Monosaccharides with an aldehyde carbonyl group are called:
A — Aldoses have an aldehyde (CHO) at C-1. Ketoses have a ketone (C=O) at C-2. Hexose/pentose refer to chain length (6 or 5 carbons), not carbonyl type. Glucose and galactose are aldoses; fructose is a ketose. Lehninger Biochemistry 7e §7.1; McMurry & Ballantine 8e §25.1
Question 2
Glucose is correctly classified as:
B — Aldohexose. Glucose has 6 carbons (hexose) and an aldehyde at C-1 (aldo-). Fructose = ketohexose; ribose = aldopentose; ribulose = ketopentose. Lehninger 7e §7.1; McMurry 8e §25.1
Question 3
Fructose is classified as:
C — Ketohexose. Fructose (fruit sugar) has 6 carbons and a ketone carbonyl at C-2. It is a monosaccharide, not a disaccharide. It is the sweetest common sugar. Lehninger 7e §7.1; McMurry 8e §25.1
Question 4
In Fischer projection, the D/L designation is defined by the –OH group on the __________ chiral carbon pointing to the right (D) or left (L):
B — Highest-numbered stereogenic centre. For glucose (C6 aldose), D/L is determined at C-5 (the penultimate carbon). D-glucose: C-5 –OH points right. This mirrors D/L glyceraldehyde (C-2 –OH right = D). C-1 is the aldehyde carbon (not chiral in the open chain). Lehninger 7e §7.1; McMurry 8e §25.1
Question 5
Galactose is an epimer of glucose differing at carbon:
A — C-4. Galactose and glucose differ only in the orientation of –OH at C-4 (galactose: C-4 OH axial; glucose: C-4 OH equatorial). This single stereocenter difference makes them C-4 epimers. Mannose is the C-2 epimer of glucose. Lehninger 7e §7.1; McMurry 8e §25.1
Question 6
In converting a Fischer projection to a Haworth pyranose ring, a –OH group that was on the RIGHT in Fischer becomes:
B — Right in Fischer = below the ring in Haworth (for C-2, C-3, C-4). Left in Fischer = above the ring. Exception: C-6 (–CH₂OH) is always above the ring for D-sugars because the chain folds upward. Memorise: right→down, left→up. Lehninger 7e §7.1; McMurry 8e §25.2
Question 7
Mutarotation is the change in optical rotation due to:
C — Anomer interconversion. In solution, the pyranose ring opens transiently to the open-chain aldehyde, then re-closes with either α or β configuration at C-1. This equilibration changes the specific optical rotation over time until a constant value is reached. Catalysed by acid, base, or the enzyme mutarotase. Lehninger 7e §7.1; McMurry 8e §25.2
Question 8
In the Haworth projection of α-D-glucopyranose, the C-1 –OH group is:
D — Below the ring in Haworth. In α-D-glucose, C-1 –OH is trans to the C-6 –CH₂OH reference group (which is above). In the chair conformation, this C-1 –OH is axial. In β-D-glucose, C-1 –OH is above the ring (equatorial in the chair) — the more stable anomer. Lehninger 7e §7.1; McMurry 8e §25.2
Question 9
At equilibrium in aqueous solution, the α:β ratio of D-glucopyranose is approximately:
A — 36% α, 64% β. The β-anomer predominates because its C-1 –OH is equatorial (less steric strain) in the chair form. About <0.003% exists as the open-chain form at equilibrium. Specific rotation: α = +112°; β = +18.7°; equilibrium = +52.7°. Lehninger 7e §7.1; McMurry 8e §25.2
Question 10
A reducing sugar is one that:
B — Free anomeric –OH. The hemiacetal at the anomeric carbon can open to the free aldehyde, which reduces Cu²⁺ (Benedict's/Fehling's → brick-red Cu₂O) or Ag⁺ (Tollens' → silver mirror). All monosaccharides are reducing sugars. Sucrose is NOT because both anomeric carbons are locked in the glycosidic bond. Lehninger 7e §7.1; McMurry 8e §25.3
Question 11
Sucrose is a non-reducing sugar because:
C — Both anomeric carbons are locked. In sucrose, C-1 of glucose (α) and C-2 of fructose (β) are both used in the glycosidic bond. Neither hemiacetal –OH is free, so the ring cannot open to a free aldehyde/ketone. This is why sucrose gives a negative Benedict's/Fehling's test. Lehninger 7e §7.2; McMurry 8e §25.4
