AP Bio- Chapter 11

4 Aspects of Fine Tuning a Response

1) amplification of the signal 2) specificity of the response 3) overall efficiency of response, enhanced by scaffolding proteins 4) termination of the signal

what triggers apoptosis?

1) extracellular death-signaling ligand 2) DNA damage in the nucleus 3) protein misfolding in the ER

3 types of membrane receptors

1. G protein-coupled receptors 2. Receptor tyrosine kinases 3. Ion channel receptors

G-protein coupled receptors

Largest family of cell surface receptors A plasma membrane receptor that works with the help of a G Protein

local signaling

animal cells may communicate by direct contact, or cell-cell recognition

activated hormone-receptor complex

can act as a transcription factor, turning on specific genes

what connects the cytoplasms of adjacent cells (in plants and animals)?

cell junctions

Inactivation mechanisms

essential aspect of cell signaling; if ligand concentration falls, fewer receptors will be bound and they will go into an inactive state

capases

main proteases (enzymes that cut up proteins) that carry out apoptosis

Receptor Tyrosine Kinases (RTKs)

membrane receptors that attach phosphates to tyrosines can trigger multiple signal transduction pathways at once

local regulators

messenger molecules that travel only short distances

what relays a signal from a receptor?

mostly proteins; activates each other like dominoes; is transduced into a different form

other pathways

regulate the activity of enzymes, not their synthesis

signaling pathways

regulate the synthesis of enzymes or other proteins, usually by turning genes on or off in the nucleus; final activated molecule may function as a transcription factor; can change shape and affect behavior

Caenorhabditis elegans (C. elegans- worms)

studied the role of apoptosis in embryonic development; apoptosis results when proteins that accelerate it override those that stop it

output response

the cell's response to an extracellular signal

first messenger

the extracellular signal molecule that binds to the receptor

multistep signal transduction pathways

can amplify a signal and can provide more opportunities for coordination and regulation of cellular response

first sign of transduction of a signal

change in shape

hormones

chemicals used by plant and animal cells to help with long distance signaling

Earl W. Sutherland

discovered how the hormone epinephrine acts on cells

where does the response to a signal transduction pathway occur?

either in the cytoplasm or the nucleus

Cells communicate via

electrical and chemical signals

a cell can only respond to a specific signal if...

it has a receptor specific to that signal

protein phosphatases

remove phosphates from proteins (dephosphorylation)

What activates the receptors?

small or hydrophobic chemical messengers that can cross membrane

protein kinases

transfer phosphates from ATP to protein (phosphorylation)

3 stages of cell signaling

1. Receptor Activation 2. Signal Transduction 3. Cellular Response

The figure above represents a generalized hormone-signaling pathway. Briefly explain the role of each numbered step in regulating target gene expression.

1. The ligand binds to the receptor, triggering signal reception 2. break down in the cell that transfers the signal from the plasma membrane to the nucleus 3. stimulation of the transcription of target genes

ligand

A molecule that binds specifically to a receptor site of another molecule

The tumor suppressor protein p53 binds to DNA and activates target genes, which results in the synthesis of p21, CD95, and other proteins. The p21 protein promotes cell-cycle arrest, whereas the CD95 protein promotes apoptosis. Which of the following will most likely result from a loss of p53 function? A Rapid cell growth without cell division B Immediate activation of apoptosis pathways C Uncontrolled cell proliferation D Increased expression of p53 target genes

C

The epinephrine signaling pathway plays a role in regulating glucose homeostasis in muscle cells. The signaling pathway is activated by the binding of epinephrine to the beta-2 adrenergic receptor. A simplified model of the epinephrine signaling pathway is represented in Figure 1. Figure 1. A simplified model of the epinephrine signaling pathway in muscle cells A researcher claims that the epinephrine signaling pathway controls a catabolic process in muscle cells. Which of the following statements best helps justify the researcher's claim? A Epinephrine is a signaling molecule that binds to a transmembrane protein. B The G protein in the epinephrine signaling pathway consists of three different subunits. C Phosphorylase kinase catalyzes the hydrolysis of ATP. D Glycogen phosphorylase catalyzes the conversion of glycogen to glucose-1-phosphate.

D

components of cAMP pathways

G proteins, G protein-coupled receptors, and protein kinases

Epinephrine is a protein hormone found in many animals. Epinephrine stimulates a signaling pathway that results in the breakdown of glycogen to glucose in the liver cells. Which of the following describes the initial steps in the process whereby epinephrine stimulates glycogen breakdown? A Epinephrine binds to a cell-surface receptor; the activated receptor stimulates production of the second messenger, cAMP B Epinephrine binds to a cell-surface receptor; the activated receptor catalyzes the conversion of glycogen to glucose. C Epinephrine diffuses through the plasma membrane; the hormone dimerizes in the cytosol. D Epinephrine is taken into the cell by endocytosis; glycogen is converted to glucose in the endocytic vesicle.

