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Lecture 5
Membranes
Why does osmosis matter?
http://www.livescience.com/37227-man-overdoses-on-soy-sauce.html?cmpid=514645
Yesterday’s Exit Ticket
Prokaryotes
Animals
Plants
No nucleus
True nucleus
True nucleus
Differences
Cell wall
(featuring
peptidoglycan)
No cell wall
Cell wall
(featuring cellulose)
No membranebound organelles
Membrane-bound
organelles (including
mitochondria, but NOT
chloroplasts or vacuole)
Membrane-bound
organelles (mitochondria,
chloroplasts, vacuole)
Similarities
DNA
DNA
DNA
Ribosomes
Ribosomes
Ribosomes
Cytoplasm
Cytoplasm
Cytoplasm
Cell Membrane
Cell Membrane
Cell Membrane
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Key Themes
(2) “Think Like a Biologist”: Understand What Life Is.
“Unity” of life: What are the common features of all life?
• Structure and function of biological membranes
• Maintenance of a suitable internal environment
at the cost of energy input
Today’s agenda:
• Fun with membranes
• Review of the key concepts for the
exam
Membrane Structure and Function
Key Functions of Membranes
Which macromolecules do which?
1) Provide a barrier around cells & sub-cellular spaces
Phospholipid bilayer provides ±impenetrable barrier
2) Provide controlled passageways for wanted &
unwanted substances
Proteins provide selective & controllable passageways
(“selective permeability”)
1. Be able to relate the basic structure of biological
membranes to their principal functions
Phospholipid bilayer as the basic membrane structure
Phospholipids have hydrophilic
& hydrophobic regions.
Fig. 7.2
Fluid-Mosaic Membrane
• Membranes: mosaic of phospholipids & proteins
• Membranes: typically “fluid” with consistency of salad
oil (fluidity level varies with temperature!)
Phospholipid
bilayer
Fig. 7.3
Hydrophobic regions
of protein
Hydrophilic
regions of protein
The effect of unsaturated versus saturated
phospholipids on membrane fluidity
In organisms that do not regulate body temperature
(microorganisms, plants, & non-regulating animals)
Fig. 7.5 (b)
Fluid
Unsaturated hydrocarbon
tails with kinks
Viscous
Saturated hydrocarbon tails
2. Be able to identify factors affecting membrane
fluidity in various organisms
3. Be able to relate saturated and unsaturated fatty
acids to the ecology of organisms
Temperate
Walnut
Northeast US & N Europe
Temperate
Mediterranean
Tropical
http://www.ecoworld.com/maps/world-ecoregions.html
Canola
&
Olive oil
versus
Palm & coconut oil
Macademia nut
Australia & Hawaii
Role of cholesterol in animal membranes
Acts as a “temperature buffer”
• Prevents hydrophobic chains from packing too
closely together: increases fluidity at low
temperatures
• Limits lateral phospholipid movement & stabilizes
membranes at high temperatures
Fig. 7.5 (c)
Cholesterol
Fig. 5.15
4. Be able to predict the passage of hydrophilic (polar) and
hydrophobic (nonpolar) molecules through biological membranes
Passage of Molecules across the
Plasma Membrane
Hydrophobic, non-polar molecules cross
membranes with ease.
Hydrophilic molecules cannot slip through
hydrophobic core of membrane: Require help
of proteins that span the entire membrane.
http://www.colorado.edu/ebio/genbio/07_11_MembraneSelectivity_A.html
Passage of Molecules across Membranes
Hydrophilic, polar molecules cannot slip through
membrane; their transport requires help of proteins
that span entire membrane.
Extracellular
fluid
Cytoplasm
Solute
Fig. 7.15 (a)
5. Structure and function of membrane channels:
Be able to predict where amino acids with hydrophilic versus
hydrophobic rest groups are found in transport proteins
Predict which portions of a membrane-spanning protein
(allowing passage of polar or charged ions/molecules)
are hydrophilic:
Predict which portions are hydrophobic:
Hydrophobic regions
(R groups!) of protein
Hydrophilic regions Fig. 7.15 (a)
(R groups!) of protein
Amino acid R (rest) groups
Fig. 5.17(a)
Nonpolar R groups: hydrophobic
Glycine
Methionine
Alanine
Phenylalanine
Valine
Leucine
Tryptophan
Isoleucine
Proline
Fig. 5.17(b&c)
Polar R groups: hydrophilic
Serine
Threonine
Cysteine
Tyrosine
Asparagine Glutamine
Electrically
Charged
R groups:
hydrophilic
Aspartic acid Glutamic acid
Lysine
Arginine
Histidine
5. Structure and function of membrane channels
(example aquaporins)
Aquaporins:
Membrane-spanning
protein channels
allowing (polar)
water to move
across
(hydrophobic) lipid
membranes
http://www-als.lbl.gov/als/science/sci_archive/54aquaporin.html
Two aspects of movement across
membranes:
• Predict when a protein is needed for movement:
For small non-polar, hydrophobic substances?
For polar, hydrophilic substances?
No
Yes
• Predict when ATP energy is needed for movement:
When substances move from high to low
concentration, i.e. along their concentration
gradient?
