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Example Questions
Example Question #1 : Cell Signaling
Listed below are events that occur during a signal transduction pathway.
I. The plasma membrane receptor interacts with an effector protein
II. Second messenger molecules are released
III. Ligand binds to the plasma membrane receptor
Which of the following lists these events in the correct order?
II, I, III
I, III, II
III, I, II
II, III, I
III, I, II
Signal transduction involves transmission of signals between cells. In a normal signal transduction pathway, a ligand (such as a hormone and neurotransmitter) binds to a cell membrane receptor on the extracellular side. Ligand binding initiates a response on the intracellular side. One such response includes the binding of the intracellular side to an effector protein. Binding of the receptor to an effector protein releases second messenger molecules that propagate and amplify the signal, often influencing transcription factors and gene expression.
There are several signal transduction pathways, but the ligand always binds to the extracellular side of the receptor and initiates a response on the intracellular side of the receptor.
Example Question #1 : Cell Signaling
Which of the following is true regarding a transmembrane receptor?
It has both hydrophobic and hydrophilic regions and the ligand binds on the hydrophilic regions
It has exclusively hydrophobic regions
It has both hydrophobic and hydrophilic regions and the ligand binds on the hydrophobic regions
It has exclusively hydrophilic regions
It has both hydrophobic and hydrophilic regions and the ligand binds on the hydrophilic regions
A transmembrane receptor, by definition, is a molecule that is inserted into a membrane (such as the plasma membrane of the cell). Recall that a membrane found in a cell is made up of a phospholipid bilayer, which contains both hydrophobic (nonpolar) and hydrophilic (polar) regions. The hydrophobic regions are found on the inside and the hydrophilic regions are found facing either the cytoplasm or the extracellular space.
To insert into a phospholipid bilayer, transmembrane receptors must have both hydrophobic and hydrophilic regions. The hydrophobic portion of the receptor is found inside the hydrophobic core of the membrane and the hydrophilic iregion s found facing the cytoplasm or extracellular space. A ligand typically contacts a transmembrane receptor from the outside or from the cytoplasm; therefore, the ligand typically binds to the hydrophilic portion of the receptor.
Example Question #1 : Cell Signaling
A type III secretion system is a mechanism several bacteria use to evade the immune system. They insert a syringe-like structure into a nearby host cell and secrete effector proteins that kill the host cell. What term best describes this kind of signaling?
Juxtacrine
Endocrine
Paracrine
Autocrine
Juxtacrine
The question states that the type III secretion system involves a bacterium making contact with a nearby host cell. Recall that if a cell makes contact with the nearby cell for signal transduction, then the signal is characterized as a juxtacrine signal. In paracrine signaling, on the other hand, the signaling cell secretes a chemical signal such as a neurotransmitter. This chemical binds to receptors on a nearby cell and transmits the signal; in paracrine signaling the two cells never come in contact.
Endocrine signaling involves release of a chemical hormone that enters the bloodstream and signals cells elsewhere in the body. Autocrine signaling involves a cell releasing a molecule that binds to receptors on its own surface; therefore, in autocrine signaling the signaling cell and the target cell are the same.
Example Question #5 : Cell Signaling
Which of the following is false regarding cellular signaling?
I. Transmembrane receptors are found on both plasma membranes and nuclear membranes
II. Transmembrane receptors are always ion channels
III. A ligand can be polar or nonpolar
I only
II only
I and II
II and III
II only
A transmembrane receptor is any receptor that inserts itself into a membrane. A cell has a phospholipid bilayer membrane surrounding most of the organelles and the cell itself (the plasma membrane). Transmembrane receptors can be found on all of these membranes because the receptors are essential for receiving signals from the outside environment; therefore, you can find transmembrane receptors on the plasma membrane and on the nuclear membrane. Statement I is true.
A prototypical receptor has a ligand-binding site. Once the ligand binds, the receptor signals the cell accordingly. This signal induction can occur via several ways. One way is for a receptor itself to be an ion channel; upon ligand binding the channel opens and transports ions across the plasma membrane, which induces a signal inside the cell. Another way is for a receptor to be a G protein coupled receptor, which induces a signaling cascade that leads to the activation of the second messenger signaling molecule cAMP. There are several ways receptors can induce a signal; therefore, not all receptors are ion channels. Statement II is false.
A ligand can be either polar or nonpolar. The classic example is hormones. Recall that there are both polar and nonpolar (steroid) hormones. Polar hormones, such as the ones released from the pituitary, can’t traverse the hydrophobic core of the phospholipid bilayer; therefore, their receptors are found on the plasma membrane. Steroid hormones, released from the adrenal glands and gonads, can pass through the hydrophobic core and enter the cell. Upon entering the cell, the steroid hormone binds to receptors in the cytoplasm or on the nuclear membrane. Statement III is true.