A

Researchers have discovered details about apoptosis (programmed cell death) by studying embryologic development of a nematode worm, Caenorhabditis elegans. Apoptosis is a normal developmental process in C. elegans. They found several genes involved in apoptosis, including ced−9 and ced−3 . The ced−3 gene was found to promote cell death, and ced−9 to inhibit it. The ced−9 gene serves as a regulator that prevents apoptosis in the absence of a signal promoting apoptosis. Which of the following statements best justifies the claim that changes in the expression of ced−9 in C. elegans can affect regulation of apoptosis in the cell? A An experiment showed that a mutation in the ced−9 gene led to excessive cell death in C. elegans. B An experiment showed that the ced−9 gene normally produces a protein that promotes excessive cell death in C. elegans. C A mutation in ced−3 will cause ced−9ced−9 to be incorrectly transcribed. D Apoptosis is dependent on a signal from the ced−9 gene in C. elegans.

A

signal transduction pathway

A series of steps in which a signal on a cell's surface is converted into a specific cellular response; leads to regulation of one or more cellular activities

A person's blood glucose level fluctuates during the day, as represented in Figure 1. Two hormones, insulin and glucagon, are directly involved in regulating the blood glucose level to maintain a healthy level. Insulin acts to lower the blood glucose level, and glucagon acts to increase the blood glucose level. Figure 1. Blood glucose fluctuations of an individual Which of the following best predicts what will happen to the blood glucose level if the person has another meal at 5 p.m.? A Immediately after the meal, the blood glucose level will decrease because of the increase in glucagon levels. B Immediately after the meal, the blood glucose level will increase, and then insulin will be secreted to counter the increase. C Several hours after the meal, the blood glucose level will increase sharply because of an increase in the amount of glucagon secreted. D The blood glucose level will not change after the 5 p.m. meal because the person has already consumed two meals and the blood glucose level has been adjusted to a steady-state level.

B

The graph above shows changes in glucagon and insulin secretions at different concentrations of blood glucose. Which of the following feedback mechanisms is best supported by the data? A falling glucagon level causes a rise in the insulin level, which maintains equal amounts of both hormones in the blood. B A high glucagon level causes a rise in the insulin level, which maintains high levels of both hormones in the blood. C A low glucose level causes the release of glucagon, which stimulates the release of more glucose from tissues, which in turn lowers the amount of glucagon being released. D A low glucose level causes the release of insulin, which stimulates the release of more glucose from tissues, which in turn increases the amount of insulin being released.

C

Which of the following steps in a signaling pathway typically occurs first once a chemical messenger reaches a target cell? A Specific genes are activated. B A second messenger molecule is produced. C A ligand binds to a receptor. D Specific proteins are synthesized.

C

Blood clots are formed by a positive feedback loop. Two pathways exist, the extrinsic and intrinsic pathways, which converge during clot formation. There are many clotting factors involved, most of which are proteins. Vitamin K is required for the formation of the active form of several of the clotting factors, including Factor X. A simplified model of the blood clotting process is shown in Figure 1. Figure 1. Simplified model of clotting cascade Warfarin is a drug used to treat certain blood clots. Warfarin blocks the formation of the active form of vitamin K-dependent clotting factors. Based on the model, which of the following best predicts the effects of warfarin on a patient? A Fibrinogen will form fibrin, but the clot will not form because Factor XIII will not be synthesized. B The intrinsic pathway will take over because the clotting factors are part of that pathway. C Thrombin will be converted to prothrombin because Factor XX will reverse the reaction. D Factor X will not be activated, which will prevent thrombin from forming.

D

G proteins are a family of receptor proteins that are involved in transmitting signals from outside a cell to inside a cell. When a signaling molecule binds to a G protein, the G protein is activated. The G protein then activates an enzyme that produces a second messenger called cAMP. Which of the following describes a critical role of cAMP during the transduction stage of a G protein signal transduction pathway? A cAMP carries the signal to the nucleus of the cell and results in new sequences of nucleotides being added to the cell's DNA. B cAMP binds the extracellular signal molecule and carries it to the intracellular target specified by the signal. C cAMP modifies a specific monomer so that it can be added to an elongating structural macromolecule. D cAMP results in the activation of an enzyme that amplifies the signal by acting on many substrate molecules.

D

Acetylcholine receptor (AChR) proteins are found at the synapse between neurons and skeletal muscle cells. Acetylcholine released from neurons binds to a specific site on the receptor proteins, which causes an ion channel in the receptors to open and allow sodium ions (Na+) to enter muscle cells. The resulting depolarization of muscle cells initiates muscle contractions. Another molecule, nicotine, can also bind to certain types of AChR proteins and activate the receptors. A researcher is investigating two different types of AChR proteins: type 1 and type 2. To determine which stimuli activate the receptors, the researcher exposes muscle cells expressing the different types of receptor proteins to stimuli and observes the results indicated in Table 1. Acetylcholinesterase is an enzyme that breaks down acetylcholine in the synapse. Describe the effect of inhibiting acetylcholinesterase on the muscle cells with AChR type 2.