Question 12
The glycosidic bond in maltose is:
D — α-1,4 glycosidic bond. Maltose = glucose–α(1→4)–glucose. It is produced by amylase hydrolysis of starch and is a reducing sugar (C-1′ –OH of the second glucose is free). Hydrolysed by maltase. Lehninger 7e §7.2; McMurry 8e §25.4
Question 13
The glycosidic bond in lactose is:
A — β-1,4 (galactose–glucose). Lactose = galactose–β(1→4)–glucose. It is found in milk, is a reducing sugar, and is hydrolysed by lactase (β-galactosidase). The β configuration is why humans lacking lactase cannot digest it. Lehninger 7e §7.2; McMurry 8e §25.4
Question 14
The glycosidic bond in sucrose is:
B — α-1,2-β linkage. The anomeric C-1 of α-D-glucose is joined to the anomeric C-2 of β-D-fructose. This locks both anomeric carbons, making sucrose non-reducing. Hydrolysed by sucrase (invertase) to give glucose + fructose (“invert sugar”). Lehninger 7e §7.2; McMurry 8e §25.4
Question 15
Lactose intolerance is caused by deficiency of:
C — Lactase deficiency. Lactase (brush-border enzyme of small intestine) cleaves the β-1,4 bond of lactose. Without it, undigested lactose passes to the colon where bacteria ferment it → gas (bloating, flatulence) + osmotic diarrhoea. Symptoms: cramping, diarrhoea after dairy ingestion. Lehninger 7e §7.2; Harper's Biochemistry §19
Question 16
Starch (amylose + amylopectin) is digestible by human amylase because it contains:
D — α-1,4 and α-1,6 bonds. Human salivary and pancreatic α-amylase cleaves internal α-1,4 bonds. Debranching enzyme hydrolyses α-1,6 bonds. Cellulose (β-1,4) is indigestible because humans lack β-glucosidase. The α-configuration allows the chain to coil into a helix accessible to amylase. Lehninger 7e §7.3; McMurry 8e §25.5
Question 17
Cellulose is indigestible by humans because it has:
A — β-1,4 linkages; no human cellulase. The β configuration causes cellulose chains to form straight, parallel sheets stabilised by inter-chain hydrogen bonds → rigid, insoluble fibres. Ruminants (cows, termites) use gut bacteria with cellulase to digest it. Cellulose serves as dietary fibre in humans. Lehninger 7e §7.3; McMurry 8e §25.5
Question 18
Glycogen differs from starch primarily in:
B — More frequent branching. Glycogen has α-1,4 backbone and α-1,6 branches every 8–12 glucose residues (amylopectin branches every ~24–30). This high branching creates many non-reducing ends for rapid glucose release by glycogen phosphorylase — essential for rapid energy mobilisation in muscle and liver. Lehninger 7e §7.3; Harper's Biochemistry §19
Question 19
Von Gierke disease (Glycogen Storage Disease Type I) involves deficiency of:
C — Glucose-6-phosphatase deficiency. Without this enzyme, glucose-6-phosphate cannot be converted to free glucose for export from liver → massive glycogen accumulation in liver and kidney, severe fasting hypoglycaemia, hepatomegaly, lactic acidosis, hyperlipidaemia. Treatment: frequent cornstarch feeds to maintain blood glucose. Lehninger 7e §15.4; Harper's Biochemistry §19
Question 20
The Benedict's/Fehling's test is positive for glucose but negative for sucrose because:
D — Sucrose is non-reducing. Glucose has a free C-1 hemiacetal –OH that opens to the free aldehyde → reduces Cu²⁺ to Cu⁺ (brick-red Cu₂O precipitate). Sucrose's anomeric carbons are both locked in the α-1,2-β bond, so no free aldehyde/ketone is available. Test is positive for all monosaccharides and most disaccharides (maltose, lactose) but NOT sucrose. Lehninger 7e §7.1; McMurry 8e §25.3
1. Aldose▼
