Biology · Body Fluids and Circulation · NEET
Thrombin is an enzyme (a proteolytic enzyme). It is formed from an inactive plasma protein called prothrombin. Do not confuse it with fibrinogen, which is the plasma protein substrate. In short: prothrombin (inactive protein) becomes thrombin (active enzyme), and thrombin then acts on fibrinogen.
Fibrinogen is a soluble plasma protein that floats in the blood. Fibrin is the insoluble thread form. Thrombin converts fibrinogen into fibrin. Fibrin threads form the mesh of the clot. NCERT is clear: fibrin is produced FROM fibrinogen, never the reverse. This exact reversal is a favourite NEET trap.
The enzyme thrombin. NEET 2021 asked this directly. Options like thrombokinase, epinephrine and renin are traps. Thrombokinase (thromboplastin) acts one step earlier to convert prothrombin into thrombin. The enzyme acting directly on fibrinogen is thrombin.
The coagulum is a network of FIBRIN threads, not thrombin threads. NEET 2022 used exactly this line as a wrong statement. Thrombin is only the enzyme; it is not part of the clot mesh. The dead and damaged blood cells get trapped in the fibrin net to form the clot.
Prothrombin is an inactive plasma clotting factor. An enzyme complex called thrombokinase (thromboplastin), released with the help of platelets and injured tissue, converts prothrombin into thrombin. This step needs calcium ions (Ca2+). So the order is: thrombokinase + Ca2+ turns prothrombin into thrombin, and thrombin turns fibrinogen into fibrin.
Calcium ions (Ca2+) are needed for the clotting reactions, especially the conversion of prothrombin to thrombin. This is why removing calcium (for example with an anticoagulant) stops blood from clotting. Calcium is a helper for the cascade, not the enzyme itself.
Which enzyme is responsible for the conversion of inactive fibrinogens to fibrins?
Which of the following statements is incorrect?
Statement I: The coagulum is formed of a network of threads called thrombins. Statement II: Spleen is the graveyard of erythrocytes. Choose the correct option.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Thrombin is the enzyme that converts soluble fibrinogen into insoluble fibrin threads. It is formed from inactive prothrombin by thrombokinase in the presence of calcium ions.
Fibrinogen is a soluble plasma protein that acts as the raw material for the clot. Thrombin cuts it into fibrin threads, which form the mesh that traps blood cells and stops bleeding.
The clot, or coagulum, is a network of fibrin threads. Thrombin is only the enzyme that makes fibrin; it is not part of the clot mesh.
Fibrinogen. Among plasma proteins, fibrinogen is needed for clotting, globulins for defence, and albumins for osmotic balance (NEET 2018).
Plasma contains clotting factors like fibrinogen in inactive form. Plasma without the clotting factors is called serum. So serum lacks fibrinogen and other clotting factors.
