Section 4 - Membrane Structure and Transport Flashcards
True or false: a cell can exist without a membrane
False: it needs a boundary to distinguish itself
True or false: for proper functioning, the membrane must be immpermeable
False: it must be permeable, although this can be controlled
What is the purpose of the cell membrane?
Maintain the difference between inside and outside, and facilitate transport
What kinds of transport does the cell membrane facilitate?
Transfer of material, information, and energy
What is the structure of the cell membrane?
A think film of lipids and proteins
What holds together the cell membrane?
Noncovalent interactions
True or false: the cell membrane is static
False: it is a very dynamic structure
What do the proteins on the cell membrane do?
Function in signaling, transport, and connection to the cytosol
What is the average thickness of a cell membrane?
~5 nm
What is the cell membrane impermeable to?
Water-soluble molecules
What is the typical structure of a transmembrane protein?
Tend to have a large section on one end of the membrane, and barely poking out on the other side
How much lipid (by mass) composes the cell membrane?
~50%
True or false: all lipids in the cell membrane are amphiphilic
True: they all have hydrophobic regions and hydrophilic regions
What does amphiphilic mean?
A molecule has both hydrophobic regions and hydrophilic regions
What are the majority of the lipids in the cell membrane?
Phospholipids
What is the structure of a phospholipid?
Two hydrocarbon tails and a polar head group are connected via glycerol
What are the majority of the phospholipids (in terms of their tails)?
Have one saturated tail and one unsaturated tail
What does an unsaturated lipid tail look like?
Kinked
What does a saturated lipid tail look like?
Straight
How long is the average carbon chain in a lipid tail?
16-18 carbons long
What happens if a cell membrane has only saturated phospholipids?
It increases packing density, thus decreasing fluidity
What is the significance of having some unsaturated lipid tails in the cell membrane?
Helps reduce packing density, and thus increases fluidity
What is the most common head group on phospholipids?
Choline
What percentage of phospholipids in the cell membrane are phosphatidylcholine?
60-70%
What is the net charge on a choline side chain?
Positive
Where does the positive net charge on a choline come from?
A nitrogen with three methyl groups and another bond
What is the net charge of phosphatidylcholine?
Neutral
What is the second most common head group on phospholipids?
Serine
What percentage of phospholipids in the cell membrane are phosphatidylserine?
30%
What is the net charge on a serine side chain?
Neutral
Where does the neutral net charge on a serine come from?
A negative carboxyl group, and a positive amino group
What s the net charge of phosphatidylserine?
Negative
What is the third most common head group on phospholipids?
Ethanolamine
What is the net charge of an ethanolamine side chain?
Positive
Where does the net positive charge on an ethanolamine come from?
A positive amino group
What is the net charge of phosphatidylethanolamine?
Neutral
What is a membrane potential?
A separation of charge across a cell membrane
What is a typical value of a membrane potential?
-70 mV
Which leaflet is PS found?
The inner leaflet
Why is PS found on the inner leaflet?
It contributes to the negative membrane potential
What is a marker of cell death?
PS found on the outer leaflet of the cell membrane
What does cholesterol do?
Helps to locally increase the rigidity of the membrane
Where is cholesterol found in the cell membrane?
Between the phospholipids
What happens if there is no cholesterol in the cell membrane?
The membrane bursts
Why does a cell membrane burst if there is no cholesterol?
The membrane is too flimsy without cholesterol providing local rigidity
What phospholipid motions are possible in the cell membrane?
Rotate (spin), flex (open/close tails), lateral diffusion (side to side), and flip-flop (switch leaflets)
Which phospholipid motion is the most rare?
Flip-flopping
Why is flip-flopping of phospholipids rare?
Need to bring the hydrophilic head group through a hydrophobic cell membrane
What is needed to help phospholipids flip-flop?
Proteins
What does scramblase do?
Moves a phospholipid from the inner leaflet to the outer leaflet
What does flippase do?
Moves a phospholipid from the outer leaflet to the inner leaflet
Which enzyme moves a phospholipid from the inner leaflet to the outer leaflet?
