IB Biology Quiz: Apply Chemical Signalling
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Apply Chemical SignallingQuestion 1 of 19

Some signal transduction pathways exhibit 'crosstalk', where the components of one pathway can influence the components of another. For example, Pathway 1 activates Protein A, while Pathway 2 activates Protein B. Protein B, when active, acts as an inhibitor of Protein A.

If a cell is stimulated with signals for both Pathway 1 and Pathway 2 simultaneously, what is the expected outcome for the activity of Protein A?

The activity of Protein A will be higher than if the cell were stimulated by Pathway 1 alone.
The activity of Protein A will be lower than if the cell were stimulated by Pathway 1 alone.
The activity of Protein A will be unchanged because the two pathways are independent.
Protein B will be inhibited by Protein A, leading to a positive feedback loop.
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IB Biology Quiz

IB Biology Quiz: Apply Chemical Signalling

Practice Apply Chemical Signalling in IB Biology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Apply Chemical Signalling, giving you a quick way to practice the rules, question types, and explanations that matter most for IB Biology.

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Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

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Question 1

Some signal transduction pathways exhibit 'crosstalk', where the components of one pathway can influence the components of another. For example, Pathway 1 activates Protein A, while Pathway 2 activates Protein B. Protein B, when active, acts as an inhibitor of Protein A.

If a cell is stimulated with signals for both Pathway 1 and Pathway 2 simultaneously, what is the expected outcome for the activity of Protein A?

  1. The activity of Protein A will be higher than if the cell were stimulated by Pathway 1 alone.
  2. The activity of Protein A will be lower than if the cell were stimulated by Pathway 1 alone. (correct answer)
  3. The activity of Protein A will be unchanged because the two pathways are independent.
  4. Protein B will be inhibited by Protein A, leading to a positive feedback loop.
Explanation: This scenario describes inhibitory crosstalk. Pathway 1 activates Protein A. Simultaneously, Pathway 2 activates Protein B. The problem states that active Protein B inhibits Protein A. Therefore, the activation from Pathway 1 will be counteracted by the inhibition from Pathway 2, resulting in a net activity for Protein A that is lower than if only Pathway 1 were active.

Question 2

In a signal transduction cascade, protein kinases phosphorylate and activate other proteins, while protein phosphatases remove phosphate groups, inactivating them. The balance between kinase and phosphatase activity determines the strength and duration of a signal.

A cell is stimulated by a growth factor that activates a kinase cascade. If a drug that inhibits all protein phosphatase activity is added simultaneously, what would be the result?

  1. The cell would undergo apoptosis due to conflicting signals between phosphorylation and dephosphorylation.
  2. The signal would be blocked immediately because kinases require phosphatases to function correctly.
  3. The signal would be amplified and prolonged because phosphorylated proteins would not be dephosphorylated. (correct answer)
  4. There would be no effect on the signalling pathway, as phosphatases only act after the signal is removed.
Explanation: Protein phosphatases act as the 'off-switch' in phosphorylation-based signalling cascades. By inhibiting them, the phosphorylated (and typically active) forms of the signalling proteins would accumulate and persist, leading to a much stronger and longer-lasting response than normal, even after the initial stimulus is gone.

Question 3

A signal transduction pathway is initiated when a ligand binds to a receptor (R), which activates a G-protein (G). The active G-protein then stimulates an effector enzyme (E), which produces a second messenger (SM) that leads to a cellular response.

A newly discovered toxin is found to irreversibly modify the G-protein, preventing it from hydrolyzing GTP to GDP. What is the most likely immediate consequence for the cell's signalling pathway after initial stimulation?

  1. The G-protein will remain active, leading to continuous production of the second messenger SM. (correct answer)
  2. The receptor R will be unable to bind the ligand, preventing any signal from being initiated.
  3. The effector enzyme E will be permanently inhibited, leading to a decrease in the level of SM.
  4. The G-protein will be unable to dissociate from the receptor, blocking the activation of the effector enzyme E.
Explanation: The hydrolysis of GTP to GDP is the 'off switch' for the G-protein. If this is prevented, the G-protein remains bound to GTP and thus stays in its active state, continuously stimulating the effector enzyme (E) and leading to an overproduction of the second messenger (SM).

Question 4

In many cells, epinephrine (adrenaline) binding to its receptor leads to the activation of adenylyl cyclase, which converts ATP to cyclic AMP (cAMP). cAMP acts as a second messenger, activating protein kinase A (PKA). PKA then phosphorylates various target proteins.

A cell line is engineered to have a form of protein kinase A (PKA) that is constitutively active, meaning it is active even in the absence of cAMP. What would be the expected phenotype of these cells, even without epinephrine stimulation?

