Two enzymes and one metal ion needed in blood clotting

The common pathway begins with the activation of factor X. In the presence of calcium ions and other clotting factors, factor X activates an enzyme called prothrombin activator. This enzyme them converts the plasma protein prothrombin into thrombin. Thrombin is an enzyme that, in turn, converts fibrinogen to fibrin.

the dark variety, as they are not visible on the dark tree barks

explanation:

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i guess you forgot putting it.