Membranes
Chapter 5
Adapted by G. Cornwall, Ph.D.
From Raven’s Biology, McGraw Hill Publishing
Membrane Structure
• Phospholipids arranged in a bilayer
• Globular proteins inserted in the lipid bilayer
– Integral membrane proteins
– Peripheral membrane proteins
• Fluid mosaic model – mosaic of proteins floats in
or on the fluid lipid bilayer like boats on a pond
2
• Cellular membranes have 4 components
1. Phospholipid bilayer
•
Flexible matrix, barrier to permeability
2. Transmembrane proteins
•
Integral membrane proteins
3. Interior protein network
•
Peripheral membrane proteins
4. Cell surface markers
•
Glycoproteins and glycolipids
3
• Both transmission electron microscope (TEM) and
scanning (SEM) used to study membranes
• One method to embed specimen in resin
– 1µm shavings
– TEM shows layers
4
• Freeze-fracture visualizes inside of
membrane
5
Phospholipids
• Structure consists of
– Glycerol – a 3-carbon
polyalcohol
– 2 fatty acids attached to
the glycerol
• Nonpolar and hydrophobic
(“water-fearing”)
– Phosphate group attached
to the glycerol
• Polar and hydrophilic
(“water-loving”)
6
• Spontaneously forms a bilayer
– Fatty acids are on the inside
– Phosphate groups are on both surfaces
7
• Bilayers are fluid
• Hydrogen bonding of
water holds the 2 layers
together
• Individual phospholipids
and unanchored
proteins can move
through the membrane
8
• Environmental influences
– Saturated fatty acids make the membrane
less fluid than unsaturated fatty acids
• “Kinks” introduced by the double bonds keep them
from packing tightly
• Most membranes also contain sterols such as
cholesterol, which can either increase or decrease
membrane fluidity, depending on the temperature
– Warm temperatures make the membrane
more fluid than cold temperatures
• Cold tolerance in bacteria due to fatty acid
desaturases
9
Membrane Proteins
• Various functions:
1.
2.
3.
4.
Transporters
Enzymes
Cell-surface receptors
Cell-surface identity
markers
5. Cell-to-cell adhesion
proteins
6. Attachments to the
cytoskeleton
10
Structure relates to function
• Diverse functions arise from the diverse
structures of membrane proteins
• Have common structural features related
to their role as membrane proteins
• Peripheral proteins
– Anchoring molecules attach membrane
protein to surface
11
• Anchoring molecules are modified lipids with
1. Nonpolar regions that insert into the internal portion
of the lipid bilayer
2. Chemical bonding domains that link directly to
proteins
12
• Integral membrane proteins
– Span the lipid bilayer (transmembrane
proteins)
• Nonpolar regions of the protein are embedded in
the interior of the bilayer
• Polar regions of the protein protrude from both
sides of the bilayer
13
– Transmembrane domain
• Spans the lipid bilayer
• Hydrophobic amino acids arranged in α helice
• Proteins need only a single transmembrane
domain to be anchored in the membrane, but
they often have more than one such domain
14
• Bacteriorhodopsin has 7 transmembrane
domains forming a structure within the
membrane through which protons pass
during the light-driven pumping of protons
15
• Pores
– Extensive nonpolar regions within a
transmembrane protein can create a pore
through the membrane
– Cylinder of  sheets in the protein secondary
structure called a -barrel
• Interior is polar and allows water and small polar
molecules to pass through the membrane
16
Card Quiz A
Which part of the phospholipid is responsible for H
bonding with water?
 Fatty acid chains
 Phosphate head
 Glycerol molecule
 Non polar tails
Card Quiz A
What type of amino acids would you expect to find in
the transmembrane portion of a membrane protein?
 Charged amino acids
 Polar amino acids
 Non-polar amino acids
 Hydrophilic amino acids
Card Quiz A
How are transmembrane proteins held in the correct
position in the membrane?
