What are the primary functions of cellular membranes?
1. Compartmentalization 2. Scaffold for biochemical acivities 3. Providing selectively permeable barrier 4. Transporting solute 5. Responding to external stimuli 6. Intervellular interaction 7. Energy transduction
What is the benefit of membrane Compartmentalization within the cell?
Enables different cellular processes to occur without external interference (can be regulated independently)
How do cell membranes act as a scaffold for biochemical activities
Basically, molecules can be kept inside membranes to increase the chances of certain biochemical interactions occurring (anchoring them)
How do plasma membranes in cells transport solutes
Often membranes are barriers between areas of high and low solute concentration. Transport machinery in the membrane allow the space inside the membrane to accumulate materials without them floating away
How do cellular plasma membranes respond to external stimuli
In a process known as signal transduction. Receptors on the membrane can react to environment factors (or molecules), causing an internal response (ex cell lysing)
How do cellular plasma membranes take part in Intercellular interaction
Allows cells to recognize one another, adhere or exchange materials.
How do cellular plasma membranes take part in energy transducton
The membranes of mitochondria and chloroplasts hold all the enzymes that break down carbs/fats and sunlight energy!
Which type of human cells are most advantageous for studying plasma membranes and why?
Red blood cells because they are anucleate and do not contain organelles. They are also very easy to obtain
What did Gorter and Grendel do in 1925 when they measured the surface area of the lipids in red blood cells? What conclusion could that lead to?
When extracting the lipids and measuring the surface area covered by the lipids they realized it was double the surface area of the cell itself. One can infer that there must then be two lipid layers.
Why must the plasma membrane of the cell be dynamic?
Movement could not occur otherwise. As well, the cell wouldn't be able to divide.
What type of molecule is this? Name the components labelled.
A- Hydrophylic head
B - Hydrophobic tail
C - Choline group
D - Phosphate group
E - Glycerol
F - Fatty acid "legs"
Why is it assumed that the lipid bi-layer must have polar and non-polar ends?
Because simple non-polar fatty acids wouldn't bond to the aqueous environment of the cell (they are hydrophobic). Therefore, the two hydrophobic fatty acid layers face each other with the hydrophilic ends (phosphate ends) facing out
TRUE OR FALSE: Phosphate is non polar
FALSE. it is polar
What gives the lipid bi-layers it's polar ends?
The presence of the polar molecule Phosphate
How does the cohesion of bilayers help with the functions of the cell
Fatty acids naturally want to stick together in an aqueous environment, and therefore cells can easily mould and shape themselves to their environment, fuse together, and put themselves back together after breaking
If plasma membranes are so good at cohesion, how do the membranes allow molecules through
Through pores/channels lined with polar proteins, preventing the fatty acids on either side from joining together
What is the "unit membrane" concept?
The idea that all membranes have the same basic structure: made of phospholipids with proteins lining them (though ratios/compositions may change)
What is the evidence to suggest that the proteins of plasma membranes are EMBEDDED in the membrane as opposed to lining it. How was that information acquired?
- Proteins isolated have hydrophobic regions
- Some proteins have been found to have portions that stick through BOTH sides of the lipid bilayer (transmembrane proteins)
- Some proteins have been shown to be mobile in the membrane - Images of the membrane show transmembrane proteins This research was performed by analyzing the individual proteins (and amino acids) present in the plasma membrane, identifying things such as hydrophobic amino acid sequences
What is the "fluid mosaic model"
The idea that the core lipid bilayer exists in the fluid state, capable of movement, with proteins penetrating the lipids
Lipid and protein components of the plasma membrane are bound by _____ bonds
What does it mean to say that membranes are amphipathic?
Have hydrophilic AND hydrophobic elements
Describe the three main types of membrane lipids
Phosphoglycerides - Diacylglycerides with small functional head groups linked to the glycerol backbone by phosphate ester bonds
Sphingolipids - ceramides formed by the attachment of sphingosine to fatty acids
Cholesterol - smaller and less amphipathic lipid (only found in animals)
What is the function of cholesterol in the membrane? Where are they found?
