Fibrinogen and prothrombin play important roles in the

1. Analyze the question The question asks about the function of two substances, fibrinogen and prothrombin. Both are proteins found in blood plasma.

2. Explain the concept Blood clotting (coagulation) stops bleeding when a blood vessel is damaged. It involves several plasma proteins called clotting factors:

• Prothrombin is an inactive plasma protein made in the liver (it needs vitamin K for its production).
• Fibrinogen is a soluble plasma protein, also made in the liver.

3. Step-by-step solution

1. When a blood vessel is injured, platelets and damaged tissue release thromboplastin (thrombokinase).
2. In the presence of calcium ions, thromboplastin converts prothrombin into thrombin (an active enzyme).
3. Thrombin converts soluble fibrinogen into insoluble fibrin threads.
4. The fibrin threads form a mesh that traps red blood cells and platelets, forming a clot that seals the wound.

Now check the other options:

• A. Deamination of proteins: the removal of the amino group from amino acids. It happens in the liver and has nothing to do with these two proteins.
• C. Detoxication: the liver breaks down harmful substances. Not related.
• D. Storage of vitamins: the liver stores vitamins such as A, D and B12. Not related.

4. Answer Fibrinogen and prothrombin are both clotting factors, so they play important roles in the clotting of blood.

5. Exam tip Remember the chain: Prothrombin → Thrombin → converts Fibrinogen → Fibrin (clot). A quick memory aid: "Fibrin is the fiber (net) that holds the clot together." Any time you see fibrinogen, prothrombin, platelets, vitamin K or calcium ions in a question, think blood clotting.