Membrane Transport and Membrane Potential Flashcards
How does the permeability of the lipid membrane differ for different types of molecules
Small hydrophobic molecules (O2, CO2, N2, steroid hormones) can pass through the bilayer no problem
Small uncharged polar molecules (H2O, urea, glycerol) will not typically cross unassisted (exception: aquaporins)
Large uncharged polar molecules (glucose, sucrose) need help crossing
Ions cannot pass through the membrane unassisted at all
What are the different major types of transport proteins?
Channel mediated proteins and transporter mediated proteins
What percentage of all membrane proteins are transport proteins?
15-30%
How do passive transport and active transport differ?
Passive transport relies on diffusion of molecules along their concentration gradients
Active transport requires energy input in order to move a molecule against its gradient
What factors contribute to the combinatorial gradient across the membrane?
The concentration gradient and the electrical gradient
What are the different types of ion channels?
Voltage gated
Ligand gated (can be extracellular or intracellular)
Mechanically gated
True or false: If the membrane potential is above a specified threshold, all channels will be open.
Fale
Channels are not simply all open or all closed, the percentage of open channels increases with membrane voltage
What mechanism do transporter proteins utilize to transport solutes into/out of the cell?
Conformational changes allow the exchange of material between the sides of the lipid membrane
What is the difference between primary and secondary active transport?
Primary: uses ATP directly
Secondary: uses energy stored in electrochemical gradients to power movement of molecules
What are the different types of transport molecules?
Uniport
Symport
Antiport
What is the most commonly used ion for coupled transport?
Sodium (Na+)
How is glucose transported into the cell?
Glucose is actively transported into the cell against its own gradient by a transporter protein powered by the sodium electrochemical gradient
What are the three classes of ATP-driven pumps and how do they differ?
P-type pumps: phosphorylated during ion transport
F-type/V-type pumps: F make ATP from the gradient, V hydrolyze ATP to pump H+
ABC transporters: pump molecules while hydrolyzing ATP
How is the Na+ gradient established and maintained?
The Na+/K+ pump hydrolyzes ATP to move 3Na+ out of the cell in exchange for 2K+ into the cell
How much of our energy is devoted to the Na+/K+ pump?
About 1/3
Describe the pumping cycle of the Na+/K+ pump.
1) Na+ binds, ATP hydrolysis, phosphorylation of cytoplasmic residues
2) Conformational change, Na+ transfer
3) K+ binds on extracellular face, dephosphorylation
4) K+ transfer with dephosphorylation induced conformational change
How do ABC transporters function?
ABC = ATP Binding Cassettes
These transporters require the hydrolysis of ATP in order to power a conformational change that allows non-ionic small molecules to pass through the membrane
What are some examples of ABC transporters in disease?
Multi-drug resistance proteins (MDR) pump numberous drugs out of the cytosol. Increased expression is responsible for drug resistance in many cases
CFTR: a Cl- channel that is defective in cystic fibrosis
Flasmodium falciparum: agent of malaria that uses ABC transporter to pump out antimalarial drugs
What is cystinuria?
A genetic defect in an amino acid transporter responsible for removing cystine from urine and the intestine
Individuals devlop cystine stones in their bladders and kidneys
How does glucose get from the intestinal lumen into the extracellular fluid?
It passes through a sodium-glucose symport on the apical side of the epithelium (against its concentration gradient)
On the basal side, glucose passively diffuses out of the cell into the extracellular fluid
True or False: Inside the cell, [Na+] is low and [K+] is high
True
Intracellular [Na+] = 5-15 mM
Intracellular [K+] = 140 mM
True or false: extracellular [Na+] is low and [K+] is high
False.
extracellular [Na+] is high (145mM)
[K+] is low (5mM)
What is the Nernst equation?
For a given ion, here potassium
Vk = (RT/zF)*ln(Ko/Ki)
RT/zF=61.5mV for K
What is the average resting membrane potential for a cell?
-70 mV
True or false: A positive voltage means that a +ion is leaving the cell or a - ion is entering the cell.
True
Positive voltage means positie current and positive current is defined as either a +ion leaging the cell of a -ion entering the cell
If Vk >Vm, a positive ion will ____ the cell and/or a negative ion will _____ the cell
If Vk >Vm, a positive ion will enter the cell and/or a negative ion will leave the cell
What are the Nernst potentials for calcium, sodium, chlorine and potassium?
Calcium = 125 mV
Sodium = 70 mV
Chlorine = -65 mV
Potassium = -90 mV