Almost all amylose is a long straight chain without branches. Only a few have short branches. Therefore, comparing glycogen with amylose is meaningless. The comparison should focus on amylopectin and glycogen. Although both are polysaccharides that store energy in organisms, glycogen is even called animal starch, but their structure, properties, and functions are very different.
Starch (amylopectin) and glycogen: Source of glucose
Glycogen exists in almost all organisms, even the ancient bacteria and archaea. Glucose is their unit that comes from various sources. Herbivores obtain it from plants cellulose and starch. Carnivores obtain it from fats and proteins in their prey. Bacteria take in organic matter from extracellular environment to make glucose. In plants and certain bacteria carbohydrates are produce from sunlight, carbon dioxide, and water. Starch only exists in plants and protists.
Synthesis and degradation
In cells, both glycogen and starch tend to be broken down into glucose-1-phosphate. However, branch points in glycogen and starch can only be hydrolyzed into glucose by a protein called debranching enzyme. Starch is hydrolyzed into glucose by hydrolase when seeds germinate. In the digestive systems of multicellular animals, they are both common substrates for amylase.
Chloroplasts are the site of starch synthesis. The triose produced by photosynthesis is converted into glucose, amylose, and amylopectin. Some starch is also synthesized in amyloplasts. Sucrose is transported to amyloplasts and hydrolyzed into fructose and glucose. Starch synthase links glucose into a linear chain. Branching enzyme cuts the long glucose chain (α-1,4-glycosidic bond) and transfers a segment to nearby site to form an α-1,6-glycosidic bond that creates a branched structure. The proportion of plant amylopectin is about 70%.
Glycogen synthesis is similar, but it can’t be synthesized from scratch. Glucose can only be added to existing glycogen or a protein core.
Structure
They are both highly branched polysaccharides, and glycogen is often referred to as animal starch. However, significant difference is present.
Glycogen granule has more branches than amylopectin. Every chain has about 13 units averagely. Branches grow from a protein core and other branches to form a glycogen β-particle that is a sphere with up to 12 tiers. These particles have surface proteins for synthesis and degradation. In the liver, 20-50 β-particles aggregate into larger glycogen α-particles that resemble broccoli.
The branch of amylopectin has 20-30 glucose units on average. Every branch in glycogen always contains 9-14 glucoses, whereas the chain length in amylopectin is much uneven. Some chains are very short, buy others have hundreds of glucose units. Amylopectin is more like a tree rather than a sphere. Its branches are parallel to the main chain and intertwined into double helices. Most of the area in starch granule is crystalline amylopectin. It allows plants to store more energy and substance in limited space. Proteins for metabolism are also attached to surface of starch granules.
Function
The distinct structures of glycogen and amylopectin indicate their very different functions. Glycogen granule is designed for rapid energy release. It is particularly important in muscle cells where glycogen granule was broken down to glucose for anaerobic glycolysis as quickly as possible when phosphocreatine are depleted (within about 10 seconds of intense activity, such as sprinting).
In plants, starch serves as both a short-term and a long-term energy source. Some starch produced during the day is temporarily stored in chloroplasts where it is broken down to supply aerobic respiration at night. Other stored in amyloplasts acts like fat and is mobilized only in the harsh times. The very strong double helix structure means that it cannot be degrade very quickly.