The pair in which both species have iron is

Haemoglobin, cytochromes

The correct answer is option 4.  Haemoglobin, cytochromes.

Let us delve into the explanation of why haemoglobin and cytochromes are the correct pair where both species involve iron:

Haemoglobin:

Function: Haemoglobin is a protein found in red blood cells that is responsible for transporting oxygen from the lungs to tissues throughout the body.

Structure: Each haemoglobin molecule contains four heme groups, and each heme group consists of an iron ion \((Fe^{2+})\) bound to a porphyrin ring.

Iron Role: The iron ion in haemoglobin is essential for binding oxygen molecules. Oxygen binds to the iron atom in the heme group to form oxyhemoglobin, facilitating oxygen transport in the bloodstream.

Cytochromes:

Function: Cytochromes are a class of proteins involved in electron transport chains in cellular respiration and photosynthesis.

Structure: Cytochromes also contain heme groups, each with an iron ion (\(Fe^{2+}\) or \(Fe^{3+}\)) coordinated by a porphyrin ring.

Iron Role: The iron ions in cytochromes undergo reversible oxidation and reduction reactions, accepting and donating electrons as part of the electron transport chain. This process is essential for generating ATP (energy currency of the cell) during cellular respiration or for converting light energy into chemical energy during photosynthesis.

Conclusion: Both haemoglobin and cytochromes contain heme groups, each of which includes an iron ion at its center. This iron ion plays a critical role in the function of both proteins: in haemoglobin for oxygen binding and transport, and in cytochromes for electron transport and energy conversion processes. Therefore, the correct pair where both species involve iron is indeed: Haemoglobin, cytochromes.