Inhibiting the acetylcholinesterase on the muscle cells with AChR type 2 would cause the cell to be unable to break down acetylcholine. The lack of acetylcholine would cause an excess production of it to compensate. This would cause an overstimulation of the receptor proteins and cell stress. The acetylcholinesterase allows the muscle to contract, but the lack of it cause relaxation. However, because the cell wouldn't be able to break down the acetylcholine, the overstimulation of the receptor proteins would cause more muscle contractions.

phosphorylation and dephosphorylation

acts as a molecular switch, turning activities on and off

G Protein

acts as an on/off switch: (If GDP is bound to the G protein, the G protein is inactive)

why would calcium increase in the cytosol?

a signal relayed by a signal transduction pathway may trigger the increase

Calcium Ions

act as a second messenger in many pathways; cells can regulate its concentration

cells have different proteins:

allow cells to detect and respond to different signals; same signal can have different effect on certain cells

enzyme cascades

amplify the cell's response to the signal; number of activated products is greater than the preceding step

adenylyl cyclase

an enzyme in the plasma membrane, converts ATP to cAMP in response to an extracellular signal

intracellular receptor proteins

found in the cytosol or nucleus of target cells

Pathway branching and "cross-talk"

further help the cell coordinate incoming signals

scaffolding proteins

large relay proteins to which several other relay proteins are attached; can increase the signal transduction efficiency by grouping together proteins; can activate some of the relay proteins

Cylic AMP (cAMP)

one of the most widely used second messengers

Apoptosis

programmed or controlled cell suicide; components of the cell are chopped up and packaged into vesicles that are digested by scavenger cells; prevents enzymes from leaking out of a dying cell and damaging neighboring cells

ancenstrial signal molecules developed in

prokaryotes then were modified in eukaryotes

G protein systems

provides further regulation of cell metabolism; inhibits adenylyl cyclase

ligand-gated ion channel

receptor acts as a gate when the receptor changes shape

second messenger

small, non-protein water-soluble molecules or ions that spread throughout a cell by diffusion; participate in pathways initiated by GPCRs and RTKs ex: cAMP and Ca2+

protein kinase A

usually activated by cAMP; phosphorylates other proteins

Adjacent plant cells have narrow channels called plasmodesmata that pass through the cell walls of the connected cells and allow a cytoplasmic connection between the cells. Which of the following statements best describes a primary function of plasmodesmata? A They allow the movement of molecules from one cell to another, enabling communication between cells. B They prevent the cell membrane from pulling away from the cell wall during periods of drought. C They eliminate the need to produce signaling molecules and eliminate the need for cells to have receptors for signaling molecules. D They increase the surface area available for attachment of ribosomes and thus increase protein synthesis.

A

An antigen can induce an immune response in a host organism. Antigens are targeted by antibodies that are produced by the organism's immune system in response to contact with the antigen. Antibodies are specific to antigens. Many different cell types are involved in immune responses. Which of the following best describes the role of cell-to-cell communication during a response to an invasion of antigens? A Chemicals that are secreted from antigen-presenting cells then activate helper T cells. B A macrophage cell engulfs a pathogen in the blood. C Antigens attaching to receptors on memory B cells stimulate the memory B cells to become plasma cells. D Antigen-presenting cells engulf antigens at the first exposure.

A

Antidiuretic hormone (ADH) is important in maintaining homeostasis in mammals. ADH is released from the hypothalamus in response to high tissue osmolarity. In response to ADH, the collecting duct and distal tubule in the kidney become more permeable to water, which increases water reabsorption into the capillaries. The amount of hormone released is controlled by a negative feedback loop. Based on the model presented, which of the following statements expresses the proper relationship between osmolarity, ADH release, and urine production? A As tissue osmolarity rises, more ADH is released, causing less water to be excreted as urine. B As tissue osmolarity rises, less ADH is released, causing less water to be excreted as urine. C As tissue osmolarity rises, more ADH is released, causing more water to be excreted as urine. D As tissue osmolarity rises, less ADH is released, causing more water to be excreted as urine.

A

During a fight-or-flight response, epinephrine is released into the body's circulatory system and transported throughout the body. Some cells exhibit a response to the epinephrine while other cells do not. Which of the following justifies the claim that differences in components of cell signaling pathways explain the different responses to epinephrine? A Cell signaling depends on the ability to detect a signal molecule. Not all cells have receptors for epinephrine. Only cells with such receptors are capable of responding. B Cell signaling depends on the transduction of a received signal by the nervous system. Not all cells are close enough to a synapse to receive the signal and respond. C Cell signaling depends on the signal being able to diffuse through the cell membrane. Epinephrine is incapable of diffusing through some plasma membranes because of the membrane's phospholipid composition. D Cell signaling requires reception, transduction, and response. All cells can receive epinephrine, all cells respond with a pathway, but only select cells have the proper coding in their DNA to respond.

A

Fibroblast growth factor receptors (FGFRs) are transmembrane proteins that regulate cellular processes such as cell proliferation and differentiation. The extracellular domains of FGFR proteins bind specifically to signaling molecules called fibroblast growth factors. The intracellular domains of FGFR proteins function as protein kinases, enzymes that transfer phosphate groups from ATP to protein substrates. FGFR activation occurs when binding by fibroblast growth factors causes FGFR proteins in the plasma membrane to become closely associated with each other. The association of two FGFR proteins stimulates protein kinase activity, which triggers the activation of intracellular signaling pathways. A simplified model of FGFR activation is represented in Figure 1. Figure 1. A simplified model of FGFR activation Which of the following changes in the FGFR signaling pathway is most likely to result in uncontrolled cell proliferation? A The irreversible association of FGFR proteins B The loss of the FGFR protein kinase function C A decrease in the intracellular concentration of ATP D A decrease in the extracellular concentrations of fibroblast growth factors

