I was taught that a simple sugar, for example, always has a carbon ring comprised of 6 carbons. My wife and friends in the sciences (but not bio majors) say they were taught the same thing.
Now, tutoring a boy in biology, his teacher draws the ring with 5 carbons and an oxygen. The sixth carbon is linked off the carbon on the upper left of the ring.
What’s the Straight Dope? Was the 6-carbon ring an over-simplification of real story? (And my advanced bio course was no cake walk!)
Glucose and fructose certainly have a 6 carbon chain. Drawn in the Fisher method, it looks like this.
This method is called Haworth, and it still has the 6 carbon chain, but is the 5th carbon is bonded to the oxygen on the first carbon.
Don’t think it is that new, though I could be wrong.
Oops, fructose obviously has a slightly different structure, though it isn’t included, didn’t mean to be unclear.
The boy’s teacher has the right of it, all simple sugars like glucose have a six membered ring made of five carbons and one oxygen atom. The ring is called a pyran. The six-membered ring made of carbon idea is not correct at all, this compound would not be classed as a carbohydrate.
The confusion may be arising from not being able to see the relationship between glucose in the straight chain form (Fischer projection provided by Epimetheus) and the cyclic form. Both forms are in equilibrium in solution through intramolecular addition of one of the hydroxyl groups to the aldehyde group, to form what the chemists call an hemi-acetal. The equilibrium is overwhelmingly on the side of the cyclic form. It is not possible to make a six membered carbocycle from straight-chain glucose without doing a chemical reaction that fundamentally alters the properties of the molecule.
There are some unusual carbohydrate sugars that do have a 6 carbon ring such as inositol. However, the vast majority do not
You are probably remembering wrong.
Glucose, fructose, galactose, and most other common sugars are all 6-carbon sugars (they have 6 carbons). Ribose and a few other common sugars have 5 carbons.
Glucose, galactose (among others) have 6-membered rings, but those rings are made of 5 carbons and one oxygen. Fructose and ribose have 5-membered rings (4 carbons and 1 oxygen).
What you’re probably remembering is that glucose has 6 carbons, and that it has a 6-membered ring, and just assumed that the ring was all carbons. There are certainly many carbohydrates that have 6-membered all-carbon rings (quinic acid, for example), but the common ones don’t.
Carbohydrates
4 carbon ketoses: Erythrose, Threose
5 carbon ketoses: Ribose, Arabinose, Xlose, Lyxose
6 carbon ketoses: Allose, Altrose, Mannose, Glucose, Gulose, Idose, Galactose, Talose
