Fox, Chapter 6, plus some references back to earlier and later chapters
and to Freeman
There's enough lipid to make two layers
shows how red blood cells react to hypertonic, isotonic and hypotonic solutions.
Get a good source of membranes:
red blood cells (erythrocytes) from adult human have only plasmalemma.
Gorter and Grendel showed in1925 that there was enough lipid to make two
Put red blood cells into distilled water, they burst from hyposmotic shock
and become only "ghosts" - membrane only.
Blood cell counts, and geometry solves for membrane surface.
Extracted lipids on a surface have an increased lateral stability when they
reach a monolayer which, when measured is twice the membrane area.
Here is a snapshot
I took of oil on a road after rain - when oil is multiple layers, you see
color, and layers slip, when oil is one layer, it is black.
(To understand how hypotonic shock burst the erythrocyte, I introduce a
fundamental concept, osmosis)
Osmosis - water moves passively from where water is at a higher concentration
(for instance pure water) to where water is at a lower concentration (where
organic chemicals are dissolved in it)
through a semipermeable membrane (i.e. a membrane which passes water but
not the organic molecules).
(Glucose transport across the cell membrane is so fundamental that it is
in the introductory book)
(Review from an introductory text)
Glucose transporter from your textbook
shows bilayer of lipids with protein in it
Notice that the lipid molecules are drawn in this "cartoon" as
a ball with two sticks.
Most membrane lipids are phospholipids with:
(1) a polar (hydrophilic) head group
(2) hydrophobic fatty acid (acyl) tails
Another example allows introduction of another fundamental molecule:
(Review from an introductory text)
Here is a famous membrane protein, rhodopsin, the molecule we see with,
and how 7 hydrophobic alpha helices of the protein fit into the hydrophobic
part of the membrane (the milieu created by the fatty acid tails). For future
reference, retinal is the chromophore, the component (chromophore) that
makes the protein [proteins are otherwise not colored] into a pigment. Retinal
is a derivative of vitamin A. Rhodopsin is in the membranes of rods and
cones, visual receptor cells, shown in the diagram. Rhodopsin is the prototypical
G protein-coupled receptor (GPCR), and GPCRs are used for hormones, neurotransmitterss,
olfaction, taste and others.
Electron microscopy (EM)
Fig. 3.2, Fox
Robertson did work that led to earlier bilayer model.
He saw 2 "electron dense" (dark) lines in EM when stained osmium,
an electron dense heavy metal. Davson and Danielli developed a membrane
model from Robertson's vistas.
Fluid mosaic Singer and Nicolson the more modern version
Picture I made freeze
fracture replicas with this apparatus. Specimen is prepared, frozen to liquid
nitrogen temperature, put inside a vacuum, smashed with a razor (membranes
break down the middle between the fatty acid tails), blasted from an angle
with a platinum gun (to shadow protein with electron dense heavy metal),
blasted from above with a carbon gun (to hold replica together), then the
tissue is dissolved away.
Here, from my research,
is an example of how things look. Picture shows visual membranes in Drosophila.
High vitamin A flies have membranes full of protein (the same rhodopsin
I mentioned above) while vitamin A deprivation decreases this protein.
Membrane lipids are composed of:
(1) Phospholipids such as phosphatidylcholine (lecithin)
I did some research on
the phospholipids of the Drosophila head. Using radioactively lbeled phosphate,
many different phospholipids are visualized after they have been separated
on a TLC (thin layer chromatography)
(3) Glycolipids such as one that accumulates in Tay-Sachs, a hereditary
lysosomal storage disease,1/30 American Jews carry, recessive, fatal at
6 mo - 5 yr
Alexa B. Serfis in SLU's Chemistry
Department studies membrane lipids and their proteins
Relevant to physiology, if the membrane had only lipids, it would have extremely
high resistance. This is because the hydrophobic milieu in the center of
the membrane does not allow water, a polar solvent, or ions which carry
current. The membrane is only permeable because some of the proteins are
channels that pass ions. Also, there is high capacitance. The concepts of
resistance and capacitance will be dealt with shortly.
Lipid makes a barrier to anything polar or big like protein hormone or epinephrine
(This receptor is the GPCR, mentioned above.)
Steroid hormones can go in
It used to be thought that lipids just sit there. In the 1980's it became
clear that they turn over metabolically and that some products of membrane
lipid turnover are important mediators of intracellular signalling. This
is very fundamental and will come up repeatedly in later.
Hormone -> receptor protein (GPCR) -> G-protein -> cascade makes
second messengers (IP3 and DAG [diacyl glycerol, not in your diagram]) from
the membrane lipid PIP2 [phosphatidylinositol-4,5-bisphosphate, not in your
diagram], note that calcium ion Ca2+) becomes a next messenger in the cascade.