Scramblase
Which enzyme moves a phospholipid from the outer leaflet to the inner leaflet?
Flippase
What is the overall fluidity of the cell membrane influenced by?
Composition (cholesterol), types of side chains (saturated vs. unsaturated), and temperature
If the membrane is thicker, what can you say about the lipid tails?
They are saturated
What is a lipid raft?
A specialized structure that are fairly rigid
What is contained in a lipid raft?
Cholesterol, transmembrane proteins, and anchoring proteins
True or false: lipid rafts are rigid structures
True: they have many proteins and cholesterol which makes them rigid
Why are many anchoring proteins found on a lipid raft?
This is a rigid structure within the more fluid cell membrane
True or false: proteins are only found in lipid rafts
False: they can be in other areas of the cell membrane too
True or false: the leaflets of the cell membrane are symmetrical
False: they are asymmetrical
What comprises the majority of the outer leaflet?
Phosphatidylcholine
What comprises the majority of the inner leaflet?
Phosphatidylserine
What is the purpose of sugar linked phospholipids?
Self recognition
Where are sugar linked phospholipids found?
The outer leaflet
Which phospholipid is important in cell signaling?
Phosphatidylinositol
What does PI3K do?
Adds phosphate groups to PI to make PIP3
What does phospholipase do?
Cleave PIP2 (head group) to create other second messengers
What is the importance of phosphorylating PI?
This can lead to protein docking sites or creating of second messengers, both for cell signaling
What are glycolipids?
Lipids that contain sugar groups
What is the function of glycolipids?
Alter the charge distribution, protect from harsh environments (pH, shear), and self recognition
True or false: membrane bound proteins are highly regular among different cell types
False: they are highly variable in type and quantity among different cell types
True or false: membrane proteins are highly regulated
True: this is similar to gene expression and protein synthesis
What is a transmembrane protein?
A protein that passes through the membrane
What is a single-pass protein?
A protein that passes through the cell membrane once
What is a multi-pass protein?
A protein that passes through the cell membrane multiple times
True or false: transmembrane proteins must be amphipathic
True: they must have hydrophilic regions and hydrophobic regions
Why do transmembrane proteins need to be amphipathic?
They need to interact with both the aqueous environment and the cell membrane environment
True or false: multi-pass proteins can be parallel or antiparallel
False: they can only be antiparallel
What is a beta barrel?
A protein channel composed of criss-crossing beta sheets
True or false: not all membrane proteins pass through the membrane
True: some are found only in the interior or exterior of the cell
How are proteins that are found only in the interior or exterior of the cell linked to the cell membrane?
Through covalent linkages, or their amphipathic nature
True or false: peripheral proteins directly interact with the cell membrane
False: they are attached through linker proteins
True or false: peripheral proteins can be released from the membrane
True: this can be through chemical or mechanical stimulation
True or false: integral proteins directly interact with the cell membrane
True: they typically modify the bilayer
True or false: integral proteins can be released from the membrane
False: integral proteins stay associated with the cell membrane
What structures are commonly seen in transmembrane proteins?
Alpha helices and beta sheets
What are hydropathy plots?
Plots that show regions of hydrophobicity and hydrophilicity in a protein
What is the purpose of a hydropathy plot?
Determine the number of passes a transmembrane protein makes
True or false: hydropathy plots are step functions
False: there are changes in polarity close to the membrane pass
True or false: multiple alpha helices can dictate the function of a protein
True: they can come together to dictate this function
How can multiple alpha helices come together to dictate the function of a protein?
They can form a channel to regulate movement, or set up charge distributions
What is the function of beta barrels?
Act as transport proteins
True or false: beta barrels are not active
True: they are fairly rigid
How come beta barrels are not active?
Their rigid structure prevents conformational changes
True or false: alpha helices are not active
False: they can move into active and inactive conformations
How come alpha helices are active?
They can move about each other for functional changes
Where are beta barrels localized?