A monosaccharide whose carbonyl group is an aldehyde (CHO) at C-1. All carbons below C-1 may be chiral. Named by chain length: triose (C3) — glyceraldehyde; pentose (C5) — ribose, deoxyribose; hexose (C6) — glucose, galactose. Contrast with ketose (ketone at C-2, e.g. fructose). The free aldehyde can reduce Cu²⁺ → all aldoses are reducing sugars. Lehninger 7e §7.1; McMurry 8e §25.1
2. Anomer▼
Two stereoisomers that differ only at the anomeric carbon (C-1 for aldoses, C-2 for ketoses) generated when the open-chain sugar cyclises to a hemiacetal. In the α-anomer, the C-1 –OH is axial (below the ring in Haworth for D-sugars); in the β-anomer, it is equatorial (above the ring). The anomeric carbon is the former carbonyl carbon. Anomers interconvert in solution by mutarotation. The β-anomer of D-glucose is more stable (C-1 –OH equatorial). Lehninger 7e §7.1; McMurry 8e §25.2
3. Mutarotation▼
The spontaneous interconversion of α and β anomers in solution via the open-chain (free aldehyde) form. Observed as a change in specific optical rotation over time until equilibrium is reached. For D-glucose: pure α starts at +112°, pure β at +18.7°, equilibrium mixture = +52.7° (36% α, 64% β). Catalysed by acid, base, or the enzyme mutarotase. Sucrose does NOT show mutarotation because both anomeric carbons are locked in the glycosidic bond. Lehninger 7e §7.1; McMurry 8e §25.2
4. Reducing sugar▼
A sugar with a free hemiacetal (free anomeric –OH) that can open to a free aldehyde/ketone and thereby reduce Cu²⁺ (Benedict's/Fehling's → brick-red Cu₂O) or Ag⁺ (Tollens' → silver mirror). All monosaccharides are reducing. Among disaccharides: maltose and lactose are reducing (one free anomeric –OH); sucrose is NOT. Polysaccharides have negligible reducing power (only one free end per chain). Lehninger 7e §7.1; McMurry 8e §25.3
5. Glycosidic bond▼
An acetal linkage formed between the anomeric –OH of one sugar and a hydroxyl group of another molecule (sugar or alcohol), with loss of water. Designated by the anomeric configuration (α or β) and the carbon numbers involved (e.g. α-1,4; β-1,4). Once formed, the bond fixes the anomer and prevents ring-opening at that carbon. Hydrolysed by specific glycosidases (maltase, lactase, sucrase, amylase). Lehninger 7e §7.2; McMurry 8e §25.4
6. Glycogen▼
The animal storage polysaccharide of glucose, found in liver and skeletal muscle. Structure: α-1,4-linked glucose backbone with α-1,6 branches every 8–12 residues (more frequent than starch amylopectin at 24–30 residues). This high branching creates many non-reducing ends for rapid phosphorolysis by glycogen phosphorylase. Liver glycogen maintains blood glucose during fasting; muscle glycogen fuels muscle contraction. Defects in glycogen metabolism cause glycogen storage diseases (e.g. Von Gierke, Pompe, McArdle). Lehninger 7e §7.3; Harper's §19
1. Convert D-glucose from Fischer projection to Haworth pyranose form, explaining the rules. State which groups go above/below the ring. (7 marks)7 marks
Step 2 — Ring formation (pyranose):
The C-5 –OH attacks the C-1 aldehyde intramolecularly → six-membered ring (pyranose). C-1 becomes the anomeric carbon with a new –OH (hemiacetal).
Step 3 — Fischer → Haworth conversion rules:
Right in Fischer → below the ring in Haworth (for C-2, C-3, C-4).
Left in Fischer → above the ring in Haworth.
C-6 (CH₂OH) is always above the ring for D-sugars (the chain folds up).
Result for α-D-glucopyranose (Haworth):
C-1 OH below; C-2 OH below (was right); C-3 OH above (was left); C-4 OH below (was right); C-5 H below, C-6 CH₂OH above.
Cite: Lehninger 7e §7.1; McMurry 8e §25.2
2. Explain mutarotation of D-glucose. Define α and β anomers, state equilibrium percentages, and explain why sucrose does not show mutarotation. (6 marks)6 marks
Definition: Mutarotation is the spontaneous change in optical rotation of a sugar solution over time due to interconversion of α and β anomers via the open-chain form.
