BCMB 311 Exam 3 – Study Guide, Practice Questions & Revision Notes
Some small molecules can cross a cell membrane without help. For the groups: small nonpolar
molecules, small polar molecules (uncharged), larger polar molecules (uncharged) and ions; be
able to rank how easily these can pass through membranes and why (size, hydrophobicity). -
ANS ✔✔- small nonpolar > small, uncharged, polar > large, uncharged, polar >> ions
- charged molecules are repelled by lipid bilayer hydrophobic tails, large molecules cannot
normally fit
Transmembrane proteins have evolved to assist and often regulate transport of specific solutes
across a membrane. What are the major differences between channel proteins and
transporters? - ANS ✔✔- channel proteins: either always open or always closed; always passive
- transporters: conformational change allows for passage of specific solute; passive or active
Appreciate that an electrochemical gradient is a form of potential energy, like water behind a
dam. - ANS ✔✔electrochemical gradient: the sum of the concentration gradient of the solute
and the membrane potential
Explain the difference between passive transporters, primary active transporters ("pumps") and
secondary active transporters (also called "gradient-driven pumps"). Which of these can only
transport down the electrochemical gradient and which can go against? Where does the energy
come from? - ANS ✔✔- passive: allows a solute to move down its concentration gradient
without using energy
- primary active: pumps a solute against its concentration gradient using ATP
- secondary active: couples movement of solute against its concentration gradient with the
movement of a solute down its concentration gradient
Describe the general mechanism (binding events and conformational changes) of a passive
transporter such as a GLUT glucose transporter. - ANS ✔✔glucose enters into a GLUT
transporter and binds, creating a conformational change that allows the glucose to exit on the
other side of the membrane, down its concentration gradient
,Describe the general mechanism (binding events, phosphorylation events, and conformational
changes) of the Na+/K+ pump. In what direction are these solutes moving? - ANS ✔✔- Na+
binds to the pump, hydrolyzing ATP and auto-phosphorylating the pump; this triggers a
conformational change, releasing Na+ and allowing K+ to bind; the pump is dephosphorylated,
triggering a conformational change that allows K+ to be released and returns the pump to its
original conformation
- 3 Na+ is pumped out, 2K+ is pumped in
Calcium is removed from the cytosol by an ATP-driven Ca2+ pump. This allows calcium influx to
act as an intermediate event in cell signaling. Two Ca2+ ions efflux at the expense of 1 ATP,
which is used to phosphorylate an aspartic acid to induce the conformational change that opens
the extracellular side (or sarcoplasmic reticulum luminal side) of the transporter to release the
calcium ions. Hydrolysis by a phosphatase of the Pi group (enzymatic removal) allows the
transporter to return to its starting conformation to bind more calcium ions. - ANS ✔✔- at rest,
Ca2+ pumps actively transport 2 Ca2+ per ATP into the sarcoplasmic reticulum
- acetylcholine opens calcium channels during an action potential, increasing cytosolic Ca2+ and
inducing muscle contraction
Be comfortable with secondary active transporters and the difference between symporters and
antiporters. - ANS ✔✔- secondary active transport: couples movement of solute against its
concentration gradient with the movement of a solute down its concentration gradient
- symporter: both solutes move in the same direction
- antiporter: solutes move in opposite directions
Understand that animal cells tend to use a Na+ gradient to drive import of solutes, while plants,
fungi, and bacteria use H+ gradients. - ANS ✔✔.
The glucose-Na+ symporter is critically important for intestinal cells and kidney cells. Why is
this? Describe how these work. - ANS ✔✔- glucose and Na+ are brought into the cell through
the use of a symporter (both bind to create a conformational change) and secondary active
transport
, - this is important in intestinal cells and kidney cells in order to provide glucose for breakdown
and other cellular functions
What is the role of tight junctions in gut epithelial cells with respect to glucose transporter
diffusion? Where would you find active vs passive glucose transporters in the plasma
membranes of these cells? How does this support the cell's function? - ANS ✔✔- tight junctions
prevent glucose or Na+ from diffusing into adjacent cells
- glucose-Na+ symporter actively brings glucose and Na+ into intestinal epithelial cells, glucose
passively diffuses out into the extracellular fluid, Na+/K+ pump actively pumps out Na+ and
brings K+ in
- this allows glucose to be transported to wherever it is needed
What is a membrane potential? - ANS ✔✔a voltage or electrical charge across the plasma
membrane; the distribution of ions on either side of a cell membrane
Where would you find the higher concentration of Na+, K+, Cl-, and Ca2+ ions in a typical resting
cell? The extracellular space? The cytosol? Somewhere else (sarcoplasmic reticulum)? - ANS
✔✔- Na+ is higher outside the cell/extracellular space
- K+ is higher inside the cell/cytosol
- Cl- is higher outside the cell/extracellular space
- Ca2+ is higher inside the sarcoplasmic reticulum (and lower in the cytosol)
Will a typical cell will have a resting membrane potential that is negative or positive? What does
this mean with respect to the relative abundance of positive vs negative charges on which sides
of the membrane? - ANS ✔✔a typical cell usually has a negative resting membrane potential,
meaning more negative charges are inside the cell and more positive charges are outside
What can the Nernst equation tell you? What is the net movement of an ion when at its
equilibrium potential? - ANS ✔✔- V = 62mV*log(Co/Ci)
- the Nernst equation calculates the contribution to the resting membrane potential of an ion
