Monosaccharides
High-Yield Summary
- Aldoses oxidize at the aldehyde to form aldonic acids (D-glucose → D-gluconic acid). Sugars capable of this are reducing sugars.
- Benedict's test: Cu²⁺ (basic solution) is reduced to Cu₂O (red precipitate) by a reducing sugar's aldehyde. Tollens' test: Ag⁺ is reduced to Ag⁰ (silver mirror).
- Ketoses (e.g. fructose) can act as reducing sugars too, but only after tautomerizing into an aldose form first.
- Reduction of an aldose's aldehyde → alditol (sugar alcohol). Replacing an OH with H → deoxy sugar (e.g. deoxyribose in DNA).
- Phosphate esters: glucose + ATP → glucose-6-phosphate + ADP, catalyzed by hexokinase (glucokinase in liver/pancreatic beta cells) — traps glucose in the cell to start glycolysis.
- Glycosidic bond: anomeric carbon's OH is replaced by an alkoxy group from an alcohol, forming an alpha or beta glycoside — this is how monosaccharides link into di-/polysaccharides.
Key Terms
- Aldonic acid
- Product of oxidizing an aldose's aldehyde to a carboxylic acid.
- Reducing sugar
- A sugar with a free (or freeable) aldehyde group that can reduce Cu²⁺ or Ag⁺; tests positive on Benedict's/Tollens'.
- Alditol
- Sugar alcohol produced by reducing an aldose's aldehyde to an alcohol.
- Deoxy sugar
- Sugar with an OH replaced by H; e.g. deoxyribose.
- Glycosidic bond
- Bond formed when the anomeric carbon's OH is replaced by an alkoxy group; links sugars into di-/polysaccharides.
- Furanoside / pyranoside
- Glycoside derived from a furanose ring / pyranose ring.
Oxidation vs. Reduction vs. Deoxy Substitution
| Reaction | Product (Example) |
|---|---|
| Oxidation (aldehyde → COOH) | Aldonic acid (D-glucose → D-gluconic acid) |
| Reduction (aldehyde → alcohol) | Alditol (sugar alcohol) |
| Deoxy substitution (OH → H) | Deoxy sugar (deoxyribose) |
Trapping Glucose in the Cell (Phosphorylation)
- 1Glucose enters the cell via a glucose transporter.
- 2Hexokinase (or glucokinase in liver/pancreatic beta cells) transfers a phosphate from ATP to glucose's C6.
- 3Glucose-6-phosphate forms; ATP is converted to ADP.
- 4The phosphate charge traps glucose inside the cell, committing it to glycolysis.
Common MCAT Trap
- Fructose (a ketose) still tests positive on Benedict's/Tollens' — it just has to tautomerize to an aldose form first. Don't assume only aldoses are reducing sugars.
- Benedict's reduces Cu²⁺ → Cu₂O (red ppt); Tollens' reduces Ag⁺ → Ag⁰ (silver mirror) — easy to swap which metal goes with which test.
- Hexokinase vs. glucokinase: same reaction, but glucokinase is tissue-specific (liver, pancreatic beta cells) and has different kinetics/regulation — a classic MCAT distinction pair.
Quick Recall
What does a positive Benedict's test look like, and why?
Why can fructose act as a reducing sugar despite being a ketose?
What enzyme phosphorylates glucose to glucose-6-phosphate in most tissues?
What bond type links monosaccharides into disaccharides?