plasma membrane is determined when the
membrane is built by the ER and Golgi
apparatus
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ER 1
Transmembrane glycoproteins
Secretory protein
Glycolipid
2 Golgi apparatus
Vesicle
3
4
Secreted protein
Transmembrane glycoprotein
Plasma membrane:
Cytoplasmic face
Extracellular face
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
• Plasma membranes are selectively
permeable, regulating the cell’s
molecular traffic
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The Permeability of the Lipid Bilayer
• Hydrophobic (nonpolar)
molecules, such as
hydrocarbons, can
dissolve in the lipid
bilayer and pass through
the membrane rapidly
• Polar molecules, such
as sugars, do not cross
the membrane easily
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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
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EXTRACELLULAR
FLUID
Channel protein Solute
CYTOPLASM
Channel Transport Proteins
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Channel Transport Proteins
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• 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
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Carrier Transport Proteins
Carrier protein Solute
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Carrier Transport Proteins
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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 exhibit a net
movement in one direction.
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• Substances diffuse down their concentration
gradient, the difference in concentration of a
substance from one area to another
• No work must be done to move substances down the concentration gradient
• The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen
Diffusion
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Types of Membrane Transport
• Passive processes
– No cellular energy (ATP) required
– Substance moves down its concentration
gradient (i.e from high to low concentration)
• Active processes
– Energy (ATP) required
– Occurs only in living cell membranes
– Substance moves against its concentration
gradient (i.e from low to high concentration)
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Types of Passive Processes
• Simple diffusion
• Facilitated diffusion:
–Carrier-mediated facilitated diffusion
–Channel-mediated facilitated
diffusion
• Osmosis
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Passive Processes: Simple Diffusion
• Nonpolar lipid-soluble
(hydrophobic, such as O2,
CO2) substances diffuse
directly through the
phospholipid bilayer
Molecules of dye Membrane (cross section)
WATER
Net diffusion Net diffusion Equilibrium
Simple Diffusion of one solute
•At dynamic equilibrium, as many molecules cross
one way as cross in the other direction
LE 7-11b
Net diffusion Net diffusion Equilibrium
Simple Diffusion of two solutes
Net diffusion Net diffusion Equilibrium
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Passive Processes: Facilitated Diffusion
• Certain lipophobic molecules (e.g.,
glucose, amino acids, and ions) use
carrier proteins OR channel proteins,
both of which:
–Exhibit specificity (selectivity)
–Are saturable; rate is determined by
number of carriers or channels
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Facilitated Diffusion Using Carrier Proteins
• Transmembrane integral proteins transport
specific polar molecules (e.g., sugars and
amino acids)
• Binding of substrate causes shape change in
carrier
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 3.7b
Lipid-insoluble
solutes (such as
sugars or amino
acids)
(b) Carrier-mediated facilitated diffusion via a protein
carrier specific for one chemical; binding of substrate
causes shape change in transport protein
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Facilitated Diffusion Using Channel Proteins
• Aqueous channels formed by transmembrane
proteins.
• Two types:
– Leakage channels
• Always open
– Gated channels
• Controlled by chemical or electrical signals
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 3.7c
Small lipid-
insoluble
solutes
(c) Channel-mediated facilitated diffusion
through a channel protein; mostly ions
selected on basis of size and charge
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Channel-mediated facilitated diffusion
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Effects of Osmosis on Water Balance
• Osmosis is the diffusion of
water across a selectively
permeable membrane
• The direction of osmosis is determined only by a difference in total solute concentration
• Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration
LE 7-12 Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
Same concentration
of sugar
Selectively
permeable mem-
brane: sugar mole-
cules cannot pass
through pores, but
water molecules can
H2O
Osmosis
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Water Balance of Cells
• Tonicity is the ability of a 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
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Water Balance of Cells
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
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• Animals and other organisms without rigid cell walls have osmotic problems in either a hypertonic or hypotonic environment
• To maintain their internal environment, such organisms must have adaptations for osmoregulation, the control of water balance
• The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump to get rid of excess water.
Water Balance of Cells Without Cell Walls
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 3.9
Cells retain their normal size and
shape in isotonic solutions (same
solute/water concentration as inside
cells; water moves in and out).
Cells lose water by osmosis and
shrink in a hypertonic solution
(contains a higher concentration
of solutes than are present inside
the cells).
(a) Isotonic solutions (b) Hypertonic solutions (c) Hypotonic solutions
Cells take on water by osmosis until
they become bloated and burst (lyse)
in a hypotonic solution (contains a
lower concentration of solutes than
are present in cells).
LE 7-14
Filling vacuole 50 µm
50 µm Contracting vacuole
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Water Balance of Cells with Cell Walls
• Cell walls help maintain water balance
• A plant cell in a hypotonic solution swells until the wall opposes uptake; 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), and the plant may wilt
• In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis
LE 7-13
Animal cell
Lysed
H2O H2O H2O
Normal
Hypotonic solution Isotonic solution Hypertonic solution
H2O
Shriveled
H2O H2O H2O H2O Plant cell
Turgid (normal) Flaccid Plasmolyzed
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Concept 7.4: Active transport uses energy to move solutes against their gradients
• Facilitated diffusion is still passive because the
solute moves down its concentration gradient
• Some transport proteins, however, can move
solutes against their concentration gradients
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Summary of Passive Processes
Process Energy
Source
Example
Simple diffusion Kinetic energy Movement of O2 through
phospholipid bilayer
Facilitated
diffusion
Kinetic energy Movement of glucose into cells
Osmosis Kinetic energy Movement of H2O through
phospholipid bilayer or AQPs
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The Need for Energy in Active Transport
• Active transport moves substances
against their concentration gradient
• Active transport requires energy,
usually in the form of ATP
• Active transport is performed by specific proteins embedded in the membranes.
