Q.It is known that exposure to carbon monoxide is harmful to animals because

Oxygen is carried through the blood mainly bound to haemoglobin inside the red blood cells, with each haemoglobin molecule able to hold at most four oxygen molecules. Carbon monoxide is known to bind at the very same haemoglobin binding sites that oxygen would otherwise occupy, and it does so far more readily, so its presence effectively takes up sites that oxygen needs. This directly cuts into how much oxygen haemoglobin can pick up and carry, reducing oxygen transport to the tissu …

Oxygen does not travel freely in the blood; the great majority of it is carried bound to haemoglobin, a red, iron-containing protein packed inside the red blood cells, with each haemoglobin molecule able to bind up to four oxygen molecules at a time to form oxyhaemoglobin.

Carbon monoxide competes for these same binding sites on haemoglobin, and it does so with a strong affinity, effectively displacing or blocking oxygen from attaching. Because haemoglobin's oxygen-carrying capacity is finite (only four sites per molecule), any sites occupied by carbon monoxide are sites unavailable to oxygen, so the overall amount of oxygen the blood can pick up and deliver to the tissues falls. …

Method: Identifying What a Competing Molecule Displaces on a Transport Carrier

Use this approach for any question about a foreign substance interfering with a normal transport molecule.

Steps

Step 1: Identify the normal carrier and its cargo

Haemoglobin normally binds O2 at a limited number of sites per molecule.

Step 2: Identify how the interferent behaves at the same sites

Carbon monoxide binds those identical sites, with a much stronger affinity than O2, so it occupies sites O2 would otherwise use.

Step 3: Conclude the functional consequence …