Important points that will come up repeatedly:
Phospholipase C is the enzyme [and I have a research interest in PLC]
IP3 is a "ligand" for a calcium channel.
Ca2+ is sequestered inside endoplasmic reticulum.
Inside a cell's cisterns is tantamount to outside the cell.
Ca2+ is high outside and low inside, like Na+ (sodium ion) unless deliberateluy
Ca2+ levels are so important that 3 hormones regulate blood Ca2+, parathormone,
calcitonin and vitamin D.
Nicotinic Acetylcholine receptor [More on this later])
Acetylcholine is a ligand (neurotransmitter), nicotine is a pharmnacological
This receptor is a channel (for ions, giving the membrane electrical conductance
Channel is ligand gated.
Sodium (Na+) and potassium (K+) shown going through pore in membrane that
can be open or closed.
Sodium, higher outside the cell, is likely to go in.
Potassium, high inside the cell is likely to go out.
Nobel prize in physiology and medicine was awarded to prize was awarded
jointly to: ERWIN NEHER and BERT SAKMANN; they developed patch clamping
that allowed electrical recording from single channels.
the Nobel prize was awarded jointly to: SIR JOHN CAREW ECCLES , SIR ALAN
LLOYD HODGKIN and SIR ANDREW FIELDING HUXLEY for their discoveries concerning
the ionic mechanisms involved in excitation and inhibition in the peripheral
and central portions of the nerve cell membrane; Hodgkin and Huxley worked
on the voltage gated channels of the axon's action potential and Eccles
worked on the neurotransmitter gated channels at synapses.
In summary, the topic of ion channels is pretty fundamental.
Also holes in membranes from one cell to another are important:
Gap junctions - 2 hexamers in register of connexin protein
This is a very big channel.
Important in many places, especially connecting one heart muscle (myocardial)
cell to another electrically.
A large fraction of the cell's energy (ATP) goes to pumping ions (active
This creates an ion imbalance, sodium Na+ high outside cell, potassium K+
This gives rise to the membrane electrical potential (voltage) important
in nerve and muscle cells.
phagocytosis - cell eating
pinocytosis - cell drinking
Receptor mediated endocytosis - clathrin coated pits turn to vesicles, clathrin
is a protein that makes vesicles look fuzzy.
Receptor mediated endocytosis is important in clearing lipoproteins, LDL
and HDL, from blood (later), and, of course, a receptor protein in the membrane
is important in the transport.
From my research, a coated
Functions of membrane proteins
(2) many enzymes are on the membrane
(3) receptors for hormones, neurotransmitters and developmental signals
are on the membrane.
(4) cells are joined by proteins
(5) cells communicate by proteins
(6) cells hook to extracellular proteins by proteins
E. Gorter and F. Grendel, On bimolecular layers of lipoids on the chromocytes
of the blood, J. Exp. Med. 41, 439-443, 1925
Exam questions from 2004 - 2011 relevant to this outline
The introductory book's transparency indicated that "GLUT-1 facilitates
glucose diffusion." Answer either why they selected the words (1) "facilitates,"
or (2) "diffusion."
(1) glucose needs a path across the membrane, (2) but it is not a pump that
In the phosphoinositide signal transduction cascade, IP3 gates a channel.
Answer either (1) Where (specifically)? Or (2) For what ion?
(1) a smooth endoplasmic reticulum (2) calcium ions
Answer either (1) Why are there rainbow colors on a drop of oil on a rain-wet
road? or (2) Why are rhodopsin or hemoglobin colored molecules?
(1) from optics of layers that are different thicknesses (2) the protein
has a chromophore that makes the protein colored
"A semipermeable membrane is part of the explanation for osmosis."
Permeable to what and not permeable to what?
permeable to water but not to a larger molecule like sugar
"Picture an area where the gravel has been lightly shadowed by a light
snow blowing from one direction." Answer either (1) What is this technique
called? (2) What do the P-face particles represent? or (3) What heavy metal
is used for shadowing?
1 freeze fracture 2 membrane proteins 3 platinum
A slide was shown where all the phospholipids of the Drosophila headwhich
had been separated were clearly visualized. Answer either (1) What is the
name of this technique? or (2) How come these phospholipids could be clearly
1 autoradiography (thin layer chromatography) 2 they radioactive and the
radioactivity exposes photographic film
Tay Sachs disease: Answer either (1) Why is it called a lysosomal storage
disease? (2) What molecule fails to turn over? or (3) Why would the eye
and brain be affected first?
1 the substance the lysosomes fail to degrade accumulate in lysosomes 2
a glycolipid 3 there are lots of membranes with lots of membrane lipids
In terms of how they mediate signal transduction, state, in simple terms,
the fundamental difference between a G protein coupled receptor and the
nicotinic acetylcholine receptor.
