Week 7 Textbook Flashcards
plasma membrane
a protein - fatty - thin, contains components inside
- 2 layers of lipid molecules with proteins inserted inside
- allows import/export, deformation, self-healing,
how did scientists find the lipid bilayer
since the layer can dissolve in organic solvents, they used benzene to extract all the lipids from the plasma membrane of RBCs - spread on a film and saw that they formed a continuous sheet which was one molecule thick
- discovered the hydrophilic head and hydrophobic tails
definition of lipid bilayer
a pair of sheets composed mainly of phospholipid molecules that forms the structural basis of all cell membranes
phospholipids
a major type of lipid molecule in cell membranes
- composed of 2 fatty acid tails and one phosphate group = polar
how is phosphatidylcholine a phospholipid
it is the most common bc it has 2 hydrocarbon tails + glycerol + phosphate group + choline on the end (still makes the head polar)
what does it mean to be amphipathic and give an example
molecules that are both hydrophilic and hydrophobic
- cholesterol which is the membrane of animal cells - has a hydrocarbon tail and a polar head
- glycolipids - sugar apart of their hydrophilic head
explain how hydrophilic molecules interact with water molecules
hydrophilic = polar, ex, acetone which has a dipole.
the positive carbon will interact with the negative end of the water (oxygen) and the oxygen on the acetone will interact with the water-positive hydrogens
explain how hydrophobic molecules interact/format themselves in water
they cannot form favourable interactions with water so they have to force water molecules to form a cage-like structure around them
- this cage structure is more highly ordered so it needs Gibbs free energy
- this is why the fats/oils in water form large fat droplets
what does a triacylglycerol
glycerol (head) with a hydrocarbon tail - the 2 parts are attached by a C=O
- they are the main constituents of animal fats and plant oils = entirely hydrophobic
explain what would happen if there was a tear in the bilayer
hydrophobic tails facing inside the membrane away from the water
hydrophilic heads facing outside the membrane
any tear in the sheet will create a free edge that is exposed to water - this is energetically unfavourable - they will spont rearrange to eliminate the free edge
- if the tear is large, then the sheet may begin to fold in on itself and break up into separate closed vesicles
why is a planar amphipathic sheet unfavourable and how can it become favourable
it is unfavourable because it has edges that allow water to be exposed from the hydrophobic tails
- they bend and seal forming a closed space
- makes the cells circular
the circle = energetically favourable
t/f the lipid bilayer is flexible and fluid
true
when using laser tweezers, the components do not leak out
what is a liposome
pure phospholipids will form closed spherical vesicles
what are the 4 movements that the lipid molecules can do in the bilayer
they can rotate, lateral diffusion (switch places with adjacent molecules), flexion (move tails right and left), flip-flop (rarely occurs but moves the top bilayer molecule to the other end of the bilayer)
because of these movements, the bilayer behaves as a 2D fluid
t/f the more closely packing the tails and composition of the bilayer is, the more/less viscous/fluid the bilayer will be
true
their length and number of double bonds in the tails affect
and the cholesterol available in the bilayer
what are the 2 major properties of hydrocarbon tails
length
- a shorter chain length reduces the tendency of the hydrocarbon tails to interact with each other and form LDFs - short chains = increase the fluidity of the bilayer
number of double bonds
- double bond = not the maximum, number of hydrogen bonds = unsaturated = forms a kink in the tail = this makes it difficult for the tails to pack against one another and form LDFs this makes the membrane more fluid and dynamic
- all single bonds = saturated = more stiff, less molecules can move thru this
t/f bacterial and yeast cells are constantly adjusting their tails
true
they do it in different conditions - varying temperatures
at high temps they make tails that are long and have less double bonds
what does it mean to hydrogenate
vegetable oils can be turned into margarine by hydrogenation
- it is the addition of hydrogens - this can remove the double bonds and make them more saturated
- this makes the oil solid into butter at room temperature (less fluid/dynamic)
what is cholesterol
the membrane fluidity is modulated by the inclusion of the sterol cholesterol
- it can fill in the spaces of phospholipids that have kinks in their tails due to unsaturation
- cholesterol can stiffen the bilayer - making it less flexible and less permeable = stiff
what is the use of having fluidity in the membrane
for proteins to go in and out
cell signalling
ensures that membrane molecules are distributed evenly between daughter cells when a cell divide
where are new phospholipids manufactured in the cell
made by enzymes in the cytosolic surface of the endoplasmic reticulum
- these enzymes deposit the newly made phospholipids only in the cytosolic half of the bilayer
how do new phospholipids make it to the opposite monolayer?
