Video Transcript
Both of these proteins are soluble
in plasma, but when the activated form of blank activates blank, the latter becomes
insoluble. (A) Prothrombin, fibrinogen; (B)
fibrinogen, prothrombin; (C) thromboplastin, fibrinogen; or (D) fibrin,
thrombin.
When blood flows normally through
blood vessels, blood platelets are in their inactive state. Blood vessels are lined by cells
called endothelial cells. When these cells are damaged due to
injury, platelets are attracted to the site of damage. Once the platelets adhere at the
site of damage, they become activated. Activated platelets release factors
that attract more platelets, causing these platelets to adhere and become activated
too.
The formation of the platelet plug
triggers a cascade of biochemical reactions. The damaged blood vessels and the
tissues surrounding blood vessels express the tissue factor thromboplastin. This factor is responsible for
converting a protein called prothrombin, which is produced by the liver, into
thrombin, which is an active enzyme. This reaction takes place if
calcium ions are present. Blood also contains a protein
called fibrinogen, which is produced by the liver and is soluble in blood
plasma.
When the enzyme thrombin is formed
from prothrombin, it acts upon fibrinogen and converts it into fibrin, which is
insoluble in blood plasma. The fibrin precipitates out of the
blood as a network of microscopic fibers. The strands of fibrin formed in
this way will reinforce the platelet plug by forming a net or mesh, which traps red
blood cells and more activated platelets.
Let’s have a look again at our
question. We will consider the second gap
first. Let’s ask ourselves, which protein
becomes insoluble once it has been activated? We have learnt that this is true
for the protein fibrinogen. With this information, we can
exclude answer options (B) and (D).
Now, we have to work out which
protein activates fibrinogen. We have already learnt that an
enzyme called thrombin activates fibrinogen, converting it into its active form
called fibrin. The inactive form of thrombin is
called prothrombin. So the correct words to fill in the
blanks in our statement are given in answer choice (A). Both of these proteins are soluble
in plasma, but when the activated form of prothrombin activates fibrinogen, the
latter becomes insoluble.