Example Question #42 : Cellular Processes And Functions
cAMP is an important molecule that is part of several signaling cascades. It is classified as a __________ messenger and it can __________ the signal.
first . . . attenuate
second . . . amplify
first . . . amplify
second . . . attenuate
second . . . amplify
To answer this question you need to know the difference between a first and a second messenger molecule. First messenger molecules are extracellular molecules such as peptide hormones that bind to the receptor and induce a signal. Second messenger molecules, however, are intracellular molecules that are released and activated by first messenger molecules. cAMP is an intracellular molecule that is activated by signaling from a G-protein coupled receptor; therefore, it is a second messenger molecule. cAMP plays an important role in activation of several signal transduction pathways.
An extracellular signal needs to be amplified by the cell so that the signal reaches all of the target regions. Second messenger molecules, such as cAMP, play an important role in this amplification.
Example Question #1 : Cell Signaling
Which organelle is primarily responsible for ATP production in eukaryotic cells?
Chloroplasts
Mitochondria
Lysosomes
Ribosomes
Mitochondria
Eukaryotic cells contain mitochondria. The inner membrane of the mitochondrion houses ATP synthase proteins, which generate molecular energy via oxidative phosphorylation. This is the primary source of cellular energy in the form of ATP.
Chloroplasts are another eukaryotic organelle involved in energy production. However, the primary function of the chloroplast is to use light energy to generate glucose from carbon dioxide. This glucose is then metabolized via cellular respiration, utilizing the mitochondria for the majority of ATP synthesis.
Example Question #1 : Cell Signaling
The force generated by a muscle when it contracts involves muscle proteins within muscle cells, namely actin and myosin. Beginning with the arrival of an action potential from the motor neuron’s axon, muscles generate force through a cascade of electrical and biochemical events. The release of acetylcholine at the presynaptic membrane into the synaptic cleft is caused by the action potential which opens calcium channels. Temporary binding of neurotransmitter at the postsynaptic membrane with the muscle’s acetylcholine receptors leads to depolarization of the postsynaptic membrane and opening of calcium channels. Twisting of tropomyosin to expose myosin attachment sites on actin is the result of calcium released from the sarcoplasmic reticulum and binding to troponin molecules. two strands of protein, myosin and actin, attach to each other by forming a cross-bridge which allows them to slide relative to each other to shorten the muscle and generate force. When depolarization ends, is pumped back into the sarcoplasmic reticulum and actin- myosin cross-bridges can no longer form resulting in relaxation.
When a motor neuron is electrically stimulated with a single impulse, a muscle innervated by that neuron produces a force called a twitch. Whereas the impulse might be 1 to 3msec in duration, the twitch is 10 to 100msec long. This is because it takes a long time for the to be pumped back into the sarcoplasmic reticulum. When the rate of impulses is low, the twitches have time to relax (Figure 1A). When the rate of simulation is high, the twitches fuse and the force in the muscle sums (Figures 1B and 1C). Maximal tension in the muscle, a condition known as tetanus (Figure 1D), is generated when the frequency of action potential is raised to the point when all cross- bridge binding sites are continuously activated and force output no longer shows any ripples.
Figure 1
Myasthenia gravis (MG) is a disease in which the number of acetylcholine receptors at the postsynaptic neuromuscular junctions becomes greatly reduced. What is the expected difference between contraction of the muscle of the MG patient and that of a healthy person in response to stimulation by a neuron?
Muscle of the MG patient will contract less strongly than the muscle of a healthy person because in the patient with MG, the number of troponin molecules bound to tropomyosin will be greater
Muscle of the MG patient will contract less strongly than the muscle of a healthy person because in the patient with MG, less will be released from the sarcoplasmic reticulum in response to neural stimulation
Muscle of the MG patient will contract more strongly than the muscle of a healthy person because in the patient with MG, a larger number of actin binding sites will be exposed on myosin
Muscle of the MG patient will contract more strongly than the muscle of a healthy person because in the patient with MG, acetylcholine will not be sequestered by the receptors
None of these
Muscle of the MG patient will contract less strongly than the muscle of a healthy person because in the patient with MG, less will be released from the sarcoplasmic reticulum in response to neural stimulation
This question asks how the response of muscle of an MG patient would differ from the response of muscle of a healthy person to stimulation by a neuron. This is a question that can stand alone from the passage; no information or data from the passage is required to answer the question.
When acetylcholine binds its receptor on a muscle cell it produces a depolarization wave that opens channels in the plasma membrane and sarcoplasmic reticulum. As a result, flows out into the sarcoplasm where it stimulates the interaction of actin and myosin and the sliding of the filaments. Since a patient with myasthenia gravis will have a reduced number of functional acetylcholine receptors, the depolarization signal will be smaller, less will be released and fewer actin-myosin cross bridges will form. This sequence of events will result in the weaker contraction in the muscle of the myasthenia gravis patient. The number of troponin molecules bound to tropomyosin does not change during contraction. Troponin is bound to tropomyosin when the muscle is at rest. When is released from the sarcoplasmic reticulum, it binds to troponin and causes it to twist the tropomyosin enough to expose the actin myosin binding sites. Since troponin is bound to tropomyosin at rest and during contraction, there shouldn’t be any difference in the number of troponin-tropomyosin interactions in patients with myasthenia gravis as compared with normal individuals.