Embedded in lipid bilayer, provide structural integrity to the cell
If a lipid bilayer is present (isolated) in an aqueous solution, it will form a ____ called a _____
Sphere called a liposome
Why are liposomes a good tool for drug delivery, particularly in cases of cancer or HIV?
They protect the drug, either keeping it in the aqueous fluid within the liposome, or in the bilayer (if the drug is lipid soluble), or the outside of the liposome can be tagged with an antibody. If you tag the liposome with an antibody to attack a specific cell, it will only attack the correct type of cell
Membrane lipids containing a phosphate group are called _____
Membrane phospholipids built on a glycerol backbone are referred to as _____
Sphingolipids are derivatives of ______. What are they composed of?
Sphingosine. Composed of a sphinosine linked to a fatty acid by its amino group (this molecule type is referred to as a ceramide)
Cholesterol is a lipid component found only in ____ membranes
What enzymes can be used to cleave lipids? Phospholipids?
If cells are incubated with phospholipases, what will happen?
The phospholipases will start to chew away at the outer leaflet of the plasma membrane
How does one determine how much of one type of membrane lipid is present in a certain membrane?
Experimentally apply single doses of phospholipases to cleave specific phospholipids. Compare ratios of cleaved lipids
TRUE OR FALSE: Phospholipid composition differs between the inner and outer leaflets of the plasma membrane. Why is this important?
TRUE. Important because membrane "sidedness" allows for proper function, especially in golgi bodies and other small vesicles
Membrane carbohydrates are _____ linked to lipids/proteins on the extracellular surface of the bilayer
If N-Acetylglucosamine attaches to it's Asparagine by a Nitrogen, what is it referred to as?
Are the proteins attached to the bilayer symmetrical across both sides?
No, some are transmembrane and some are peripheral on only one side
What are the three types of membrane proteins?
Integral membrane proteins, peripheral membrane proteins, GPI-anchored protein
What are the basic features of integral membrane proteins?
Must be amphipathic (hydrophobic in bilayer and hydrophilic outside bilayer)
What are channel proteins?
Proteins creating channels in the membrane (technically integral membrane proteins), Which have hydrophilic cores that form aqueous channels
How can one analyze integral membrane proteins?
Using freeze fracture analysis. Cell is frozen and phospholipid leaflets are divided. Integral membrane proteins appear as bumps/pits using electron microscope.
How can one determine which integral membrane proteins are present on which leaflet?
Using nonpenetrating agents that label the proteins (ex. antibodies)
Receptors for hormones are _____ proteins
Trans membrane proteins
What is the best way to study structure and properties of integral membrane proteins?
Extracting the proteins is very hard, so one must do so by using detergent to break apart the plasma membrane, then Crystallize it to learn structure (crystallography). But that is SUPER DUPER DUPER FUCKING HARD. It's also possible to learn the genome of the protein (and corresponding amino acid sequence). You can then plot them on a hydropathy plot to see if some amino acids will be hydrophobic and some hydrophilic.
How does one study the spatial relationships within an integral membrane proteins
Sit-directed mutagenesis: can replace specific amino acids with others and see how the cell functions
What part of the red blood cell determines human blood type?
The carbohydrates of the glycolipids on the plasma membrane
How to the enzymes of those with varying blood types differ?
People with A blood have an enzyme that attaches N-acetylgalactosamine to the end of the antigen chain, people with B blood have an enzyme that attaches galactose. People with AB blood have both and people with O blood have neither
What are the two phases of membrane lipids?
Gel and liquid-crystal
The temperature at which the liquid crystal transitions into gel phase is called the _____
Why is the fluidity of the membrane beneficial?
Truly rigid membranes are not able to properly assemble and grow, and cannot allow proteins through
_____ fatty acids have a lower transition temperature
How is membrane fluidity maintained by organisms in varying temperatures?