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Active Transport
• Active transport allows cells to maintain
concentration gradients that differ from
their surroundings
• The sodium-potassium pump is one
type of active transport system
LE 7-16
Cytoplasmic Na+ bonds to
the sodium-potassium pump
CYTOPLASM Na+
[Na+] low
[K+] high
Na+
Na+
EXTRACELLULAR
FLUID [Na+] high
[K+] low
Na+
Na+
Na+
ATP
ADP
P
Na+ binding stimulates
phosphorylation by ATP.
Na+ Na+
Na+
Phosphorylation causes
the protein to change its
conformation, expelling Na+
to the outside.
P
Extracellular K+ binds
to the protein, triggering
release of the phosphate
group.
P P
Loss of the phosphate
restores the protein’s
original conformation.
K+ is released and Na+
sites are receptive again;
the cycle repeats.
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Active Transport / The sodium-potassium pump
LE 7-17
Diffusion Facilitated diffusion
Passive transport
ATP
Active transport
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Membrane Potential
• 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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Diffusion of Ions (charged atoms or molecules) Across a Membrane
• Two forces, collectively called the electrochemical gradient, drive the diffusion of ions (charged atoms or molecules) across a membrane:
– A chemical force (the ion’s concentration gradient)
– An electrical force (the effect of the membrane potential, differences in charge distribution across the membrane, on the ion’s movement)
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• An electrogenic pump is a transport protein that
generates the charage differential (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
LE 7-18
H+
ATP
CYTOPLASM
EXTRACELLULAR
FLUID
Proton pump
H+
H+
H+
H+
H+
+
+
+
+
+
–
–
–
–
–
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Cotransport: Coupled Transport by a Membrane Protein
• Cotransport occurs when active transport of a
solute indirectly drives transport of another solute
• Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell
LE 7-19
H+
ATP
Proton pump
Sucrose-H+
cotransporter
Diffusion
of H+
Sucrose
H+
H+
H+
H+
H+
H+
+
+
+
+
+
+
–
–
–
–
–
–
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Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
• Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins
• Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles.
• Bulk transport requires energy
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Exocytosis
• In exocytosis, transport vesicles migrate to the
membrane, fuse with it, and release their contents
• Many secretory cells use exocytosis to export their products
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Endocytosis
• In endocytosis, the
cell takes in
macromolecules by
forming vesicles from
the plasma membrane
• Endocytosis is a reversal of exocytosis, involving different proteins
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• Three types of endocytosis:
– Phagocytosis (“cellular eating”): Cell engulfs particle in a vacuole. The vacuole fuses with a lysosome to digest the particle
– Pinocytosis (“cellular drinking”): Cell creates vesicle around fluid. Molecules are taken up when extracellular fluid is “gulped” into tiny vesicles.
– Receptor-mediated endocytosis: Binding of ligands to receptors triggers vesicle formation. A ligand is any molecule that binds specifically to a receptor site of another molecule
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Phagocytosis
PHAGOCYTOSIS
CYTOPLASM EXTRACELLULAR
FLUID Pseudopodium
“Food” or
other particle
Food vacuole
Food vacuole
Bacterium
An amoeba engulfing a bacterium
via phagocytosis (TEM)
Pseudopodium
of amoeba
1 µm
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Pinocytosis
PINOCYTOSIS
Plasma membrane
Vesicle
0.5 µm
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM)
LE 7-20c
Receptor
Ligand
Coated
pit
Coated
vesicle
Coat protein
Coat
protein
A coated pit and a coated
vesicle formed during
receptor-mediated
endocytosis
(TEMs).
Substances taken up by this
method include enzymes, insulin
and some other hormones, LDL
cholesterol, and iron.
Flu viruses, diphtheria, and
cholera toxins use this route to
enter and attack our cells.
Receptor-mediated endocytosis
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Fig. 7-UN3
Environment:
0.01 M sucrose
0.01 M glucose
0.01 M fructose
“Cell”
0.03 M sucrose
0.02 M glucose
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You should now be able to:
1. Define the following terms: amphipathic
molecules, aquaporins, diffusion
2. Explain how membrane fluidity is influenced
by temperature and membrane composition
3. Distinguish between the following pairs or
sets of terms: peripheral and integral
membrane proteins; channel and carrier
proteins; osmosis, facilitated diffusion, and
active transport; hypertonic, hypotonic, and
isotonic solutions
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You should now be able to:
4. Explain how transport proteins facilitate
diffusion
5. Explain how an electrogenic pump creates
voltage across a membrane, and name two
electrogenic pumps
6. Explain how large molecules are transported
across a cell membrane