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Is it right that are all sugars are carbohydrates but not all carbohydrates are sugars?

If so, which carbohydrates are sugars and which aren't and why all sugars are carbohydrates?

If I had to answer I'd say there are carbohydrates of three kinds: sugars, starch, fiber and from which we can deduce that "not all carbohydrates are sugars".

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  • $\begingroup$ there's also glucose and fructose i think... $\endgroup$
    – Charon
    Aug 23 '16 at 21:53
  • $\begingroup$ Sugar is a common name for the scientific word carbohydrate. Carbohydrate are of four types basically on the basis of the number of units: Monosaccharide(1unit of sugar molecule), Disaccharide (2 units) Oligosaccharide (3-10 sugar units) and Polysaccharide(more than 10 units of sugar molecule). $\endgroup$
    – Tyto alba
    Aug 24 '16 at 6:09
  • $\begingroup$ @SanjuktaGhosh: Does that mean all carbohydrates are sugars? $\endgroup$
    – cpx
    Aug 24 '16 at 14:18
  • $\begingroup$ @cpx I won't say that. I'd say sugar is another name for carbohydrate. $\endgroup$
    – Tyto alba
    Aug 24 '16 at 18:03
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You need to make a distinction between polymers and monomers (or dimers).

Typically, sugars are monomers like glucose, maltose, or fructose. Sucrose (table sugar) is a dimer. These are all carbohydrates (made up of carbon, oxygen, and hydrogen).

Polymers of these sugars are also carbohydrates. For example, starch, glycogen, and cellulose.

Cellulose, which you called fibre, tends to be structural, and is only found in plants (think celery, or wood, or paper). Starch, is also synthesized by plants, and could be used for energy storage (as I recall). In animals, monomeric glucose is stored as glycogen in skeletal muscle, where it becomes available as energy for short bursts of energy.

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  • $\begingroup$ Doesn't answer the question. $\endgroup$
    – cpx
    Aug 24 '16 at 14:16
  • $\begingroup$ I'd edit your last sentence to stop after the first mention of the word glycogen. It is stored in liver and brain as well as muscle, and you must surely regret writing "energy for short bursts of energy". $\endgroup$
    – David
    Aug 28 '16 at 17:54
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The statement in your question is correct. All sugars are carbohydrates but not all carbohydrates are sugars.

In fact, sugars are by definition carbohydrates. Wikipedia defines sugars as :

Sugar is the generalized name for sweet, short-chain, soluble carbohydrates, many of which are used in food. They are composed of carbon, hydrogen, and oxygen

Not all carbohydrates are sugars. In addition to those you have named, there exist chemicals such as inositol, chitin, and various biologically important carboxylic acids.

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Although one might naïvely imagine that the term carbohydrate is used for a molecule containing carbon, hydrogen and oxygen, this is not the case as, if so, it would include compounds like fatty acids. According to the Wikipedia entry it is more restricted, although still somewhat imprecise:

A carbohydrate is a biological molecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water); in other words, with the empirical formula Cm(H2O)n (where m could be different from n). Some exceptions exist; for example, deoxyribose, a sugar component of DNA has the empirical formula C5H10O4. Carbohydrates are technically hydrates of carbon; structurally it is more accurate to view them as polyhydroxy aldehydes and ketones.

By this definition all sugars are carbohydrates if one accepts the authoratitive definition of a sugar by Ron on SE Chemistry:

Simple sugars have the molecular formula Cn(H2O)n, where n is at least 3. They also have to be capable of forming an aldehyde or ketone carbonyl group.

He uses the qualifier of being able to form an aldehyde or ketone to exclude certain carbohydrates such as inositol from being classified as sugars:

…compounds in the inositol family [that] fit the molecular formula requirement, but are not considered to be sugars because they are incapable of forming a carbonyl.

So, yes, all sugars are carbohydrates, but not all carbohydrates are sugars. (Which, despite the unsatisfactory nature of the definitions, is what is generally accepted.)

Disclaimer

This answer describes the terminology used in biological sciences. Because it exists one has to work with it, but one should realize that it is not chemically exact, as illustrated by the following.

Chemists (and biochemists) refer to reducing sugars — and hence by implication there must be non-reducing sugars. The disaccharide sucrose — which is what most of us think of as sugar — is a non-reducing sugar. But the reducing property is a feature of the aldehyde or ketone group, which by Ron’s definition (which I emphasize is that accepted by biologists) would exclude sucrose from being a sugar! Clearly there is a conflict here.

The Wikipedia entry for Reducing Sugars includes the following:

A reducing sugar is any sugar that is capable of acting as a reducing agent because it has a free aldehyde group or a free ketone group. All monosaccharides are reducing sugars, along with some disaccharides, oligosaccharides, and polysaccharides.

and

Disaccharides are formed from two monosaccharides and can be classified as either reducing or nonreducing. Nonreducing disaccharides like sucrose and trehalose have glycosidic bonds between their anomeric carbons and thus cannot convert to an open-chain form with an aldehyde group; they are stuck in the cyclic form. Reducing disaccharides like lactose and maltose have only one of their two anomeric carbons involved in the glycosidic bond, meaning that they can convert to an open-chain form with an aldehyde group.

The way out of the dilemma available to chemists (but not to dieticians) would seem to be not to talk about sugars and carbohydrates at all, but refer to specific chemical types: monosaccharides, disaccharides etc. and their derivatives; and for other dietary/storage molecules refer to fatty acids, triglycerides etc.

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  • $\begingroup$ Interesting, I had not heard of the aldehyde/ketone thing before, but it makes sense --- all sugars I know of are classified as either aldose (aldehyde forming) or ketose (ketone forming). Some refer to inositol (and sorbitol, mannitol, xylitol ... ) as sugar alcohols, as they form only alcohol (hydroxyl) gruops. Perhaps this distinction is important in that aldehyde and ketone groups permit certain reaction mechanisms? $\endgroup$
    – Roland
    Aug 28 '16 at 19:40
  • $\begingroup$ @Roland I'm no expert (my chemistry degree was years ago, and I never 'practiced') but on thinking about the question was uncertain about the situation and googled a bit. However it all seems to be a bit of a 'fudge' as the term 'reducing sugar' suddenly welled up in my memory from the distant past. I'll add a footnote to my answer, but in brief the reducing sugars are the ones that are the aldehydes or ketones, so there must be non-reducing sugars, which then violate the definition. I think the get-out is to say that chemists would talk about monosaccharides and (in this case) disaccharides. $\endgroup$
    – David
    Aug 28 '16 at 20:38
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Sugar is a very casual term used for monosaccharides and oligosaccharides, which normally tastes sweet.

The polysaccharides (starch, cellulose etc) usually does not considered as sugar since they are normally tasteless.

Monosaccharide, Oligosaccharide and Polysaccharide, all three make the chemical-class carbohydrate. So Polysaccharide is the group of carbohydrate that is not sugar.

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