Scientific Foundations

Recall Questions

1. Which of the following enzymes catalyzes the first commitment step of glycolysis?

   1. Hexokinase
   2. Phosphofructokinase-1 (PFK-1)
   3. Aldolase
   4. Pyruvate kinase

2. In glycolysis, what is the net gain of ATP molecules per molecule of glucose?

   1. 1
   2. 2
   3. 4
   4. 6

3. Which coenzyme is reduced during glycolysis and later used in oxidative phosphorylation?

   1. NADP⁺
   2. NAD⁺
   3. FAD
   4. Coenzyme A

4. What is the fate of pyruvate under anaerobic conditions in muscle cells?

   1. It is converted to acetyl-CoA.
   2. It is converted to lactate.
   3. It enters the citric acid cycle.
   4. It is converted to ethanol.

5. The Cori cycle allows the liver to produces glucose from which of the following?

   1. Acetyl-CoA
   2. Lactate
   3. Ribose 5-phosphate
   4. Glycogen

6. Which of the following enzymes is responsible for the breakdown of glycogen into glucose-1-phosphate?

   1. Glycogen synthase
   2. Glycogen phosphorylase
   3. Glucokinase
   4. Phosphoglucomutase

7. Which of the following hormones stimulates glycogen breakdown in the liver?

   1. Insulin
   2. Epinephrine
   3. Glucagon
   4. Cortisol

8. Which of the following molecules directly inhibits glycogen phosphorylase in muscle cells?

   1. Glucose-6-phosphate
   2. AMP
   3. ATP
   4. Calcium

9. In glycogen synthesis, what is the activated form of glucose that is added to the growing glycogen chain?

   1. Glucose-1-phosphatee
   2. Glucose-6-phosphate
   3. UDP-glucose
   4. Fructose-6-phosphate

10. What enzyme activates glycogen phosphorylase?

    1. Protein kinase A
    2. Glycogen synthase kinase
    3. Phosphoprotein phosphatase
    4. Phosphorylase kinase

11. Which precursor for gluconeogenesis is produced in muscles during anaerobic respiration?

    1. Glycerol
    2. Lactate
    3. Acetyl-CoA
    4. Fructose-2,6-bisphosphate

12. Which enzyme catalyzes the final step of gluconeogenesis, converting glucose-6-phosphate to free glucose?

    1. Hexokinase
    2. Phosphofructokinase-1
    3. Glucose-6-phosphatase
    4. Aldolase

13. Which cofactor is required by pyruvate carboxylase during the conversion of pyruvate to oxaloacetate in gluconeogenesis?

    1. Biotin
    2. NADH
    3. FAD
    4. TPP (Thiamine pyrophosphate)

14. The oxidative phase of the pentose phosphate pathway generates which of the following?

    1. NADH
    2. FADH₂
    3. NADPH
    4. ATP

15. What is the main purpose of NADPH produced in the pentose phosphate pathway?

    1. ATP synthesis
    2. Oxidative phosphorylation
    3. Reductive biosynthesis and maintaining glutathione in its reduced form
    4. Glycogen synthesis

16. Which of the following tests is used to diagnose an active SARS-CoV-2 infection?

    1. PCR (Polymerase Chain Reaction) test
    2. ELISA (Enzyme-Linked Immunosorbent Assay)
    3. Western blot
    4. Rapid strep test

17. What is the primary mechanism of action of mRNA vaccines against COVID-19?

    1. Directly inactivates the virus
    2. Inhibits viral replication
    3. Neutralizes toxins produced by the virus
    4. Stimulates an immune response by translation of the viral spike protein

18. Which of the following is the primary role of the tricarboxylic acid cycle (TCA cycle) in cellular metabolism?

    1. To produce ATP directly through substrate-level phosphorylation
    2. To generate high-energy electron carriers for oxidative phosphorylation
    3. To synthesize glucose from fatty acids
    4. To catalyze the breakdown of amino acids into ammonia

19. Which coenzyme is essential for the function of the α-ketoglutarate dehydrogenase complex?

    1. Biotin
    2. Pyridoxal phosphate
    3. Thiamine
    4. Vitamin K

20. In the context of the TCA cycle, which molecule is regenerated to continue the cycle after the production of oxaloacetate?

    1. Acetyl-CoA
    2. Succinate
    3. Fumarate
    4. Isocitrate

21. Which of the following enzymes is a part of the TCA cycle and also plays a role in the electron transport chain?

    1. Succinate dehydrogenase
    2. Citrate synthase
    3. Malate dehydrogenase
    4. Aconitase

22. Mutations in enzymes that catalyze reactions in the TCA cycle often lead to lactic acidosis. Why do cells produce lactate if pyruvate is the end product of glycolysis?

    1. Pyruvate is less soluble than lactate
    2. Lactate is more easily converted to glucose in the liver
    3. The conversion of pyruvate to lactate generates NAD⁺
    4. Cells can more easily convert lactate to fatty acids

23. Which of the following is the primary function of the electron transport chain (ETC) in oxidative phosphorylation?

    1. Conversion of glucose to pyruvate
    2. Production of NADH and FADH₂ from acetyl-CoA
    3. Transfer of electrons from NADH and FADH₂ to oxygen, creating a proton gradient
    4. Direct synthesis of ATP from ADP and inorganic phosphate

24. Which of the following molecules is the final electron acceptor in the electron transport chain?

    1. NADH
    2. FADH₂
    3. Oxygen
    4. Carbon dioxide

25. Which of the following best characterizes a biochemical reaction with positive ΔG in metabolic pathways?

    1. Never happens
    2. Generates ATP
    3. Coupled with reaction that has a negative ΔG
    4. Don't require enzymes

    Show Explanation

    Reactions with a positive ΔG are energetically unfavorable and will not proceed unless coupled to a reaction that has a negative ΔG. Importantly, the sum of the ΔGs for both reactions must be negative.