phospholipids are transferred by a protein called scramblase - this is a type of transporter protein that removes randomly selected phospholipids from one half of the lipid bilayer and inserts them in the other
what does flippase do
it is apart of the Golgi membrane
- contains flippase which is the phospholipid-handling transporter
- they use ATP to transfer SPECIFIC phospholipids from one side of the bilayer to the other (moving lipids from the exterior monolayer to the interior)
= this maintains the aymmetrc arrangement of phospholipids that is characteristic of the animal cell membranes
cytosolic and noncytosolic monolayer
cytosolic layer faces the cytosol
noncytosolic monolayer faces the lumen which is the interior space of the organelle
how do glycolipid molecules position themselves in the membrane
they have sugar groups on the ends
the sugars are placed only to the exterior of the cell with the extracellular space
- glycolipids are distributed asymmetrically in the lipid bilayer of the animal cell plasma membrane
what are the functions of membrane proteins
transport certain nutrients (Na+ pump)
receptors that detect chemical signals in the cells environment (platelet driver growth factor - PDGF)
enzymes to catalyze specific reactions at the membrane
anchors (integrins)
explain how transmembrane proteins interact with the bilayer
has both hydrophobic/philic regions
the phobic regions lie in the interior and the philic are exposed to the aqueous environment
what are 4 types of interactions that proteins can have with their bilayer
- transmembrane proteins
- proteins that are associated with the systolic half of the bilayer by an amphipathic alpha helix
- some lie entirely outside the bilayer on the other side - they are attached to the membrane by covalently attached lipid groups
- some proteins are only held in place by their interactions with other membrane proteins nd face only one way
what are integral membrane proteins and how can they be removed
they are the proteins that are directly attached to the bilayer
- we remove only by disrupting the bilayer with detergents
- the rest of the proteins are called peripheral membrane proteins which can be removed with simpler extraction procedures that leave the bilayer intact
what is the transmembrane protein’s orientation and attachments
connected to the specialized membrane-spanning segments of the polypeptide chain
these segments run through the hydrophobic (interior of the bilayer) - composed largely of amino acids with hydrophobic side chains
- bc these side chains cannot interact with the water molecules they prefer to interact with the hydrophobic tails of the lipid molecules
t/f the peptide bonds that join the amino acids in proteins are normally polar
true
making it hydrophilic
bc water is absent in the hydrophobic part of the bilayer, the atoms that are part of the polypeptide backbone are thus driven to form h bonds with one another
- H bonds are maximized if the polypeptide chain forms a regular alpha helix
in the spinning helix, the hydrophobic lipid tails are on the outside (between the phospholipid tails) and the hydrophilic polypeptide backbones form H bonds with one another within the helix
how do multipass transmembrane proteins arrange themselves in the membrane
usually amphipathic formed from alpha helices
- hydrophobic side chains fall on the inside of the circular arrangement
- the hydrophilic side chains are concentrated on the other side
- form into a ring structure
T/F the most common form that a polypeptide chain crosses the lipid bilayer is beta sheets
false - it is alpha sheets
how do beta sheets arrange themselves as transmembrane proteins
beta-sheet is rolled into a cylinder = beta-barrel
the amino acid side chains that face the inside of the barrel = aqueous channel = hydrophilic and the outside lining of the beta barrel is in contact with the hydrophobic tails
what are porin proteins
they have a lot of beta-barrel structures
they form large, water-filled pores in mitochondrial and bacterial outer membranes
porins allow the passage of small nutrients, metabolites and inorganic ions across their outer membranes while preventing unwanted larger molecules from crossing
how do detergents work to examine the lipid bilayer in detail
it is a soapy substance used to solubilize lipids and membrane proteins
- disrupting the hydrophobic associations
the detergents are small amphipathic lipid-like molecules that only have a single hydrophobic tail - in water these molecules tend to go into small irregular clusters called micelles
- when you mix it, the hydrophobic ends of detergent molecules interact with the membrane-spanning hydrophobic regions of the transmembrane proteins and the phobic tails of the phospholipids (disrupts bilayer)
what happened when scientists fused a mouse cell and a human cel together and watched the activity of the plasma membrane?