  1. The cellular effects normally mediated by PKA will occur continuously. (correct answer)
  2. The levels of cAMP will be extremely high due to a positive feedback loop.
  3. Adenylyl cyclase will be inhibited by the constitutively active PKA.
  4. Epinephrine will be unable to bind to its receptor on the cell surface.
Explanation: If PKA is constitutively active, it will phosphorylate its target proteins regardless of whether the upstream components of the pathway (receptor, G-protein, adenylyl cyclase, cAMP) are active. This bypasses the need for the signal and second messenger, leading to a continuous response.

Question 5

Some signalling pathways use calcium ions (Ca²⁺) as a second messenger, released from the endoplasmic reticulum into the cytosol. For the signal to be terminated, these ions must be actively transported back into the endoplasmic reticulum. Which substance would most directly interfere with the termination of a Ca²⁺-mediated signal?

  1. A chemical that blocks ATP synthesis in the mitochondria. (correct answer)
  2. A chemical that increases the fluidity of the plasma membrane.
  3. A chemical that competitively inhibits the initial hormone receptor.
  4. A chemical that enhances the activity of protein phosphatases in the cytosol.
Explanation: Active transport requires energy in the form of ATP. Pumping Ca²⁺ ions back into the endoplasmic reticulum against their concentration gradient is a form of active transport. Therefore, a chemical that blocks ATP synthesis would prevent this pumping action, causing Ca²⁺ levels in the cytosol to remain high and prolonging the signal.

Question 6

Propranolol is a drug that acts as an antagonist for beta-adrenergic receptors, to which epinephrine normally binds. It is often prescribed for conditions like high blood pressure and anxiety. Which statement correctly explains its mechanism at the molecular level?

  1. Propranolol binds to the receptor but does not activate the associated G-protein, blocking epinephrine's effect. (correct answer)
  2. Propranolol binds to epinephrine in the bloodstream, preventing it from reaching the receptor.
  3. Propranolol enhances the breakdown of the second messenger cAMP inside the target cell.
  4. Propranolol inhibits the synthesis of epinephrine in the adrenal glands, reducing its overall levels.
Explanation: An antagonist is a molecule that binds to a receptor but fails to elicit the normal biological response. By occupying the binding site, it prevents the natural ligand (in this case, epinephrine) from binding and activating the receptor, thus blocking the signal transduction pathway at its first step.

Question 7

A key feature of many signal transduction cascades is amplification. Which step in a typical G-protein coupled receptor (GPCR) pathway contributes most significantly to this amplification?

  1. A protein kinase phosphorylating a single molecule of its target protein.
  2. A single ligand molecule binding to a single receptor protein on the cell surface.
  3. The hydrolysis of a single GTP molecule to GDP by an activated G-protein.
  4. An activated enzyme, such as adenylyl cyclase, producing many molecules of a second messenger. (correct answer)
Explanation: Amplification occurs when one active molecule leads to the creation or activation of many downstream molecules. While an activated receptor can activate several G-proteins, the most significant amplification step is when one active enzyme (like adenylyl cyclase) can catalyze the conversion of many substrate molecules (ATP) into many product molecules (cAMP), vastly increasing the number of signalling molecules.

Question 8

Paracrine signalling involves a cell secreting a signalling molecule that acts on nearby target cells. This contrasts with endocrine signalling, where hormones travel through the bloodstream to act on distant cells.

A researcher observes that cell type X releases a chemical that causes a rapid response in adjacent cell type Y, but not in distant cells of the same type. The effect is blocked when the extracellular fluid is rapidly diluted. This is most likely an example of which type of signalling?

  1. Autocrine signalling, because dilution of the fluid would affect any extracellular signal.
  2. Endocrine signalling, because a chemical is released to cause a response in another cell.
  3. Synaptic signalling, because the response is rapid and involves two different cell types.
  4. Paracrine signalling, because the signal is local and concentration-dependent. (correct answer)
Explanation: The key features described are local action ('adjacent cell') and the signal being diluted in the extracellular fluid, which prevents it from reaching distant targets. This is the definition of paracrine signalling. Endocrine signalling is long-distance via the bloodstream. Synaptic signalling is a specialized form of paracrine signalling over a very short distance (synaptic cleft) and is not the best general description. Autocrine signalling involves the cell signalling to itself.

Question 9

Glucagon is a peptide hormone that raises blood glucose levels, while insulin lowers them. Both are produced in the pancreas. How is the secretion of glucagon typically regulated in a healthy individual?

