Transport: Breaking the Barrier Flashcards
Lecture 17
What is relative permeability of the phospholipid bilayer based on?
concentration
size
hydrophobicity
charge
Are gases (like CO2, N2, and O2) permeable, slightly permeable, or impermeable?
Permeable
Are small uncharged polar molecules (like ethanol, urea, and water) permeable, slightly permeable, or impermeable?
Some are permeable, others aren’t. Ethanol is permeable, but urea and water are only slightly permeable.
Are large uncharged polar molecules (like glucose and fructose) permeable, slightly permeable, or impermeable?
Impermeable
Are ions permeable, slightly permeable, or impermeable?
Impermeable
Are charged polar molecules (like amino acids, ATP, glucose 6-phosphate, proteins, and nucleic acids) permeable, slightly permeable, or impermeable?
Impermeable
What substances can be transported via simple diffusion? Why?
Nonpolar molecules and very small polar molecules (water, glycerol, and ethanol)
Due to the hydrophobic interior of the membrane
What are the three passive transport mechanisms?
Simple Diffusion
Facilitated Diffusion with pores and channels
Facilitated Diffusion with transporters (carriers)
Describe the rate of influx of simple diffusion.
Linear, solely based on going down a concentration gradient
Describe the rate of influx of facilitated diffusion.
Linear towards the beginning, but begins to plateau towards the end as the extracellular concentration decreases
Describe the rate of influx of facilitated diffusion using carriers.
Linear towards the beginning, but plateaus at Vmax, when all carrier molecules are occupied
Does passive transport require an energy source?
No
Does active transport require an energy source?
Yes
Does active transport go up or down a concentration gradient?
Up
Does passive transport go up or down a concentration gradient?
Down
What are the two ways active transport can find the energy to proceed?
Directly, via use of ATP, redox reactions, or light
Indirectly by coupling to concentration gradient
How many ions can move through an ion channel per second?
10^7-10^8 ions per second
What are the 4 types of ion channels in mammal cells?
Voltage-gated
Ligand-gated w/extracellular ligand
Ligand-gated w/intracellular ligand
Mechanically-gated
How many molecules move through transporters (carriers) per second?
100-10,000 (10^2-10^4) molecules per second
What is a uniporter?
a uniporter is a transporter that only transports one solute across a membrane
What is a symporter?
a symporter is a transporter that transports two things in the same direction simultaneously; the transport of either is reliant on the presence of the other; needed for indirect active transport
What is an antiporter?
an antiporter is a transporter that transports two things in the opposite directions simultaneously; the transport of either is reliant on the presence of the other; needed for indirect active transport
What are the the two kinds of transporters used for coupled transport? Which is the only transporter not used for coupled transport?
Symporters and antiporters are used for coupled transport.
Uniporters are not.
What is a coupled transport?
When two solutes are transported simultaneously and their transport is coupled such that transport of either stops if the other is absent
What is an example of a uniporter?
GLUT1 transports glucose down its concentration gradient, from the exterior of the cell into the cytosol.
Describe how glucose enters the cell.
- Glucose arrives at a transport site in GLUT 1.
- An in-ward facing conformation occurs on the cytosolic side of a cell.
- The glucose enters the cell.
- There’s an outward-facing conformation that opens the transport site to more glucose.
Describe transepithelial transport of sodium and glucose.
Uses a 2 Na+/glucose symporter to transport from the apical membrane into the membrane.
Uses GLUT2 to transport glucose further into the basolateral membrane and a Na+/K+ ATPase to transport sodium into the basolateral membrane and potassium out of it.
How many ions are transported via ATP-powered pumps per second?
1-1000 (10^0 to 10^3) ions per second
What is the general concentration state of a cell’s glucose and sodium levels?
There is more sodium outside of the cell.
There is more glucose inside of the cell.
What are the four classes of pumps?
P-Class
V-Class
F-Class
ABC Class
What are ATP-powered pumps?
Pumps that use the energy of ATP hydrolysis to transport molecules against their concentration gradients (active transport)
Which pumps can run backwards?
F-Class pumps
What is osmosis? When does it occur?
simple diffusion of water toward equilibrium, from low solute concentration to high solute concentration; occurs when a membrane is not permeable to the dissolved solute
Why does osmosis occur?
When water has few solutes to interact with, it forms very highly-ordered structures with itself and is in a state of high free energy. The presence of a solute interrupts the interactions of water molecules, leading to an increase in entropy and a state of lower free energy (an equilibrium point). Water equalizes the concentration of solutes on both sides of a selectively-permeable membrane.
What happens to a cell in a hypertonic solution?
It shrivels (becomes crenated) and water leaves the cell.
What is a hypertonic solution?
One in which there is a higher concentration of solute in the solvent than in the cell
What is an isotonic solution?
a solution in which there is an equal concentration of solute in the solvent and in the cell
What happens to a cell in an isotonic solution?
Water moves in and out, but there is no net movement in either direction
What happens to a cell in a hypotonic solution?
Water moves into the cell, causing it to lyse (burst).
What is a hypotonic solution?
A solution in which there is a lower solute concentration in the solvent and a higher concentration inside of a cell
Which forms of transport requires a membrane protein?
Facilitated Diffusion
Active Transport
What determines the movement of a solute without a net charge?
concentration gradient across the membrane
What is a concentration gradient?
magnitude of difference in concentration of a substance on opposite sides of a membrane; larger concentration difference = larger gradient = larger driving force
What determines the movement of a charged solute?
electrochemical potential
What is the electrochemical potential?
the sum of the chemical and electrical driving forces; combined effect of the concentration gradient and the net difference in charge across the membrane
Can simple diffusion occur down an electrochemical potential?
No, ions cannot simply diffuse. Must use facilitated diffusion or active transport methods.
Explain how electrochemical gradients work with sodium ions. What impact does the charge gradient have? What impact does the concentration gradient have?
Many cells have mostly negative charges inside. If Na+ were in equal concentration inside and outside the cell, the charge gradient would drive the Na+ inside (where the negative charge is), causing the concentration inside the cell to become higher than outside. The concentration gradient would begin favoring the outward movement of Na+. These two forces together would balance the charge gradient, favoring inward movement.
What is a membrane potential (Vm)? What creates it?
the charge gradient across a membrane created by ion gradients
created by active transport
What is the inside of a cell’s charge like, relative to the outside?
The inside of a cell is generally negatively charged compared to the outside.
What is the Vm of a resting nerve cell? What does it do to the movement of cations and anions?
-60 mV, causing cations to move inside and anions to move outwards; also opposes the outward movement of cations and the inward movement of anions
What do pores/channels do?
allow water and ions to enter and leave the cell rapidly in response to cell needs
How does simple diffusion work?
O2, CO2, and H20 diffuse directly across the plasma membrane in response to relative concentrations inside and outside the cell without using transport proteins or extra energy.
What is simple diffusion?
unassisted net movement of a solute from a region where its concentration is higher to a region where its concentration is lower
How does facilitated diffusion using carrier proteins work? What are two examples?
A carrier protein (like GLUT1) transports a molecule inside the cell, where the concentration of the molecule is lower. An anion exchange protein transports Cl- and HCO3- in opposite directions.
How does facilitated diffusion using channel proteins work? Give an example.
Aquaporin channel proteins can facilitate the rapid inward or outward movement of water depending on the relative solute concentration on opposite sides of the membrane.
How does active transport using ATP-requiring pumps work? Give an example.
Driven by the hydrolysis of ATP, the sodium-potassium pump moves sodium ions outward and potassium ions inward, establishing an electro-chemical potential for both ions.