Altering the composition of membrane lipids by saturation or desaturation of acyl chains
How does one verify the importance of fatty acid modifications in the plasma membrane?
Create mutants that cannot modify fatty acids. They will be unable to carry out desaturation reactions in response to cold
Describe lipid rafts
Highly ordered microdomains formed by packed together cholesterol and sphingolipids. They provide a favourable surface for cell surface receptors and GPI anchored proteins
How can cell fusion be induced in experimental environments?
Using certain viruses (ex. sendai virus) or polyethylene glycol
How can one study how membrane proteins interact after fusion?
Label membrane proteins (secondary antibodies against the other cell) then induce two different cells to fuse (ex. mouse and human). Observe that membrane proteins distribute themselves throughout the new fused cell
What is the full name of the FRAP technique? What about SPT
Fluorescence recovery after photobleaching. Single particle tracking
How is FRAP performed
Proteins are labelled with fluorescent dye, small area of cell is bleached using a laser beam (no longer has colour), and then that spot is observed for molecule movement
How is SPT performed?
Tagging one specific protein, observing movement which is evidence of the fluid nature of the plasma membrane
Membrane proteins can move in different fashions. How is this possible?
Movement may be restricted by range or simply because there are too many proteins in one area. They may be held in the substrate or only able to move along cytoskeleton
Are membrane protein movements slower or faster than expected?
Slower, especially given protein size and membrane viscosity
How does the structure of membrane protein affect mobility?
The two leaflets of the membrane have differing compositions, which can provide mobility problems for some proteins
How is membrane LIPID mobility determined?
It is restricted to within the bilayer, and they are confined to certain areas for short periods followed by random movement. Integral membrane proteins can restrict movement of lipids along membrane skeleton
Give an example of a human cell type that has a membrane serving varying functions on different sides. Describe the functions
Ex. GI tract epithelium. For example, the apical (top) plasma membrane performs nutrient/water intake, secretion, protection. The lateral sides are involved in adhesion to other cells and communication between them. The basal membrane generates ion gradients and attaches the cell to the substratum
What are "ghost" red blood cells?
Red blood cells that have been prepared using hemolysis (lysing cells)
When performing SDS-PAGE electophoresis to obtain red blood cell membrane proteins, how does one open up the cell membrane
red blood cell proteins?
Red blood cell peripheral membrane proteins Ankyrin and Spectrin alpha and beta do what? What else does Ankyrin bond to?
maintain structural integrity for those moments when the red blood cell must go through capillaries smaller than itself. Ankyrin also bonds noncovalently to Band 3 proteins
Red blood cell membrane protein Glycophorin A is what kind of molecule?
A dimer with 16 oligosaccharide chains which may prevent red blood cells from clumping
If someone's red blood cells have a mutated Glycophorin A molecule what occurs?
THEY CAN'T GET MALARIA
Red blood cell membrane proteins in Band 3 serve what function?
Exchange Cl- and HCO3- across the membrane
The major component of the internal membrane skeleton in red blood cells is ____
_____ and ____ are cytoplasmic proteins which are linked to the Spectrin protein in the membrane of red blood cells
Flux across the plasma membrane can occur by what methods?
Passive diffusion and/or active transport
TRUE OR FALSE: Glucose does not need a transporter to enter the cell
FALSE. It typically needs a sodium ion
Addition of a carbohydrate is referred to as ____
The carbohydrate of glycoproteins exists as short, branched ______. ``
Give an example of a glycolipid carbohydrate that alters the function of the human body
ABO blood types are dependent on the carbohydrate present on the glycolipids of the red blood cell membrane
Active transport is accompanied by hydrolysis of ____ to ____
ATP -> ADP
Delta G indicates....?
Gibbs free energy
If DeltaG is negative the reaction is _____
______ is the spontaneous movement of material from a region of high concentration to low
If concentration of a non-electrolyte molecule inside over concentration outside the cell equals less than 1, is it a negative DeltaG value?
