cell bio 11&12

Which of the following would yield the most highly mobile phospholipid (listed as number of carbons and number of double bonds, respectively)?

15 carbons with two double bonds

Water molecules readily form hydrogen bonds with other polar molecules, and when they encounter nonpolar molecules they must form hydrogen-bonding networks with neighboring water molecules. Which of the following molecules will cause a "cage" of water to form?

2-methylpropane

Membranes undergo spontaneous rearrangement if torn. Which of the following would happen if a cell membrane underwent a large tear?

C

You have generated antibodies that recognize the extracellular domain of the Ca2+-pump. Adding these antibodies to animal cells blocks the active transport of Ca2+ from the cytosol into the extracellular environment. What do you expect to observe with respect to intracellular Ca2+?

Ca2+-pumps in the endoplasmic reticulum membrane keep cytosolic calcium levels low.

Consider the apical location of a particular protein expressed in epithelial cells, illustrated in part A of Figure 11-36. When a molecule that chelates calcium is added to the cell culture medium, you observe a redistribution of that protein around the entire cell, shown in part B of Figure 11-36. Which is most likely to be true about the role of calcium in maintaining an apical distribution of protein A?

Calcium is required to maintain the structural integrity of the junctional complex.

We can estimate the relative mobility of a population of molecules along the surface of a living cell by fluorescently labeling the molecules of interest, bleaching the label in one small area, and then measuring the speed of signal recovery as molecules migrate back into the bleached area. What is this method called? What does the abbreviation stand for?

FRAP

Cytosolic Ca2+ concentration is kept low by the use of chelators such as EDTA.

False. Ca2+ concentrations in the cytosol are kept low by the action of ATP-driven calcium pumps in the endoplasmicreticulum membrane and the plasma membrane.

Aquaporin channels are found in the plasma membrane, and allow the rapid passage of water molecules and small ions in and out of cells.

False. Charged molecules (even protons, which are very small) are not able to pass through aquaporins.

Gap junctions are large pores that connect the cytosol to the extracellular space.

False. Gap junctions are used to connect the cytosol of adjacent cells, allowing the sharing of ions and small metabolites. Because gap junctions are large channels, if they were open while facing the extracellular environment, the ability of the plasma membrane to serve as a permeability barrier would be greatly reduced.

Membrane proteins that pump ions in and out of the cell are classified as enzymes.

False. Membrane proteins that pump ions in either direction across the membrane are in the functional class of transporters.

Ion channels are classified as membrane transport proteins. They discriminate between ions based on size and charge. In addition to Na+, which one of the following ions would you expect to be able to freely diffuse through a Na+ channel? Explain your answer.

H+

We know the detailed molecular structure and mechanism of action of the transmembrane protein bacteriorhodopsin. This protein uses sunlight as the source of energy to pump __________ out of the cell.

H+

The amino acid sequences below represent the sequences of transmembrane helices. The characteristics of α helices that form a channel are different from those that form a single transmembrane domain. Select the helix that forms a single transmembrane domain.

IMIVLVMLLNIGLAILFVHF

If Na+ channels are opened in a cell that was previously at rest, how will the resting membrane potential be affected?

It becomes more positive.

Which of the following best describes the behavior of a gated channel?

It opens more frequently in response to a given stimulus.

Cells use membranes to help maintain set ranges of ion concentrations inside and outside the cell. Which of the following ions is the most abundant outside a typical mammalian cell?

Na+

Cells make use of H+ electrochemical gradients in many ways. Which of the following proton transporters is used to regulate pH in animal cells?

Na+-H+ exchanger

Which mechanism best describes the process by which nutrients are taken up selectively at the apical surface of the epithelial cells that line the gut and released from their basal and lateral surfaces?

Protein movement is limited by the presence of a diffusion barrier.

Which mechanism best describes the process by which a budding yeast cell designates the site of new bud formation during cell division?

Proteins are tethered to the cell cortex.

Which mechanism best describes the process by which focal adhesions are formed to promote cell motility?

Proteins are tethered to the extracellular matrix.

Which mechanism best describes the process by which an antigen-presenting cell triggers an adaptive immune response?

Proteins interact with the proteins on the surface of another cell.

Which mechanism best describes the process by which neutrophils are recruited by endothelial cells?

Proteins interact with the proteins on the surface of another cell.

It is possible to follow the movement of a single molecule or a small group of molecules. This requires the use of antibodies linked to small particles of gold, which appear as dark spots when tracked through video microscopy. What is this method called? What does the abbreviation stand for?

