6) In detective novels, “the odor of bitter almonds” is the tell-tale clue to murder by cyanide poisoning. Cyanide works by attacking the enzyme that transfers electrons from the respiratory electron transport chain to O2. Why is it not possible for the victim to survive by using anaerobic respiration? Why is cyanide poisoning almost immediately fatal?

It is not possible for the victim to survive by using anaerobic respiration in the case of cyanide poisoning due to the fact that cyanide specifically targets the enzyme involved in transferring electrons from the respiratory electron transport chain to molecular oxygen (O2). This enzyme, called cytochrome c oxidase or complex IV, plays a critical role in the final step of aerobic respiration, where O2 serves as the final electron acceptor, allowing the production of cellular energy in the form of adenosine triphosphate (ATP).

In the absence of an available oxygen molecule, the respiratory electron transport chain cannot function properly. Anaerobic respiration usually relies on alternative electron acceptors, such as nitrate or sulfate, to continue producing ATP. However, cyanide specifically disrupts the transfer of electrons to O2, preventing the use of any other electron acceptors.

Cyanide poisoning is almost immediately fatal because the lack of functional cytochrome c oxidase effectively halts cellular respiration. As a result, cells are unable to generate sufficient ATP, which is essential for various cellular processes. Vital organs like the brain and heart heavily rely on aerobic respiration and oxidative phosphorylation to meet their high energy demands. Without an adequate energy supply, these organs quickly become compromised, leading to rapid organ failure and ultimately death.

To understand why surviving through anaerobic respiration is not possible in cyanide poisoning, we need to understand the role of cyanide in inhibiting the electron transport chain and why it leads to immediate fatality.

Cyanide works by binding to and inhibiting an enzyme called cytochrome c oxidase, also known as Complex IV, in the electron transport chain. This enzyme is responsible for transferring electrons from the respiratory chain to oxygen, which allows for the generation of ATP, the energy currency of cells. Without this enzyme functioning properly, cells are unable to produce energy efficiently.

Anaerobic respiration is an alternate metabolic pathway that some cells can use in the absence of oxygen. It involves the breakdown of glucose without the involvement of oxygen, producing a small amount of ATP. However, even though anaerobic respiration can provide a limited energy supply, it cannot sustain the body in the presence of cyanide poisoning for several reasons:

1. Inhibition of Complex IV: Cyanide inhibits the cytochrome c oxidase enzyme, which is essential for the last step of aerobic respiration. Without this enzyme, the electron transport chain is disrupted, limiting the production of ATP.

2. Oxygen deprivation: Cyanide prevents the utilization of oxygen by cells, leading to a lack of oxygen supply to various tissues and organs. Anaerobic respiration does not provide sufficient oxygen to compensate for this deficiency, as it relies on glucose breakdown in the absence of oxygen. Without oxygen, cells cannot generate enough ATP to meet their energy demands, resulting in cellular dysfunction and ultimately organ failure.

3. Accumulation of toxic byproducts: Anaerobic respiration produces lactic acid as a byproduct, which can build up and cause metabolic acidosis. This further impairs cellular function and can be detrimental to the body's overall physiology.

As cyanide poisoning inhibits the crucial step of transferring electrons to oxygen, cells rapidly lose their ability to produce adequate energy, leading to severe and immediate consequences. The brain and heart, which are highly dependent on ATP production, are particularly vulnerable, contributing to the rapid fatality associated with cyanide poisoning.

In summary, although anaerobic respiration can provide limited energy in the absence of oxygen, it cannot sustain the body in cyanide poisoning due to the inherent disruption of the electron transport chain and the subsequent lack of oxygen supply to vital tissues and organs.