Biology · Body Fluids and Circulation · NEET
Work in the direction the reaction happens: (1) Injury makes platelets and injured tissues release factors. (2) These, with calcium ions, form the enzyme complex thrombokinase. (3) Thrombokinase converts inactive prothrombin into active thrombin. (4) Thrombin converts inactive fibrinogen into insoluble fibrin threads. (5) Fibrin threads form a mesh that traps blood cells = the clot. NCERT (Ch 15) describes it exactly in this backward-naming style, so learn both directions.
Fibrinogen is a soluble, inactive plasma protein floating in the blood. Fibrin is the insoluble, thread-like active form. The enzyme thrombin cuts fibrinogen into fibrin. NEET tests the direction: fibrin is made FROM fibrinogen, never the reverse. So the statement 'fibrinogen is produced from fibrin' is WRONG.
The enzyme complex thrombokinase (also called thromboplastin) converts inactive prothrombin into active thrombin. Thrombokinase is not present ready-made; it is built by a cascade of linked enzyme reactions using clotting factors from plasma, plus factors from platelets and injured tissue, and it needs calcium ions.
NCERT states 'Calcium ions play a very important role in clotting.' Ca2+ is needed to form thrombokinase and to run the cascade steps. This is why blood banks add substances like citrate or oxalate that bind (remove) calcium to STOP stored blood from clotting. No calcium = no clot.
Two triggers. First, an injury stimulates platelets to release factors that begin coagulation. Second, injured tissues at the wound site also release factors that can start coagulation. Both feed into the same cascade that builds thrombokinase. So both platelets and tissue factors initiate clotting.
Thrombin is a proteolytic enzyme (a biocatalyst). NEET has directly asked to match 'Thrombin' with 'Biocatalyst'. It is stored as inactive prothrombin and only becomes an active enzyme during clotting to act on fibrinogen.
Which one of the following statements about blood coagulation is INCORRECT?
Match List I with List II: (a) Glycogen (b) Globulin (c) Steroids (d) Thrombin :: (i) Hormone (ii) Biocatalyst (iii) Antibody (iv) Storage product
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the clotting of blood after a cut or injury. Blood turns from liquid into a solid mesh of fibrin threads that plugs the wound and stops excess blood loss.
A clot (coagulum) is mainly a network of fibrin threads. Dead and damaged formed elements of blood (like blood cells) get trapped inside this fibrin mesh, forming the dark reddish-brown scum you see over a wound.
Thrombokinase is an enzyme complex built by the cascade. It converts inactive prothrombin into active thrombin. It is formed from platelet factors, tissue factors and clotting factors in plasma, and needs calcium ions.
Anticoagulants like sodium citrate or oxalate are added, which bind calcium ions. Since Ca2+ is essential for the clotting cascade, removing it prevents coagulation and keeps the stored blood liquid.
Haemophilia is a sex-linked recessive disorder in which one protein of the clotting cascade is defective. A simple cut then leads to non-stop bleeding because the fibrin clot cannot form properly.