Electrolyte molecules are otherwise known as _____
Describe the difference between the free energy change during diffusion of electrolytes vs non electrolytes
Non electrolytes move according to the concentration gradient Electrolytes move dependent on the electrochemical gradient
TRUE OR FALSE: The more lipid soluble an electrolyte molecule is, the more easily it can pass through the cell
Uncharged small molecules pass more easily into the cell. Give examples of some of these molecules
O2, CO2, NO, H2O
Lipid permeability is determined by what?
Partition coefficient, molecule size, polarity
Diffusion of water through a semipermeable membrane is called ______
In a hypotonic solution, cells ____
In a hypertonic solution, cells _____
In an isotonic solution, cells _____
stay the same
What do plant cells do in hypotonic solutions?
Develop internal turgor pressure (cell walls prevent swelling)
What do plant cells do in hypertonic solutions?
What are plant cell aquaporins?
Protein channels that allow passive movement of water.
How were aquaporin channels discovered and tested?
They were discovered through electrophoresis and western blot, then they injected the protein into frog eggs, which swelled and became permeable to water!
Ions can only pass plasma membranes through _____, in the direction of _____
Ion channels, the electrochemical gradient
How does one analyze ion channel using patch clamping
Patch clamping is when a micropipette has a small electrode in it of a specific charge, which will analyze the charge of the channels
List the different types of Gated ion channels
Voltage gated channels, lingand gated channels, mechanogated channels
Describe voltage gated channels
Conformational state depends on the difference of ionic charge on the two sides of the membrane
Describe ligand gated ion channels
Depends on the binding of a specific molecule (ligand) which usually isn't the thing passing through the channel
Describe mechano-gated ion channels
Depends on mechanical forces applied to the membrane
Describe the voltage gated potassium channel and its filtration system
It is made of 4 subunits arranged to create an ion conducting pore. The subunits form rings of oxygens to filter in K by forming bonds with 8 oxygens (other ions, for example sodium, couldn't form the bonds necessary to pass through the channel)
The Eukaryotic potassium channel is divided into many segments. Describe them
6 membrane associated helices, divided into two domains: Pore domain - permits selective passage of K ions Voltage sensing domain - Consists of helices S1-S4 that senses the voltage across the membrane
Do Eukaryotic Potassium channels stay open forever?
Over 10 million K ions can pass through per second, so the channel closes after a few milliseconds in a process known as inactivation (inactivation peptide simply clogs the pore).
How do large/hydrophilic substance cross the membrane?
Require a facilitative transporter
What are the important features of facilitated diffusion?
It is passive, specific to certain molecules, saturable (Can reach too many), and regulated (to prevent that)
How does insulin stimulate glucose uptake?
Causing vesicles to insert into the cell membrane
Describe a glucose co-transporter
Glucose and a sodium ion (which is along the concentration gradient) are transported inside of the cells of the lumen (of the GI tract). The sodium ions are sent out of the cell AGAINST the concentration gradient so more glucose may be transported into the cell. The pump brings in K+ ions and out Na+ ions
How is active transport of K+/Na+ related to ATP hydrolysis?
ATPase breaks down ATP, where phosphorylation causes changes in ion affinity that allow transport against gradients. 3 NA+ for every 2 K+, which creates a charge (potential difference)
What is the difference between configuration and conformation?
Configuration - Change in covalent bonds to rearrange molecule
Conformation - Change in 3D structure (does not require energy to change)
How does the Na+/K+ ATPase pump function?
Na+ ions bind to the inside of the pump, which triggers ATP to phosphorylate to ADP. This triggers the other end of the pump to face (open up) outside the cell. IT loses its affinity for NA+ ions and they are released. The new conformation (E2) causes an affinity for K+ ions, which come in from outside the cell and trigger hydrolysis in the cell (phosphate released). IT is now once again in closed conformation (E1) and K+ is released into the cytoplasm
The NA+/K+ ATPase pump is referred to as a _____ type pump. Why is it called this?
P-type pump, named for "phosphorylation"
How do V-type pumps differ from P type pumps?
