Biological molecules MCQs
MDCAT · Biology · 8 questions
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PMDC MDCAT syllabus: Biological molecules
Four groups of molecules do almost everything in a cell. Carbohydrates: monosaccharides (glucose, fructose, galactose) join by glycosidic bonds into disaccharides (sucrose = glucose + fructose, lactose = glucose + galactose, maltose = glucose + glucose) and polysaccharides (starch in plants, glycogen in animals and fungi, cellulose in plant walls, the most abundant organic compound on Earth). Lipids: a triglyceride is one glycerol plus three fatty acids joined by ester bonds; unsaturated fatty acids have C=C double bonds and form oils. Proteins: amino acids join by peptide bonds; the α-helix and β-sheet (secondary structure) are held by hydrogen bonds, while tertiary structure also uses ionic bonds and disulphide bridges. Nucleic acids: nucleotides join by phosphodiester bonds; DNA has thymine where RNA has uracil. Most questions ask which bond joins which units, so learn the bond names as pairs.
Questions with answers
Written by Parcha from the syllabus. Samples until a subject reviewer checks them; report anything wrong from the practice screen.
Q1 · Parcha original · Sample
Amino acids in a protein are joined by:
- A. Glycosidic bonds
- B. Peptide bonds
- C. Ester bonds
- D. Phosphodiester bonds
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Answer: B. Peptide bonds
A peptide bond forms between the carboxyl group of one amino acid and the amino group of the next, releasing a water molecule (condensation). Glycosidic bonds join sugars, ester bonds join fatty acids to glycerol, and phosphodiester bonds join nucleotides.
Open this question on its own pageQ2 · Parcha original · Sample
Sucrose is made of which two monosaccharides?
- A. Glucose and galactose
- B. Glucose and glucose
- C. Glucose and fructose
- D. Fructose and galactose
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Answer: C. Glucose and fructose
Sucrose (table sugar) is glucose joined to fructose by a glycosidic bond. Lactose is glucose + galactose, and maltose is glucose + glucose.
Q3 · Parcha original · Sample
The most abundant organic compound on Earth is:
- A. Cellulose
- B. Glycogen
- C. Starch
- D. Chitin
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Answer: A. Cellulose
Cellulose, the main material of plant cell walls, is the most abundant organic compound on Earth. It is a polymer of β-glucose. Starch and glycogen are storage polysaccharides; chitin is found in fungal walls and insect skeletons.
Q4 · Parcha original · Sample
One triglyceride molecule is formed from one glycerol and how many fatty acids?
- A. One
- B. Two
- C. Four
- D. Three
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Answer: D. Three
Each of glycerol's three –OH groups forms an ester bond with one fatty acid, so a triglyceride has three fatty acids and three water molecules are released.
Q5 · Parcha original · Sample
Which nitrogenous base is found in RNA but not in DNA?
- A. Thymine
- B. Uracil
- C. Adenine
- D. Cytosine
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Answer: B. Uracil
RNA uses uracil where DNA uses thymine. Adenine, guanine and cytosine are found in both.
Q6 · Parcha original · Sample
The α-helix of a protein's secondary structure is held mainly by:
- A. Disulphide bridges
- B. Ionic bonds
- C. Hydrogen bonds
- D. Peptide bonds
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Answer: C. Hydrogen bonds
In an α-helix, the C=O of one peptide bond forms a hydrogen bond with the N–H four amino acids further along. Disulphide and ionic bonds matter more in tertiary structure; peptide bonds make the primary chain itself.
Q7 · Parcha original · Sample
An unsaturated fatty acid contains:
- A. At least one C=C double bond
- B. Only single C–C bonds
- C. A phosphate group
- D. Nitrogen in its chain
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Answer: A. At least one C=C double bond
Unsaturated fatty acids have one or more carbon–carbon double bonds, which put kinks in the chain and keep such fats liquid at room temperature (oils). Saturated fatty acids have only single bonds.
Q8 · Parcha original · Sample
Glycogen is the main storage carbohydrate in:
- A. Plants
- B. Animals and fungi
- C. Bacterial cell walls
- D. Insect exoskeletons
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Answer: B. Animals and fungi
Animals (in liver and muscle) and fungi store glucose as glycogen, a highly branched polymer of α-glucose. Plants store starch. Bacterial walls are made of peptidoglycan, and insect skeletons of chitin.