 Covalent bonding to the phosphate group
 Hydrophobic domain is held in place by
hydrophobic exclusion
 Cytoskeleton filaments keep them in place
 The cell wall keeps the proteins in place
Card Quiz Answers
Yellow
Red
Blue
Passive Transport
• Passive transport is movement of molecules
through the membrane in which
– No energy is required
– Molecules move in response to a concentration
gradient
• Simple Diffusion is movement of molecules
from high concentration to low concentration
across a semipermeable membrane
– A form of Passive Transport
– Will continue until the concentration is the same in all
regions
Annimation
21
• Major barrier to crossing a biological membrane
is the hydrophobic interior that repels polar
molecules but not nonpolar molecules
– Nonpolar molecules will move until the concentration
is equal on both sides
– Limited permeability to small polar molecules
– Very limited permeability to larger polar molecules
and ions
22
• Facilitated diffusion
– Molecules that cannot cross membrane easily
may move through proteins
– Move from higher to lower concentration
– Channel proteins
• Hydrophilic channel when open
– Carrier proteins
• Bind specifically to molecules they assist
• Membrane is selectively permeable
23
Channel proteins
• Ion channels
– Allow the passage of ions
– Gated channels – open or close in response
to stimulus (chemical or electrical)
– 3 conditions determine direction
• Relative concentration on either side of membrane
• Voltage differences across membrane
• Gated channels – channel open or closed
24
Carrier proteins
• Can help transport both ions and other solutes,
such as some sugars and amino acids
• Requires a concentration difference across the
membrane
• Must bind to the molecule they transport
– Saturation – rate of transport limited by number of
transporters
25
Osmosis
• Cytoplasm of the cell is
an aqueous solution
– Water is solvent
– Dissolved substances
are solutes
• Osmosis – net diffusion
of water across a
membrane toward a
higher solute
concentration
26
Osmotic concentration
• When 2 solutions have different osmotic
concentrations
– Hypertonic solution has a higher solute
concentration
– Hypotonic solution has a lower solute concentration
• When two solutions have the same osmotic
concentration, the solutions are isotonic
• Aquaporins facilitate osmosis
27
Osmotic pressure
• The amount of water that
enters a cell depends on
the difference in solute
concentration between
the cell and the
extracellular fluid
• Cell in a hypotonic
solution gains water
causing cell to swell –
creates pressure
28
• If membrane strong
enough, cell reaches
counterbalance of osmotic
pressure driving water in
with hydrostatic pressure
driving water out
– Cell wall of prokaryotes,
fungi, plants, protists
• If membrane is not strong,
may burst
– Animal cells must be in
isotonic environments
29
Maintaining osmotic balance
• Some cells use
extrusion in which
water is ejected
through contractile
vacuoles
30
• Isosmotic regulation
involves keeping cells
isotonic with their
environment
– Marine organisms adjust
internal concentration to
match sea water
– Terrestrial animals
circulate isotonic fluid
• Plant cells use turgor
pressure to push the cell
membrane against the
cell wall and keep the
cell rigid
31
Card Quiz B
Permeability refers to –
 the movement of molecules from an area of
greater concentration to an area of lower
concentration
 the amount of solute in a solution
 the extent a membrane allows a substance to
pass through
 the state of being permanent
Card Quiz B
The movement of molecules from an area of high
concentration to an area of low concentration is –
 Osmosis
 Active transport
 Solubility
 Diffusion
Card Quiz B
A cell is placed in an hypotonic solution. Which way will
the water move?
 Into the cell
 Out of the cell
 No net movement
Card Quiz B
If a blood cell (0.9% NaCl) shrivels, what type of
solution was it placed in?
 0.8%
 0.1%
 0.9%
 4%
Card Quiz Answers
Blue
Blue
Blue
Yellow
Active Transport
• Some molecules need to be moved
against their concentration gradient
• Requires energy – ATP is used directly or
indirectly to fuel active transport
• Moves substances from low to high
concentration (against the gradient)
• Requires the use of highly selective carrier
proteins
37
• Carrier proteins used in active transport
– Uniporters – move one molecule at a time
– Symporters – move two molecules in the
same direction
– Antiporters – move two molecules in opposite
directions
38
Sodium–potassium (Na+–K+) pump
• Direct use of ATP for active transport
• Uses an antiporter to move 3 Na+ out of the cell
and 2 K+ into the cell
– Against their concentration gradient
39
• ATP energy is used to change the conformation
of the carrier protein
• Affinity of the carrier protein for either Na+ or K+
changes so the ions can be carried across the
membrane
40
Coupled transport
• Uses ATP indirectly (2° Active Transport)
• Uses the energy released when a molecule
moves by diffusion to supply energy to active
transport of a different molecule
41
• Symporter is used
• Glucose–Na+ symporter captures the energy
from Na+ diffusion to move glucose against a
concentration gradient
42
Bulk Transport
•
Endocytosis
–
–
–
Movement of substances into the cell
Phagocytosis – cell takes in particulate matter
Pinocytosis – cell takes in only fluid
43
Receptor-mediated endocytosis – specific molecules are
taken in after they bind to a receptor
• In the human genetic disease familial hypercholesterolemia, the LDL
receptors lack tails, so they are never fastened in the clathrin-coated
pits and as a result, do not trigger vesicle formation. The cholesterol
stays in the bloodstream of affected individuals, accumulating as
plaques inside arteries and leading to heart attacks.
44
•
Exocytosis
–
–
–
Movement of materials out of the cell
Used in plants to export cell wall material
Used in animals to secrete hormones,
neurotransmitters, digestive enzymes
45
Card Quiz C
A macrophage engulfing a bacterium is an example of –




Exocytosis
Pinocytosis
Chemocytosis
Phagocytosis
Card Quiz C
What type of transport protein can move 2 different
molecules in the same direction?
 Uniporter
 Antiporter
 Symporter
 Multiporter
Card Quiz C
How is the sodium-potassium pump able to move
ions against their concentration gradients?
 Facilitated diffusion
 Expending ATP
 Ion channels
 Bulk transport
Card Quiz Answers
Green
Blue
Red