Casein separates out of milk when the milk is acidified to about pH 4.6. The reason is that at this pH casein:

Has no net charge and is least soluble

Has its disulphide bonds broken by acid

Is hydrolysed into free amino acids

Is salted out by the high ionic strength

A 3-week-old girl has had severe watery diarrhoea since birth on breast milk and on glucose-based oral rehydration solution. Her stool is acidic and positive for reducing substances. The diarrhoea stops on a formula whose only carbohydrate is fructose. Which transporter is defective?

Sodium-glucose cotransporter SGLT1

Chemistry of biomolecules and cell membranes

41 Kerala MSc MLT Entrance practice questions on Chemistry of biomolecules and cell membranes, part of Biochemistry. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A fat sample from a Kerala market gives: saponification number 255, iodine number 9, Reichert-Meissl number 7. Which fat fits these values best?
1. ACoconut oil
2. BGhee (butter fat)
3. CSunflower oil
4. DGroundnut oil
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Answer: A · Coconut oil
A very high saponification number means short average chain length, and a very low iodine number means highly saturated fatty acids. Coconut oil, rich in lauric and other medium-chain saturated acids, has a saponification number of about 250-260 and an iodine number below 12. Ghee also has short-chain acids but a much higher Reichert-Meissl number (about 25-35) from butyric acid, and an iodine number near 30. Sunflower and groundnut oils have saponification numbers near 190 and high iodine numbers.
- B: Ghee would show a Reichert-Meissl number above 20 and an iodine number near 30.
- C: Sunflower oil is highly unsaturated, with an iodine number above 120.
- D: Groundnut oil has an iodine number near 90 and a saponification number near 190.
Question 2 · Hard
The saponification number is the mg of KOH (molar mass 56.1) needed to saponify 1 g of fat. What is the saponification number of pure tripalmitin (molar mass 807)?
1. A149
2. B219
3. C209
4. D70
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Answer: C · 209
One mole of a triacylglycerol releases three fatty acids, so it needs three moles of KOH. 1 g of tripalmitin is 1/807 mol and needs 3/807 x 56.1 g = 0.2085 g, or 209 mg of KOH. Fats with shorter chains have a smaller molar mass and so a higher saponification number. This is why coconut oil has a higher value than most seed oils.
- A: 149 uses the molar mass of NaOH (40) instead of KOH.
- B: 219 uses the molar mass of one palmitic acid (256) instead of the whole triacylglycerol.
- D: 70 forgets that one triacylglycerol takes three molecules of KOH.
Question 3 · Medium
Casein separates out of milk when the milk is acidified to about pH 4.6. The reason is that at this pH casein:
1. AHas its disulphide bonds broken by acid
2. BHas no net charge and is least soluble
3. CIs hydrolysed into free amino acids
4. DIs salted out by the high ionic strength
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Answer: B · Has no net charge and is least soluble
pH 4.6 is the isoelectric point of casein. At this pH positive and negative charges balance, so the molecules no longer repel each other and they aggregate and precipitate. Proteins are least soluble at their isoelectric pH. This is used to separate proteins by isoelectric precipitation.
- A: Dilute acid does not break disulphide bonds; reducing agents do.
- C: Mild acid at room temperature does not hydrolyse peptide bonds.
- D: Salting out needs high concentrations of salts such as ammonium sulphate.
Question 4 · Medium
The Sakaguchi reaction, with alpha-naphthol and alkaline hypobromite, gives a red colour with arginine. Which group of arginine is responsible?
1. AImidazole group
2. BIndole group
3. CPhenolic group
4. DGuanidino group
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Answer: D · Guanidino group
The Sakaguchi test is specific for the guanidino group, which in proteins is found only in arginine. Alpha-naphthol and hypobromite react with it in alkali to give a red product. Each side-chain colour test picks out one group: Pauly for imidazole (histidine), Hopkins-Cole for indole (tryptophan) and Millon for phenol (tyrosine). These tests identify amino acids in practical classes.
- A: The imidazole group of histidine gives Pauly's test.
- B: The indole group of tryptophan gives the Hopkins-Cole (glyoxylic acid) test.
- C: The phenolic group of tyrosine gives Millon's test.
Question 5 · Hard
A 3-week-old girl has had severe watery diarrhoea since birth on breast milk and on glucose-based oral rehydration solution. Her stool is acidic and positive for reducing substances. The diarrhoea stops on a formula whose only carbohydrate is fructose. Which transporter is defective?
1. ABrush border enzyme lactase
2. BSodium-glucose cotransporter SGLT1
3. CFacilitative transporter GLUT2
4. DFacilitative transporter GLUT5
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Answer: B · Sodium-glucose cotransporter SGLT1
SGLT1 in the intestinal brush border takes up glucose and galactose together with sodium, by secondary active transport. When it fails, both sugars stay in the gut lumen, causing osmotic diarrhoea, and bacteria ferment them to acids. Fructose enters by GLUT5 and leaves the cell by GLUT2, so it is absorbed normally. Lactase deficiency would not cause diarrhoea with glucose solution.
- A: Lactase deficiency would not explain diarrhoea on glucose-based solution.
- C: GLUT2 also carries fructose out of the enterocyte, so a GLUT2 defect would not spare fructose.
- D: GLUT5 carries fructose, which this baby absorbs normally.
Question 6 · Medium
The Na+/K+-ATPase is described as electrogenic. This is because each cycle:
1. AMoves equal numbers of Na+ and K+
2. BMoves three Na+ out and two K+ in
3. CMoves two Na+ out and three K+ in
4. DMoves Na+ and K+ in the same direction
Answer and explanation in LearnFree account
Question 7 · Easy
The sugar-phosphate backbone of a DNA strand is held together by which linkage between successive nucleotides?