Bacteria, mitochondria, and chloroplasts
What are the characteristics of the inside of the pore?
Hydrophilic
Why are the insides of pores hydrophilic?
They need to interact with water to let molecules pass through
What are the characteristics of the exterior of the beta barrel?
Hydrophobic
Why are the exteriors of beta barrels hydrophobic?
They have to interact with the lipophilic cell membrane
How are amino acids arranged in alpha helices (in terms of transmembrane proteins)?
One side of the turn has hydrophobic amino acids, and the other side of the turn has hydrophilic amino acids
How are amino acids arranged in beta sheets (in terms of transmembrane proteins)?
One zig has hydrophobic amino acids, while the zag has hydrophilic amino acids
True or false: proteins that face the external environment are rarely glycosylated
False: they are commonly glycosylated with oligosaccharides
What is a common example of a glycoprotein?
The glycocalyx
What is the function of the glycocalyx?
Used for cell signaling and cell recognition
What is the structure of the sugars on glycoproteins?
Highly branched with many sugar molecules
True or false: most membrane proteins function as a single unit
False: most membrane proteins function as a complex
What comprises the membrane protein complexes?
Transmembrane proteins, cytosolic proteins, and extracellular proteins
True or false: membrane protein complexes are static structures
False: they can have movement within the membrane
What type of movement is possible for membrane protein complexes?
Rotation and lateral translation
What type of movement is not possible for membrane protein complexes?
Flip-flopping
True or false: proteins that are found in lipid rafts stay in lipid rafts
False: proteins can enter and leave lipid rafts
Why is flip-flopping not possible for membrane proteins?
Need to move large hydrophilic regions through the cell membrane
True or false: if a membrane protein could flip-flop, it would function similarly
False: it would probably not function
Why would flipping a membrane protein not result in a similar function?
It may only work when certain molecules are on one particular side of the membrane (and attaches to a specific domain of the protein)
True or false: membrane proteins are uniformly distributed along the cell membrane
False: proteins can be localized in specific portions of the cell membrane
What is the consequence of membrane proteins being localized to a specific region of the cell membrane?
This can lead to the cell membrane performing different functions
What is an example of a cell that localizes its cell membrane proteins to achieve a unique function?
Epithelial cells
What is the role of an epithelial cell?
Act as a barrier
What is the function of the tight junctions in epithelial cells?
Prevent movement of fluids and cell membrane proteins between the apical and basolateral domains
What is the purpose of the cytoskeleton?
Give membrane its strength
What gives the cell membrane its strength?
The cytoskeleton
How does the cytoskeleton give the membrane its strength?
Providing a site for membrane components to anchor to
What is an example of the cytoskeleton giving the membrane its shape?
Red blood cells have a unique shape (concave disc), mediated by the cytoskeleton
What molecules are excluded due to the cell membrane?
Most polar molecules
Why are transport proteins needed?
Many polar molecules that cannot cross the cell membrane are necessary to survive
True or false: some cells can spend 2/3 of their metabolic energy on transport
True: these cells are usually excitable
Why are there no “oxygen transporters” in the cell membrane?
Oxygen is nonpolar, and thus can cross the cell membrane without the need of a transport protein
What is the consequence of recreational drugs being small and nonpolar?
They can diffuse through the membrane easily
What kinds of cells spend a lot of energy on transport?
Excitable cells
Why do excitable cells spend a lot of energy on transport?
Need proper ion concentration gradients function properly when excited
When happens if ion gradients are not maintained through transport?
The cell will die
True or false: given enough time, almost any molecule can cross the bilayer down its concentration gradient
True: however, this can be a very slow and inefficient process
What are the fastest molecules that can diffuse through the cell membrane?
Small, lipophilic molecules
What types of proteins do membrane transport proteins transport?
Polar molecules
True or false: all membrane transport proteins are multi-pass proteins
True: this structure is needed for proper transport
Why are all membrane transport proteins multi-pass proteins?
They need regions of both hydrophobicity and hydrophilicity in order to function
What is another name for a carrier?
A permease
How do carriers work?
