granules

Meaning: A granule is a small visible particle in the cytoplasm that stores material for later use. In secretory cells, proteins are made on the rough ER, modified in the Golgi, and packaged into vesicles that mature into granules. A signal, often a rise in Ca²⁺, triggers fusion with the plasma membrane and exocytosis. Zymogen granules in pancreatic acinar cells, which hold inactive digestive enzymes, are the standard example. In white blood cells, granules hold enzymes and mediators. Neutrophils (bacteria), eosinophils (parasites and allergic responses) and basophils (histamine) are the granulocytes. Glycogen granules are a different case: clusters of glycogen in liver and muscle cytoplasm with no surrounding membrane.

On the MCAT: Expect questions that sort leukocytes. Granulocytes belong to the myeloid lineage and the innate response, while lymphocytes and monocytes are agranular. Passages may also trace the secretory pathway (rough ER → Golgi → vesicle → plasma membrane) and ask what accumulates if one step, such as the Golgi or exocytosis, is blocked.

Common confusion: Students assume a cell with granules is a lymphocyte, or that every granule has a membrane. Lymphocytes (B and T cells) and monocytes are the agranulocytes, and glycogen granules lack a membrane. Granule contents are also not made in the granule: the ER and Golgi make them.