SN1
SHORT NOTE
5M
★ High-yield
Define reducing sugars. Describe the Benedict test and its clinical significance.
Definition
Reducing sugars = sugars possessing a free (or potentially free) aldehyde or ketone group, which can reduce metal ions such as Cu2+ in alkaline solution.
Which Sugars Reduce
| Reducing | Non-reducing |
|---|---|
| All monosaccharides — glucose, fructose, galactose, mannose, ribose | Sucrose |
| Maltose, lactose, isomaltose | Trehalose |
| — | Starch, glycogen, cellulose (only one free end) |
- Ketoses also reduce, because alkali converts them to aldoses by enolisation — fructose gives a positive Benedict test
- Sucrose does not reduce because both anomeric carbons are engaged in the glycosidic bond
Benedict Test
Sugar + Benedict reagent (copper sulphate, sodium citrate, sodium carbonate) → Heat in a boiling water bath for 2 minutes → Alkali converts the sugar to a strong reducing enediol → Cu2+ (blue) → Cu+ → Red precipitate of cuprous oxide
| Colour | Approximate sugar | Grading |
|---|---|---|
| Blue (no change) | Nil | Negative |
| Green | < 0.5% | Trace / + |
| Yellow | 0.5–1% | ++ |
| Orange | 1–2% | +++ |
| Brick red | > 2% | ++++ |
- Sodium citrate keeps copper in solution, preventing precipitation as hydroxide — the advantage over Fehling reagent
- It is semi-quantitative and non-specific
Other Sugar Tests
| Test | Detects | Principle |
|---|---|---|
| Barfoed test | Monosaccharide vs disaccharide | Acidic copper; only monosaccharides reduce it within 2 minutes |
| Seliwanoff test | Ketoses (fructose) | Rapid cherry-red with resorcinol and HCl |
| Bial test | Pentoses | Green colour with orcinol |
| Osazone formation | Identifies individual sugars | Glucose and fructose give identical needle-shaped crystals; lactose gives powder-puff; maltose gives sunflower |
| Iodine test | Polysaccharides | Starch blue, glycogen red-brown |
Applied Aspects
- Benedict test is not specific for glucose — a positive result in an infant may mean galactosaemia, fructosuria or pentosuria. This is why the glucose oxidase strip, which IS specific, must be used alongside
- A positive Benedict with a negative glucose oxidase strip in a newborn is the classical screening finding in galactosaemia
- False positives — ascorbic acid, salicylates, homogentisic acid, and several antibiotics reduce copper
- Lactosuria is normal in late pregnancy and lactation and must not be mistaken for diabetes
- Urine testing has largely been replaced by blood glucose and HbA1c, since the renal threshold varies between individuals
All questions in Cell, Biomolecules & Chemistry of Carbohydrates
- Q1Describe the structure and functions of the cell organelles. Give their marker enzymes and the principle of subcellular fractionation. ★
- Q2Describe the structure of the plasma membrane. Explain the mechanisms of transport across it. ★
- Q3Define and classify carbohydrates with suitable examples. Add a note on the derived carbohydrates. ★
- Q4Describe the isomerism shown by carbohydrates. Explain epimers, anomers and mutarotation with their significance. ★
- Q5Classify polysaccharides with examples. Compare starch, glycogen and cellulose. ★
- SN1Define reducing sugars. Describe the Benedict test and its clinical significance. ★
- SN2Describe the glycosidic bond with its types and significance. ★
- SN3Define dietary fibre. Classify it and describe its physiological effects. ★
- SN4Describe the structure and functions of lysosomes. Add a note on the lysosomal storage diseases. ★
- SN5Describe the structure and functions of glycoproteins. Add a note on the blood group antigens.
- SN6Describe the properties of water and the distribution of body water. ★
- SN7Define buffers. Describe the buffer systems of the body and the Henderson–Hasselbalch equation. ★