A

Insulin, a hormone secreted by pancreatic cells, stimulates glucose uptake in skeletal muscle cells by mobilizing glucose transporter proteins (GLUT4) to the plasma membrane. As depicted in Figure 1, binding of insulin to the insulin receptor triggers an intracellular signaling cascade in which certain molecules activate other molecules in a relay of the hormone signal to cell targets. One outcome of the signaling cascade is mobilization of GLUT4 from vesicle storage sites in the cytoplasm to sites at the cell surface, where GLUT4 allows glucose to enter the cell. In type 2 diabetes, the cellular response to insulin is disrupted, and individuals with type 2 diabetes cannot properly regulate their blood glucose levels. In an investigation of the insulin signaling pathway, samples of skeletal muscle were isolated from individuals who have type 2 diabetes and from individuals who do not. The results of several experiments that were performed on the muscle samples are shown in Figure 2, Figure 3, and Figure 4. Which of the following is a valid interpretation of the experimental results that explains how individuals with type 2 diabetes differ from individuals without diabetes? A The relatively low levels of glucose uptake in individuals with type 2 diabetes indicate that mobilization of GLUT4 to the cell surface is reduced in muscle cells of those individuals. B The relatively low levels of glucose uptake in individuals with type 2 diabetes indicate that no functional GLUT4 protein is produced in the muscle cells of those individuals. C The absence of activated insulin receptors in individuals with type 2 diabetes indicates that no insulin is secreted by the pancreatic cells of those individuals. D The absence of activated IRS-1 in individuals with type 2 diabetes indicates that no functional insulin receptor protein is produced in the muscle cells of those individuals.

A

The epinephrine signaling pathway plays a role in regulating glucose homeostasis in muscle cells. The signaling pathway is activated by the binding of epinephrine to the beta-2 adrenergic receptor. A simplified model of the epinephrine signaling pathway is represented in Figure 1. Figure 1. A simplified model of the epinephrine signaling pathway in muscle cells Which of the following outcomes will most likely result from the irreversible binding of GDP to the G protein? A The intracellular concentration of glycogen will increase. B The intracellular concentration of activated protein kinase A will increase. C The intracellular concentration of cyclic AMP will increase. D The intracellular concentration of glucose-1-phosphate will increase.

A

Ethylene is an organic compound produced by ripening fruits. In a controlled experiment, researchers found that ethylene gas stimulated the ripening process in newly harvested fruits. Which of the following describes the most likely connection between natural ethylene production and fruit ripening? A As a result of metabolic inactivity, newly harvested fruits are unable to absorb ethylene gas from the atmosphere. B Ethylene gas is a chemical signal through which ripening fruits trigger the ripening process in other fruits. C Because of normal phenotypic variation, only some of the fruits in a given generation are expected to produce ethylene gas. D The rate of ethylene gas production by ripening fruits is an indicator of the relative age of an ecosystem.

B

Figure 1 represents the relative time and sequence of the phases of the cell cycle. Figure 1. Representation of the cell cycle and identification of the G1/S checkpoint Which statement best predicts why a cell's progression through the cell cycle might be halted at the G1/S checkpoint? A Spindle fibers have not correctly attached to chromosomes. B There are not enough nucleotides available to construct new DNA. C Damage occurred to DNA when it was being copied in G1G1. D Proteins necessary for M phase of the cell cycle have not been produced.

B

In mammals, an increase in the concentration of sodium in the blood triggers the release of antidiuretic hormone (ADH) from the pituitary gland. As the concentration of sodium in the blood returns to previous levels, the release of ADH from the pituitary gland is reduced. Based on the information presented, which of the following describes the most likely role of ADH in maintaining blood osmolarity? A ADH promotes an increase in the movement of sodium into the bloodstream. B ADH promotes an increase in the movement of water into the bloodstream. C ADH promotes an increase in the excretion of water from the body. D ADH promotes an increase in the secretion of additional ADH from the pituitary gland.

B

Metformin is a drug used to treat type 2 diabetes by decreasing glucose production in the liver. AMP-activated protein kinase (AMPK) is a major cellular regulator of glucose metabolism. Metformin activates AMPK in liver cells but cannot cross the plasma membrane. By blocking AMPK with an inhibitor, researchers found that AMPK activation is required for metformin to produce an inhibitory effect on glucose production by liver cells. Which of the following best describes the component that metformin represents in a signal transduction pathway that regulates glucose production in the liver? A It is a secondary messenger that amplifies a signal through a cascade reaction. B It is a ligand that activates the signal transduction pathway of the activation of AMPK. C It is an allosteric regulator that binds to a crucial section of the DNA that makes the enzymes needed for glucose uptake. D It is a competitive inhibitor that binds to glucose and prevents it from entering the cell.

B

Phosphofructokinase (PFK) is a key enzyme in glycolysis. ATP is one of the two substrates for the reaction catalyzed by PFK. ATP is also an allosteric regulator of PFK. Figure 1 shows the enzyme-substrate interactions of PFK. Figure 1. The enzyme-substrate interactions of PFK A researcher found a mutation that resulted in the PFK enzyme being unable to bind ATP to the allosteric site. Which of the following best predicts the effect of the mutation? A The activity of the enzyme will not be affected because the active site is not involved in substrate binding at the allosteric site. B Negative feedback regulation does not occur, so the enzyme will be active when glycolysis is not needed. C Positive feedback does not occur, and the activity of the enzyme will decrease when glycolysis is needed. D The activity of the enzyme will fluctuate independent of the ATP concentration.