SPT

In a method called patch-clamping, a glass capillary can be converted into a microelectrode that measures the electrical currents across biological membranes. Which of the following statements about the patch-clamp method is FALSE?

Single-channel patch-clamp recordings have demonstrated that gated membrane channels will only open and close in response to specific stimuli.

Which of the following statements is TRUE?

Striated muscle cells are long and cylindrical with many nuclei.

Which of the following statements is TRUE?

The Na+-K+ pump helps to keep both Na+ and Cl− ions out of the cell.

In the photosynthetic archaean Halobacterium halobium, a membrane transport protein called bacteriorhodopsin captures energy from sunlight and uses it to pump protons out of the cell. The resulting proton gradient serves as an energy store that can later be tapped to generate ATP. Which statement best describes how bacteriorhodopsin operates?

The absorption of sunlight triggers a shift in the conformation of the protein's seven, membrane-spanning α helices, allowing a proton to leave the cell.

Which of the following statements about the carbohydrate coating of the cell surface is FALSE?

The arrangement of the oligosaccharide side chains is highly ordered, much like the peptide bonds of a polypeptide chain.

When the net charge on either side of the plasma membrane is zero, what else is true?

The electrochemical potential across the membrane is zero.

Which of the following is most likely to occur after the lipid bilayer is pierced?

The membrane reseals

When using the Nernst equation to calculate membrane potential, we are making several assumptions about conditions in the cell. Which of the following is NOT a good assumption?

The plasma membrane is primarily permeable to Na+.

The Na+-K+ ATPase is also known as the Na+-K+ pump. It is responsible for maintaining the high extracellular sodium ion concentration and the high intracellular potassium ion concentration. What happens immediately after the pump hydrolyzes ATP?

The pump is phosphorylated.

Which of the following statements about resting membrane potential is FALSE?

The resting membrane potential for most animal cells is negative, because the inside of the cell is more negatively charged than the outside of the cell.

Both glycoproteins and proteoglycans contribute to the carbohydrate layer on the surface of the cell. Which of the following statements about glycoproteins is FALSE?

They have long carbohydrate chains.

K+ leak channels are found in the plasma membrane. These channels open and close in an unregulated, random fashion. What do they accomplish in a resting cell?

They keep the electrochemical gradient for K+ at zero.

Which of the following statements about GABA receptors is FALSE?

They promote neuronal uptake of Na+.

Formation of a lipid bilayer is energetically favorable. How does this arrangement result in higher entropy for the system, and thus make bilayer formation energetically favorable?

Water molecules form cage-like structures around hydrophobic molecules.

Three phospholipids, X, Y, and Z, are distributed in the plasma membrane as indicated in Figure 11-10. For which of these phospholipids does a flippase probably exist?

X and Y

Pumps are transporters that are able to harness energy provided by other components in the cells to drive the movement of solutes across membranes, against their concentration gradient. This type of transport is called

active transport.

Diversity among the oligosaccharide chains found in the carbohydrate coating of the cell surface can be achieved in which of the following ways?

all of these answers are correct

Both excitatory and inhibitory neurons form junctions with muscles. By what mechanism do inhibitory neurotransmitters prevent the postsynaptic cell from firing an action potential?

by opening K+ channels d. by opening Cl− channels

Which of the following membrane lipids does not contain a fatty acid tail?

cholesterol

New membrane phospholipids are synthesized by enzymes bound to the __________ side of the __________membrane.

cytosolic; endoplasmic reticulum

Which of the following substances is most commonly used to help purify a membrane protein?

detergent

Below is a list of molecules with different chemical characteristics. Knowing that all molecules will eventually diffuse across a phospholipid bilayer, select the option below that most accurately predicts the relative rates of diffusion of these molecules (fastest to slowest).

estrogen > propanol > sodium > alanine

Which of the following occur WITHOUT coupling transport of the solute to the movement of a second solute?

export of Ca2+ from the cytosol

We can test the relative permeability of a phospholipid bilayer by using a synthetic membrane that does not contain any protein components. Some uncharged, polar molecules are found to diffuse freely across these membranes, to varying degrees. Which of the following has the lowest rate of diffusion across an artificial membrane? Why?

glucose

Some lipases are able to cleave the covalent bonds between the glycerol backbone and the attached fatty acid. What final products do you expect to accumulate through the action of the enzyme monoacylglycerol lipase?

