What is the role of thrombin in the
process of blood clotting? (A) To form a net to trap red blood
cells that aggregate into a clot. (B) To catalyze the conversion of
soluble fibrinogen into insoluble fibrin. (C) To initiate the conversion of
the protein prothrombin into the enzyme thrombin. Or (D) to initiate the release of
calcium ions to encourage the aggregation of blood cells.
When a blood vessel is damaged, a
few things happen. Platelets are attracted to the site
of the damage and adhere to the damaged tissues. At the same time, the damaged
tissues express the tissue factor thromboplastin. This factor, in the presence of
calcium ions, converts prothrombin, a protein produced in the liver, to thrombin, an
active enzyme. Thrombin catalyzes the conversion
of fibrinogen, a blood component, into fibrin threads. Fibrin precipitates at the site of
the injury and forms a net. The net traps more platelets and
nearby red blood cells, creating the final blood clot.
So, looking back at our question,
we know it is fibrin, not thrombin, that creates the net. Therefore, we can eliminate
(A). We also know that it is
thromboplastin, not thrombin, that converts prothrombin into thrombin, so we can
eliminate (C). And also calcium is required for
the production of thrombin, not the other way around. So we can eliminate (D).
Thrombin is an enzyme. This means it catalyzes a
reaction. The particular reaction it is a
catalyst for is fibrinogen to fibrin. While fibrinogen is soluble, the
fibrin threads are not soluble, which is why they precipitate to form a fibrin
net. So the correct answer is (B).
The role of thrombin in the process
of blood clotting is to catalyze the conversion of soluble fibrinogen into insoluble