B

Signal transduction may result in changes in gene expression and cell function, which may alter phenotype in an embryo. An example is the expression of the SRY gene, which triggers the male sexual development pathway in mammals. This gene is found on the Y chromosome. Which statement provides the evidence to justify the claim that signal transduction may result in an altered phenotype? A If the SRY gene is absent or nonfunctional, the embryo will exhibit male sexual development. B If the SRY gene is absent or nonfunctional, the embryo will exhibit female sexual development. C An embryo with a male sex chromosome will always exhibit male sexual development. D An embryo with two male sex chromosomes will always exhibit male sexual development.

B

The beta-2 adrenergic receptor is a membrane-bound protein that regulates several cellular processes, including the synthesis and breakdown of glycogen. The receptor binds specifically to the hormone epinephrine. The binding of epinephrine to the beta-2 adrenergic receptor triggers a signal transduction cascade that controls glycogen synthesis and breakdown in the cell. A simplified model of the signal transduction cascade is represented in Figure 1. Figure 1. A simplified model of the signal transduction cascade triggered by epinephrine binding to the beta-2 adrenergic receptor Which of the following outcomes will most likely result from the inactivation of the beta-2 adrenergic receptor? A The cellular concentration of cyclic AMP will increase. B The enzymatic activity of protein kinase A will increase. C The activation of glycogen phosphorylase will increase. D The rate of glycogen synthesis in the cell will increase.

B

The diagram above represents a model of signal transduction pathways (I and II) in a cell that is targeted by two different hormones (H1 and H2). The components of the signal transduction pathways are identified in the figure legend. Each cellular molecule in both pathways can exist in an inactive or active form. When the components in pathway I are sequentially activated, the TAP molecules promote cell division. When the components in pathway II are sequentially activated, downstream signaling by the G protein is inhibited. Based on the model, which of the following mutations is most likely to result in a cell that will generate a cancerous tumor? A A mutation in the gene encoding PP that results in a nonfunctional protein B A mutation in the gene encoding G-PIP that results in a nonfunctional protein C A mutation in the gene encoding R1 so that it is inactive even in the presence of H1 D A mutation in the gene encoding R2 so that it is active even in the absence of H2

B

The endocrine system incorporates feedback mechanisms that maintain homeostasis. Which of the following demonstrates negative feedback by the endocrine system? A During labor, the fetus exerts pressure on the uterine wall, inducing the production of oxytocin, which stimulates uterine wall contraction. The contractions cause the fetus to further push on the wall, increasing the production of oxytocin. B After a meal, blood glucose levels become elevated, stimulating beta cells of the pancreas to release insulin into the blood. Excess glucose is then converted to glycogen in the liver, reducing blood glucose levels. C At high elevation, atmospheric oxygen is more scarce. In response to signals that oxygen is low, the brain decreases an individual's rate of respiration to compensate for the difference. D A transcription factor binds to the regulatory region of a gene, blocking the binding of another transcription factor required for expression.

B

The epinephrine signaling pathway plays a role in regulating glucose homeostasis in muscle cells. The signaling pathway is activated by the binding of epinephrine to the beta-2 adrenergic receptor. A simplified model of the epinephrine signaling pathway is represented in Figure 1. Figure 1. A simplified model of the epinephrine signaling pathway in muscle cells Based on Figure 1, which of the following statements best describes the epinephrine signaling pathway? A It involves the opening and closing of ion channels. B In involves enzymes activating other enzymes. C It involves changes in the expression of target genes. D It involves protons moving down a concentration gradient.

B

The insulin receptor is a transmembrane protein that plays a role in the regulation of glucose homeostasis. The receptor's extracellular domain binds specifically to the peptide hormone insulin. The receptor's intracellular domain interacts with cellular factors. The binding of insulin to the receptor stimulates a signal transduction pathway that results in the subcellular translocation of GLUT4, a glucose transport protein that is stored in vesicles inside the cell. A simplified model of the insulin receptor-signaling pathway is shown in Figure 1. Figure 1. A simplified model of the insulin receptor-signaling pathway Which of the following statements best predicts the effect of a loss of function of the insulin receptor's intracellular domain? A The stimulation of the signal transduction pathway will increase. B The storage of GLUT4 in vesicles inside the cell will increase. C The number of GLUT4 molecules in the plasma membrane will increase. D The concentration of glucose inside the cell will increase.

B

The model shown in the figure represents the role of two hormones, calcitonin and parathyroid hormone (PTH), in maintaining normal blood calcium levels in humans. If a dietary change results in an increase in blood calcium concentration above normal levels, which of the following is the most likely effect on calcium homeostasis? A Calcitonin levels will decline, thus stimulating the release of PTH. B Calcitonin levels will rise, thus promoting the deposit of calcium into bones. C PTH levels will decline, thus stimulating the loss of calcium from bones. D PTH levels will increase, thus preventing the release of calcitonin.

