Cell Biology

What acts as an anchor for both ends of the microtubule?

??? The minus end of a microtubule is usually anchored to the MTOC (microtubule organizing center) and the plus end is usually anchored to the cell membrane by proteins.

Consider the following statements about the GTP cap in a microtubule. Which of these statements are true? A. GTP cap in a microtubule is found only on the plus end of the microtubule. B. GTP cap in a microtubule is spontaneously lost over time by hydrolysis of GTP to GDP in the tubulin subunits. C. GTP cap in a microtubule promotes the elongation of the microtubule in which it is found.

A. True. The GTP cap is found on the positive end. B. True. This is the cause of dynamic instability. C. False. The GTP cap is not necessary for elongation so it does not promote it. Although, the plus end which has the GTP cap does grow faster than the minus end.

To make a branch off an existing actin filament, cells use the protein _____.

ARP 2/3 complex

Cells can continue to import glucose molecules even wen the cytoplasmic concentration is very high. This would be an example of _____ type of transport.

Active transport (usually secondary active transport for glucose)

Explain the mechanism of a voltage-gated channel.

Changes in a membrane's electromagnetic field caused by a change in the voltage across the membrane lead to conformational changes in the transmembrane segments of a gated channel, which causes the gate to open.

A _____ is a membrane protein structure that allows ions to flow freely in or out through an opening of a particular size.

Channel

What is the difference between channels and carriers?

Channels are open at both ends, like a tunnel and allow for much more rapid transport. Carriers are always closed at one end and open at the other. Carriers are also much slower than channels.

Co-transport of nutrients across the intestinal cell membranes is an active process that can move glucose against a concentration gradient. What is the energy requiring step for co-transport?

Co-transport, aka secondary active transport, uses the concentration gradient of another molecule to pump something against its concentration gradient.

The binding of _____ forms bundles of parallel actin filaments.

Cross-link proteins (Villin and Fimbrin)

Passive carriers transport sugars, active carriers transport ions. True of false.

False. Most carriers are specific to what they transport, but the molecules that they translate do not classify them as active or passive. There are both active and passive glucose transporters for example.

Passive carriers consume ATP energy when they transport ions, active carriers create ATP. True or false.

False. Passive carriers do not use ATP. Active carriers hydrolyze ATP for energy (like gas in a car)

K+ ions have a smaller hydration shell than Na+ ions, allowing the passage of hydrated K+, but not Na+, through the selectivity filter of the K+ channel. True or False

False. The hydration shell of Na is actually smaller than the hydration shell of K. The hydration shell of Na uses more energy than the K channels have in order to dehydrate it so this is the reason that Na does not pass through K channels.

What serves as a site for microtubule and microfilament assembly?

MTOC (microtubule organizing center); usually the centrosome

What are the differences between passive transport and simple diffusion?

Simple diffusion is the movement of particles from a high concentration area to an area of low concentration. Passive transport is the movement of molecules down their concentration gradient, but across a membrane which may or may not require the use of proteins.

The microtubule-organizing center (MTOC) nucleates microtubule assembly. True or false.

The MTOC functions to nucleate microtubules; the most common MTOC in animals is the centrosome.

By changing the proteins within its membrane, our cells can be made to function very differently. True or False

True. The proteins determine how the cell interacts with the outside world.

The action of the Na+/K+ ATPase pumps maintain an excess of Na+ ions outside the cells and an excess of K+ ions inside the cell. The K+ ion channels are unidirectional and only allow the transport of ions out of the cell. True or false.

True. This is because the K channel cannot perform active transport. It can only do passive transport of the K ions down their concentration gradient. Since the inside of the cell has a high K concentration and the outside has a low concentration, K will always move out of the cell through the K channels.

What acts as a motor for driving chromosomes apart during mitosis?

??? Microtubules attach to the chromosomes and then shorten to pull them apart.

When is ATP hydrolyzed by actin?

ATP is hydrolyzed after the actin polymer is formed. Polymerization does not require the hydrolysis of ATP.

What is an antiporter?

Antiporters are secondary active transporter proteins that move two different ions (or other molecules) in opposite directions. One moves into the cell while one moves out of the cell.

Concerning ion transport, how can passive carriers be distinguished from active transporters (pumps)?

Anytime ATP is required or an ion is moving from low concentration to high concentration, it is active transport. Passive transport will always move ions down the concentration gradient.

Microtubules have a _____ arrangement in the main body of a flagellum?

Circular, 9 pairs in the outer layer and 1 pair in the inner layer; often called the "9+2" arrangement.

According to the current model, what is the sequence of microtubule assembly?

Dimers joint to form oligomers > protofilaments > sheets of protofilaments > closing of microtubule > elongating microtubule

What is the composition of F-actin filaments?