GPCR leads to transduction cascade and "second messenger" while
NAChR is a channel
In "receptor mediated endocytosis," the pit (that becomes a vesicle)
has a receptor (like the receptor for LDL) plus what famous protein that
makes these pits and vesicles look the way they do in the electron microscope?
"In an isotonic milieu like the blood stream, a red blood cell's shape
remains unchanged." To make red blood cell ghosts, Gorter and Grendel
Answer one of the following: (1) They moved the cells from an isotonic solution
to what kind of solution? (2) What famous membrane transport process mediated
the net influx of water into the cell?
hypotonic (distilled water), osmosis
In the third lecture, the importance of G protein coupled receptors (GPCR)
was introduced. Where, in the ultrastructural anatomy of either a rod cell
or a cell involved in phosphoinositide signaling, would you find a GPCR?
membrane disks in rod, on the plasmalemma (cell membrane
How much energy, delivered by ATP, is used by the GLUT-1 transporter to
get glucose into the cell?
In contrast with the G protein coupled receptor, why is the nicotinic receptor
an ion channel is more direct than a signal transduction cascade
In addition to glycolipids and cholesterol, what is the major lipid constituent
of the cell membrane?
In addition to the receptor, in receptor-mediated endocytosis, what famous
protein is seen in transmission electron micrographs that gives coated pits
and coated vesicles their names?
How much ATP would a cell use to open and close the nicotinic receptor?
"IP3 is a ligand for a calcium channel." Answer one of the following:
(1) Where is this Ca2+ channel? (2) Where was the precursor of IP3 (the
molecule from which IP3was formed by the action of PLC [phospholipase C])?
(1) on a smooth endoplasmic reticulum, a calcium sequestering cistern (2)
PIP2 is a membrane phospholipid
"Vitamin A deprivation decreases the number of bumps seen in fruit
fly photoreceptor membranes (transmission electron microscopy of freeze
fracture replicas)." These bumps are a visualization of what molecule?
Note that ATP is not used in a channel like the nicotinic receptor that
can be closed or, when opened, passes K+ and Na+ (causing depolarization).
Where, if at all, is ATP utilized in generating membrane potentials based
on K+ and Na+?
Working in the background, the Na+ - K+ ATPase establishes Na+ and K+ gradients
Why do you need lead, osmium, uranium or platinum to see aspects of membrane
structure in the electron microscope?
heavy metals are electron dense
What did hypotonic shock do to what kind of cells to allow Gorter and Grendel
to show that there was enough lipid in the membrane to make two layers?
burst red blood cells to make red blood cell ghosts with a measured membrane
Why do you need a chromophore (such as retinal for rhodopsin and heme for
hemoglobin) to make a protein into a pigment?
proteins do not absorb visible light
How does a steroid hormone get into a cell?
that can pass the lipid barrier
How do sodium ions get forced out of the cell?
active transport with a protein that uses ATP
The nicotinic receptor is a cation channel for what two ions?
K+ and Na+
Rhodopsin and neurotransmitter and hormone receptors interact with what
downstrean heterotrimeric protein?
the G protein
What do you call electrical junctions from cell to cell with channels composed
of hexamers of connexin protein in register?
What must be bound to the G-protein-coupled-receptor protein to make the
fully-functional rhodopsin molecule that absorbs light?
As a result of phospholipase C (PLC) activation, what ion is released into
the cytoplasm from smooth endoplasmic reticulum?
In what fundamental way does the location of a steroid hormone receptor
differ from that of the receptor for epinephrine?
steroid receptor is in cell, epinephrine receptor is in membrane
In a "cartoon" of a membrane phospholipid, there is a ball with
two tails. The ball is the polar (hydrophilic) head group. What are the
fatty acids (acyl groups)
Which direction does the ATPase pump sodium ions?
out of the cell
Epinephrine binds one G protein-coupled receptor (GPCR). What other GPCR,
used for vision, is a pigment that contains a form of vitamin A?
What is the name of the electrical connection between myocardial cells composed
of connexin proteins?