B

The relative amounts of DNA present in the nucleus of a cell at four different stages of the life cycle are shown in Figure 1. Figure 1. Relative amounts of DNA present in the nucleus of a cell Based on Figure 1, which of the following statements correctly links a stage of the cell cycle with the event occurring at that stage? A Stage II represents the G2 phase of the cell cycle. B Synthesis of sufficient DNA for two daughter cells occurs in stage II. C Stage IIIIII includes mitosis. D The replication of genetic material occurs in stage IVIV.

B

Which of the following statements best describes how a growth factor stimulates cell division from outside a cell? A The growth factor binds to other cells in the same area and holds them together to form a large, multicellular structure. B The growth factor binds to receptors on the cell surface, initiating a signal transduction pathway that activates specific target genes. C The growth factor binds to sugar molecules in the extracellular fluid and provides them to the cell as a source of energy. D The growth factor binds to phospholipids in the plasma membrane, creating a channel through which substances enter the cell.

B

A hydrophilic peptide hormone is produced in the anterior pituitary gland located at the base of the brain. The hormone targets specific cells in many parts of the body. Which of the following best explains a possible mechanism that would enable the hormone to efficiently reach all of the target cells in the body? A The hormone interacts with the nerves at the base of the brain and directs signals to the target cells through the nervous system. B The hormone diffuses into target cells adjacent to the anterior pituitary gland, where the hormone is degraded. C The hormone is released into the bloodstream where it can be transported to all cells with the correct receptors. D The hormone moves through cytoplasmic connections between cells until it has reached all cells with the correct intracellular binding sites.

C

A model of the typical life cycle of a cell is shown in Figure 1. Figure 1. Typical life cycle of a eukaryotic cell Scientists have estimated that it takes yeast cells approximately 20 hours to complete the entire cycle. Table 1 shows the amount of time in each phase of the life cycle for yeast cells. Table 1. Amount of time spent in each stage of the cell cycle by yeast cells StageMG1SG2Time (hours)21053 Based on Table 1, what percent of the life cycle of yeast cells is spent in DNA replication? A 5 percent B 10 percent C 25 percent D 50 percent

C

A student used microscopy to investigate the relative lengths of the different stages of mitosis. The student prepared slides of cells isolated from a growing onion root tip and viewed the slides under a dissecting microscope. The student then made diagrams of cells that were in different stages of mitosis and counted the number of cells that were in each of those stages. The student's data are presented in Table 1. Table 1. Number of cells in each of four different stages of mitosis Based on the data, the percent of the mitotic cells that were in metaphase is closest to which of the following? A 5% B 11% C 18% D 66%

C

Cell communication is critical for the function of both unicellular and multicellular eukaryotes. Which of the following is likely true of cell signaling? A Cell signaling uses the highest molecular weight molecules found in living cells. B Cell signaling has largely been replaced by other cell functions in higher mammals. C Similar cell signaling pathways in diverse eukaryotes are evidence of conserved evolutionary processes. D Cell signaling functions mainly during early developmental stages.

C

Excess intracellular iron is toxic to cells (iron-induced toxicity). Ferritin is an intracellular iron storage protein that binds excess iron. The presence of ferritin can protect cells from iron-induced toxicity. In an experiment to investigate the effects of dietary iron intake on ferritin synthesis, rats were given food containing different amounts of iron. Subsequently, the levels of ferritin protein in the liver were measured. The results are shown in Figure 1. Based on these and other data, researchers have developed the following model demonstrating how ferritin synthesis is regulated by iron. When iron levels are low, a repressor of translation, iron response protein (IRP), binds to an iron response element (IRE), which is a stem-loop structure near the 5¢ end of ferritin mRNA. When iron levels are high, intracellular iron binds to the IRP, and the iron-IRP complex dissociates from the IRE, permitting ribosomes to proceed with the translation of ferritin mRNA. Figure 2 represents the model of the regulation of ferritin mRNA translation by iron. Figure 2. Model of regulation of ferritin synthesis by iron Based on the model of ferritin synthesis presented in Figure 2, which of the following best describes the mechanism whereby iron most likely regulates ferritin production? A Translation occurs under low intracellular iron concentration when the IRP recruits ribosomes to the ferritin mRNA. B Translation occurs under low intracellular iron concentration when the IRP stabilizes the stem-loop structure in the ferritin mRNA. C Translation occurs under high intracellular iron concentration when the IRP-iron complex dissociates from ferritin mRNA, permitting ribosomes access to the ferritin coding region. D Translation occurs under high intracellular iron concentration when the IRP-iron complex brings the 5¢ end of the mRNA closer to the ferritin coding region.

C

Insulin, a hormone secreted by pancreatic cells, stimulates glucose uptake in skeletal muscle cells by mobilizing glucose transporter proteins (GLUT4) to the plasma membrane. As depicted in Figure 1, binding of insulin to the insulin receptor triggers an intracellular signaling cascade in which certain molecules activate other molecules in a relay of the hormone signal to cell targets. One outcome of the signaling cascade is mobilization of GLUT4 from vesicle storage sites in the cytoplasm to sites at the cell surface, where GLUT4 allows glucose to enter the cell. In type 2 diabetes, the cellular response to insulin is disrupted, and individuals with type 2 diabetes cannot properly regulate their blood glucose levels. In an investigation of the insulin signaling pathway, samples of skeletal muscle were isolated from individuals who have type 2 diabetes and from individuals who do not. The results of several experiments that were performed on the muscle samples are shown in Figure 2, Figure 3, and Figure 4. Based on the experimental results, which of the following describes the most likely defect in muscle cells of patients with type 2 diabetes? A Insulin receptor proteins do not reach the cell surface. B Insulin does not activate its receptor. C IRS-1 activation is reduced at high insulin concentrations. D GLUT4 blocks glucose from entering cells.