Double helical filament structure.

What is dynamic instability?

Dynamic instability allows the cell to rapidly reorganize the cytoskeleton when necessary.

Describe the concept of "dynamic instability" as it applies to the microtubules.

Dynamic instability allows the cell to rapidly reorganize the cytoskeleton when necessary. This is important because cells change/divide frequently. This can occur several different ways, but essentially is the result of a lost GTP cap which results in peeling.

What is facilitated diffusion?

Facilitated diffusion is a passive transport process that moves molecules down their concentration gradient that are too large to pass through the cell membrane, so they use transport proteins.

The central cavity of the K+ channel can only accommodate hydrated K+ but not hydrated Na+. True or false.

False

Passive carriers contain beta sheets in their transmembrane domains, active carriers contain alpha helices. True or false.

False. Alpha helices and beta sheets are found in both passive and active carriers.

GLUT2 initially transports glucose into the pancreas after eating, allowing insulin to be released into the bloodstream. True or false.

False. GLUT2 exports glucose out of glucose producing organs like the liver.

GLUTs are uniporters that use ATP hydrolysis for energy. True or False

False. GLUTs are uniporters that mediate facilitated transport of glucose down its concentration gradient. Na/glucose cotransporters on the other hand do use secondary active transport, but not ATP.

The K+ channel is gated closed to Na+ and only opens when it "senses" K+. True or false.

False. The K channel is always open, but it has a selectivity pore that will only allow K to pass through it. Na cannot pass through it because it does not fit the selectivity pore. The K channel does not "sense" K.

The Na/K ATPase pumps Na from the cell into the lumen of the intestine. True or false.

False. The Na/K pump moves Na out of the cell and into the interstitial fluid (or the blood) but not into the intestinal lumen.

The permease (transporter) pumps glucose from the cell into the blood requires ATP. True or false.

False. The cell has a higher concentration of glucose than the blood because glucose is actively pumped into cells by secondary active transport. Therefore, glucose will move to the blood through permease transporters by passive transport (facilitated diffusion).

The permease (transporter) allows glucose and Na+ into the cell requires ATP. True or false.

False. The permease transporter is a secondary active transporter that uses the gradient of Na to move glucose into cells.

Phospholipids containing ______ are always found exclusively on the exterior side of a membrane.

Glycolipids (carbohydrate chains)

Membrane proteins called _____ channels open to allow ions to flow in and out of the cell when the concentration of ions nearby is changed.

Ion channels.

What are the common features of the motor proteins kinesin and dynein?

Kinesin and dynein are both motor proteins that walk along the microtubules. They are different in structure and the direction they walk however. Kinesins move toward the plus end (the membrane) and the dyneins move toward the negative end (the MTOC or center).

The definition of ______ is a measure of ionic imbalance across the plasma membrane, caused by unequal concentrations of ions in the cytosol vs. the extracellular space; measured in millivolts.

Membrane potential

In a typical animal cell, which of the following types of transport would increase at a linear rate (no saturation) proportional to the concentration gradient? Roughly draw Na+, Ca2+, Cl-, and K+ distributions inside and outside a cell.

Movement of Na+ into a cell via a channel

Concerning ion transport, how can passive carriers be distinguished from active transporters (pumps)?

Passive carriers dissipate concentration gradients, active transporters build concentration gradients

What types of transport would increase at a linear rate (no saturation) proportional to the concentration gradient?

Passive transport (at least until the gradient is destroyed) Note: active transport cannot and any receptor mediated transport essentially would become saturated (reach a maximum transport level).

Under normal physiologic conditions during elongation, the barbed end of an actin filament is called _____, is capped by _____ and structurally different than the _____ end.

Positive end; ATP-actin; pointed (minus) end

The Na/K transporter is well-studied example of _____ type of transport.

Primary active, antiporters

What is a motor protein?

Proteins that use repeated cycles of ATP hydrolysis to power continuous movements along the side of microtubules; like walking.

What is resting potential?

Resting potential is the normal electrical potential that exists across a cell membrane due to a more positive charge on the outside of the cell and a less positive (considered negative) charge inside the cell caused by the relative concentrations of Na and K. The resting potential is -70 mV.

Co-transport of nutrients across the intestinal cell membranes is an active process that can move glucose against a concentration gradient. The energy requiring step for co-transport is:

The Na+/K+ ATPase that pumps Na+ from the cell into the blood, maintaining low Na+ levels in the cell

What is the definition of critical concentration of purified tubulin dimers?

The critical concentration of tubulin dimers is the concentration of the dimers left in the solution when the maximum amount of polymer is formed, and the steady state is reached.