Application Questions

1. The following three questions refer to the image below of a cross-section of a peripheral nerve. What best describes the structure seen in the image?

   

   1. Myelinated nerve
   2. Unmyelinated nerve
   3. Neuron cell body
   4. Axon terminus

   Show Explanation

   Note the large nucleus of a cell which appears to encapsulate several small structures. The small structures are axons in cross-section. This indicates the cells is a Schwann cell. The Schwann cells does not form a thick wrapping around the axons (myelin) so this is an unmyelinated nerve.

2. In the image above, what is arrow 1 one pointing to?

   1. Elastic fibers
   2. Proteoglycans
   3. Neuron cell body
   4. Collagen fibrils

   Show Explanation

   Note the arrow is pointing to the extra cellular matrix. In cross section, collagen fibrils appear round and show a regular spacing between each other.

3. In the image above, what protein would be expect to find in the cell membrane of the region indicated by arrow 2?

   1. Integrins
   2. Cadherins
   3. Claudins
   4. Connexins

   Show Explanation

   Note the arrow is pointing the section of the Schwann cell membrane that faces the extracellular matrix. Cells use integrins to interact with components of the extracellular matrix.

4. You are studying different muscle tissue and measuring the rate at which each contracts. To initiate contraction, you apply an electrical stimulus to each muscle type. You notice that smooth muscle starts to contract much longer after stimulation than skeletal and cardiac muscle. What accounts for this delayed response in smooth muscle cells?

   1. Slower rate of myosin activation
   2. Slower rate of calcium channels opening
   3. Slower rate of tropomyosin movement
   4. Slower rate of sodium channels opening

   Show Explanation

   Contraction of smooth muscle is triggered through a signaling pathway that leads to a phosphorylation of myosin light chains. Phosphorylated light chains activate the myosin motor which can generate tension on actin filaments and lead to cell contraction. In skeletal and cardiac muscle, the myosin motor is already in active state and only requires a small shift in the position of tropomyosin along actin filaments to initiate contraction. The process of shifting tropomyosin is much faster than the several steps that lead to phosphorylation of myosin light chain.

5. You are working with a neurologist to identify biopsies that show damage to motor neurons that innervate skeletal muscle cells. The biopsies show axons of several different types, and you must quickly identify axons which are most likely part of a motor neuron. Which type would you look for?

   1. 1 µm diameter, unmyelinated
   2. 1 µm diameter, myelinated
   3. 10 µm diameter, myelinated
   4. 10 µm diameter, unmyelinated

   Show Explanation

   Motor neurons that innervate skeletal muscle tissue must be able to generate action potentials with fast conduction velocities. Conduction velocity increases with an increase in axon diameter and with myelination of the axon, but myelination has much larger impact on conduction velocity than axon diameter.

6. In patients with cystic fibrosis, the concentration of sodium in their sweat higher than normal. What explains the inability of the epithelium in the sweat gland to absorb sodium?

   1. Reduced activity of the sodium-potassium pump
   2. Fewer sodium channels in apical membrane
   3. Depolarization of apical membrane
   4. More restrictive tight junctions

   Show Explanation

   Cystic fibrosis patients have mutations in the CFTR channel that reduces the concentration and/or activity of the channel in the cell membrane of their epithelial cells. The epithelium of sweat glands uses the CFTR channel to absorb chloride from the lumen while sodium is absorbed through ENaC. If the cells have fewer CFTR channels, then they will absorb less chloride from the lumen. As sodium enters the cell, the apical membrane will depolarize (membrane potential becomes more positive). As a result, less sodium will diffuse through sodium channels in the apical membrane, increasing the sodium concentration in the sweat.

7. A patient presents with muscle weakness and cramps. Physical exam finds an arrhythmia. Blood tests show an elevated potassium concentration. What best explains the patient's muscle weakness?

   1. Hyperpolarization of membrane potential
   2. Inactivated sodium channels
   3. Inactivated potssium channels
   4. Increase in threshold potential

   Show Explanation

   Elevated extracellular potassium concentration would increase membrane potential (make it more positive). While the increase in membrane potential would make triggering an action potential more likely, it would also reduces the probability that sodium channels are reactivated after the action potential. Recall that during an action potential sodium channels are inactivated after a short period of time. Those channels remain closed until membrane potential becomes sufficiently negative. The higher membrane potential in the patient would lead to an accumulation of inactive sodium channels and reduce the likelihood of cells generating an action potential.

8. A 40 year old male who weighs 75 kg has a serum sodium concentration of 190 meq/L. What is the water deficit in the patient?

   1. 8 liters
   2. 10 liters
   3. 12 liters
   4. 16 liters

   Show Explanation

   First calculate total body water = 75 * 0.6 = 45 liters. Next, calculate the water deficit:

   Water deficit = TBW_t=0(([Na⁺]_t=0/[Na⁺]_normal) - 1)

   Water deficit = 45((190/140) - 1) = 16.07