When substances are moved from low to
high concentration, i.e. “uphill” against the
concentration gradient?
No
Yes
6. Be able to predict when when ATP energy is needed to fuel active transport
Overview of the two possibilities
“Downhill” Passive transport
Active transport
“Uphill”
ATP
Diffusion
Facilitated diffusion
Fig. 7.17
ex. fructose, H2O
Passive transport
Facilitated
diffusion
Predict how glucose moves from the gut into intestinal
cells when the glucose concentration in the gut is
higher than in the intestinal cells after a meal:
A) by passive transport
B) by active transport
Think-Pair-Share
Fig. 7-11a
Molecules of
dye
Membrane (cross section)
WATER
Net diffusion
(a) Diffusion of one solute
Net diffusion
Equilibrium
7. Be able to predict the
direction of water movement via osmosis
Water crosses
membranes by
OSMOSIS
down its
concentration
gradient
http://isite.lps.org/sputnam/Biology/U3Cell/Unit3Notes_cell.htm
Salt (Na+) retention & high blood pressure
Fig. 7-12
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
H2O
Selectively
permeable
membrane
Osmosis
Same concentration
of sugar
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
Osmosis = passive (net!) movement of water across
membranes along/down the concentration gradient
Net water movement follows only the water
gradient (regardless of what kinds of dissolved
compounds are involved)
Intravenous saline solution
(1) with similar concentration of all dissolved
compounds, like salts & sugars, combined as the
blood plasma
Net water movement into or out of red blood cells?
(2) Intravenous “solution” of pure water
Net water movement into or out of red blood cells?
(3) Intravenous solution more concentrated in
salt & sugars
Net water movement into or out of red blood cells?
• The Crash Course for Membranes is
particularly good!!
http://www.youtube.com/watch?v=dPKvHr
D1eS4&list=PL3EED4C1D684D3ADF
3:07-3:43
5 minute break
30
Overview of the two possibilities
“Downhill” Passive transport
Active transport
“Uphill”
Na+
K+/Na+
pump
ATP
Diffusion
Fig. 7.17
Facilitated diffusion
K+
Na+/K+ Pump
Na+
ATP
K+
• Cells want to pump Na+ out
• Cells want to pump K+ in
8. Be able to apply the principal features and functions
of an ATP-fueled ion pump to the Na+/K+ pump
Active transport and
the sodium-potassium pump
Both Na+ and K+ are moved
AGAINST their concentration gradient
See Fig. 7.16 for a six panel, blow-by-blow
description of the sodium-potassium pump.
http://www.colorado.edu/ebio/genbio/07_16ActiveTransport_A.html
Fig.8.7
http://onlinephys.com/circuit1.html
Cotransport: Using potential energy
Na+
ATP fuels the Na+/K+ pump
Na+ accumulates “on top of the hill”
(against its concentration gradient)
Na+ flows downhill again
ATP
Releasing useful energy
Cotransport: Using potential energy
This potential energy
can be used…
To transport other molecules
AGAINST their concentration
gradient
The Na+ gradient built up by the Na+/K+ pump
also fuels the secondary active transport
of glucose (& other substances)
AGAINST their concentration gradient
Via Na+/glucose cotransport, where
Na+ flows back downhill & drags
glucose uphill AGAINST its
concentration gradient
https://www.youtube.com/watch?v=LyvmM1lKtWs
https://www.youtube.com/watch?v=svAAiKsJa-Y
Predict how glucose moves into intestinal cells
when glucose concentration is lower in the gut
than in the intestinal cells:
A) through a glucose channel
B) directly through the lipid bilayer
C) via Na+-glucose cotransport fueled by the
Na+/K+ pump
D) directly through the ATP-fueled Na+/K+
Think-Pair-Share
pump
Passive transport
Facilitated
diffusion
Predict how glucose moves into intestinal cells when
glucose concentration is higher in the gut than in the
intestinal cells after a meal:
A) through a glucose channel
B) directly through the lipid bilayer
C) via Na+-glucose cotransport fueled by the Na+/K+
pump
D) through the ATP-fueled Na+/K+ pump
Think-Pair-Share
Membrane Bioflix
Exo- and Endocytosis
Signaling molecule
Receptor
ATP
(a) Transport
Signal transduction
(c) Signal transduction
Fig. 7.9
Overview of
functions of
membrane
proteins
10. Be able to predict the principal differences in signal transduction
of a protein hormone versus a steroid hormone
Let’s look at the two major classes of hormones:
Protein hormones and steroid hormones
Predict which hormones can pass directly
through the lipid bilayer of membranes:
A) Protein hormones
B) Steroid hormones
Think-Pair-Share
(a) Water-soluble
protein hormones
relay message via
signal transduction
pathway to a gene
regulatory protein.
Eighth ed. = Fig. 45.5
(a) Water-soluble
protein hormones
relay message via
signal transduction
pathway to a gene
regulatory protein.
(b) Lipid-soluble
(e.g. steroid)
hormones move
into nucleus &
bind directly to
gene regulatory
protein.
Eighth ed. = Fig. 45.5
See also Fig. 11.8
Open Forum
Questions leading up to Monday’s exam?
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