A solute binds, causing a conformational change to transport the solute
How do channels work?
Solutes go through the channel down their concentration gradient
Which membrane proteins are easiest to control?
Carriers and pumps
What is facilitated diffusion?
Molecules move through a channel protein down its concentration gradient
What is active transport?
Energy is required to move a molecule up its concentration gradient
What is the electrochemical gradient?
The electrical potential difference also determines how solutes will move across a membrane
What types of proteins utilize passive transport?
Channels
What types of proteins utilize active transport?
Pumps
What is the energy source for active transport?
ATP, or an electrochemical gradient
True or false: the solute is changed when it moves through a transporter
False: the solute does not change shape, the protein does
True or false: the conformational changes of the transporter are reversible
True: this allows for the protein to transport solutes multiple times
What are the types of active transporters?
Coupled transporters, ATP-driven pumps, and light-driven pumps
How do coupled transporters work?
The uphill transfer of one solute is coupled to the downhill transfer of another solute
How do ATP-driven pumps work?
ATP hydrolysis is coupled with a solute moving up its concentration gradient
How do light-driven pumps work?
Input of energy from light is coupled with a solute moving up its concentration gradient
Why are coupled transporters used a lot in the cell?
They are an indirect use of energy (concentration gradient)
What is a uniport transporter?
Moves one solute up its concentration gradient
What is a symport transporter?
Two molecules move together across the cell membrane
What is an antiport transporter?
Two molecules move to opposite sides of the cell membrane
Which cells use a lot of symports and antiports?
Epithelial cells
What is an example of a cotransporter?
The glucose transporter
How does the glucose transporter work?
Sodium moving down its concentration into the cell is coupled to glucose moving up its gradient into the cell
What kind of coupled transporter is the glucose transporter?
Symport
How do epithelial cells achieve transcellular transport?
Through distributing transporters non-uniformly
What transporter is seen at the apical domain of the epithelial cell?
A glucose-sodium symport, bringing both of these solutes into the cell
What transporter is seen at the basolateral domain of the epithelial cell?
A glucose channel, and a sodium-potassium pump
How does transcellular transport in epithelial cells work?
- A glucose-sodium symport brings sodium and glucose into the cell (down sodium’s concentration gradient, and up glucose’s concentration gradient) at the apical domain
- Glucose is released through a glucose channel at the basolateral domain (down its concentration gradient)
- Sodium is pumped back out of the cell through a sodium/potassium pump (up its concentration gradient)
At the apical domain, how is glucose moving (in terms of its concentration gradient and direction)?
Up its concentration gradient into the cell
At the apical domain, how is sodium moving (in terms of its concentration gradient and direction)?
Down its concentration gradient into the cell
At the basolateral domain, how is glucose moving (in terms of its concentration gradient and direction)?
Down its concentration gradient out of the cell
At the basolateral domain, how is sodium moving (in terms of its concentration gradient and direction)?
Up its concentration gradient out of the cell
What drives glucose into the epithelial cell?
The movement of sodium down its concentration gradient
What drives sodium into the epithelial cell?
Its concentration gradient
What drives glucose out of the epithelial cell?
Its concentration gradient
What drives sodium out of the epithelial cell?
An ATP-driven pump
How come the glucose-sodium symport in epithelial cells cannot be flipped?
There is little sodium inside the cell to drive the transport
How come the glucose channel cannot be at the apical domain?
This will cause the glucose to leave the cell, reversing the work of the glucose-sodium symport
What is another name for ATP pumps?
Transport ATPases
How do transport ATPases get their energy to function?
Through the hydrolysis of ATP into ADP and Pi
What are the three classes of ATP pumps?
P-type, F-type, and ABC transporter
What are P-type pumps?
ATP pumps that can phosphorylate themselves
What are P-type pumps used for?
Maintaining ion gradients across the cell membrane
What are F-type pumps?
ATP pumps that use a proton gradient to produce ATP
Where are F-type pumps found?
In the inner matrix of the mitochondria
What are ABC transporters?