C

Researchers tracked the amount of DNA (measured in picograms) over time beginning with a single cell and continuing through several rounds of cell division. The researchers observed threadlike chromosomes prior to cell division. The threadlike chromosomes disappeared from view shortly after each division. The amount of DNA in picograms per cell over several rounds of cell division is shown in Figure 1. Figure 1. Amount of DNA in picograms per cell over several rounds of cell division Which of the following statements is consistent with the data in Figure 1? A The cells have a haploid chromosome number of 3. B The cells have a diploid chromosome number of 6. C There is a change from 3 to 6 picograms of DNA because DNA is replicated before each round of cell division. D There is a change from 6 to 3 picograms of DNA after each cell division because the chromosomes lengthen following cell division.

C

The Hedgehog protein (Hh) plays a critical role during a certain period of embryo development, but it normally has no role in adults except for the maintenance of adult stem cells. However, the Hedgehog protein has been detected in 70 percent of pancreatic cancer cell samples. As illustrated in the figures below, the Hedgehog protein binds to an integral membrane protein receptor known as Patched (Ptc), thus initiating a pathway of gene expression. When Hedgehog is absent, Ptc inhibits another protein known as Smoothened (Smo), which, in turn, blocks the activation of a group of proteins collectively known as the Hedgehog signaling complex (HSC). The inactivation is the result of proteolytic cleavage of one component of the HSC complex, a transcription factor known as Cubitus interruptus (Ci). When Hedgehog is present, it binds to Ptc, which prevents the inhibition of Smo by Ptc. The result is that Ci remains intact and can enter the nucleus, where it binds to and activates certain genes. One approach to treating patients with pancreatic cancer and other cancers in which the Hedgehog protein is detected is to modify the Hedgehog signaling pathway. Which of the following is the most useful approach? A Treating patients with a molecule that is structurally similar to Hedgehog and that will bind to and interact with Ptc in the same fashion as Hedgehog B Injecting patients with embryonic cells so that Hedgehog will bind to those cells instead of the cancer cells C Treating patients with a membrane-soluble compound that can bind to Smo and block its activity D Injecting patients with a preparation of purified membrane-soluble Ci that will enter the nuclei of the cancer cells and induce gene transcription

C

The epinephrine signaling pathway plays a role in regulating glucose homeostasis in muscle cells. The signaling pathway is activated by the binding of epinephrine to the beta-2 adrenergic receptor. A simplified model of the epinephrine signaling pathway is represented in Figure 1. Figure 1. A simplified model of the epinephrine signaling pathway in muscle cells Cyclic AMP phosphodiesterase is an enzyme that catalyzes the conversion of cyclic AMP to a different molecule. Which of the following best predicts the effect of inhibiting cyclic AMP phosphodiesterase in a muscle cell stimulated by epinephrine? A The concentration of cyclic AMP will decrease because adenylyl cyclase will no longer be activated. B The G protein will diffuse out of the cell because it will no longer bind to the plasma membrane. C Phosphorylase kinase will remain active because protein kinase A will no longer be deactivated. D Glycolysis will stop because epinephrine signaling will no longer stimulate glycogen breakdown.

C

The epinephrine signaling pathway plays a role in regulating glucose homeostasis in muscle cells. The signaling pathway is activated by the binding of epinephrine to the beta-2 adrenergic receptor. A simplified model of the epinephrine signaling pathway is represented in Figure 1. Figure 1. A simplified model of the epinephrine signaling pathway in muscle cells Which of the following statements best describes the role of adenylyl cyclase in the epinephrine signaling pathway? A It converts a polymer to its monomer subunits. B It moves substances across the plasma membrane. C It accelerates the production of a second messenger. D It transfers phosphate groups from ATP to protein substrates.

C

Cortisol is a hormone produced in response to stress, including starvation, in humans. Which of the following is most likely an immediate effect of a starvation-induced increase in cortisol secretion? A Increased activation of the immune system B Increased urine production by the kidneys C Increased bone and collagen formation D Increased mobilization of fatty acids from fat cells

D

A cell culture commonly used in research was selected to study the effect of a specific virus on the timing of cell cycle phases. Two separate cultures were started, one untreated and one inoculated with the virus. Both cultures were incubated under identical conditions. After a period of time, 200 cells from each culture were observed and classified as shown in Table 1. Table 1. Number of normal and infected cells found in three phases of the cell cycle Which of the following most accurately describes an observation and an effect of the viral infection indicated by the data in Table 1? A Normal cells spend 98 percent of their time cycling in and out of interphase. The virus reduces this to 5 percent of the time. B Twenty percent of the virus-infected cells are in interphase. These cells are no longer part of the cell cycle. C Forty percent of the virus-infected cells are in interphase. These cells are preparing for replication of genetic material. D Seventy-five percent of the virus-infected cells are found in mitosis. The virus stimulates frequent cell division.

