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Biological molecules and enzymes
8 MDCAT Biology questions with answers and explanations.
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A peptide bond joins:
- 1 Two glucose molecules
- 2 Two amino acids
- 3 Two nucleotides
- 4 A fatty acid and glycerol
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Correct answer: Two amino acids
A peptide bond forms between the carboxyl group of one amino acid and the amino group of the next, releasing water.
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Which of the following is a disaccharide?
- 1 Glucose
- 2 Fructose
- 3 Sucrose
- 4 Ribose
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Correct answer: Sucrose
Sucrose is made of glucose and fructose. Glucose, fructose and ribose are monosaccharides.
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Enzymes speed up reactions by:
- 1 Raising the temperature of the reaction
- 2 Lowering the activation energy
- 3 Increasing the amount of substrate
- 4 Changing the final products
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Correct answer: Lowering the activation energy
An enzyme gives the reaction an easier path with a lower activation energy. It is not used up and does not change the products.
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Which lipid is the main structural component of the plasma membrane?
- 1 Triglycerides
- 2 Waxes
- 3 Steroids only
- 4 Phospholipids
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Correct answer: Phospholipids
Phospholipids form the bilayer of the membrane, with water-loving heads facing outward and fatty tails facing inward.
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Cellulose is a polymer of:
- 1 Alpha glucose
- 2 Beta glucose
- 3 Fructose
- 4 Amino acids
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Correct answer: Beta glucose
Cellulose is made of beta-glucose units joined by beta 1,4 glycosidic bonds. Starch and glycogen use alpha glucose.
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The primary structure of a protein is the:
- 1 Alpha helix
- 2 Folding into a 3D shape
- 3 Sequence of amino acids
- 4 Joining of several polypeptide chains
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Correct answer: Sequence of amino acids
The primary structure is the order of amino acids in the chain. Helices and sheets are secondary structure, and the 3D fold is tertiary.
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Nucleotides in one strand of DNA are joined by:
- 1 Hydrogen bonds
- 2 Peptide bonds
- 3 Phosphodiester bonds
- 4 Glycosidic bonds between sugars only
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Correct answer: Phosphodiester bonds
A phosphodiester bond links the phosphate of one nucleotide to the sugar of the next, forming the backbone. Hydrogen bonds hold the two strands together.
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The active site of an enzyme is the region where:
- 1 The enzyme is made
- 2 The substrate binds
- 3 The enzyme is stored
- 4 The coenzyme is destroyed
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Correct answer: The substrate binds
The substrate binds to the active site, where the reaction is catalysed. Its shape fits the substrate.