Membrane Transport Flashcards
How is chemical disequilibrium maintained across the plasma membrane?
The result of the permeability porperties of the plasma membrane and the actions of specific transport proteins
How does cholesterol affect membrane permeability to water?
Low cholesterol = Higher water permeability
What four things affect the rate of diffusion of a molecule through a lipid bilayer?
Membrane permeability
Surface area
Thickness
Concentration gradient
What is an electrochemical gradient?
The combined effects of the concentration and electrical gradients across a membrane
What are the three functional categories of membrane transport proteins?
Channels
Transporters
ATP-powered pumps
What is the structure and function of a channel?
Transmembrane protein that forms a water-filled pore
Selective
Rapid
Gated
Flow occurs down the concentration gradient
What is the structure and function of transporters (Carrier Proteins)
Bind substrate with high specificity
Do not form direct connection
Conformational change responsible for transport
Facilitated diffuciton
What are uniporters?
Carriers that bind and transport only one type of substrate.
Form of facilitated diffusion
What are cotransporters and how do they function?
Protein carriers that move more than one substrate at a time.
Often couple transport down a gradient with transport against a gradient
Secondary active transport
What are the two types of cotransporters and what is the difference between them?
Symporters - move substrates in the same direction
Antiporters - move substrates in opposite directions
Describe the function of ATP-powered pumps.
Use energy from ATP to move substrates against their concentration gradient.
Primary active transport
What are aquaporins and what is their function
Family of integral membrane channels that selectively transport water
What are the two types of ion channels?
Leak channels - always open
Gated channels - open and close in response to a stimulus
What are the three ways ion channels are gated?
Voltage
Ligand
Mechanically
How does the selectivity filter on voltage gated K channels work?
The hydration shell of K is displaced as K interacts with carbonyl oxygen groups lining the pore, allowing K to pass.
Na is too small and tends to remain hydrated
How does the selectivity filter of a Na channel work?
Essentially a size filter unique to the ionic radius of Na
What are three properties of carrier-mediated transporters?
Specificity
Competition (e.g. glucose and galactose
Saturation
What is Kt in the rate equation for the transport of glucose through GLUT1?
Used to describe the affinity fo the glucose transporter for glucose
What is GLUT4 regulated by and how does this affect transporter kinetics
GLUT4 is regulated by insulin —> induces translocation of GLUT4 to the plasma membrane
Increases Vmax
What is SGLT1 an example of?
A symporter.
Transports glucose and sodium from the intestinal lumen into the cell
Glucose moves against its concentration gradient
How is the Na gradient maintained for use by SGLT1?
Na/K ATPase at the basal surface of the plasma membrane
The Chloride-Bicarbonate exchanger is an example of what kind of transporter?
Antiporter
What are P-class pumps and what is an example?
P-class pumps are ion pumps made up of two a subunits, one of which gets phosphorylated during the transport cycle.
What is the function digoxin and ouabain
Inhibit Na/K ATPase
What is the function and role of V-class pumps?
Pump protons against their electrochemical gradient from the cytosol into intracellular organelles via ATP hydrolysis.
Lower the pH within the lumen of organelles.
E.g. osteoclasts use V-pumps to promote bone resorption
What is the function and role of F-class proton pumps?
They utilize potential energy from the movement of protons down their electrochemical gradient to generate the formation of ATP.
Also know as ATP synthase
What is the general structure of ABC transporters?
Two transmembrane domains that form pathways through which solutes cross and determine specificity
Two nucleotide binding domains or (ATP-binding cassette domains) that face the cytosolic side
What function do ABC transporters usually serve in eukaryotes?
Exporters
What does the exporter function of ABC transporter do for the cell?
Protect cells from the toxic effects various endogenous metabolites and xenobiotics
What is the effect of multidrug resistant transporters?
The bestow multidrug resistance to tumor cells because they transport the drug out of cell, thus increasing the necessary dose of the drug to kill the cells.
What are three factors that channel selectivity can depend on?
Charge of the ion
Size of the ion
Amount of water attracted to and retained by the ion (hydration shell)