Potassium ion (K+) channels are very selective for K+, although the ion sodium (Na+) has the same charge as K+ and is even smaller. What feature of the K+ ion channel explains this selectivity?

The dehydration step. The dehydration of Na ions is energetically unfavorable for K channels because the dehydrated Na ion is too small to coordinate with the channel's carbonyl oxygen atoms.

What is treadmilling of microtubules?

The plus end of the microtubule grows longer while the minus end shortens at the same rate. This creates a conveyor belt effect and can only occur at critical concentration of tubulin heterodimers (Cc). This process can be used to transport vesicles, etc. across the cell.

What is treadmilling of an actin filament?

The simultaneous polymerization of the barbed end and depolymerization of the pointed end of an actin filament, which creates a constant flux without changing concentrations in the cell. This is regulated by proteins and can only occur when the concentration is between the Cc of both ends.

In vitro and under steady state conditions, the critical concentration (Cc) of the barbed end of a filament is 0.1 um, and the Cc of the pointed end of this filament is 0.6 um. If the concentration of actin-ATP monomers is 0.3 um, what would happen to this actin filament?

This induces a treadmilling effect when the barbed end will grow and the pointed end will shrink simultaneously because of the difference in their Cc.

Tubulin dimers may assemble at the plus end and disassemble at the minus end simultaneously in a process known as _____.

Treadmilling. The truth is that addition and removal of heterodimers is constantly occurring at both ends but the plus end grows and the minus end shortens at the same rate, creating a conveyor belt effect.

Both types of motor proteins step from one tubulin to another, an 8 nm "step." True or False

True. Both types of motor proteins walk in 8 nm steps, which is the length of a single tubulin heterodimer.

Actin exists in cells in two major forms called G acting and F actin. True or false.

True. G-actin is an actin monomer (G = globular) and F-actin is a linear polymer of actin monomers (F = filamentous). There are 3 isoforms of actin however. Alpha, beta, and gamma. Alpha is in the muscle cells, while beta and gamma are not.

GLUT1 transports glucose across the blood brain barrier. True or false.

True. GLUT1 moves glucose from blood to endothelial cells to astrocytes and into the brain.

After insulin signaling, GLUT4 is transported to the membrane of various cells to increase glucose transport in tissues (But not if you have Type II Diabetes). True or False

True. GLUT4 is added to the membranes of adipose and muscle cells to increase glucose uptake in response to insulin signals after eating. In type II diabetes, the transport of GLUT4 to the membrane is inhibited.

The Na/K ATPase pumps Na+ from the cell into the blood, maintaining low Na+ levels in the cell. True or false.

True. Na/K ATPase pumps move 3 Na out of the cell and 2 K into the cell creating an electrochemical gradient.

Passive carriers dissipate concentration gradients, active transporters build concentration gradients. True or false.

True. Passive creating a concentration gradient requires energy, while diffusion does not.

If the plasma membrane of an animal cell was made entirely permeable to Na+, the Na/K pump would continue to pump ions and hydrolyze ATP, but not build a Na+ gradient. True or false.

True. The Na/K ATPase would continue to pump the ions, but since the Na is able to pass right back through the membrane, the gradient would be destroyed at the same instant that it was created.

Passive carriers are localized to the basolateral membrane of epithelial cells, active transporters are localized to the apical membrane. True or false.

True. The apical membrane is oriented toward the lumen of the intestine, while the basolateral membrane is oriented away from the lumen. We have to actively pump things from the lumen into the cell then passively move them out of the cell and into the blood etc.

The actin cytoskeleton generates cell movement in the following way: Directed polymerization of actin filaments against the plasma membrane and the interaction of actin filaments and the motor protein myosin. True or false.

True. The cell migrates by directed polymerization of actin filaments and muscle cells shorten, causing the muscle to contract by actin and myosin interactions.

Specific amino acids that line the selectivity filter of the K+ channel can dehydrate K+ and allow passage of that ion but cannot coordinate the dehydration of Na+. True or false.

True. The dehydration of a Na ion is energetically unfavorable because it is too small to fit in the K channel selectivity filter which is lined with carbonyl oxygens from select AAs. For this reason, it will not dehydrate Na, but it will dehydrate K.

The force generating powerstroke of myosin along an actin filament occurs when myosin _____.

When myosin releases phosphate. Note: when ATP binds, the myosin head lets go of the actin. Then the ATP is hydrolyzed to cock the head (reverse powerstroke) and bind to actin again. Then the phosphate group is released and the myosin head powerstrokes. During the powerstroke, ADP also falls off.

Do motor proteins undergo conformational changes when the motor travels?

Yes, this is how the protein puts one "foot" in front of the other to walk forward.