Example Question #44 : Cellular Processes And Functions
Both the sympathetic and the parasympathetic nervous systems are essential for homeostasis and for survival. For example, when we are trying to run away from a threat, the sympathetic nervous system is in full effect to allow us to escape from danger. However, when there is no obvious threat, the parasympathetic nervous system tends to be more in control.
There are similarities and differences between the sympathetic and the parasympathetic nervous systems. In preganglionic nerve fibers, both the sympathetic and the parasympathetic nervous system utilize the neurotransmitter acetylcholine. Closer to the target organ, the parasympathetic nervous system remains dependent on acetylcholine whereas norepinephrine and epinephrine are the predominant neurotransmitters utilized by the sympathetic nervous system.
When norepinephrine and epinephrine bind to their receptors, different effects are carried out based on the type of receptor, affinity, and location of the receptor. For example, epinephrine has a higher affinity for the beta-2 receptor. When epinephrine binds to the beta-2 receptor, common effects include vasodilation and bronchodilation. Norepinephrine has a stronger affinity for the alpha-1, alpha-2 and beta-1 receptors. When norepinephrine binds to its receptor, common effects on the body include vasoconstriction (alpha-1), increased heart rate (beta-1) and uterine contraction (alpha-1).
Patient A accidentally overdosed on a drug that activates the alpha-1, beta-1 and beta-2 receptors and is now experiencing a severe asthma attack. Which of the following second messengers should be regulated to treat the asthma attack?
I. Decrease CAMP level
II. Decrease adenylate cyclase activity
III. increase phospholipase C activity
II only
None of these
III only
I only
I and II
None of these
During a severe asthma attack, one should administer cpinephrine. Epinephrine binds to the beta-2 receptor. As mentioned in the passage, the activation of the beta-2 receptor will activate intercellular level of cyclic AMP. Cyclic AMP will then activate protein kinase A, which will then phosphorylate various proteins. Therefore, we want to increase cAMP levels, adenylate cyclase activity and decrease phospholipase C activity.
Example Question #11 : Cell Signaling
Sildenafil (commonly called Viagra) is a common drug used to treat erectile dysfunction and pulmonary arterial hypertension. Sildenafil's effect comes from its ability to cause vasodilation in smooth muscle cells. For this problem, we're only going to consider its effects on erections in males.
Erectile dysfunction is a common medical problem in older men. Its most significant effect is the prevention of erections. Erections occur when there is an increase in blood flow via enlargement of an artery (vasodilation). Understanding the mechanism by which vasodilations occur is important in order to treat erectile dysfunction.
Erections occur when nitric oxide is released from an area in the penis and binds to guanylate cyclase in other cells of the penis, which creates cyclic guanosine monophosphate (cGMP) from GTP. cGMP causes a relaxation of the arterial wall in order to increase blood flow to the region, thereby causing an erection. cGMP is broken down over time by cGMP-specific phosphodiesterase type 5 (PDE5) into GTP, which reverses the effect and causes vasoconstriction on the arterial wall. Combatting this effect is the major method by which Viagra functions.
Which of the following is not a possible mechanism by which Sildenafil treats erectile dysfunction?
Increased breakdown of nitric oxide
Inhibition of PDE5 activity
Increase in cGMP production
Increase in nitric oxide release
Decrease in cGMP breakdown
Increased breakdown of nitric oxide
For this question we have to select an answer choice that would decrease the prolonging of vasodilation.
The only answer choice that decreases vasodilation is by increasing the breakdown of nitric oxide, which is the first messenger in this signal transduction cascade to cause vasodilation. If nitric oxide breakdown is increased, vasodilation would decrease.
Increase in cGMP production, decrease in cGMP breakdown, inhibition of PDE5 activity, and increase in nitric oxide release would all prolong vasodilation.
Example Question #12 : Cell Signaling
Sildenafil (commonly called Viagra) is a common drug used to treat erectile dysfunction and pulmonary arterial hypertension. Sildenafil's effect comes from its ability to cause vasodilation in smooth muscle cells. For this problem, we're only going to consider its effects on erections in males.
Erectile dysfunction is a common medical problem in older men. Its most significant effect is the prevention of erections. Erections occur when there is an increase in blood flow via enlargement of an artery (vasodilation). Understanding the mechanism by which vasodilations occur is important in order to treat erectile dysfunction.
Erections occur when nitric oxide is released from an area in the penis and binds to guanylate cyclase in other cells of the penis, which creates cyclic guanosine monophosphate (cGMP) from GTP. cGMP causes a relaxation of the arterial wall in order to increase blood flow to the region, thereby causing an erection. cGMP is broken down over time by cGMP-specific phosphodiesterase type 5 (PDE5) into GTP, which reverses the effect and causes vasoconstriction on the arterial wall. Combatting this effect is the major method by which Viagra functions.
Nitric oxide is which of these types of signals?
Endocrine signal
Autocrine signal
Paracrine signal
Growth hormone
Neurotransmitter
Paracrine signal
Nitric oxide, as stated in the passage, is a signal that is sent from an area in the penis to another area within the penis. Since this is signaling to nearby cells, it is an example of paracrine signaling.
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