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a. Iron (Fe) ion

b. Haemoglobin

Explanation:

Red blood cells (RBC), also known as erythrocytes, are one of the cells found in the blood. Red blood cells are the major components of the blood involved with the transport of oxygen throughout the body. Red blood cells are able to do this because they contain proteinous molecules called HAEMOGLOBIN.

Haemoglobin is a molecule that contains hemic iron groups. Haemoglobin is composed of an IRON ION at the core of its structure, which binds to oxygen atom. Hence, according to this question, haemoglobin is a proteinous molecule that contains iron-binding heme groups in the red blood cells.