BCMB 311 Exam 3 Questions with Verified Correct
Answer
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).
- 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?
- 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.
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?
- 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.
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?
- 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.
- 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.
- 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.
.
The glucose-Na+ symporter is critically important for intestinal cells and kidney cells. Why
is this? Describe how these work.
- 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
Answer
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).
- 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?
- 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.
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?
- 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.
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?
- 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.
- 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.
- 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.
.
The glucose-Na+ symporter is critically important for intestinal cells and kidney cells. Why
is this? Describe how these work.
- 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
