What are the membrane-enclosed fragments of specialized cells in the blood?

Platelets are indeed membrane-enclosed fragments of specialized cells in the blood. Specifically, they are derived from larger cells known as megakaryocytes in the bone marrow. When megakaryocytes undergo a process called thrombopoiesis, they release small, disc-shaped fragments that circulate in the bloodstream. These fragments play a crucial role in hemostasis, the process that prevents excessive bleeding when blood vessels are injured.

In contrast, red blood cells are fully formed cells responsible for transporting oxygen and carbon dioxide but are not fragments of another cell. White blood cells are also complete cells that perform various immune functions, including fighting off infections. Plasma proteins, while crucial for functions such as coagulation and immune responses, are not cell fragments; they are large molecules found dissolved in the plasma component of blood.

Understanding the unique roles and characteristics of each type of blood component helps to clarify why platelets, being membrane-enclosed fragments, are distinct from the other listed options.