the 2 cells remained on either side of their halves, but after time, the two sets of proteins became evenly mixed over the entire x2 sized cell
what are membrane domains
functionally and structurally specialized regions in the membrane of a cell or organelle - typically characterized by the presence of specific proteins
what are the ways lateral mobility of plasma membrane proteins can be restricted
- proteins can be tethered to the cell cortex inside the cell
- proteins can be tethered to just the extracellular matrix molecules outside the cell
- the proteins on the surface, interact with the adjacent cell
- proteins can have diffusion barriers which can restrict proteins to a particular membrane domain
what is the basal lamina
it is a mat of extracellular matrix that supports all epithelial sheets
how do we measure and study the fluidity of cell membranes
- FRAP - fluorescence recovery after photobleaching
labelled components of the cell membrane with a fluorescent marker - monitored in a fluorescent microscope - measuring the amount of time the proteins take to migrate into the bleached region of the membrane
DRAWBACK
- monitors the movement of fairly large populations of proteins so there’s no way to track the motion of individual molecules -
- they have developed methods to label individual molecules = SPT single particle tracking microscopy - tagging the molecules with antibodies
- mild detergents can be used to solubilize and reconstitute functional membrane proteins - artificial lipid bilayers generally diffuse more freely and rapidly
- it is bc in the real cell membrane they are surrounded by more types of proteins and contain a greater variety of lipids than an artificial lipid bilayer
whats the difference between an artificial bilayer and a cell membrane bilayer
the artificial one is protein free = liposome = impermeable to most water soluble molecules
the real cell membrane has transport proteins to transfer specific molecules by facilitated transport = those who cannot diffuse on its own
what kinds of molecules can diffuse into the membrane easily
small, nonpolar molecules and small uncharged polar molecules
= concentration gradient - goes from high to low concentration
nonpolar:
steroids, horomes, CO2, O2 - simple diffusion all hydrophobic moloecules = faster diffusion
uncharged polar:
H2O, ethanol, glycerol
- the small the better but most of it cant go thru
what kinds of molecules need membrane proteins to go into the membrane
larger uncharged polar molecules
- amino acids, glucose, nucleosides
a very tiny amount can but most cannot
large charged polar molecules (ions)
- H+, K+, Cl-
- none can go thru
what are the 2 main classes for membrane transport proteins
- channel
- selective about size and electric charge
- transient interactions with the channel - van der waals, h bonds, etc
- there is no conformation changes, always stay the same shape = open channel - transporter
- the solute can be any kind just need to fit inside binding site
- conformational changes - can only take one solute at a time so needs to close and open on the other end
what falls under passive transport
channel mediated, and transport mediated proteins
driven by concentration gradient from high to low
what is active transport
pushes solutes against the conc gradient - from low to high
needs ATP to open and close and release contents
concentration gradient + membrane potential = ____
electrochemical gradient
what does the electrochemical gradient induce
passive transport
when the voltage (charges on either side) and the concentration gradients work in the same direction, what happens?
theres is a fast diffusion
if its + on the outside and - on the inside, and the solute has a + charge it will repel the + on the outside and have a large driving force that pushes to the negative side where the +/- have attraction
when the voltage (charges on either side) and the concentration gradients work in the opposite direction, what happens?
