MUSCLE CONTRACTION QUESTION

Question 1

Define “competitive antagonist” and “non-competitive antagonist”

Answer 1

Competitive antagonist:
Binds reversibly to the same active site on the receptor as the agonist (acetylcholine in this case).

Can be overcome by increasing agonist concentration.

Shifts the dose-response curve to the right, no reduction in maximal response.

Non-competitive antagonist:
Binds to a different (allosteric) site or irreversibly to the active site.

Cannot be displaced by more agonist.

Reduces the maximal response even at high agonist concentrations.

Dose-response curve is depressed downward, not just shifted.

Question 2

Briefly describe the signalling pathway by which epinephrine stimulates an increase in the concentration of cyclic AMP.

Answer 2

Epinephrine binds to GPCR

GPCR activates G_s protein, which activates adenylyl cyclase.

Adenylyl cyclase converts ATP to cyclic AMP (cAMP).

cAMP activates PKA (protein kinase A), which triggers cellular responses.

Question 3

Briefly describe the events, triggered by mitosis promoting factor that lead to the separation of sister chromatids at the metaphase to anaphase transition.

Answer 3

MPF (CDK1-cyclin B) triggers mitosis entry.

APC/C (anaphase-promoting complex) is activated by MPF.

APC/C ubiquitinates securin, causing its degradation.

This frees separase, which cleaves cohesin, allowing sister chromatid separation.

Question 4

Outline the steps in the apoptosis pathway triggered in the absence of trophic factors

Answer 4

Trophic factors absent → reduced survival signalling.

Pro-apoptotic Bcl-2 proteins (e.g. Bax, Bak) activated.

Mitochondria release cytochrome c.

Cytochrome c binds Apaf-1, forming the apoptosome.

Caspase-9 activated, which activates executioner caspases (e.g. caspase-3) → cell death.