Test 2 Ch 12 Flashcards
Cell membranes and Transport
Cell membranes contain specific proteins that regulate molecule passage.
Transport occurs through:
Facilitated diffusion (passive, no energy required).
Active transport (requires ATP).
Plasma Membrane Transport
-Controls what enters/exits the cell.
-Examples of transported molecules:
-Nucleotide – essential for DNA/RNA synthesis.
-Sugar – used for energy.
-Amino acid – building blocks of proteins.
-Na⁺ / K⁺ – essential for nerve signals and cell function.
Lysosome Transport
-Contains an H⁺ pump that maintains an acidic pH by pumping in hydrogen ions (H⁺).
-Important for breaking down waste and recycling cellular materials.
Mitochondrial Transport
-Pyruvate (from glycolysis) enters mitochondria for energy production.
-ATP/ADP exchange:
-ATP (energy carrier) moves out of the mitochondrion.
-ADP enters to be converted back into ATP.
Inner Mitochondrial Transport
-Specialized transport proteins regulate movement across the membrane.
-Essential for cellular respiration and ATP production.
Passive Transport
Passive Transport
- Direction:
from High to low concentration, down/with concentration gradient
Energy required: No
- How pass-through membrane:
- Diffusion through lipid bilayer
- Transporter or channel proteins (facilitated diffusion)
Active Transport
-Direction:
- Low to high concentration, against/up concentration gradient
- Energy required: Yes
How pass through membrane: Transporter proteins (pumps)
Osmosis
diffusion of water
across a membrane.
(Passive through aquaporin)
-Osmosis is always passive
-Have to compare solute concentration to determine where water will go
hypertonic
solution is more concentrated than the cell
isotonic
solution is balanced between the cell and outside solution
Hypotonic
solution is less concentrated than the cell
Water moves from
an area of lower solute to higher solute
What are aquaporins
Channel proteins that facilitate water movement across the cell
How do aquaporins help with water movement?
They allow water molecules to pass through the plasma membrane via osmosis
What type of transport do aquaporins use?
passive transport (osmosis), meaning no energy (ATP) is required
Why are aquaporins important for cells
Water is polar, and the cell membrane is hydrophobic, making water diffusion slow. Aquaporins allow water to pass efficiently through a hydrophilic channel
What is structural composition of aquaporins?
Aquaporins are alpha-helical transmembrane proteins, forming a narrow, hydrophilic pore for selective water transport
beta barrels are basically found
in porins of bacterial outer membranes, allowing passage of larger molecules
Aquaporins are
highly selective, only allowing water molecules through while excluding ions like H+, preventing pH imbalances
Factors effecting passive transport
- Size of molecule
- Polarity, solubility in lipids (more polar have a harder time passing through membrane)
- Charge on molecule (charged molecules don’t pass through the bilayer need a transport protein)
- Difference in concentration across membrane concentration
gradient (Steepness of concentration gradient affects how quickly it goes) - Difference in charge across membrane (electrical or voltage
gradient)
What is an electrochemical gradient?
it is the combined effect of:
1. Concentration gradient – the difference in the concentration of a charged molecule across a membrane.
- Electrical gradient – the difference in charge across the membrane (positive outside negative inside)
What happens when the electrical and concentration gradients work in the same direction?
When the voltage (electrical gradient) and concentration gradient work together, the charged molecules move more easily across the membrane.
Example: If the inside of the cell is negative and the outside is positive, positively charged ions (e.g., Na⁺) will flow inside due to both charge attraction and concentration difference.
What happens when the electrical and concentration gradients work in opposite directions?
When the electrical gradient opposes the concentration gradient, movement is more difficult and may require active transport.
Example: If the inside of the cell is negative but there are already many positive ions inside, the electrical gradient favors entry, but the concentration gradient pushes ions outward.
Passive transporters
- Channels: Allow charged ions to get through the bilayer they’re selective, gated, and move very fast
-Transporter: Take mostly everything else through the membrane. They conduct small molecules across lipid bilayers via a series of conformational changes
What is a uniporter?
A uniporter is a transport protein that moves one type of solute across the membrane, following its concentration gradient. This process does not require energy (ATP) and is a form of passive transport.
How does a uniporter facilitate glucose transport?
- Glucose binds to the glucose transporter on one side of the membrane.
- The transporter undergoes a conformational change, moving glucose across.
