Chapter 4: Carbohydrate Structure and Function
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1. (4.1) A monosaccharide with a ketone functional group is classified as a(n):
B. Monosaccharides are classified by their carbonyl group: aldoses have an aldehyde (at C1), ketoses have a ketone (usually at C2). Fructose is the canonical ketose; glucose is an aldose.
2. (4.1) Glucose (C6H12O6) is classified as a(n):
C. Glucose has 6 carbons (hexose) and an aldehyde group at C1 (aldose) = aldohexose. Fructose has 6 carbons and a ketone at C2 = ketohexose. Ribose is an aldopentose.
3. (4.2) In a Fischer projection, the D/L designation of a sugar is determined by:
A. D/L refers to the penultimate (highest-numbered) chiral carbon. If the -OH on that carbon points RIGHT in Fischer, it's D; LEFT = L. Nature overwhelmingly uses D-sugars (and L-amino acids).
4. (4.2) A Haworth projection represents:
D. Haworth projections show the ring form. The ring is drawn flat (hexagonal for pyranoses, pentagonal for furanoses). Substituents point up or down. Fischer = open-chain; Haworth = closed ring.
5. (4.3) Two sugars that differ in configuration at only one chiral center are called:
C. Epimers differ at exactly ONE chiral center. Glucose and galactose are C4 epimers; glucose and mannose are C2 epimers. Anomers are a special case - epimers at the anomeric carbon (C1 for aldoses). Enantiomers differ at ALL stereocenters.
6. (4.3) Glucose and mannose differ only in the stereochemistry at C2. They are:
B. Differ at one chiral center (C2) = C2 epimers. Glucose/galactose are C4 epimers. Remember: anomers differ at the anomeric carbon only (C1 for aldoses).
7. (4.4) When an aldose forms a ring, the carbonyl carbon becomes the anomeric carbon and can exist in two configurations, called:
A. Ring closure generates a new chiral center at C1 (the anomeric carbon). In D-sugars drawn as Haworth: α-OH points DOWN (trans to the CH2OH at C5), β-OH points UP (cis). Interconversion = mutarotation.
8. (4.4) Mutarotation refers to:
D. In solution, cyclic sugars slowly open to the straight-chain aldehyde and re-close as either the alpha or beta anomer. The optical rotation changes as the ratio equilibrates ("mutarotates").
9. (4.5) Which sugar is the body's primary blood-circulating fuel?
B. Blood glucose is tightly regulated near 5 mM (~90 mg/dL). Fructose is dietary (fruits, HFCS) and primarily metabolized in the liver. Galactose comes from lactose digestion. Ribose is a 5-carbon sugar in nucleotides.
10. (4.5) Ribose and deoxyribose differ in that:
C. Both are pentoses; both are aldoses. The only difference: ribose has -OH at C2, deoxyribose has -H ("deoxy" = missing an oxygen). Ribose is in RNA; deoxyribose is in DNA.
11. (4.6) Lactose is a disaccharide composed of:
A. Lactose = galactose-β(1→4)-glucose (milk sugar). Sucrose = glucose-α(1→2)-fructose (table sugar). Maltose = glucose-α(1→4)-glucose (from starch digestion). Cellobiose = glucose-β(1→4)-glucose (indigestible).
12. (4.6) Sucrose is not a reducing sugar because:
D. A reducing sugar has a free anomeric carbon that can open to the aldehyde/ketone form. Sucrose's α,β(1→2) linkage ties up BOTH anomeric carbons - no free form possible. Lactose and maltose retain one free anomeric carbon and ARE reducing sugars.
13. (4.7) The bond formed between the anomeric carbon of one sugar and a hydroxyl oxygen of another sugar is called a(n):
B. The glycosidic bond is the -O- linkage between sugars, formed by condensation (release of water). Specified as α or β (anomeric configuration) and numbered by the two carbons joined, e.g., α(1→4).
14. (4.7) Humans cannot digest cellulose because:
C. Humans make amylase (breaks α-1,4) and debranching enzymes (α-1,6) but no cellulase. Ruminant bacteria make cellulase. Fiber passes through us undigested because the β bond is stereochemically incompatible with our enzymes' active sites.
15. (4.8) Glycogen differs from amylopectin (a component of starch) in that glycogen has:
A. Both are α-1,4 glucose polymers with α-1,6 branches. Glycogen is MORE branched. More branches = more non-reducing ends = more simultaneous access points for rapid mobilization (important for animals needing fast energy release).
16. (4.8) The structural polysaccharide in arthropod exoskeletons is:
D. Chitin is a polymer of N-acetylglucosamine (GlcNAc) joined by β(1→4) bonds - like cellulose but with an acetamido group replacing one OH. Found in insect shells, crab shells, and fungal cell walls.
17. (4.9) Glycoproteins have carbohydrate chains attached via:
C. N-linked glycosylation: sugar attached to the amide nitrogen of asparagine (consensus sequence Asn-X-Ser/Thr). O-linked: sugar attached to the -OH oxygen of serine or threonine. Glycosylation happens in the ER and Golgi.
18. (4.9) Glycolipids are most commonly found:
B. Glycolipids (and glycoproteins) concentrate on the extracellular face of the plasma membrane, forming the glycocalyx - a carbohydrate-rich coat involved in cell recognition, lubrication, and immune signaling.
19. (4.10) The ABO blood group antigens are defined by:
A. The ABO antigens differ only in a single terminal sugar. Type A adds GalNAc to the H antigen; Type B adds galactose; Type O has just the H antigen (no additional sugar). AB individuals have both A and B antigens. This is a textbook example of how a one-sugar change creates a biologically meaningful signal.
20. (4.10) A person with type O blood can safely donate to all blood types because:
D. Type O red cells display only the base H antigen - no A or B added. Any recipient's pre-existing anti-A or anti-B antibodies have no target, so no agglutination. Universal donor = universal red-cell donor (plasma is a different story; type AB plasma lacks anti-A and anti-B).
Sugar is not one molecule. It is a family of thousands of molecules, built by snapping together simple monosaccharides in every possible combination. Your body uses them for quick energy (glucose), long-term energy storage (glycogen), structural reinforcement (cellulose in plants, chitin in insects), cell recognition (blood group antigens), and signaling (glycoproteins on every cell surface).
The MCAT focuses on recognizing sugar structures and linkages: aldoses vs. ketoses, D vs. L, alpha vs. beta, Fischer vs. Haworth. Master the vocabulary first, and individual sugars become easy.
Sugar Cubes
Keep in mind that monosaccharides are the bricks. Disaccharides are two bricks cemented together. Polysaccharides are whole walls or towers of bricks. The cement between bricks is the glycosidic bond. The kind of cement (alpha-1,4 vs. beta-1,4 vs. alpha-1,6) decides what kind of structure the bricks form - a digestible helix or an indigestible flat sheet.
In This Chapter
- 4.1 Monosaccharide Classification
- 4.2 Fischer and Haworth Projections
- 4.3 Sugar Stereochemistry
- 4.4 Ring Formation and Mutarotation
- 4.5 Important Monosaccharides
- 4.6 Disaccharides
- 4.7 Glycosidic Bonds
- 4.8 Polysaccharides
- 4.9 Glycoproteins and Glycolipids
- 4.10 Cell Signaling and Blood Typing
- Section Test