V (vacuolar) type pumps use ATP but there is no phosphorylation during pumping
Describe ABC transporters
ATP-binding cassette transporters have regulatory ATP binding sites
H+ pumps are also known as ____
How does Zantac prevent acid reflux?
Binds to histamine receptor on parietal cells of stomach, preventing the proton pump from releasing acid
How does the archaebacterial protein bacteriorhodopsin function?
Absorbs light energy to transport protons out of the cell (gradient used to make ATP)
Neurons: What kind of membranes do Schwann cells have?
High lipid to protein ratio - more hydrophobic, more insulation
____ are specialized cells for information transmission using changes in membrane potentias
Describe briefly the parts of the neuron
Dendrite - recieves information Cell body with nucleus Axon - long extension for conducting outgoing impulses Myelin-sheath - wraps around cell
_____ is the membrane potential of a nerve or muscle cell
What is responsible for resting potential of a nerve?
K+ gradients maintained by Na+/K+-ATP-ase
____ is used to calculate the voltage equivalent of the concentration gradients for specific ions.
The resting membrane potential for K+ ions is near ______ and for Na+ ions is near _____
K+ - negative Nenrst potential Na+ - positive Nernst potential
When Na+ channels open, the membrane of nerve cells are _____, triggering _____
Depolarized, triggering the action potential (AP)
After Na+ channels are inactivated following action potential, a _____ occurs
The Na+/K+ action potential channels in neurons are _____ gated channels
Action potentials produce local membrane currents that depolarize adjacent membrane regions, resulting in ______
Velocity of a nerve impulse is increased by the presence of myelin sheath how?
Charges must hop over the myelin sheath which starts much faster movement than what would occur if the charges could simply move along the axon
_______ neurons communicate with ______ neurons at the _____, across a gap called the ______. ______ released from this gap diffuse to receptors on the other cell
Presynaptic neurons, postsynaptic neurons, synapse, synaptic cleft. Neurotransmitters
When a neurotransmitter jumps over the synaptic cleft, what can it do?
Depolarize (excite) or hyperpolarize (inhibit) the cell.
How is neurotransmitter action terminated
by reuptake or enzymatic breakdown
How do Ca2+ ions trigger AcCh?
A nerve impulse stimulates the voltage gated Ca2+ gates to open. Intake of Ca2+ into the axon causes the production and release of AcCh, which is bound in a synaptic vesicle that must be released from the presynapic membrane. The released AcCh binds to the AcCh receptors on the post synaptic membrane. Ligand-gated anion or cation channel opens (dependent on the nature of the channel).
How does "action potential" work?
A stimulus is applied to an axon. Parts of the membrane depolarize. If the stimulus isn't enough, the membrane won't reach the "threshold" and therefore the depolarization (and subsequent influx of Na+ ions) will be temporary. If it surpasses the threshold, the voltage-gated sodium ion channels open and they diffuse freely into the cell. This causes an action potential
When an axon is interacting with a muscle cell, which is the presynaptic cell and which is the post synaptic?
Presynaptic - neuron Postsynaptic - muscle
If a hydropathy plot shows a protein spanning 12 transmembrane domains, what can you infer?
IT could either be a series of channels or a wavy protein sticking out of the cell then back in repeatedly
Label/describe the steps in this process. What is this process called?
Called The Na+/K+ - ATPase pump.
1. 3 sodium ions bind to a protein inside the membrane (ATP is hydrolyzed)
2. Conformation changes so top is opened to external environment
3. Na+ ions expelled to external space and 2 K+ ions bind
4. Phosphate group removed
5. Protein returns to original conformation
6. Potassium ions enter interior of cell
What is happening in this photo? Name the five ion channels and the 1 neurotransmitter
It is a muscular synapse (demonstrating muscular action).
1. Voltage gated Ca2+ ion channel
2. Acetylcholine - gated cation channel
3. Voltage-gated Na+ channe;
4. Voltage-gated Ca2+ channel
5. Gated Ca2+ release channel