- Glucose is released on the other side (cytosol), following the concentration gradient (high to low)
This process is passive transport, meaning it occurs without energy input.
What drives the movement of glucose through a passive transporter?
The movement is driven by the concentration gradient—glucose moves from a higher concentration (extracellular space) to a lower concentration (cytosol).
No ATP is needed since the transport occurs downhill in terms of energy.
Passive Transporters (uniporters)
-Transmembrane transporter proteins
-They have three conformations, flip between randomly
- Specific molecules bind most readily on side with higher concentration
- Move down concentration gradient (and voltage gradient if molecule is charged)(if molecule has a charge it’s voltage gradient will impact speed aswell)
- Channel proteins:
-Carry ions - They’re Selective
- They move down electrochemical gradient
What is active transport, and how does it differ from passive transport?
Active transport moves molecules against their concentration or electrochemical gradient, requiring energy input. Unlike passive transport, which occurs naturally, active transport needs external energy sources such as ATP, ion gradients, or light.
What are the three types of active transport pumps?
-Gradient-Driven Pump – Uses the energy from one molecule moving down its concentration gradient to power the movement of another molecule against its gradient.
-ATP-Driven Pump – Directly uses ATP hydrolysis to transport molecules against their gradient.
-Light-Driven Pump – Uses light energy (e.g., in bacteria and some cells) to drive transport against a gradient.
How does a gradient-driven pump work?
-Uses the movement of one solute down its gradient to drive the transport of another solute against its gradient.
-Example: Na⁺/glucose symporter, where Na⁺ moves down its gradient, bringing glucose into the cell against its concentration gradient.
How does an ATP-driven pump function?
-ATP is hydrolyzed into ADP + P, releasing energy.
-This energy changes the protein’s shape, allowing molecules to be transported against their gradient.
Example: Na⁺/K⁺ pump, which moves Na⁺ out and K⁺ in, maintaining cellular ion balance.
What is a light-driven pump?
-Uses light energy to move molecules against their gradient.
-Common in bacteria and some plant cells.
Example: Bacteriorhodopsin, which pumps H⁺ (protons) out of the cell using light energy.
What is Na+/K+ Pump (sodium-potassium pump)?
It is the most important ATP driven pump in animal cells
-The Na⁺/K⁺ pump is an active transport mechanism that moves 3 Na⁺ out of the cell and 2 K⁺ into the cell using ATP.
Steps of the sodium-potassium pump
- 3 Na⁺ binds to the 3 binding sites on the pump inside the cell.
- Pump phosphorylates itself, hydrolyzing ATP.
- Phosphorylation causes a conformational change, ejecting 3 Na⁺ outside.
4.2 K⁺ binds to 2 K+ binding sites from the extracellular space. - Pump dephosphorylates itself, restoring its shape.
- 2 K⁺ is released inside the cell, and the cycle repeats. A complete cycle takes about 10 milliseconds
Why is the sodium-potassium pump important?
It helps maintain the resting membrane potential, regulates cell volume, and creates an electrochemical gradient necessary for nerve signaling and muscle contraction.
Does the sodium potassium pump require energy?
Yes, it uses ATP to move ions against their concentration gradients. So, it is active transport
How many Na+ and K+ ions are moved per cycle?
3 Na⁺ are pumped out, and 2 K⁺ are pumped in.
What gradient does the Na+/K+ pump create?
It establishes a high Na⁺ concentration outside and a high K⁺ concentration inside, which is crucial for processes like nerve impulses.
How does the Na⁺/K⁺ pump contribute to the resting membrane potential?
By maintaining an uneven charge distribution, with more positive ions outside the cell, it keeps the inside of the cell negatively charged.
