- Storage polysaccharides are large carbohydrate molecules that serve as the primary energy reserves in living organisms, enabling them to store glucose in compact, insoluble, and metabolically accessible forms.
- Their structure is characterized by long chains of glucose units linked by glycosidic bonds, with varying degrees of branching depending on the type of polysaccharide. Because free glucose is highly soluble and would cause osmotic imbalances if stored directly, organisms instead package it into these polymers, which can be broken down when energy is needed. The two most important storage polysaccharides are starch in plants and glycogen in animals and fungi.
- Starch is the major carbohydrate reserve in plants and is composed of two distinct glucose polymers: amylose and amylopectin. Amylose is mostly linear, consisting of α-1,4-glycosidic bonds, and tends to form helical structures. Amylopectin, on the other hand, is highly branched, with α-1,4-linked chains and α-1,6 branch points occurring about every 24–30 glucose units. Together, these molecules are stored in semi-crystalline granules within plant plastids (chloroplasts and amyloplasts). Starch serves not only as a vital energy supply for plants during dark periods and germination but also as a major dietary carbohydrate for animals and humans. Its digestibility and physicochemical properties depend heavily on the relative proportions of amylose and amylopectin.
- Glycogen, the primary storage polysaccharide in animals, fungi, and many bacteria, is structurally similar to amylopectin but far more densely branched, with α-1,6 linkages occurring roughly every 8–12 glucose units. This dense branching creates a highly compact structure and provides numerous non-reducing ends, allowing rapid release of glucose when metabolic demand spikes. In mammals, glycogen is stored predominantly in the liver (to maintain blood glucose homeostasis) and in skeletal muscle (to provide energy for contraction). The liver can mobilize glycogen to release free glucose into circulation, while muscle glycogen is used locally as glucose-6-phosphate for ATP generation.
- The mobilization of storage polysaccharides involves tightly regulated enzymatic pathways. In plants, starch breakdown (starch degradation) occurs through enzymes such as α-amylase, debranching enzymes, and glucosidases, especially during germination or nighttime respiration. In animals, glycogenolysis is carried out by glycogen phosphorylase and the glycogen debranching enzyme, with hormonal regulation from glucagon, epinephrine, and insulin ensuring that glycogen reserves are used appropriately. This regulatory complexity underscores the essential role of storage polysaccharides in balancing energy supply with physiological needs.
- Beyond starch and glycogen, some organisms utilize other polysaccharides for storage. For example, many red algae store floridean starch, a polymer similar to amylopectin, while some protozoa and bacteria synthesize paramylon or other β-linked glucans. These variations reflect evolutionary adaptations to different ecological niches, but the unifying principle is the same: glucose is stored in a polymerized, osmotically stable, and easily mobilized form.