glycerol and free fatty acid

Which of the following gated ion channels are involved in inhibitory synaptic signaling?

glycine-gated Cl− channels

The acetylcholine receptor in skeletal muscle cells is a/an __________ ion channel.

ligand-gated

Cholesterol serves several essential functions in mammalian cells. Which of the following is NOT influenced by cholesterol?

membrane thickness

Membrane lipids are capable of many different types of movement. Which of these does not occur spontaneously in biological membranes?

moving between lipid layers

The endothelial cells found closest to the site of an infection express proteins called lectins. Each lectin binds to a particular __________ that is presented on the surface of a target cell

oligosaccharide

Ca2+-pumps in the plasma membrane and endoplasmic reticulum are important for

preventing Ca2+ from altering the activity of molecules in the cytosol.

A bacterium is suddenly expelled from a warm human intestine into the cold world outside. Which of the following adjustments might the bacterium make to maintain the same level of membrane fluidity?

produce lipids with hydrocarbon tails that are shorter and have more double bonds

Figure 12-32 illustrates changes in membrane potential during the formation of an action potential. What membrane characteristic or measurement used to study action potentials is indicated by the arrow?

resting membrane potential

Which of the following functions of the plasma membrane is possible without membrane proteins?

selective permeability

Membrane curvature is influenced by the differential lipid composition of the two membrane monolayers. Which factor do you think has the largest impact on the curvature of biological membranes?

size of the lipid head group

Some cells express aquaporin proteins—they are channel proteins that facilitate the flow of water molecules through the plasma membrane. What regulates the rate and direction of water diffusion across the membrane?

solute concentrations on either side of the membrane

Transporters, in contrast to channels, work by

specific recognition of transport substrates.

Red blood cells have been very useful in the study of membranes and the protein components that provide structural support. Which of the following proteins is the principal fibrous protein in the cortex of the red blood cell?

spectrin

Consider the apical location of a particular protein expressed in epithelial cells, illustrated in part A of Figure 11-35. Which type of defect described below is the most likely to cause the redistribution of that protein around the entire cell, shown in part B of Figure 11-35

the deletion of a junctional protein

Approximately, how many distinct synapses are established on the dendrites and cell body of a motor neuron in the spinal cord?

thousands

Epithelial cell membranes are asymmetric, and proteins from the apical side of the cell membrane cannot diffuse into the basal side of the membrane.

true

Glycolipids lack the glycerol component found in phospholipids.

true

Most ion channels are gated, which allows them to open and close in response to a specific stimulus, rather than allowing the constant, unregulated flow of ions.

true

Neurotransmitters are small molecules released into the synaptic cleft after the fusion of synaptic vesicles with the presynaptic membrane.

true

The electrochemical gradient for K+ across the plasma membrane is small. Therefore, any movement of K+ from the inside to the outside of the cell is driven solely by its concentration gradient.

true

The extracellular concentration of Ca2+ is approximately 104-fold higher than the concentration of Ca2+ in the cytosol.

true

The low cytosolic Ca2+ concentration sensitizes the cell to an influx of Ca2+, ensuring a rapid response to environmental stimuli.

true

The speed of fluorescent signal recovery during a FRAP assay is a measure of lateral mobility for the molecule of interest

true

Transporters undergo transitions between different conformations, depending on whether the substrate-binding pocket is empty or occupied.

true

Voltage-gated Na+ channels become automatically inactivated shortly after opening, which ensures that the action potential cannot move backward along the axon.

true

Although the extracellular environment has a high sodium ion concentration and the intracellular environment has a high potassium ion concentration, both must be neutralized by negatively charged molecules. In the extracellular case, what is the principal anion?

Cl−

Cells use membranes to help maintain set ranges of ion concentrations inside and outside the cell. Which of the following negatively charged ions is NOT primarily used to buffer positive charges inside the cell?

Cl−

Negatively charged ions are required to balance the net positive charge from metal ions such as K+, Na+, and Ca2+. Which of the following negatively charged ions is the most abundant outside the cell and which ion most often neutralize (written in parentheses)?

Cl− (Na+)

Action potentials are usually mediated by voltage-gated Ca2+ channels.

False. Action potentials are usually mediated by voltage-gated Na+ channels.

When a mouse cell is fused with a human cell, the movement of the respective membrane proteins is restricted to their original locations at the time of fusion.