Importance of Na+K+ Pump
- Creates and Maintains the Na+, K+ gradients
- Na+ is high outside, low in, K + is reverse
- Uses 1 molecule of ATP per cycle, moving 3 Na+ out per 2 K+ in
- Contributes to difference in charge across membrane = Electrogenic
- More negative inside cell
-Help maintain osmotic balance - Gradient of Na+ used to drive transport of other molecules
- K+ gradient important in creating membrane potential
Coupled transport
Materials may be transported against their concentration gradients without
direct use of ATP
- Coupled transport uses gradient driven pumps to move one solute against its concentration gradient using the energy of another solute moving with its concentration gradient
Coupled transport can be either
Symport: carries 2 types of solutes through membrane both molecules move in same direction so for ex. It could move a solute down its concentration gradient of low to high while bringing another solute against its concentration gradient, but the key is they are going in the same direction
antiport: Moves 2 types of solutes in opposite directions so both molecules moving in opposite directions. So for example, the antiport will open and bind to a square molecule and take it through the membrane and then after solute is released it will use that energy to grab a solute from inside the membrane and take it outside the membrane. So solutes are going in different directions
Due to the action of the Na+ K+ pump, the ___ gradient is frequently used
as the energy source for coupled transport
Na+
Uniporters
move one TYPE of molecule selectively moving it from one side of the membrane to the other, thery’re always passive
Coupled transport
Carry 2 types of solutes through the membrane
Antiport maintains
proper intracellular pH:
- Na⁺/H⁺ Exchanger: Exports protons (H⁺) out of the cell in exchange for sodium (Na⁺), preventing acidity.
- Cl⁻/HCO₃⁻ Exchanger: The exchanger is an antiporter that typically moves Cl⁻ into the cell while simultaneously exporting HCO₃⁻ out of the cell. This process helps regulate intracellular pH by removing excess bicarbonate, which would otherwise make the cytoplasm too basic.
Role: Maintains optimal pH for cellular processes, prevents excessive acidity or alkalinity.
What is symport in glucose transport?
Symport is a process where glucose and Na⁺ are transported together into intestinal cells. Na⁺ moves with its concentration gradient, providing energy to bring glucose inside against its gradient.
How does Na⁺ contribute to glucose transport in intestinal cells?
Na⁺ is in high concentration outside the cell and moves inside along its electrochemical gradient. This movement provides energy for glucose to be transported into the cell.
Glucose is usually
higher in concentration inside the cells than in the gut. So, transporting it up the cell requires energy
information about glucose symport to understand
- A glucose sodium symport harvest the energy stored in the sodium gradient to pump glucose into the cell
- sodium and glucose can both bind to the pump but the binding of one makes the binding of the other more effective
When the binding sites of the symport are open to the lumen of the gut the high sodium concentration will make glucose more likely which in tern causes Na+ to bind more effectively
because the conformational change of the transport will only occur when both binding sites of the symport are filled then the symport will release them in strict unison on the other side of the membrane from extracellular to the cytosolic side of the membrane
-There is plenty of glucose inside the cell but low sodium so most glucose molecules will not leave by the same route, so it is unidirectional
What happens after glucose enters the intestinal cell?
Glucose moves through the cell and is passively released into the extracellular fluid, where it can be used by other tissues.
Why doesn’t glucose diffuse laterally between intestinal cells?
Tight junctions between cells prevent lateral diffusion of glucose and other molecules, ensuring directed movement from the gut lumen to the extracellular space.
What is the role of the Na⁺/K⁺ pump in glucose transport?
The Na⁺/K⁺ pump maintains the Na⁺ gradient by actively pumping Na⁺ out of the cell, ensuring that Na⁺ continues to flow in and drive glucose uptake.
Does the sodium-potassium pump (Na⁺/K⁺ ATPase) transport glucose?
No. The sodium-potassium pump only moves Na⁺ and K⁺, not glucose. However, it maintains the Na⁺ gradient needed for the sodium-glucose cotransporter (SGLT) to work.
What happens to glucose transport if the sodium-potassium pump is blocked?
If the Na⁺/K⁺ pump is blocked:
- Na⁺ builds up inside the cell, disrupting the Na⁺ gradient.
- The sodium-glucose cotransporter (SGLT) stops working because it relies on Na⁺ moving in.
- Glucose cannot enter the cell, leading to lower glucose levels inside.
- Since glucose exits passively through a uniporter (GLUT), no glucose entry means no glucose to exit.
Why does the sodium-glucose cotransporter (SGLT) stop working if the Na⁺/K⁺ pump is blocked?
SGLT depends on Na⁺ moving into the cell to bring glucose along. If the Na⁺/K⁺ pump isn’t removing Na⁺, there is no Na⁺ gradient to drive glucose uptake.
How does glucose exit the intestinal cell after being transported in?