Mechanism:
α-D-glucopyranose → (ring opens) → open-chain aldehyde → (ring closes with either face) → α or β-D-glucopyranose.
Catalysed by acid/base or the enzyme mutarotase.
Anomer definitions:
α-anomer: C-1 –OH is trans to C-6 CH₂OH reference (below ring in Haworth; axial in chair). [α]D = +112°.
β-anomer: C-1 –OH is cis to C-6 CH₂OH (above ring in Haworth; equatorial in chair). [α]D = +18.7°. More stable.
Sucrose does NOT mutarotate because both anomeric carbons (C-1 of glucose and C-2 of fructose) are locked in the α-1,2-β glycosidic bond. The ring cannot open; no free hemiacetal exists.
Cite: Lehninger 7e §7.1; McMurry 8e §25.2
3. Compare maltose, lactose, and sucrose: monosaccharide units, glycosidic bond, reducing/non-reducing, hydrolyzing enzyme. (8 marks)8 marks
Property
Maltose
Lactose
Sucrose
Units
Glucose + Glucose
Galactose + Glucose
Glucose + Fructose
Glycosidic bond
α-1,4
β-1,4
α-1,2-β
Reducing?
Yes (free C-1′ OH on 2nd glucose)
Yes (free C-1′ OH on glucose)
No (both anomeric C locked)
Enzyme
Maltase
Lactase (β-galactosidase)
Sucrase (invertase)
Source
Starch digestion; malt
Milk
Cane/beet sugar
Cite: Lehninger 7e §7.2; McMurry 8e §25.4
4. Distinguish starch, cellulose, and glycogen: glycosidic linkage, branching, digestibility, and biological role. (7 marks)7 marks
Property
Starch (amylopectin)
Cellulose
Glycogen
Backbone bond
α-1,4
β-1,4
α-1,4
Branch bond
α-1,6 (every ~24–30 residues)
None (linear)
α-1,6 (every 8–12 residues)
Branching
Moderate
None
Highly branched
Human digestion
Yes (α-amylase + glucosidase)
No (no cellulase)
Yes (glycogen phosphorylase)
Biological role
Energy storage in plants
Structural (plant cell walls)
Energy storage in liver & muscle
Key point: The β-1,4 linkage of cellulose forces straight chains → hydrogen-bonded sheets → rigid, insoluble fibre. The α-1,4 linkage of starch/glycogen produces helical chains → compact storage form accessible to α-amylase.
Cite: Lehninger 7e §7.3; McMurry 8e §25.5
5. Explain the clinical basis of lactose intolerance and Von Gierke disease (GSD Type I): deficient enzyme, accumulated substrate, clinical consequences. (6 marks)6 marks
Lactose Intolerance:
Deficient enzyme: Lactase (β-galactosidase) in the brush border of the small intestinal mucosa.
Accumulated substrate: Undigested lactose passes intact to the colon.
Clinical consequences: Colonic bacteria ferment lactose → short-chain fatty acids + gas (CO₂, H₂, CH₄) → bloating, flatulence, cramping. Undigested lactose also exerts osmotic pressure → osmotic diarrhoea. Symptoms appear 30 min–2 h after dairy ingestion. Management: lactase supplements, lactose-free milk, calcium supplementation to prevent deficiency.
Von Gierke Disease (GSD Type I):
Deficient enzyme: Glucose-6-phosphatase (G6Pase) in liver and kidney (endoplasmic reticulum). Autosomal recessive; chromosome 17q21.
Accumulated substrate: Glucose-6-phosphate cannot be dephosphorylated → massive glycogen accumulation in liver and kidney; also ↑ lactate (glycolysis shunted), ↑ triglycerides (lipogenesis), ↑ uric acid (purine synthesis).
Clinical consequences: Severe fasting hypoglycaemia (glucose cannot be exported to blood); hepatomegaly and nephromegaly; lactic acidosis; hyperlipidaemia (xanthomas); hyperuricaemia (gout). Doll-face appearance, short stature. Treatment: frequent uncooked cornstarch feeds to maintain blood glucose; avoid fasting.
Cite: Lehninger 7e §15.4; Harper's Biochemistry §19; Robbins Pathology §9