False. After about 1 hour, the mouse and human proteins present on the surface of the fused cell are found evenly dispersed throughout the plasma membrane.

The only role of the carbohydrate layer on the cell surface is to absorb water, which creates a slimy surface and prevents cells from sticking to each other.

False. Although the absorption of water is an important role of the carbohydrates on the surface of the plasma membrane, a second critical role is that of cell-cell recognition, which is important in immune responses, wound healing, and other processes that rely on cell-type-specific interactions.

A protein can be embedded on the cytosolic side of the membrane bilayer by employing a hydrophobic α helix.

False. An embedded protein employs an amphipathic helix. The hydrophobic side interacts with the fatty acid tails of the membrane lipids, and the hydrophilic portion interacts with the aqueous components of the cytosol.

The primary mechanism by which Ca2+ acts as a signaling molecule is by increasing the net charge in the cytosol.

False. Ca2+ binds tightly to many proteins in the cell, which in turn changes their activity. This interaction is the primary mechanism by which Ca2+ signaling occurs.

The net negative charge on the cytosolic side of the membrane enhances the rate of glucose import into the cell by a uniporter

False. Glucose is an uncharged molecule, and its import is not directly affected by the voltage difference across the membrane if glucose is being transported alone. If the example given were the Na+/glucose symporter, we would have to consider the charge difference across the membrane.

Lipid-linked proteins are classified as peripheral membrane proteins because the polypeptide chain does not pass through the bilayer.

False. Lipid-linked proteins are classified as integral membrane proteins because although they are not transmembrane proteins, they are covalently bound to membrane lipids and cannot be dissociated without disrupting the membrane's integrity.

Phosphatidylserine is the most abundant type of phospholipid found in cell membranes.

False. Phosphatidylcholine is the most abundant phospholipid found in cell membranes.

SDS is a mild detergent that is useful for the reconstitution of membrane components.

False. SDS is a strong, ionic detergent that will break up membrane bilayers and also denature proteins.

The ion selectivity of a channel depends solely on the charge of the amino acids lining the pore inside the channel.

False. Selectivity depends on three parameters: the diameter, shape, and charge of the ion trying to pass through the pore of the channel.

FRAP is a method used to study the movement of individual proteins.

False. The FRAP method involves photobleaching of a small region of the membrane, which contains hundreds of target molecules, and follows the displacement of these molecules with neighboring molecules that have not been bleached.

The highly ordered structure of the lipid bilayer makes its generation and maintenance energetically unfavorable.

False. The formation of a lipid bilayer is energetically favorable.

The differences in permeability between artificial lipid bilayers and cell membranes arise from variations in phospholipid content.

False. The primary difference between cell membranes and artificial membranes is that cell membranes have proteins responsible for creating a selective permeability, which varies with the location and function of the membrane.

In order to study the activity of isolated transmembrane proteins, the membrane lipids must be completely stripped away.

False. The region of the protein that normally crosses the membrane must be stabilized by the presence of phospholipids for the purified protein to be active. For this reason, purified membrane proteins are often reconstituted into artificial lipid bilayers.

The longest carbohydrates found on the surfaces of cells are linked to lipid molecules.

False. The very long, branched polysaccharides that are attached to integral membrane proteins are much longer than the oligosaccharides covalently attached to membrane lipids.

Facilitated diffusion can be described as the favorable movement of one solute down its concentration gradient being coupled with the unfavorable movement of a second solute up its concentration gradient.

False. This describes coupled transport, which is one type of active transport. Facilitated diffusion can also be called passive transport, in which a solute always moves down its concentration gradient.

Transporters are similar to channels, except that they are larger, allowing folded proteins as well as smaller organic molecules to pass through them.

False. Transporters work by changing conformation after specific binding of the solute to be transported. Channels exclude molecules on the basis of size and charge, but do not depend on specific recognition of the molecules moving through

Voltage-gated K+ channels also open immediately in response to local depolarization, reducing the magnitude of the action potential.

False. Voltage-gated K+ channels respond more slowly than the voltage-gated Na+ channels. Because voltage-gated K+ channels do not open until the action potential reaches its peak, they do not affect its magnitude. Instead, they help to restore the local membrane potential quickly while the voltage-gated Na+ channels are in the inactivated conformation.

Cells use membranes to help maintain set ranges of ion concentrations inside and outside the cell. Which of the following ions is the most abundant inside a typical mammalian cell?