Glucose exits passively through a glucose uniporter (GLUT transporter) on the basolateral membrane into the extracellular fluid for use by the body.
inhibitors of Na+ K+ Pump
-Digitalis (digoxin)
-Ouabain
-Inhibitors of ATP Production
These materials block the function of the pump, therefore allowing the gradients of the Na+ and K+ to dissipate
Ca++ Antiporter
in cardiac muscle cells:
-Ca++ is released from the ER storage upon stimulation
- This free Ca++ stimulates contraction (to relax cell calcium has to be pumped out of cell)
- Some of the Ca++ is removed by a Na+/ Ca++ antiporter uses the energy of Na+ moving into the cell (with its concentration gradient) to pump Ca++ out
-This system does not use ATP directly but requires functioning Na+K+ Pump
How does Digitalis work
Digitalis is used for congestive heart failure by helping heart muscles pump a little longer
-Digitalis (digoxin) blocks the Na+K+ Pump
- When digitalis is present, the Na+ gradient is dissipated to some extent
- With less of a Na+ gradient, there is less energy to force Ca++ out of cells
- Ca++ stays around longer, muscle contraction is maintained for a longer time
characteristics of ion channels
ion channels are channels (made of helices) that carry ions through the membrane
theyre:
-Always passive
-selective
-Gated (closed or open)
-Fats transport (very fast) about 1000x faster than a transport protein
Ion movement controlled by:
Concentration gradient
Voltage gradient
=electrochemical gradient
What is membrane potential?
Membrane potential is the difference in voltage between the inside and outside of a cell due to the distribution of charged ions. It typically ranges from -60 to -90 mV in a resting cell.
What happens when there is an exact balance of charges on both sides of the membrane?
When charges are evenly distributed, the membrane potential is 0 (no voltage difference).
How does membrane potential become nonzero?
When some positive ions (e.g., Na⁺ or K⁺) move across the membrane, it creates an imbalance of charge, leading to a nonzero membrane potential.
Why is the resting membrane potential typically negative?
The resting membrane potential is negative because more positive ions leave the cell (e.g., K⁺ via leak channels) than enter, making the inside more negative compared to the outside.
4 Types of ion channels
- Leak Channels- gate opens and closes at random
ex: K+ leak channel - Ligand-gated – open when ligand bound to channel
ex: acetylcholine receptor - Mechanically gated – open/close upon mechanical stimulation
ex: auditory hair cells - Voltage-gated – open/close when voltage across membrane changes
in Nerve axons:
Na+ channels - fast
K+ channels - slow
What do K⁺ leak channels do?
K⁺ leak channels allow potassium ions (K⁺) to move out of the cell, helping to establish the resting membrane potential.
What happens when K⁺ leak channels are closed?
When K⁺ leak channels are closed, positive and negative charges are balanced, and the membrane potential is 0.
What happens when K⁺ leak channels open?
When K⁺ leak channels open:
K⁺ moves out of the cell due to its concentration gradient.
This creates a negative charge inside the cell, leading to a nonzero membrane potential.
Eventually, the electrical gradient (negative charge inside) balances the tendency of K⁺ to leave.
What two forces control K⁺ movement through leak channels?
1.Concentration gradient (pushes K⁺ out of the cell).
- Voltage (electrical) gradient (pulls K⁺ back into the cell).
When these forces balance, the resting membrane potential is reached.
potassium is usually higher on
the inside
Resting Membrane potential
Generally, -60 to -90 mVolts, negative inside with respect to outside
Nernest equation
Used to calculate forces on ions at
resting membrane potential
- Takes into account concentration and electrical forces acting on an ion
- For an ion with a single positive charge
at 37o C:
V= 62log(Co/Ci) - If V is negative, ion has a tendency to move out of the cell; if V is positive, ion moves in
Applying the Nernst Equation:
X+: Co = 145 mM (outside); Ci= 10 mM (inside)
V = 62 log (145/10)
V= 72 mV
Positive thus, X+ has tendency to move in
Action potential
- Excitable cells can change membrane potential locally in response to stimulus
-Changes in membrane potential (called polarization) are accomplished by opening/closing ion channels
- Voltage gated Na+ and K+ channels
Arrangement of Nerve cells
What is an action potential?
An action potential is a rapid electrical signal that travels along neurons, caused by the movement of Na⁺ and K⁺ ions across the membrane.
What are the three states of a voltage-gated Na⁺ channel?
- Closed – At resting potential, Na⁺ channels are closed.
- Open – When the neuron is depolarized, Na⁺ channels open, allowing Na⁺ to rush in.
- Inactivated – Shortly after opening, Na⁺ channels become inactivated to stop further Na⁺ entry.
What are the phases of an action potential?