1. A3',5'-phosphodiester bond
2. BN-glycosidic bond
3. CHydrogen bond
4. DPeptide bond
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Question 8 · Easy
Which amino acid has no chiral carbon and is therefore optically inactive?
1. ASerine
2. BAlanine
3. CGlycine
4. DProline
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Question 9 · Easy
What is the smallest peptide that gives a positive biuret reaction?
1. AHexapeptide
2. BTetrapeptide
3. CDipeptide
4. DTripeptide
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Question 10 · Hard
An unknown sugar solution gives: Molisch positive; Benedict positive; Barfoed red precipitate within 2 minutes; Seliwanoff no red colour within 1 minute; Bial negative; osazone needle-shaped crystals in about 5 minutes. The sugar is:
1. AFructose
2. BLactose
3. CMaltose
4. DGlucose
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Question 11 · Medium
A vegetable oil has a high iodine number. This shows that its fatty acids:
1. AHave a short average chain length
2. BInclude many volatile, water-soluble acids
3. CAre largely present in the free, unesterified form
4. DContain many carbon-carbon double bonds
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Question 12 · Medium
Four double-stranded DNA samples are analysed. Which one will have the highest melting temperature (Tm)?
1. AAdenine 15% of bases
2. BAdenine 25% of bases
3. CThymine 30% of bases
4. DAdenine 35% of bases
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Question 13 · Easy
Trans fatty acids, formed when vegetable oils are partly hydrogenated to make vanaspati and bakery shortenings, are harmful to the heart mainly because they:
1. ARaise LDL cholesterol and lower HDL cholesterol
2. BAre essential fatty acids the body cannot make
3. CLower plasma triglycerides like fish oils
4. DAre oxidised quickly to ketone bodies
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Question 14 · Hard
Serum is mixed with an equal volume of saturated ammonium sulphate solution, so the mixture is half-saturated, and then centrifuged. The supernatant gives a strong biuret reaction. Which protein makes up most of the protein left in the supernatant?
1. AHaptoglobin
2. BImmunoglobulin M
3. CAlbumin
4. DImmunoglobulin G
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Question 15 · Medium
A double-stranded DNA sample contains 30% adenine. What is its guanine content?
1. A70%
2. B30%
3. C40%
4. D20%
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Question 16 · Medium
When double-stranded DNA is heated and the strands separate, its absorbance at 260 nm rises. This hyperchromic effect is due to:
1. ABreaking of the phosphodiester bonds
2. BRelease of the deoxyribose from the strand
3. CLoss of stacking between the bases
4. DOxidation of the purine bases by heat
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Question 17 · Medium
In Seliwanoff's test, fructose gives a cherry-red colour within a minute while glucose does not. The reason is that ketohexoses:
1. AAre hydrolysed to glucose by the hot acid
2. BReduce cupric ions more strongly than aldoses
3. CAre dehydrated to hydroxymethylfurfural much faster
4. DForm furfural, which reacts only with resorcinol
Answer and explanation in LearnFree account
Question 18 · Hard
A healthy 30-year-old man has a positive Benedict's test on routine urine screening, but the glucose oxidase strip is negative. Fasting plasma glucose is 88 mg/dL. Further urine tests: Seliwanoff negative; Bial's orcinol test gives a green colour. What is the most likely condition?
1. AGalactosaemia
2. BRenal glycosuria
3. CEssential fructosuria
4. DEssential pentosuria
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Question 19 · Easy
With ninhydrin, most amino acids give a purple colour. Which one gives a yellow colour instead?
1. AProline
2. BLysine
3. CHistidine
4. DGlycine
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Question 20 · Easy
The Liebermann-Burchard reaction, with acetic anhydride and sulphuric acid giving a green colour, detects:
1. AFree fatty acids
2. BPhospholipids
3. CGlycerol
4. DCholesterol
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Question 21 · Medium
Amylose gives a deep blue colour with iodine, while glycogen gives only a red-brown colour. Amylose gives the blue colour because:
1. AIt is made of beta-glucose units joined by 1-4 bonds
2. BIt has a free reducing end that reacts with iodine
3. CIts many branch points bind iodine at the 1-6 links
4. DIts long unbranched helical chains hold iodine inside the helix
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Question 22 · Easy
Millon's test gives a red precipitate with a protein because of which amino acid?
1. ATryptophan
2. BTyrosine
3. CArginine
4. DCysteine
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Question 23 · Medium
A child with acute watery diarrhoea is given oral rehydration solution containing both glucose and sodium. Glucose is included in the solution because:
1. AIt blocks the chloride channel opened by the cholera toxin
2. BIt is needed to make the Na+/K+-ATPase pump work faster
3. CIt raises the osmolality so that water is held in the gut
4. DIt drives sodium uptake through the sodium-glucose cotransporter
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Question 24 · Medium
Glycine occupies every third position in the collagen chain. Why is glycine needed there?
1. AIt carries the sugar residues attached to collagen
2. BIts side chain is hydroxylated to cross-link the chains
3. CIts charge holds the three chains together by ionic bonds
4. DOnly its small side chain fits at the centre of the triple helix
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Question 25 · Hard
A mixture of aspartate (pI 2.8), glycine (pI 6.0), histidine (pI 7.6) and lysine (pI 9.7) is applied at the centre of a paper strip and electrophoresed in a buffer of pH 6.0. Which amino acid moves towards the anode?
1. AAspartate
2. BGlycine
3. CLysine
4. DHistidine
Answer and explanation in LearnFree account

