The difference between alpha and beta glucose is quite subtle.
Alpha glucose is what’s termed in chemistry a cis isomer while beta glucose is a trans isomer. Cis because the hydrogen (top, H) and hydroxyl (bottom OH) groups on the left and right of the glucose are on the same side of each other, and for beta glucose it’s trans because they are across from each other diagonally, and not on the same side.
Basic unit: α glucose
Function: the main storage molecule in plants
Structure: starch is made of two compounds – amylose and amylopectin. Both are, of course, made of α glucose, but their overall shapes differ. Amylose is a spiral, while amylopectin has branches. Combined, they give starch the appearance of a tightly wound molecule like a brush.
Crucially, starch is an excellent storage compound, so must satisfy certain requirements. Its size must be relatively big so that it is not soluble. This prevents it from causing an osmotic effect in cells whereby water floods in. The molecule must be compact in order to take up little space rather than a lot. This is achieved by the branches and spirals within starch. Finally, the branches also contribute to the readiness of the glucose molecules of being “nipped off” and quickly usable. This is because only glucose molecules at the ends of starch can be used in that way.
confers structural strength in plant cell walls
Cellulose is the only molecule in this list which is solely made of β glucose. Despite beta glucose only having one chemical group different from alpha glucose, the result is a significant overall structural difference in cellulose. Unlike amylopectin and glycogen, cellulose has a structure based upon straight chains of beta glucose units, rather than spiraling chains of alpha glucose molecules.
These straight chains laid next to one another form hydrogen bonds which strengthen them into larger sub-units called microfibrils. Microfibrils are what cellulose is made of, and what gives plant cell walls their great strength.
N-acetylglucosamine (glucose derivative)
confers structural strength in fungi cell walls and exoskeletons
Chitin is the only molecule in this list which is solely made of N-acetylglucosamine. Cellulose has a structure similar to cellulose, hence conferring it good structural properties. It features in insect and crustacean exoskeletons as well as fungal cell walls, providing strength, support and protection to these components of cells and organisms.
Due to its unique properties, chitin has been used increasingly by humans for various purposes such as a food thickener and stabiliser, and in the process of sizing and strengthening paper.
The points about starch being suitable for energy storage due to its chemical and physical properties also apply to glycogen. Cellulose and chitin are suitable for structural support rather than energy storage due to the arrangement of bonds between monomers, described in the cellulose section.