K+

Active transport requires the input of energy into a system so as to move solutes against their electrochemical and concentration gradients. Which of the following is NOT one of the common ways to perform active transport?

K+-coupled

Voltage-gated channels contain charged protein domains, which are sensitive to changes in membrane potential. By responding to a threshold in the membrane potential, these voltage sensors trigger the opening of the channels. Which of the following best describes the behavior of a population of channels exposed to such a threshold?

Some channels remain closed and some open completely.

Which of the following statements best reflects the nature of synaptic plasticity?

Synaptic response changes in magnitude depending on frequency of stimulation.

A group of membrane proteins can be extracted from membranes only by using detergents. All the proteins in this group have a similar amino acid sequence at their C-terminus: -KKKKKXXC (where K stands for lysine, X stands for any amino acid, and C stands for cysteine). This sequence is essential for their attachment to the membrane. What is the most likely way in which the C-terminal sequence attaches these proteins to the membrane?

The cysteine residue is covalently attached to a membrane lipid.

Porin proteins form large, barrel-like channels in the membrane. Which of the following statements about these channels is FALSE?

They are made primarily of α helices.

The stimulation of a motor neuron ultimately results in the release of a neurotransmitter at the synapse between the neuron and a muscle cell. What type of neurotransmitter is used at these neuromuscular junctions?

acetylcholine

Figure 12-33 illustrates changes in membrane potential during the formation of an action potential. What membrane characteristic or measurement used to study action potentials is indicated by the arrow?

action potential

Which of the following channels would not be expected to generate a change in voltage by movement of its substrate across the membrane where it is found?

an aquaporin

Membrane proteins, like membrane lipids, can move laterally by exchanging positions with other membrane components. Which type of membrane protein is expected to be the least mobile, based on their function?

anchors

Plasma membranes are extremely thin and fragile, requiring an extensive support network of fibrous proteins. This network is called the

cortex.

Membrane synthesis in the cell requires the regulation of growth for both halves of the bilayer and the selective retention of certain types of lipids on one side or the other. Which group of enzymes accomplishes both of these tasks?

flippases

Where does most new membrane synthesis take place in a eukaryotic cell?

in the endoplasmic reticulum

A hungry yeast cell lands in a vat of grape juice and begins to feast on the sugars there, producing carbon dioxide and ethanol in the process: C6H12O6 + 2ADP + 2Pi + H+ → 2CO2 + 2CH3CH2OH + 2ATP + 2H2O Unfortunately, the grape juice is contaminated with proteases that attack some of the transport proteins in the yeast cell membrane, and the yeast cell dies. Which of the following is the most likely cause of the yeast cell's demise?

inability to import sugar into the cell

Many receptors for neurotransmitters are __________ ion channels.

ligand-gated

Which type of lipids are the most abundant in the plasma membrane?

phospholipids

The stimulation of auditory nerves depends on the opening and closing of channels in the auditory hair cells. Which type of gating mechanism do these cells use?

stress-gated

__________ ion channels are found in the hair cells of the mammalian cochlea.

stress-gated

Figure 12-31 illustrates changes in membrane potential during the formation of an action potential. What membrane characteristic or measurement used to study action potentials is indicated by the arrow?

threshold potential

Which of the following phospholipid precursors is the most hydrophobic?

triacylglycerol

A protein that relies on protein-protein interactions to stabilize its membrane association is classified as a peripheral membrane protein because it can be dissociated without the use of detergents.

true

Although cholesterol is a hydrophobic molecule, it has a hydrophilic head group like all other membrane lipids

true

CO2 and O2 are water-soluble molecules that diffuse freely across cell membranes.

true

Cells expend energy in the form of ATP hydrolysis so as to maintain ion concentrations that differ from those found outside the cell.

true

Most animal fats form a solid at room temperature, while plant fats remain liquid at room temperature. Which of the following is a feature of lipids in plant membranes that best explains this difference?

unsaturated hydrocarbons

__________ ion channels in the mimosa plant propagate the leaf-closing response.

voltage-gated

__________ ion channels respond to changes in membrane potential.

voltage-gated

Which of the following is required for the secretion of neurotransmitters in response to an action potential? a. neurotransmitter receptors

voltage-gated Ca2+ channels

The Nernst equation can be used to calculate the membrane potential based on the ratio of the outer and inner ion concentration. In a resting cell, membrane potential is calculated taking only K+ ions into account. What is V when Co= 15 mM and Ci = 106 mM?

−52.7 mV