if your moving + solutes out of the cell where the + is outside and the - is inside, you will get a smaller net driving force because its going in the unfavourable direction where the + will repel the + on the outside instead of moving TOWARDS the charges that it attracst
explain the structure of channel proteins and how they work
hydrophobic core that pinches close together at a certain area to have transient interactions and be selective - this area strips the K+ ion of any other negative charge surrounding it to make sure it only transports the singular K+ = ion channel transport
- fastest transport
what is an ion channel
it is a type of channel protein
found in animals and plants that transports ions only
what are the 2 kinds of ion channels
- non gated ion channels
- always open like K+ leak channels
- moving K+ out of the cell leak - moves in the direction of the gradient
- this can generate the resting membrane potential by creating a voltage on one side - gated ion channels
- some type of signal is required to open the channel and then it stays open - requires specific kind of ion to pass thru as it has the pinching mechanism
what are the 4 kinds of gated ion channels
- mechanically gated
- the signal to open the gate would be the stretching or compressing of the membrane - ligand-gated extracellular ligand
- the signal would be neurotransmitters from outside of the cell binding in its binding spots on the outside - ligand-gated intracellular ligand
- the signal would be like an ion or nucleotide that binds to the binding site on the inside of the protein to open it - voltage-gated
- the signal would be a change in voltage across the membrane
- making the membrane depolarized (negative charges on the outside0
explain the transporter proteins
they can only bind to a specific solute that goes thru a conformational change to move the solutes to the other side
- follows the concentration gradient
- when there is more of a difference of concentration on either side = more speed in transporting to resolve
explain the Vmax of the transporter proteins
the rate of a simple diffusion channel protein is much faster and can increase in speed
but the transporter protein can only go up to Vmax bc it carries one solute at a time and gets saturated and cant move any faster
explain this type of transporter protein: uniport
one solute moves at a time
down its electrochemical gradient
works with concentration gradient and it can be reversible depending on the gradient of that solute
EX: glucose transporter (GLUT Uniporter)
- transports glucose down its gradient
- can go in or out of the cell (reversible)
what are the 3 types of active transport
- gradient driven
- symport and antiport - atp driven
- P-type, V-type, ABC transport - light driven pump
explain the types of gradient driven active transport
- symport
- can move 2 solutes at the same times in the same direction
- one goes against the gradient
- one goes towards gradient
EX; Na+/Glucose symporter - antiport
- can move two solutes in the opposite direction
- one goes against the gradient
- one goes towards the gradient
EX: Na+/H+ antiporter
- both use this principal
that the free energy from the 1st solute moving down its gradient is used to transport the 2nd solute AGAINST its gradient (coupling) - the one going against needs more free energy since its unfavourable
explain the Na+/Glucose symporter
na+ down the electrochemical gradient (high to low) provides the energy needed to move the glucose against its own concentration gradient
- the conformational change only occurs when both solutes are in the binding site
occluded empty = closed, no Na+ or glucose in binding site
occluded occupied = the brief second that both solutes are in the binding and the open ended closes while the other end is preparing to open
occluded empty again
explain the Na+/H+ antiporter/exchanger
both solutes have their own unique gradient
the na+ moves from high to low - provides free energy
the H+ gradient moves against (low to high) - uses free energy in Na+ to move against
some reasons why there is high conc of H+ on the outside of the cell is bc lysosomes release H+ and it needs to be maintained for enzyme function to occur in this environment
- the transporters maintain the pH inside the cell
- so when the pH drops (lots of H+ makes acidic) inside the cell they move H+ outside
since the gradient of Na+ drives and gives free energy to alot of other pumps, how does the Na+ gradient remain maintained?