D

Damaged tissue releases chemicals that activate platelets and stimulate the formation of blood clots. Which of the following predictions about the activity of platelets best describes a positive feedback mechanism? A Activated platelets release chemicals that inhibit blood clot formation. B Activated platelets release signaling molecules that inhibit cell division in damaged tissue. C Activated platelets constrict the blood vessels, stopping blood flow. D Activated platelets release chemicals that activate more platelets.

D

Excess intracellular iron is toxic to cells (iron-induced toxicity). Ferritin is an intracellular iron storage protein that binds excess iron. The presence of ferritin can protect cells from iron-induced toxicity. In an experiment to investigate the effects of dietary iron intake on ferritin synthesis, rats were given food containing different amounts of iron. Subsequently, the levels of ferritin protein in the liver were measured. The results are shown in Figure 1. Based on these and other data, researchers have developed the following model demonstrating how ferritin synthesis is regulated by iron. When iron levels are low, a repressor of translation, iron response protein (IRP), binds to an iron response element (IRE), which is a stem-loop structure near the 5¢ end of ferritin mRNA. When iron levels are high, intracellular iron binds to the IRP, and the iron-IRP complex dissociates from the IRE, permitting ribosomes to proceed with the translation of ferritin mRNA. Figure 2 represents the model of the regulation of ferritin mRNA translation by iron. Figure 2. Model of regulation of ferritin synthesis by iron Based on the model of ferritin synthesis presented in Figure 2, which of the following describes the role of feedback on the control of intracellular iron levels? A decrease in iron levels activates the IRP. The IRP in turn activates iron transport proteins in the cell membrane, thereby returning free iron levels to normal. B A decrease in iron levels activates synthesis of ferritin protein. Ferritin protein in turn releases bound iron, thereby returning free iron levels to normal. C An increase in iron levels activates the IRP. The IRP in turn binds iron, thereby decreasing both free iron levels and ferritin synthesis. D An increase in iron levels activates synthesis of ferritin protein. Ferritin protein in turn binds iron, thereby decreasing both free iron levels and ferritin synthesis.

D

Researchers grew seedlings of corn, Zea mays, in loose and compact sand. The researchers measured the amount of time required for the cells in the growing root tips of the seedlings to double in number. The mean cell doubling times for the two groups of seedlings are shown in Figure 1. Figure 1. Mean cell doubling times for the growing root tips of Zea mays seedlings planted in loose or compact sand Based on the sample means, which of the following conclusions about the cells in the growing root tips of Zea mays seedlings is best supported by the results of the experiment? A The cells of the root tips grow to larger sizes when the seedlings are planted in compact sand than when the seedlings are planted in loose sand. B The average rate of mitotic cell division is greater for the root tips growing in loose sand than for the root tips growing in compact sand. C The average cell cycle time is greater for the root tips growing in compact sand than for the root tips growing in loose sand. D More cells are produced per unit of time in the root tips growing in compact sand than in the root tips growing in loose sand.

D

The diagram above illustrates feedback control as exerted by the hormone thyroxine. Following surgical removal of the thyroid gland, the level of TSH in the blood will increase. Which of the following best explains this increase? A Residual blood thyroxine, from prior to thyroid gland removal, will bind to cells in the anterior pituitary, signaling more TSH secretion. B Thyroxine will remain bound to thyroxine receptors on various body cells, and these body cells will secrete additional hormones that stimulate the anterior pituitary to secrete TSH. C Thyroxine that was stored in the anterior pituitary prior to thyroid gland removal will signal more TSH secretion. D A decrease in thyroxine levels means a loss of inhibition to the hypothalamus and anterior pituitary, leading to increased TSH secretion.

D

The epidermal growth factor receptor EGFR is a cell surface receptor. When a growth factor binds to EGFR, the receptor is activated. The activated EGFR triggers a signal transduction pathway, which leads to increased frequency of cell division. Which of the following best predicts the effect of a mutation that causes EGFR to be active in the absence of a growth factor? A Increased apoptosis will lead to abnormal growth of the tissue. B Increased cell division will lead to the formation of a tumor. C Cells will exit the cell cycle, entering a non-dividing G0 phase. D Fewer cells will be in any of the stages of mitosis.

D

Acetylcholine receptor (AChR) proteins are found at the synapse between neurons and skeletal muscle cells. Acetylcholine released from neurons binds to a specific site on the receptor proteins, which causes an ion channel in the receptors to open and allow sodium ions (Na+) to enter muscle cells. The resulting depolarization of muscle cells initiates muscle contractions. Another molecule, nicotine, can also bind to certain types of AChR proteins and activate the receptors. A researcher is investigating two different types of AChR proteins: type 1 and type 2. To determine which stimuli activate the receptors, the researcher exposes muscle cells expressing the different types of receptor proteins to stimuli and observes the results indicated in Table 1. Describe the difference in the structure AND function between AChR type 1 and AChR type 2.

In the AChR type 2, it only has one binding site in which only acetylcholine can bind to, which is shown through the lack of activation of nicotine. In the AChR type 1, it has two binding sites that allow both receptor proteins to bind to the AChR protein and produce a response (activation).