Carbohydrates: Monosaccharides, Aldoses, Ketoses, and Ring Structures

Carbohydrates: Basic properties and vocabulary

Carbohydrates are made up of carbon, hydrogen, and oxygen; chemical formula pattern relates to carbon and water (CH2O units).
All sugars are polar molecules.
Some sugars can be amino sugars (contain nitrogen), but every sugar must have C, H, and O; some may also contain N.
The terms sugar and carbohydrate are interchangeable in many contexts.
The suffix -ose is common in sugar names (e.g., glucose, fructose, lactose, cellulose).
The summary idea behind the name: Carbon + Water (carbohydrate) — essentially the essential ingredients in the sugar unit.
In class coloring or labeling, categories like aldehyde groups or other functional groups were highlighted; color-coding is a study technique.

Monosaccharides are the smallest unit of sugars; they are the building blocks for disaccharides and polysaccharides.
They link together via dehydration synthesis to form larger carbohydrates (disaccharides and polysaccharides).
General formula for monosaccharides: Cn(H2O)_n
- For every carbon atom, there is one water molecule associated in the formula unit.
- Exception: deoxyribose is missing an oxygen, with formula C5H{10}O_4. This is an exception to the general rule; you don’t need to memorize every exception, just recognize the rule and be aware of the notable exception.
Common size categories of monosaccharides:
- Trioses: 3 carbons
- Pentoses: 5 carbons
- Hexoses: 6 carbons
Size and formula examples:
- For hexoses, the general formula becomes C6(H2O)6 = C6H{12}O6.
The endings of sugars: many end in -ose (e.g., glucose, fructose, lactose, cellulose).

Aldose: contains an aldehyde group (CHO) at the end of the carbon backbone.
- Examples: glucose and galactose are aldohexoses.
Ketose: contains a ketone group (C=O) in the middle of the carbon chain, not at the end.
- Example: fructose is a ketohexose.
The position of the carbonyl group changes properties and reactivity of the sugar.
Relationship among these: same molecular formula can yield different structures (isomers).

Glucose and galactose are isomers (same formula, different structure): both are hexoses with formula C6H{12}O_6.
The difference between glucose and galactose is an epimer at carbon 4: the orientation of the hydroxyl (OH) vs hydrogen (H) at C4 differs, which changes the sugar’s identity.
Fructose differs from glucose/galactose by being a ketose (C=O in the middle rather than at the end).
In a quick reference: glucose = aldohexose, galactose = aldohexose (epimer of glucose at C4), fructose = ketohexose.

Glucose is the most important monosaccharide for life, serving as a primary energy source.
It is used by cells to generate ATP (cellular energy).
Chemical formula: C6H{12}O_6.
In solution, glucose can exist in linear form or cyclic forms due to ring closure.

In aqueous solution, hexoses can cyclize to form a stable ring structure (pyranose form is common for hexoses).
The ring is formed when the aldehyde or keto group reacts with a hydroxyl group on the same molecule, creating a hemiacetal (or hemiketal in ketoses).
For glucose, cyclization occurs between the carbonyl carbon (C1) and the hydroxyl on C5, forming a six-membered ring (a pyranose ring).
The ring structure contains:
- Five carbon atoms and one ring oxygen (total of six atoms in the ring, hence a six-member ring).
- A sixth carbon (C6) remains outside the ring as a CH2OH group attached to carbon 5.
Anomerism: after ring formation, the anomeric carbon (C1) can have the hydroxyl group oriented in two different ways, giving two anomers:
- Alpha (α) anomer: the OH on C1 is oriented downward (below the plane in Haworth projection).
- Beta (β) anomer: the OH on C1 is oriented upward (above the plane in Haworth projection).
The ring can open back to the linear form and can re-close to either α or β anomer, explaining the existence of α- and β-D-glucose in solution.
In Haworth projections, the ring oxygen is typically drawn at the top-right position in the hexagon, with the substituents around the ring alternating up (above) and down (below) the ring plane.
The CH2OH group on C5 is outside the ring and typically points up or down depending on the stereochemistry (e.g., in D-glucose, the CH2OH is drawn above the ring).

When drawing monosaccharides, remember:
- The anomeric carbon is C1 (the former carbonyl carbon).
- The ring height and substituent orientation determine α vs β forms.
- The ring oxygen is part of the ring and the sixth carbon (C6) appears outside the ring as CH2OH.
- For hexoses like glucose, expect a six-member ring (pyranose) with OH and H alternating around the ring.
The difference between α and β forms is one of the key features that affect how sugars behave in biological systems and during polymerization.

Carbohydrates frequently end in -ose (e.g., glucose, fructose, lactose, cellulose).
Simple sugars are monosaccharides; some combinations form disaccharides (two sugars) or polysaccharides (many sugars).
An example catchphrase from class: “Oats” sounds like “-ose”; it helps remember the naming convention for sugars.

Carbon, hydrogen, and oxygen make up the carbohydrate backbone; the CH2O unit concept helps explain general molecular formulas.
The presence and position of carbonyl groups (aldehyde in aldose vs ketone in ketose) determine properties and metabolic roles.
Epimers (e.g., glucose vs galactose at C4) illustrate how small changes in stereochemistry lead to different molecules with distinct biological functions.
Ring formation and anomerism (α vs β) have practical consequences for how sugars polymerize and interact with other biomolecules.
The energy role of glucose underpins metabolism: glucose oxidation → ATP production, fueling cellular work and growth.

General monosaccharide formula: Cn(H2O)_n
Hexose example: C6H{12}O_6
Deoxyribose (exception to the rule; missing one oxygen): C5H{10}O_4
Common hexose sugars: glucose (aldohexose), galactose (aldohexose, C4 epimer of glucose), fructose (ketohexose)
Ring form: pyranose (six-member ring); anomeric carbon: C1; outside ring: CH2OH at C5/C6 position
Anomeric forms: α and β (OH orientation at C1 relative to ring plane)