ATP pumps that transport small molecules across the cell membrane
What is a V-type pump?
An ATP pump that uses ATP to pump protons against their concentration gradient
What type of pump is a V-type pump similar to?
An F-type pump
What is the difference between a V-type pump and an F-type pump?
The direction that they work (use ATP or make ATP)
Which pumps can phosphorylate themselves?
P-type pumps
Which pumps act like a turbine to produce ATP?
F-type pumps
Which pumps transport small molecules across a membrane?
ABC transporters
Which pumps act like a turbine that uses ATP?
V-type pumps
What engineering structure resembles the F-type pump?
A turbine
What type of pump is the calcium pump?
A P-type pump
When the calcium pump is unphosphorylated, what is its structure?
Two alpha helices are not in contact (closed state), allowing for calcium to bind
When the calcium pump is phosphorylated, what is its structure?
A conformational change pushes calcium out of the cell, against its gradient
What does the sodium-potassium ATPase do?
Moves 3 sodium out of the cell, and moves 2 potassium into the cell
True or false: the sodium-potassium ATPase uses ATP to drive both sodium and potassium up their concentration gradients
True: this resets the ion gradient, which is important for excitable cells
What kinds of cells are sodium-potassium ATPases critical for?
Excitable cells
Why are sodium-potassium ATPases critical for excitable cells?
Need to be able to reset concentration gradients that drive action potentials
What determines whether sodium or potassium will bind to the sodium-potassium ATPase?
The conformation of the pump (which changes throughout the cycle)
What is the rate limiting step for the calcium pump?
Presence of calcium
Why is the presence of calcium the rate limiting step for the calcium pump?
The cytosolic concentration of calcium is so low, so the pump is always ready
What is the rate limiting step for the sodium-potassium ATPase?
The binding of sodium
Why is the binding of sodium the rate limiting step for the sodium-potassium ATPase?
Need to bind three ions close together in the pocket of the pump
How much ATP is needed to drive an ABC transporter?
2 ATP molecules
What is the typical structure of an ABC transporter?
A dimer
What is an example of a general ion channel?
Cation / anion channels, etc.
What is an example of a specific ion channel?
Sodium / potassium channels, etc.
Which types of transporters are the fastest?
Ion channels
Why are ion channels the fastest transporter?
No conformational change is needed to function
Which is larger: sodium or potassium?
Potassium
What question can be raised about potassium channels letting sodium through?
How can the potassium ion channel exclude sodium if potassium is bigger than sodium?
What is a selectivity filter?
Interactions between the channel proteins to allow only certain ions to pass through
Compared to the ion, what size should the channel protein be?
Enough for force interactions between the ion and the channel protein (selectivity filter)
What is the state of ions in a biological environment?
Hydrated with water
What is the key to understanding the solution to the problem of sodium passing through the potassium channel?
Ions must shed all of their hydrating ions
What does it mean for an ion channel to be gated?
It can be turned on or off
How can a gated ion channel be turned on or off?
Through voltage, chemical, or mechanical stimulation
Why do ions associate with water?
It lowers the energetics of the ions (more stable)
How do voltage gated ion channels work?
They have highly charged regions, which can restrict or open the ion channel
What is the structure of the selectivity filter for potassium?
- It has negative amino acids to attract the potassium cation
- The carbonyl oxygens are spaced enough to bind to potassium when it sheds its water molecules
What is used to match the hydration of the ions?
The carbonyl oxygens on the amino acids
Why are carbonyl oxygens used in the selectivity filter?
They resemble the oxygens on water (hydration)
What happens if a hydrated sodium tries to enter a potassium channel?
It is too big to fit through the pores
What happens if a nonhydrated sodium tries to enter a potassium channel?
It is too small to interact with all of the carbonyl oxygens, and thus is energetically unfavorable (will not enter the pore)
What are the differences in structure between a cation channel and an anion channel?
- The cation channel has four subunits, while the anion channel has two
- The cation selectivity filter is at the edge, while the anion selectivity filter is in the middle