- Depolarization – Na⁺ channels open, Na⁺ enters, and the membrane potential becomes positive.
- Repolarization – K⁺ channels open, K⁺ exits, and the membrane potential returns negative.
- Threshold potential – The minimum voltage needed to trigger an action potential (~ -55mV).
How do voltage-gated K⁺ channels respond during an action potential?
K⁺ channels open with a delay after depolarization, allowing K⁺ to exit, which restores the resting membrane potential.
Why do voltage-gated Na⁺ channels inactivate?
To prevent excessive Na⁺ entry, ensuring the action potential moves in one direction and allowing time for recovery before another signal.
Steps in action potential
- Depolarization stimulus
- Membrane potential gets less negative
- Until reaches threshold potential
- Voltage gated Na+ channels open
- Na+ moves into cell, making membrane potential more positive= depolarization
- When reaches ~+40 mV, Na channels inactivated (cannot be reopened)
- Voltage gated K+ channels open
-K+ moves out of cell, making membrane potential more negative = repolarization
- When reach ~-40 mV, Na+ channels close (can be reopened)
What is action potential propagation?
Action potential propagation is the movement of the electrical signal along the axon due to the sequential opening and closing of ion channels.
How does depolarization cause propagation?
Depolarization in one part of the membrane causes nearby voltage-gated Na⁺ channels to open, allowing the action potential to travel down the axon.
What prevents the action potential from moving backward?
The inactivation of voltage-gated Na⁺ channels prevents Na⁺ from re-entering, ensuring the signal moves in one direction.
What role do K⁺ channels play in propagation?
-K⁺ channels open after Na⁺ influx, allowing K⁺ to exit the cell.
-This repolarizes the membrane, restoring resting potential and preparing the axon for the next signal.
What are the key steps in action potential propagation?
1.Resting State: Na⁺ and K⁺ channels are closed.
- Depolarization: Na⁺ channels open, Na⁺ enters, membrane becomes positive.
3.Propagation: Depolarization spreads, opening adjacent Na⁺ channels.
- Repolarization: K⁺ channels open, K⁺ exits, membrane returns negative.
- Refractory Period: Na⁺ channels inactivate, preventing backward movement.
How does an action potential trigger neurotransmitter release?
The action potential reaches the nerve terminal, opening voltage-gated Ca²⁺ channels. Ca²⁺ influx triggers synaptic vesicle fusion with the membrane, releasing neurotransmitters.
Why is calcium (Ca²⁺) important in neurotransmitter release?
Ca²⁺ binds to synaptic vesicles, causing them to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft.
What is the difference between a resting and an activated nerve terminal?
-Resting terminal: Voltage-gated Ca²⁺ channels are closed, and no neurotransmitters are released.
-Activated terminal: Action potential opens Ca²⁺ channels, leading to neurotransmitter release.
What happens to neurotransmitters after they are released into the synaptic cleft?
They diffuse across the synapse and bind to receptors on the postsynaptic cell.
How does a neurotransmitter activate the postsynaptic cell?
It binds to neurotransmitter receptors, opening ion channels that change the membrane potential, triggering an electrical signal.
What is the difference between an inactive and an activated postsynaptic cell?
Inactive cell: No neurotransmitter binding, ion channels closed.
Activated cell: Neurotransmitters bind, ion channels open, and the electrical signal is transmitted.
What happens at the synaptic cleft
Axon of nerve not directly connected to receiving cell
- In pre-synaptic cell (axon)
- Neurotransmitters stored in vesicles
- When A.P. reaches axon terminus, voltage gated Ca+ channels open
- Ca+ flows into cell
- Vesicles fuse with membrane, releasing neurotransmitter into
synaptic cleft
+ Post-synaptic cell:
- Neurotransmitter binds to ligand gated ion channel
- Channel opens
+ Excitatory neurotransmitter:
- Na+ channel, depolarization
+ Inhibitory neurotransmitter:
- Cl- channel, membrane potential more negative
- Neurotransmitter is rapidly removed or degraded
What is an excitatory synapse?
A synapse where neurotransmitters cause Na⁺ influx, depolarizing the membrane and increasing the likelihood of an action potential.
What is an inhibitory synapse?
A synapse where neurotransmitters cause Cl⁻ influx, keeping the membrane polarized and decreasing the likelihood of an action potential.
How do excitatory and inhibitory synapses regulate neural activity?