  1. The anterior pituitary gland secretes a hormone that stimulates alpha cells to secrete glucagon.
  2. High blood glucose levels stimulate alpha cells to secrete glucagon.
  3. High levels of insulin in the blood stimulate alpha cells to secrete glucagon.
  4. Low blood glucose levels stimulate alpha cells to secrete glucagon. (correct answer)
Explanation: Glucagon's role is to counteract hypoglycemia (low blood sugar). Therefore, its secretion from the alpha cells of the pancreas is stimulated directly by a drop in blood glucose concentration. This is a primary example of humoral (blood-borne) stimulation in a homeostatic feedback loop.

Question 10

Chronic exposure to a high concentration of a specific hormone can lead to receptor down-regulation in target cells. What is the primary physiological consequence of this process?

  1. The hormone is degraded more quickly in the bloodstream, reducing its overall effect.
  2. The target cells become more sensitive to the hormone, leading to an amplified response.
  3. The target cells become less sensitive to the hormone, requiring a higher concentration for the same response. (correct answer)
  4. The endocrine gland producing the hormone reduces its secretion due to positive feedback.
Explanation: Down-regulation is a homeostatic mechanism where the number of receptors on a target cell's surface is decreased in response to high levels of a signalling molecule. With fewer receptors available, the cell's sensitivity to the hormone is reduced. This is a form of desensitization to prevent overstimulation.

Question 11

At the midpoint of the menstrual cycle, high levels of estrogen from the developing follicle switch from exerting negative feedback to positive feedback on the hypothalamus and pituitary. What is the direct consequence of this switch?

  1. The immediate breakdown of the corpus luteum and the onset of menstruation.
  2. A sharp decrease in LH and FSH secretion, which prevents multiple follicles from developing.
  3. A massive surge in LH and FSH secretion, which triggers ovulation. (correct answer)
  4. A rapid increase in progesterone secretion from the follicle itself.
Explanation: The switch to positive feedback means that high estrogen stimulates, rather than inhibits, the release of GnRH from the hypothalamus and LH/FSH from the pituitary. This creates a rapid, exponential increase in LH levels (the LH surge), which is the primary trigger for the mature follicle to rupture and release the egg (ovulation).

Question 12

Tyrosine kinase receptors are a class of cell-surface receptors. When a ligand binds, two receptor monomers dimerize (join together). This dimerization activates the intrinsic kinase activity of the receptor, causing the two monomers to phosphorylate each other on tyrosine residues (autophosphorylation). What is the most likely immediate effect of a mutation that prevents dimerization?

  1. The ligand will be unable to bind to the individual receptor monomers.
  2. The receptor's kinase domain will not be activated, and intracellular proteins will not be recruited. (correct answer)
  3. The receptor will be constitutively active, leading to uncontrolled cell growth.
  4. The receptor will still autophosphorylate but will be unable to activate downstream pathways.
Explanation: Dimerization is the critical step that brings the two intracellular kinase domains close enough to phosphorylate and activate each other. If dimerization is prevented, autophosphorylation cannot occur. Without this activation step, the receptor cannot bind and activate downstream intracellular signalling proteins, and the signal is not transduced.

Question 13

Testosterone, a steroid hormone, promotes the development of male secondary sexual characteristics by altering gene expression in target cells. Which sequence correctly outlines the primary mechanism of action for testosterone?

  1. Binding to cell-surface receptor -> activation of G-protein -> production of cAMP -> activation of protein kinases.
  2. Diffusion across membrane -> binding to cytoplasmic receptor -> translocation to nucleus -> binding to DNA -> change in transcription. (correct answer)
  3. Diffusion across membrane -> binding to mitochondrial receptor -> increase in ATP production -> change in cell metabolism.
  4. Binding to cell-surface receptor -> opening of ion channels -> change in membrane potential -> muscle contraction.
Explanation: As a steroid hormone, testosterone is lipid-soluble and can diffuse directly across the cell membrane. Its receptor is located in the cytoplasm or nucleus. The hormone-receptor complex then acts as a transcription factor, binding to specific DNA sequences (hormone response elements) and altering the rate of transcription of target genes. This is the classic mechanism for steroid hormones.

Question 14

Diabetes mellitus can be caused by either a lack of insulin (Type 1) or a decreased response of target cells to insulin (Type 2). Insulin resistance in Type 2 diabetes often involves defects in the insulin receptor or downstream signal transduction. Which molecular event would be a plausible cause of insulin resistance?

  1. Increased secretion of glucagon from pancreatic alpha cells.
  2. Reduced phosphorylation of intracellular proteins after the insulin receptor is activated. (correct answer)
  3. Overexpression of GLUT4 transporters on the surface of muscle and fat cells.
  4. A mutation causing the insulin receptor to be permanently active.
Explanation: The insulin receptor is a tyrosine kinase. When insulin binds, it triggers a cascade of intracellular protein phosphorylation that ultimately leads to the translocation of GLUT4 glucose transporters to the cell membrane. If these downstream phosphorylation events are reduced or blocked, the cell will not respond properly to insulin (i.e., it will be resistant), even if insulin is present and binding to its receptor.