Why does an erythrocyte turn into an erythrocyte ghost when placed into
because of osmosis, it swells and bursts
When a membrane lipid is drawn as a ball with two sticks in a diagram, what
are the ball and sticks respectively?
polar head group, fatty acids
One signal transduction product of phospholipase C (PLC), diacylglycerol
(DAG), is in the membrane while the other, IP3, inositol trisphosphate,
goes into the cytoplasm. Where is the precursor, PIP2 (phosphatidylinositol-4,5-bisphosphate)?
it is a membrane phospholipid
When the nicotinic acetylcholine receptor channel opens, there is an efflux
of K+. Why?
because it allows Na+ and K+, so K+ goes down its chemical gradient
One membrane protein is sometimes called the Na+-K+-ATPase. What is its
pumps Na+ out, K+ in
What is the function of a hexamer of connexin proteins in one cell's membrane
in register with a similar hexamer on the adjacent cell?
gap junction connects adjacent cells' cytoplasm and passes current
How can it be that a cortisol receptor is intracellular while so many hormone
receptors, for instance for epinephrine, are on the membrane?
steroids pass through the membrane
Robertson's pioneering electron microscopy paved the way for Davson and
Danielli's bilayer membrane model and Sanger and Nicolson's fluid mosaic
model. Why were heavy metals like osmium necessary for that demonstration?
If water and ions are excluded from the center of the membrane, where fatty
acids reside, how is it that a G protein-coupled receptor can span the membrane?
There are hydrophobic amino acids
Tay-Sachs disease is a fatal autosomal lysosomal storage disease. What accumulates?
Before I introduced metabotropic receptors (G protein coupled proteins that
bind a ligand such as a neurotransmitter), I showed rhodopsin, the prototypical
G protein coupled receptor. Why doesn't rhodopsin need to have a ligand
bind to it? (i.e. What does it have that a neurotransmitter receptor does
it has retinal, a vitamin A derivative
Researchers have been able to make artificial membranes out of phospholipids
in a hole between two compartments in a water bath. Why would membrane lipids
naturally arrange themselves as they are aligned in membranes?
polar heads would orient to water and hydrophobic tails toward eachother
In the phosphoinositide signal cascade, phospholipase C (PLC) makes "second
messengers" IP3 (inositol trisphosphate, the polar head group) and
what(?) from the membrane lipid PIP2 (phosphatidylinositol-4,5-bisphosphate.)
[If you do not remember, I have put enough information into the question
that you should be able to figure it out.]
Osmosis was referred to as passive transport. Is the sodium-potassium pump
no, it is active b/c it uses ATP
After feeding radioactive phosphate, extracted lipids were visualized by
audioradiography of a TLC (thin layer chromatography) plate. Why didn't
I see the sort of lipids that accumulate in Tay Sachs disease?
it is glycolipids and would not take up phosphate
If you fracture a frozen membrane, proteins are exposed. But you cannot
see them in the transmission electron microscope unless you do something.
you need to make a replica, shadow it from an angle with an electron dense
Under what circumstances (what do you do?) does a passive process (what
process?) let you make a red blood cell ghost from a red blood cell?
Put r.b.c into distilled water, osmosis
Tell me about a famous lysosomal storage disease. Your answer can be biochemical,
cell biological, or genetic.
Tay Sachs fails to break down a glycolipid that accumulates in the cell,
autosomal recessive carried in Ashkenaze Jews
In the transmission electron microscope, what membrane specialization of
receptor mediated endocytosis is visualized?
Clathrin coated pits (vesicles)
"11-cis retinal (a bent form of the aldehyde of vitamin A) is the chromophore
of rhodopsin." In terms of the molecule's interaction with visible
light, what does this mean.
Gives the protein its property of absorbing visible light
"The two half leaflets of the membrane, as seen in freeze-fracture,
are called P- and E-faces." Answer either (1) why P and E? or (2) describe
why these two halves are distinct from each other.
1 protoplasmic (on the cell side) exoplasmic (the external side), 2 turns
out that the proteins are usually seen on the P-face
"Diacyl glycerol." Explain how this name, by itself, would tell
you what would have had to happen to a membrane phospholipid to make diacyl
2 farry acids (acyl groups) attached to a glycerol means that the polar
head group had been chopped off
Tell me how autoradiography of a TLC (thin layer chromatography) plate can
help you identify the numerous phospholipids of the cell membrane.
A TLC plate separates lipids, labeling with radioactive phosphate allows
exposure of an audorad
"IP3 is a ligand for...," answer either (1) What kind of molecule?
Or (2) Where is the molecule (that it is a ligand for)?
1 channel for calcium ion 2 on a smooth ER that sequesters calcium
Extracts from what kind of cell gave the original demonstration that there
was enough lipid in a cell membrane to make two layers?
What must be true about the amino acids of the alpha helix of rhodopsin
that would let it cross the membrane?
they would be hydrophobic
For Tay-Sach's disease, answer either (1) Why is it called a "lysosomal
storage disease?" (2) What molecule is missing because of the mutation?
Or (3) What type of molecule is not broken down?
(3) glycolipids are not broken down because of (2) an enzyme deficiency
leading to (1) accumulation
In signal transduction cascades, what protein binds to either GTP or GDP.
G protein sepcifically the alpha subunit
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