Excitatory synapses promote neuron firing, while inhibitory synapses prevent excessive activity, maintaining balance in the nervous system.
What type of receptor is the acetylcholine receptor?
It is a ligand-gated ion channel.
What happens when acetylcholine binds to its receptor?
The receptor changes conformation, opening the ion channel and allowing Na⁺ to enter the cell.
What is the effect of Na⁺ entry on the postsynaptic cell?
It depolarizes the membrane, increasing the chance of an action potential firing.
Action potentials in nerves depends on
ion gradients and local changes in voltage-gated ion channels
What triggers voltage-gated Ca²⁺ channels to open in the presynaptic terminal?
The arrival of an action potential at the nerve terminal.
What happens when Ca²⁺ enters the presynaptic terminal?
It causes synaptic vesicles to fuse with the membrane and release neurotransmitters into the synaptic cleft.
What is the role of neurotransmitters in synaptic transmission?
They act as chemical signals that bind to receptors on the postsynaptic cell to continue signal transmission.
What happens after neurotransmitters are released into the synaptic cleft?
They bind to neurotransmitter receptors on the postsynaptic cell membrane.
What is the effect of neurotransmitter binding on the postsynaptic cell?
It opens ion channels, leading to a change in membrane potential, which may trigger an electrical response.
What happens if the neurotransmitter is not removed from the synaptic cleft?
The signal would persist, leading to prolonged activation or inhibition of the postsynaptic neuron.
Do Action Potential assignment
Do Nernest Equation Assignment
A. Long and more saturated
C. Peripheral proteins on the extracellular face
E
C. Carbohydrates are added to proteins and lipids on the non-cytosolic face in the ER lumen before traveling to the PM by transport vesicles
E
A. The post-synaptic cell would be continuously depolarized
B. Inactivated
C
A
D. Active transport does not require a transport protein
A
D
Sodium ions, oxygen (O2), and glucose pass directly through lipid bilayers at dramatically different rates. Which of the following choices presents the correct order, from fastest to slowest?
A. glucose, sodium ions, oxygen
B. glucose, oxygen, sodium ions
C. sodium ions, oxygen, glucose
D. oxygen, glucose, sodium ions
E. oxygen, sodium ions, glucose
d. oxygen, glucose, sodum ions
How do transporters and channels select which solutes they help move across the membrane?
Choose one:
A. Transporters discriminate between solutes mainly on the basis of size and electric charge; channels bind their solutes with great specificity in the same way an enzyme binds its substrate.
B. Both channels and transporters discriminate between solutes mainly on the basis of size and electric charge.
C. Channels will allow the passage of any solute as long as it has an electrical charge; transporters bind their solutes with great specificity in the same way an enzyme binds its substrate.
D. Channels allow the passage of solutes that are electrically charged; transporters facilitate the passage of molecules that are uncharged.
E. Channels discriminate between solutes mainly on the basis of size and electric charge; transporters bind their solutes with great specificity in the same way an enzyme binds its substrate.
E. Channels discriminate between solutes mainly on the basis of size and electric charge; transporters bind their solutes with great specificity in the same way an enzyme binds its substrate.
In one experiment, investigators create a liposome—a vesicle made of phospholipids—that contains a solution of 1 mM glucose and 1 mM sodium chloride. If this vesicle were placed in a beaker of distilled water, what would happen the fastest?
Choose one:
A. Na+ would diffuse out.
B. H2O would diffuse in.
C. Cl– would diffuse out.
D. Glucose would diffuse out.
E. NaCl would diffuse out.
B. H2O would diffuse in.
In this experiment, the possible molecules that can move across the membrane are water, glucose, and ionized sodium chloride. Glucose requires a transporter to move across a lipid membrane because of its relatively large size and the experimental design does not include these transporter proteins in the liposome. Therefore, glucose will not leave the liposome. Sodium chloride will ionize into Na+ and Cl– when dissolved into solution. While small in size, the charge of these molecules likewise means that they cannot diffuse across the nonpolar liposome membrane without a channel protein. On the other hand, water is small enough, as well as noncharged, meaning that it can cross the membrane. Distilled water outside of the cell lacks dissolved solutes (i.e., high water concentration), whereas the interior of the liposome has a relatively high concentration of two different solutes (i.e., low water concentration). Water will follow its concentration gradient and move into the liposome.
