Advanced Biochemistry - 2022 MCQs

What is the correct order of events during an action potential?

• (A) -> (B) -> (C) -> (D)

• (D) -> (A) -> (B) -> (C)

• (D) -> (A) -> (C) -> (B)

• (C) -> (A) -> (B) -> (D)

• (A) -> (C) -> (B) -> (D)

Answer explanation

During an action potential, voltage-gated Na+ channels open first (D), leading to depolarization of the membrane (A). Once the peak is reached, the Na+ channels close (C), and the cell enters the refractory period (B) during which another action potential cannot occur immediately.

What is the biochemical mechanism behind the refractory period of electrically excitable cells?

• The cell needs time to re-establish Na+ balance.

• The Na+ channel voltage sensor blocks the channel.

• The Na+ channel voltage sensor alters the selectivity filter characteristics.

• The Ca2+ channel is activated, flooding the cytoplasm with more positive charge.

• The Na+ channel’s gate closes for a specific period of time based on the duration that it was previously open.

Answer explanation

During the refractory period, the Na+ channels remain inactivated after opening. This inactivation occurs because the voltage sensor blocks the Na+ channel, preventing further depolarization until the channel resets. This period ensures the action potential travels in one direction and prevents the immediate re-firing of another action potential.

Solute binding in transporter mediated diffusion can be blocked by ______.

• Inhibitors

• GTP

• Divalent cations

• Divalent anions

• ATP

Answer explanation

Transporter-mediated diffusion can be blocked by inhibitors that specifically bind to the transporter, preventing the solute from attaching. This is analogous to enzyme inhibition, where the presence of inhibitors prevents the normal functioning of the transporter.

______ transporters constitute the largest family of membrane transport protein.

• V-type proton pump

• K+ ion

• Ca2+ ion

• P-type pump

• ABC

Answer explanation

ABC (ATP-binding cassette) transporters are the largest family of membrane transport proteins. They use ATP to transport various molecules across membranes and are found in all organisms from prokaryotes to humans.

What is the primary characteristic of plants that are defective in chlorophyll synthesis?

• They are light sensitive.

• They are yellow and are unable to develop into mature plants.

• They accumulate aminolevulinate.

• They have abnormal leaf morphology.

• All of the above.

Answer explanation

Plants defective in chlorophyll synthesis typically appear yellow (due to the lack of green chlorophyll) and are unable to undergo proper photosynthesis, preventing them from developing into mature plants.

Mutant analysis is one method of providing information about biosynthetic pathways. Which of the following characteristics of a mutant would be most helpful in elucidating the pathway?

• The mutant accumulates a substrate in the pathway.

• Growth of the mutant on a wide range of medias.

• Sequence of the mutant’s 16S rRNA gene.

• The mutant is able to incorporate radiolabelled S³⁵-Methionine.

• None of the above.

Answer explanation

Accumulation of a substrate in the pathway suggests that the mutant is blocked at a specific step in the biosynthetic pathway, allowing researchers to identify where the mutation has affected the process. This is highly informative in understanding the pathway.

Which of the following best describes the cause of the porphyria disease group?

• Light sensitivity.

• Propensity to develop dementia.

• Defect in chlorophyll biosynthesis.

• Defect in phytochrome biosynthesis.

• Defect in heme biosynthesis.

Answer explanation

Porphyria is a group of disorders caused by a defect in the heme biosynthesis pathway. This results in the accumulation of porphyrins or their precursors, leading to symptoms such as light sensitivity and other clinical manifestations, but the underlying cause is related to heme production, not chlorophyll or phytochrome

Scientists can determine if a biosynthetic pathway is present in an organism with a sequenced genome by …

• Scanning for orthologous proteins based on known gene sequences.

• Scanning for synonymous proteins based on known gene sequences.

• Growing the organism on an alternative carbon source.

• Growing the organism on an alternative sugar source.

• None of the above.

Answer explanation

Orthologous proteins are those that are conserved across species and have similar functions. By scanning for orthologous proteins in a sequenced genome, scientists can identify genes involved in specific biosynthetic pathways based on their similarity to known genes in other organisms

Aminolevulinic acid synthesis can proceed by two pathways, the …

• Shemin pathway using acetyl-CoA and glycine substrates and the C5 pathway using glutamate substrate.

• Shemin pathway using succinyl-CoA and proline substrates and the C5 pathway using aspartate substrate.

• Shemin pathway using acetyl-CoA and glycine substrates and the C5 pathway using aspartate substrate.

• Shemin pathway using acetyl-CoA and glycine substrates and the C5 pathway using glutamate substrate.

• Shemin pathway using succinyl-CoA and glycine substrates and the C5 pathway using glutamate substrate.

Answer explanation

The Shemin pathway involves succinyl-CoA and glycine as substrates, while the C5 pathway uses glutamate. These pathways lead to the synthesis of aminolevulinic acid, a precursor in the biosynthesis of heme.

What technique is used to elucidate steps within biosynthetic pathways?

• Cloning whole organisms.

• Enzyme degradation.

• Mutant complementation.

• Isotonic tracers.

• None of the above.

Answer explanation

Mutant complementation is used to identify and elucidate steps within biosynthetic pathways by restoring the function of a defective gene in a mutant with a functional version, thus revealing the role of the gene in the pathway