Question 15

The posterior pituitary is not a true endocrine gland because it does not synthesize its own hormones. Instead, it stores and releases hormones produced elsewhere. Which statement correctly describes this relationship?

  1. It releases TSH and ACTH, which are synthesized by the hypothalamus and travel via a portal system.
  2. It releases releasing hormones, which are synthesized by the anterior pituitary to control other glands.
  3. It releases ADH and oxytocin, which are synthesized by neurosecretory cells in the hypothalamus. (correct answer)
  4. It releases melatonin, which is synthesized by the pineal gland and stored for release based on light cues.
Explanation: The posterior pituitary is an extension of the hypothalamus. Neurosecretory cells with their cell bodies in the hypothalamus synthesize ADH and oxytocin, which are then transported down the axons to the posterior pituitary for storage and release into the bloodstream. The other options describe incorrect hormone origins or functions.

Question 16

Aldosterone is a steroid hormone that acts on kidney cells to increase the reabsorption of sodium ions. It functions by binding to an intracellular receptor, which then acts as a transcription factor to increase the synthesis of sodium channel proteins. What would be the effect of a drug that non-selectively blocks all protein synthesis by inhibiting ribosomes?

  1. The drug would prevent the long-term cellular response to aldosterone. (correct answer)
  2. The drug would prevent aldosterone from entering the target cell.
  3. The drug would prevent aldosterone from binding to its intracellular receptor.
  4. The drug would accelerate the degradation of existing sodium channels.
Explanation: Aldosterone's mechanism involves stimulating the synthesis of new proteins (transcription followed by translation). A drug that blocks ribosomes would inhibit translation, thereby preventing the production of new sodium channels. This would block the cellular response to the hormone. The initial steps of the hormone entering the cell and binding its receptor would be unaffected.

Question 17

The hypothalamic-pituitary-thyroid axis regulates metabolism. The hypothalamus secretes thyrotropin-releasing hormone (TRH), which stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH). TSH then stimulates the thyroid gland to produce thyroxine, which increases metabolism. Thyroxine exerts negative feedback on both the hypothalamus and pituitary.

A patient is diagnosed with a pituitary adenoma (a benign tumour) that secretes excessive amounts of TSH, independent of TRH stimulation. Which set of hormone levels would be most consistent with this condition?

  1. Low TRH, high TSH, high thyroxine. (correct answer)
  2. High TRH, high TSH, high thyroxine.
  3. High TRH, low TSH, low thyroxine.
  4. Low TRH, low TSH, high thyroxine.
Explanation: The pituitary tumour causes high levels of TSH. This high TSH will overstimulate the thyroid, leading to high thyroxine. The high levels of thyroxine will then exert strong negative feedback on the hypothalamus, suppressing the release of TRH. The tumour itself is not responsive to this feedback, so TSH remains high.

Question 18

Steroid hormones are hydrophobic, while peptide hormones are hydrophilic. This fundamental difference has major implications for their transport and mechanism of action. Which statement correctly contrasts these two types of hormones?

  1. The receptors for peptide hormones are located in the nucleus, while receptors for steroid hormones are on the cell surface.
  2. Peptide hormones have a longer half-life in the bloodstream than steroid hormones.
  3. Steroid hormones are stored in vesicles prior to release, while peptide hormones are synthesized on demand.
  4. Steroid hormones require transport proteins in the blood, whereas peptide hormones are generally soluble. (correct answer)
Explanation: Because they are hydrophobic (lipid-soluble), steroid hormones do not dissolve well in the aqueous environment of the blood and must be bound to hydrophilic plasma proteins for transport. Peptide hormones, being hydrophilic, are readily soluble in blood plasma and do not require carrier proteins. The other options reverse the correct properties.

Question 19

Which of the following represents the most significant difference between neural signalling and endocrine signalling in terms of achieving specificity?

  1. Neural signalling relies on a network of specific 'wired' connections, while endocrine signalling relies on receptor presence. (correct answer)
  2. Neural signals are all-or-none, whereas endocrine signals are graded based on hormone concentration.
  3. Neural signalling uses chemical neurotransmitters, while endocrine signalling uses electrical impulses.
  4. Neural signalling is generally a slower process, allowing for more specific targeting than rapid endocrine responses.
Explanation: The primary basis for specificity in the nervous system is the anatomical arrangement of neurons and synapses—a signal travels down a specific axon to a specific target cell. In the endocrine system, hormones are broadcast throughout the bloodstream, and specificity is achieved because only cells with the appropriate receptor for that hormone will respond.