Most sports drinks contain both carbohydrates and salts. The carbohydrates replace glucose burned during exercise and the salts replace salts lost in sweat. The salt also helps the small intestine absorb glucose. Pick the answer that accurately describes which salt is most beneficial for glucose absorption.
Choose one:
A. HCl, because H+ is needed for glucose entry.
B. NaCl, because Na+ is needed for glucose entry.
C. KCl, because Cl– is needed for glucose entry.
D. KCl, because K+ is needed for glucose entry.
B. NaCl, because Na+ is needed for glucose entry.
Which of the following describes the resting membrane potential of a neuron?
Choose one:
A. a voltage difference of 0 millivolts (mV) across the membrane
B. a voltage difference that is chiefly a reflection of the electrochemical Na+ gradient across the plasma membrane
C. a voltage difference across the plasma membrane, with more positive membrane potential inside
D. a voltage difference across the plasma membrane when the neuron has been stimulated
E. a state in which the flow of positive and negative ions across the plasma membrane is precisely balanced
a state in which the flow of positive and negative ions across the plasma membrane is precisely balanced
A toxin present in scorpion venom prolongs the duration of action potentials in nerve cells. Which of these actions would best explain how this toxin exerts its effect?
Choose one:
A. It slows the inactivation of voltage-gated K+ channels.
B. It inhibits the opening of voltage-gated Na+ channels.
C. It accelerates the opening of voltage-gated K+ channels.
D. It slows the inactivation of voltage-gated Na+ channels.
E. It prolongs the inactivation of voltage-gated Na+ channels.
It slows the inactivation of voltage-gated Na+ channels.
The drug scopolamine is used to treat dizziness, motion sickness, and smooth muscle spasms. When isolated muscle cells are incubated with scopolamine, addition of acetylcholine no longer depolarizes the muscle cell membrane or stimulates muscle cell contraction. Which would best explain how scopolamine exerts its muscle-relaxing effects?
Choose one:
A. It inhibits the transporters that pump Ca2+ into the muscle cell cytosol during an action potential.
B. It inhibits the opening of acetylcholine-gated Na+ channels in the muscle cell membrane.
C. It inhibits the opening of Ca2+ channels in the sarcoplasmic reticulum.
D. It inhibits the opening of voltage-gated K+ channels.
E. It inhibits the transporters that pump Na+ into the muscle cell cytosol during an action potential.
B. It inhibits the opening of acetylcholine-gated Na+ channels in the muscle cell membrane.
Which of the following form tiny hydrophilic pores in the membrane through which solutes can pass by diffusion?
Choose one:
A. channels
B. liposomes
C. transporters
D. anions
E. pumps
A. channels
Which of the following inhibits inorganic ions, such as Na+ and Cl–, from passing through a lipid bilayer?
Choose one:
A. the carbohydrate layer on the surface of the lipid bilayer
B. the watery environment on either side of the lipid bilayer
C. the hydrophobic interior of the lipid bilayer
D. the hydrophilic exterior of the lipid bilayer
E. the ions’ large size
the hydrophobic interior of the lipid bilayer
In general, which of the following will diffuse across a lipid bilayer most rapidly?
Choose one:
A. water
B. a small hydrophilic molecule
C. a large hydrophobic molecule
D. a small hydrophobic molecule
E. a large hydrophilic molecule
D. a small hydrophobic molecule
Which of the following statements is true?
Choose one:
Na+ is the most plentiful positively charged ion outside the cell, while K+ is the most plentiful inside.
K+ and Na+ are both maintained at high concentrations inside the cell compared to out.
K+ and Na+ are present in the same concentration on both sides of the plasma membrane.
K+ and Na+ are both excluded from cells.
K+ is the most plentiful positively charged ion outside the cell, while Na+ is the most plentiful inside.
Na+ is the most plentiful positively charged ion outside the cell, while K+ is the most plentiful inside.
What is the voltage difference across a membrane of a cell called?
Choose one:
potential balance
membrane potential
gradient establishment
electrical current
membrane potential
The movement of an ion down its concentration gradient is called what?
Choose one:
active transport
passive transport
osmosis
pumping
passive transport
When glucose moves across a phospholipid bilayer by passive transport, which factor determines the direction of its transport?
Choose one:
A. the charge difference across the membrane
B. the amount of energy available to fuel the transport process
C. the concentrations of glucose on either side of the membrane
D. whether the cell is metabolically active or not
C. the concentrations of glucose on either side of the membrane
An electrochemical gradient has a chemical component and an electrical component. Which of the following will have the largest electrochemical gradient?
