Which one of the given proteins transports oxygen in the bloodstream?
A. Myoglobin
B. Insulin
C. Albumin
D. Haemoglobin

Hint: Proteins are vital units of living organisms which take part in every function of the cells. Proteins are biomolecules involving one or more long chains of amino acids.

Complete Step by Step Solution:
Proteins are crucial for many processes in organisms like catalysis of metabolic reactions, DNA replication, reacting to stimuli, etc.

One of the functions includes the transportation of oxygen in the bloodstream.
The red blood cells get oxygen from inhaled air in the blood vessels of the lungs.
Red blood cells deliver it to all parts of the body with the help of the bloodstream.
The cells require oxygen for metabolism.

Haemoglobin is a protein present in red blood cells.
Haemoglobin has a quaternary structure which means it comprises two or more protein chains called subunits.

Each subunit has a protein chain linked with a non-protein prosthetic heme group.
A prosthetic group denotes the non-amino acid unit of the protein.
Oxygen connects to this heme group. The capacity of oxygen to attach itself to the heme group enhances as more oxygen molecules are bonded to heme.
Haemoglobin present in blood transmits oxygen from the organs involved in respiration like the lungs or gills to the other parts of the body which are tissues.
In the tissues, it releases oxygen for aerobic respiration for providing energy to the functions of an organism.
So, option D is correct.

Additional Information:
Haemoglobin also acts as the carrier of other gases as well.
It acts as a carrier for a portion of the respiratory carbon dioxide as carbaminohemoglobin.
Carbaminohemoglobin is nothing but carbon dioxide attached to the heme protein.
The molecule acts as a carrier for a significant regulatory molecule nitric oxide which is bound to a thiol group in the globin protein.

Note: A heme group includes an iron (Fe) ion attached to a heterocyclic ring called a porphyrin. The iron ion is the site where the oxygen binds itself. This iron ion present at the centre of the ring forms coordination bonds with four nitrogen atoms which all stay in a single plane. When oxygen binds to the heme unit of the haemoglobin in red blood cells, the new structure formed is called oxyhaemoglobin.