User:BusyBee222/Granule (cell biology)

In cell biology, a granule is a small particle barely visible by light microscopy. The term is most often used to describe a secretory vesicle containing important components of cell phyisology. Examples of granules include granulocytes, platelet granules, insulin granules, germane granules, starch granules, and stress granules.

Granules in Leukocytes (Expanded & Rewritten)

A group of leukocytes, called granulocytes, are white blood cells containing enzyme granules that play a significant role in the immune system. Granulocytes include neutrophils, eosinophils, and basophils which attack bacteria or parasites, and respond to allergens. Each type of granulocyte contains enzymes and chemicals tailored to it’s function.

Neutrophils for example, contain primary granules, secondary granules, tertiary granules, and secretory vesicles. Primary vesicles, also known as azurophilic granules, secrete hydrolytic enzymes including Including elastase, myeloperoxidase, cathepsins, and defensins that aid in pathogen distruction. Secondary granules, or specific granules, in neutrophils contain ​​iron-binding protein lactoferrin. Tertiary granules contain matrix metalloproteinases.

Other immune cells, such as natural killer cells, contain granular enzymes, including perforin and proteases which can lead to the lysis of neighboring cells.  

The process by which granule contents are released is known as degranulation. This tightly controlled process is initiated by immunological stimuli and results in the movement of granules to the cell membrane for fusion and release.

Insulin Granules in Beta Cells (Expanded & Rewritten)

Beta cell with insulin granules, which are the dark black spots surrounded by a white area called a halo.

Insulin granules are a specific type of granule found in pancreatic beta cells. Insulin granules are secretory granules, which are responsible for the storage and secretion of insulin, a hormone that regulates the concentration of glucose in the bloodstream to maintain homeostasis. The release of insulin by granules is signaled by plasma glucose concentrations and the resultant influx of calcium ions in pancreatic cells, which initiate granule exocytosis. Insulin release is biphastic, as insulin is first released in the primary phase by granules closest to the plasma membrane. In the secondary phase, insulin granules are are recruited from reserves deeper in the beta cell for a slower release rate.

Insulin granules undergo a significant maturation process. First, precursor proinsulin molecules are synthesized in the endoplasmic reticulum and packaged in the golgi network. Insulin granules bud from the trans golgi network and are further sorted via clathrin-coated vesicle transport. After budding, insulin secretory granules are acidified, activating endoproteases PC1/3 and PC2 to convert proinsulin into insulin. The clatherin coating is released and the insulin secretory granules are transported across the cell via actin filaments and microtubules.

Starch Granules in Plants (Expanded & Rewritten)

Starch granules in potato cells.

Starch is an insoluble carbohydrate used for energy storage in plant cells. There are two forms of starch, transitionary starch and storage starch. Transitionary starch is synthesised via photosynthesis and found in photosythetic plant tissue cells, such as the leaves. Storage starch is reserved for longer periods of time and is found in in non-photosynthetic tissue cells such as the roots or stem. Storage starch is utilized during germination or regrowth, or when energy demands exceed net energy production from photosynthesis.

Starch is stored in granule form. Starch granules are composed of a crystalline structure of amylopectin and amylose. Amylopectin forms the structure of the starch granule, with branching and non branching A-chains, B-chains, and C-chains. Amylose fills in the gaps of the amylopectin structure. Under a microscope, starch granules look like concentric layers, referred to as “growth rings”. Starch granules also contain granule-bound starch synthase and amylopectin synthesizing enzymes. Notably, starch granules vary in size and morphology across plant tissues and species.

Stress Granules (Added)

Assembly and disassembly of stress granules.

Stress granules are composed of protein and RNA, which form from pools of mRNAs that have not started translation as a result of environmental conditions including oxidative stress, temperature, toxins, and osmotic pressure. Stress granules also contain translation initiation factors, RNA binding proteins (which account for 50% of the granule’s components), and non-RNA binding proteins. They are formed via protein-protein interactions between mRNA binding proteins and are influenced by protein methylation or phosphorylation. They contain a “core” with high concentrations of proteins and mRNA and a less-concentrated outer region. Stress granules are dynamic in structure, and can dock and exchange with p-bodies or the cytoplasm. They can also perform fusion and fission in the cytoplasm.

Stress granule assembly is dependent upon the conditions of the cell. In yeast, stress granules form under conditions of high heat. Stress granules are of significance for their roles in mRNA localization, cell signaling pathways, and antiviral processes. Once disassembled, the RNA inside stress granules can go back to translation or be removed as cellular waste. Stress granules may provide protection for mRNA from interactions with the cytosol. Moreover, mutations that affect the formation or degradation of stress granules may contribute to neurodegenerative conditions such as ALS and FTLD. However, the effects of stress granules on cell physiology are still under study.