Na+/K+ pump - ATP driven
- needed bc the symporter and antiporter proteins wont work without an Na+ gradient givning free energy
what are the 3 types of ATP driven pumps
P type
- Na+/K+
V type proton pump
ABC transporter
explain P type pumps
uses ATP hydrolysis to transport solutes
- phosphorylated during pumping cycle
ATP –> ADP + P
the P phosphorylates the transporter protein and changes conformation
- flippases that move PL to the other side = P type pump
explain the P-type pump using Na+/K+ mechanisms
uses ATP hydrolysis
the Na+ moves against the gradient - from low to high ( 3 Na+ out)
the K+ moves against the gradient - from low to high
( 2 K+ in)
Na+ gradient is useful for transporting nutrients from other pumps like glucose and maintain pH
explain in depth the pumping cycle of the Na+ K+ pump (EXAM)
K+ high in cytosol (inside)
Na+ high in EXC
the pump opens, 3Na+ goes in
- ATP –> ADP + P, phosphorylates the pump to change conformation and expels the 3Na+ outside
the pump is now exposed and open to the EXC side where 2K+ go inside
- ADP + P –> ATP (dephosphorylates - removal of P from pump)
turns the pump back to its og conformation to open the pump towards the inside and 2K+ is released
what is the use of P-type pumps
generates and maintains the electrochemical gradients
the Na+/K+ maintains the Na+ electrochemical gradient for driving antiport and symport
also plays a role int he membrane potential (overall + or - charge on one side)
H+ pump in plant cells are also P-type
maintains the H+ gradient and used to drive symport and antiport like Na+ does
also plays a role in the membrane potential
explain the ABC transporter
uses 2 ATP to pump small molecules across the cell membrane
usually its toxins out of the cell
explain the V type proton pump
uses ATp to pump H+ into the organelles to make them acidic (MOVES AGAINST GRADIENT)
this is useful for organelles like the lysosome and plant vacuole (digestion)
explain the F-type ATP synthase
structurally similar to he V type but uses H+ gradient to drive the SYNTHESIS of ATP
moves H+ from high to low so ADP + P –> ATP can occur
- this is useful in the mitochondria, chloroplasts and bacteria
explain how transporters work together to transfer glucose from the intestine to the blood
in one epithelial cell there is the gut lumen, the apical domain with the microvilli, then you have the lateral domain and then the basal lamina, then EXC of the bloodstream
there is a Na+/glucose symport pump on the microvilli to move Na+ and glucose inside (Glucose against gradient)
there is a glucose pump (GLUT transporter high to low) attached to the basal lamina to move glucose into the bloodstream but it needs the energy of the Na+/K+ pump also on the basal lamina
theres low conc of glucose in the gut lumen and a high concentration of glucose in the lateral domain of the epithelial cell where the Na+/glucose symporter work. then theres low concentration of glucose in the bloodstream
how are membrane proteins restricted to particular domains of the plasma membrane of the epithelial cells in the gut
creating boundaries with tight junctions
the apical membrane (the surface of the gut lumen) has the Na+/glucose symporter
the basolateral plasma membrane (basal lamina and the lateral domain share the same protein)
- has the GLUT uniporter (only moves glucose)
- also has the Na+/K+ to give energy
what is the significance of the tight junctions
without tight junctions the glucose or the molecules of transport wont know where to go
what is the membrane potential
it is the difference in electrical charge on both sides of the membrane
- used by the gradient driving pumps to carry out active transport like symport and antiport
how does the animal cell generate membrane potential
- K+ leak channels
- outward flow of K+ (out of cell) using gated ion channels - the signal to open the gates is when the membrane potential or voltage is off
- when the driving force due to K+ gradient = the driving force due to voltage gradient - Na+/k+ pump
- maintains Na+ low in the cell and K+ high in the cell
since 3Na+ and 2K+ there is a net 1+ ion which is pumped out making the membrane potential outside = (+) and inside = (-)
explain the 2 pumps that generate the membrane potential in animal cells
the K+ leak channels and the Na+/K+ pump gives the outside of the cell slightly more +
the K+ leak channels move the K+ out of the cell from high to low
the Na+/K+ has a net of 1+ outside the cell
makes the outside + and the inside - bc there are also Cl- fixed anions inside the cell
what is the equilibrium resting membrane potential
around -20mV to -200 mV
explain the generation of membrane potential in plant cells
the P-type H+ pump generates a H+ electrochemical gradient inside the cell
used by gradient driven pumps to carry out active transport
- regulates pH and electrical signalling