Choose one:
A. a positively charged ion, such as K+, at high concentrations inside the cell
B. a negatively charged ion, such as Cl–, at high concentrations outside the cell
C. a positively charged ion, such as Na+, at high concentrations outside the cell
C. a positively charged ion, such as Na+, at high concentrations outside the cell
Which of the following correctly describes osmosis?
Choose one:
A. the movement of water from an area of high solute concentration to an area of low solute concentration
B. the movement of water from an area of low solute concentration to an area of high solute concentration
C. the movement of water from an area of low solvent concentration to an area of high solvent concentration
D. the movement of water from an area of low water concentration to an area of high water concentration
B. the movement of water from an area of low solute concentration to an area of high solute concentration
Why do cells lack membrane transport proteins that are specific for the movement of O2?
Choose one:
A. because oxygen concentrations must be kept low inside cells to avoid creating reactive superoxide radicals that can damage DNA and proteins
B. because oxygen is transported in and out of the cell by special oxygen-binding proteins such as hemoglobin
C. because oxygen dissolves readily in lipid bilayers
D. because oxygen, dissolved in water, can enter cells via aquaporins
E. because transport of oxygen across cell membranes is energetically unfavorable
because oxygen dissolves readily in lipid bilayers
Cells lack membrane transport proteins that are specific for the movement of O2 because oxygen dissolves readily in lipid bilayers. This small, nonpolar molecule can diffuse across the cell membrane without the need for a membrane transport protein. The channels of aquaporins are lined with amino acids that provide an environment for the formation of transient hydrogen bonds that facilitate the passage of water molecules, which line up in single file. The channels of aquaporins exclude ions and most other molecules, including O2.
What condition must exist for glucose to be transported into a cell using the glucose–Na+ symport?
Choose one:
A. high Na+ concentration inside the cell
B. high Na+ concentration outside the cell
C. high ATP concentration inside the cell for phosphorylation of the glucose–Na+ symport
D. high glucose concentration outside the cell
B. high Na+ concentration outside the cell
The glucose–Na+ symport transports glucose into the epithelial cells lining the gut. How would import of glucose into the cells be affected by addition of a leaky Na+ channel to their plasma membrane?
Choose one:
A. A leaky Na+ channel would not affect glucose transport because these two transporters are unrelated.
B. Na+ transport would slow, but glucose transport would remain high because glucose could still be transported by the glucose–Na+ symport.
C. Glucose transport would increase because the Na+ gradient is strengthened by the Na+ channel.
D. Glucose transport would slow because the Na+ gradient is dissipated by the Na+ channel.
Glucose transport would slow because the Na+ gradient is dissipated by the Na+ channel.
uring the activation of a neuron, the action potential propagates in only one direction. How is this achieved in the neuron?
Choose one:
A. The Na+ channel closes during the action potential and then rapidly returns to the open state after the action potential passes.
B. The Na+ channel becomes inactivated and refractory to reopening for a short time after the action potential passes.
C. The Na+ channel becomes permanently inactivated after the action potential passes.
D. The Na+ channel remains open during the action potential and then rapidly returns to the closed state after the action potential passes.
B. The Na+ channel becomes inactivated and refractory to reopening for a short time after the action potential passes.
Tetrodotoxin is a potent toxin found in a variety of organisms including the pufferfish. The toxin binds to the extracellular side of the Na+ channel and prevents channel opening. This leads to paralysis of muscles, including the diaphragm. Death from respiratory failure can occur after ingestion of as little as 1 mg of the toxin. Why does this toxin cause paralysis?
Choose one:
A. The Na+ channels remain in the inactive, refractory state.
B. The axon membranes become over-depolarized.
C. The membrane depolarization is not amplified along the axon.
D. The Na+ channel does not open wide enough to allow enough Na+ through the channel.
C. The membrane depolarization is not amplified along the axon.
The tetrodotoxin binds to the extracellular side of the Na+ channel and prevents the channel from opening. This prevents Na+ from entering the cytosol of the cell and the subsequent depolarization of the membrane. If the membrane does not depolarize, the signal is not propagated along the axon. The muscle at the axon terminus does not receive the proper signal and remains in a relaxed state. When this occurs in the muscles required for breathing, such as the diaphragm, the victim is unable to breathe and can die from respiratory failure.