type; V is the membrane potential
Some small molecules can cross a cell membrane without help. For the groups: small nonpolar
molecules, small polar molecules (uncharged), larger polar molecules (uncharged) and ions; be
able to rank how easily these can pass through membranes and why (size, hydrophobicity). -
ANS ✔✔- small nonpolar > small, uncharged, polar > large, uncharged, polar >> ions
- charged molecules are repelled by lipid bilayer hydrophobic tails, large molecules cannot
normally fit
Transmembrane proteins have evolved to assist and often regulate transport of specific solutes
across a membrane. What are the major differences between channel proteins and
transporters? - ANS ✔✔- channel proteins: either always open or always closed; always passive
- transporters: conformational change allows for passage of specific solute; passive or active
Appreciate that an electrochemical gradient is a form of potential energy, like water behind a
dam. - ANS ✔✔electrochemical gradient: the sum of the concentration gradient of the solute
and the membrane potential
Explain the difference between passive transporters, primary active transporters ("pumps") and
secondary active transporters (also called "gradient-driven pumps"). Which of these can only
transport down the electrochemical gradient and which can go against? Where does the energy
come from? - ANS ✔✔- passive: allows a solute to move down its concentration gradient
without using energy
- primary active: pumps a solute against its concentration gradient using ATP
- secondary active: couples movement of solute against its concentration gradient with the
movement of a solute down its concentration gradient
Describe the general mechanism (binding events and conformational changes) of a passive
transporter such as a GLUT glucose transporter. - ANS ✔✔glucose enters into a GLUT
transporter and binds, creating a conformational change that allows the glucose to exit on the
other side of the membrane, down its concentration gradient
,Describe the general mechanism (binding events, phosphorylation events, and conformational
changes) of the Na+/K+ pump. In what direction are these solutes moving? - ANS ✔✔- Na+
binds to the pump, hydrolyzing ATP and auto-phosphorylating the pump; this triggers a
conformational change, releasing Na+ and allowing K+ to bind; the pump is dephosphorylated,
triggering a conformational change that allows K+ to be released and returns the pump to its
original conformation
- 3 Na+ is pumped out, 2K+ is pumped in
Calcium is removed from the cytosol by an ATP-driven Ca2+ pump. This allows calcium influx to
act as an intermediate event in cell signaling. Two Ca2+ ions efflux at the expense of 1 ATP,
which is used to phosphorylate an aspartic acid to induce the conformational change that opens
the extracellular side (or sarcoplasmic reticulum luminal side) of the transporter to release the
calcium ions. Hydrolysis by a phosphatase of the Pi group (enzymatic removal) allows the
transporter to return to its starting conformation to bind more calcium ions. - ANS ✔✔- at rest,
Ca2+ pumps actively transport 2 Ca2+ per ATP into the sarcoplasmic reticulum
- acetylcholine opens calcium channels during an action potential, increasing cytosolic Ca2+ and
inducing muscle contraction
Be comfortable with secondary active transporters and the difference between symporters and
antiporters. - ANS ✔✔- secondary active transport: couples movement of solute against its
concentration gradient with the movement of a solute down its concentration gradient
- symporter: both solutes move in the same direction
- antiporter: solutes move in opposite directions
Understand that animal cells tend to use a Na+ gradient to drive import of solutes, while plants,
fungi, and bacteria use H+ gradients. - ANS ✔✔.
The glucose-Na+ symporter is critically important for intestinal cells and kidney cells. Why is
this? Describe how these work. - ANS ✔✔- glucose and Na+ are brought into the cell through
the use of a symporter (both bind to create a conformational change) and secondary active
transport
, - this is important in intestinal cells and kidney cells in order to provide glucose for breakdown
and other cellular functions
What is the role of tight junctions in gut epithelial cells with respect to glucose transporter
diffusion? Where would you find active vs passive glucose transporters in the plasma
membranes of these cells? How does this support the cell's function? - ANS ✔✔- tight junctions
prevent glucose or Na+ from diffusing into adjacent cells
- glucose-Na+ symporter actively brings glucose and Na+ into intestinal epithelial cells, glucose
passively diffuses out into the extracellular fluid, Na+/K+ pump actively pumps out Na+ and
brings K+ in
- this allows glucose to be transported to wherever it is needed
What is a membrane potential? - ANS ✔✔a voltage or electrical charge across the plasma
membrane; the distribution of ions on either side of a cell membrane
Where would you find the higher concentration of Na+, K+, Cl-, and Ca2+ ions in a typical resting
cell? The extracellular space? The cytosol? Somewhere else (sarcoplasmic reticulum)? - ANS
✔✔- Na+ is higher outside the cell/extracellular space
- K+ is higher inside the cell/cytosol
- Cl- is higher outside the cell/extracellular space
- Ca2+ is higher inside the sarcoplasmic reticulum (and lower in the cytosol)
Will a typical cell will have a resting membrane potential that is negative or positive? What does
this mean with respect to the relative abundance of positive vs negative charges on which sides
of the membrane? - ANS ✔✔a typical cell usually has a negative resting membrane potential,
meaning more negative charges are inside the cell and more positive charges are outside
What can the Nernst equation tell you? What is the net movement of an ion when at its
equilibrium potential? - ANS ✔✔- V = 62mV*log(Co/Ci)
- the Nernst equation calculates the contribution to the resting membrane potential of an ion
type; V is the membrane potential