16 more Chemistry of biomolecules and cell membranes questions

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Chemistry of biomolecules and cell membranes

A fat sample from a Kerala market gives: saponification number 255, iodine number 9, Reichert-Meissl number 7. Which fat fits these values best?

The saponification number is the mg of KOH (molar mass 56.1) needed to saponify 1 g of fat. What is the saponification number of pure tripalmitin (molar mass 807)?

Casein separates out of milk when the milk is acidified to about pH 4.6. The reason is that at this pH casein:

The Sakaguchi reaction, with alpha-naphthol and alkaline hypobromite, gives a red colour with arginine. Which group of arginine is responsible?

A 3-week-old girl has had severe watery diarrhoea since birth on breast milk and on glucose-based oral rehydration solution. Her stool is acidic and positive for reducing substances. The diarrhoea stops on a formula whose only carbohydrate is fructose. Which transporter is defective?

The Na+/K+-ATPase is described as electrogenic. This is because each cycle:

The sugar-phosphate backbone of a DNA strand is held together by which linkage between successive nucleotides?

Which amino acid has no chiral carbon and is therefore optically inactive?

What is the smallest peptide that gives a positive biuret reaction?

An unknown sugar solution gives: Molisch positive; Benedict positive; Barfoed red precipitate within 2 minutes; Seliwanoff no red colour within 1 minute; Bial negative; osazone needle-shaped crystals in about 5 minutes. The sugar is:

A vegetable oil has a high iodine number. This shows that its fatty acids:

Four double-stranded DNA samples are analysed. Which one will have the highest melting temperature (Tm)?

Trans fatty acids, formed when vegetable oils are partly hydrogenated to make vanaspati and bakery shortenings, are harmful to the heart mainly because they:

Serum is mixed with an equal volume of saturated ammonium sulphate solution, so the mixture is half-saturated, and then centrifuged. The supernatant gives a strong biuret reaction. Which protein makes up most of the protein left in the supernatant?

A double-stranded DNA sample contains 30% adenine. What is its guanine content?

When double-stranded DNA is heated and the strands separate, its absorbance at 260 nm rises. This hyperchromic effect is due to:

In Seliwanoff's test, fructose gives a cherry-red colour within a minute while glucose does not. The reason is that ketohexoses:

A healthy 30-year-old man has a positive Benedict's test on routine urine screening, but the glucose oxidase strip is negative. Fasting plasma glucose is 88 mg/dL. Further urine tests: Seliwanoff negative; Bial's orcinol test gives a green colour. What is the most likely condition?

With ninhydrin, most amino acids give a purple colour. Which one gives a yellow colour instead?

The Liebermann-Burchard reaction, with acetic anhydride and sulphuric acid giving a green colour, detects:

Amylose gives a deep blue colour with iodine, while glycogen gives only a red-brown colour. Amylose gives the blue colour because:

Millon's test gives a red precipitate with a protein because of which amino acid?

A child with acute watery diarrhoea is given oral rehydration solution containing both glucose and sodium. Glucose is included in the solution because:

Glycine occupies every third position in the collagen chain. Why is glycine needed there?

A mixture of aspartate (pI 2.8), glycine (pI 6.0), histidine (pI 7.6) and lysine (pI 9.7) is applied at the centre of a paper strip and electrophoresed in a buffer of pH 6.0. Which amino acid moves towards the anode?