Alpha vs Beta Glucose: Starch vs Cellulose

0:28Glucose: alpha and beta isomers

Everything starts with glucose, and glucose comes in two forms, called isomers. Same atoms, different arrangement. Look at carbon one, on the right of the ring. In alpha glucose, the OH on carbon one points down, below the ring.

In beta glucose, it points up, above the ring. Remember the shorthand: alpha is down, beta is up. That one flipped OH decides everything that follows. The shape of the chain, and so the job of the molecule.

1:00Building polysaccharides

Polysaccharides are built by condensation. Two glucose molecules join, a molecule of water is released, and a glycosidic bond forms. Repeat that many times and you get a polysaccharide. The reverse reaction, using water to break the bond, is hydrolysis. Keep that word in mind, because it matters for storage.

1:21Starch: alpha-glucose storage

Starch is made from alpha glucose, and it is a mixture of two polysaccharides. Amylose is a long chain joined by one four glycosidic bonds. That chain coils into a helix, so it is compact and packs a lot of glucose into a small space. Amylopectin has the same one four chain, but with side branches joined by one six glycosidic bonds. Branches mean many free ends, so enzymes can hydrolyse off glucose quickly.

And starch is insoluble, so it does not affect water potential, and it will not diffuse out of the cell. That makes it a good store in plants. Plants make glucose in photosynthesis, store the extra as starch, and hydrolyse it back to glucose when they need energy.

2:09Glycogen: the branched store

Glycogen is also made of alpha glucose, with one four and one six bonds, but it is even more highly branched. It is the store in animals. More branches mean even more ends, so glucose is released faster, which suits the high energy demand of animals. It is compact, and insoluble too. Animals take in glucose from food and store the extra as glycogen, mostly in the liver and in muscle, and hydrolyse it when blood glucose falls or muscles work hard.

Starch plant, glycogen animal. Get that the right way round.

2:46Cellulose: beta-glucose

Cellulose is made from beta glucose. Now the problem. In beta glucose, the OH on carbon one is up, but the OH on carbon four is down. So in two beta glucose molecules lined up the same way, the two OH groups that need to react are on opposite faces, and they cannot join. So every other glucose is turned upside down, inverted, and then the bond forms.

The result is a long, straight, unbranched chain.

3:18Why cellulose is strong: hydrogen bonds

Straight chains lie side by side. Many hydrogen bonds form between the chains. Each hydrogen bond is weak, but there are thousands, so together they are strong. Bundles of chains form microfibrils, and microfibrils give the plant cell wall its strength. That strength stops the cell bursting when water enters by osmosis.

So starch is built for storage, and cellulose is built for support. Same monomer, different isomer, completely different job.

4:33Examiner trap and recap

The trap. Saying cellulose is made of beta glucose is not enough. You must say alternate molecules are inverted, and that hydrogen bonds link the chains.

Recap. Alpha glucose gives coiled, branched stores. Beta glucose gives straight chains and strong walls.