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Carbohydrates and Biomolecules Essentials
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1
Question
What is the general definition of carbohydrates as polyhydroxy compounds?
Answer
Carbohydrates are polyhydroxy aldehydes, ketones, or substances that produce such units on hydrolysis, and they are optically active.
2
Question
What is the molecular formula of glucose and fructose?
Answer
\(C_6H_{12}O_6\)
3
Question
Classify carbohydrates based on hydrolysis: monosaccharides, oligosaccharides, and polysaccharides.
Answer
Monosaccharides: cannot be hydrolyzed further (e.g., glucose); Oligosaccharides: hydrolyze to 2-10 monosaccharides (e.g., sucrose); Polysaccharides: hydrolyze to more than 10 monosaccharides (e.g., starch).
4
Question
What are reducing sugars? Give examples.
Answer
Reducing sugars have a free aldehyde or ketone group that can reduce Tollens' or Fehling's reagent. Examples: All monosaccharides and most disaccharides except sucrose; polysaccharides are non-reducing.
5
Question
Why is sucrose a non-reducing sugar?
Answer
Sucrose has no free aldehyde or ketone group; its anomeric carbons are involved in the glycosidic linkage between glucose and fructose.
6
Question
What is the open-chain structure of D-glucose?
Answer
A straight-chain aldose with six carbons: aldehyde at C1, hydroxyls on C2-C6, with specific configuration at chiral centers.
7
Question
How many chiral carbons are in D-glucose?
Answer
4
8
Question
What is the cyclic structure of glucose called, and why?
Answer
Pyranose form; it resembles a six-membered pyran ring with five carbons and one oxygen.
9
Question
Differentiate between α-D-glucose and β-D-glucose.
Answer
In α-D-glucose, the OH on C1 is below the ring plane; in β-D-glucose, it is above the ring plane in the Haworth projection.
10
Question
What is mutarotation?
Answer
The change in optical rotation of α- and β-anomers of glucose to an equilibrium mixture due to ring opening and closing.
11
Question
What is inversion of sugar?
Answer
Hydrolysis of sucrose (dextrorotatory) yields glucose and fructose, resulting in a levorotatory mixture.
12
Question
What is the structure of fructose?
Answer
A ketohexose with ketone at C2 in open chain; forms furanose or pyranose rings.
13
Question
What is a glycosidic linkage?
Answer
The ether linkage (C-O-C) formed between the anomeric carbon of one monosaccharide and a hydroxyl of another.
14
Question
Why is maltose a reducing sugar?
Answer
Maltose has one free anomeric carbon (reducing end) from its two glucose units linked by α-1,4 glycosidic bond.
15
Question
Differentiate between amylose and amylopectin in starch.
Answer
Amylose: linear chain of α-1,4 linked glucose (15-20%); Amylopectin: branched with α-1,6 linkages (80-85%).
16
Question
What is the main difference between starch and cellulose?
Answer
Starch has α-1,4 linkages (digestible); cellulose has β-1,4 linkages (indigestible by humans).
17
Question
What is glycogen, and where is it found?
Answer
Branched polymer of glucose (animal starch) stored in liver, muscles, and brain.
18
Question
What are epimers? Give an example.
Answer
Stereoisomers differing in configuration at only one chiral carbon. Example: Glucose and mannose at C2.
19
Question
What is the Barfoed's test used for?
Answer
To distinguish monosaccharides from reducing disaccharides; monosaccharides give red Cu2O precipitate quickly.
20
Question
What is the Seliwanoff's test?
Answer
Test for ketoses (e.g., fructose); forms cherry red complex with resorcinol in acid.
21
Question
What is the Molisch test?
Answer
General test for carbohydrates; violet ring at junction of H2SO4 and α-naphthol in alcohol.
22
Question
What is Lobry de Bruyn–van Ekenstein transformation?
Answer
Isomerization of aldoses and ketoses (e.g., glucose ↔ fructose) in alkaline medium via enediol intermediate.
23
Question
What is osazone formation?
Answer
Glucose reacts with three moles of phenylhydrazine to form phenylhydrazone at C1 and C2.
24
Question
What is the structure of an α-amino acid?
Answer
H2N-CH(R)-COOH, where R is the side chain; α-carbon is chiral except in glycine.
25
Question
Why is glycine optically inactive?
Answer
Glycine has H as side chain (R=H), so α-carbon has two identical H atoms, no chirality.
26
Question
Classify amino acids: essential vs. non-essential.
Answer
Essential: Cannot be synthesized (e.g., valine, leucine); Non-essential: Synthesized in body (e.g., glycine, alanine).
27
Question
Classify amino acids by nature: acidic, basic, neutral.
Answer
Acidic: More COOH (e.g., aspartic acid); Basic: More NH2 (e.g., lysine); Neutral: Equal (e.g., glycine).
28
Question
What is a zwitterion?
Answer
The dipolar form of amino acid with -COO⁻ and -NH3⁺ groups.
29
Question
What is the isoelectric point (pI)?
Answer
pH at which amino acid has no net charge and does not migrate in electric field.
30
Question
What is a peptide bond?
Answer
Amide linkage (-CO-NH-) between carboxyl of one amino acid and amino of another.