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A-Level Biology Practice Questions

Advanced practice questions covering all A-Level Biology topics. Each question includes detailed step-by-step solutions.

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Question 1
Biological Molecules AS Level

Name the monomers that make up proteins.

Solution:
  1. Proteins are polymers (large molecules)
  2. They are made from smaller repeating units called monomers
  3. These monomers contain an amino group (-NH₂) and carboxyl group (-COOH)
  4. They join together by peptide bonds in condensation reactions
Answer: Amino acids
Question 2
Cell Structure A2 Level

Explain the role of ribosomes in protein synthesis.

Solution:
  1. Ribosomes are the site of translation
  2. mRNA (messenger RNA) binds to the ribosome
  3. tRNA (transfer RNA) brings amino acids to the ribosome
  4. Ribosome reads mRNA codons (3 bases)
  5. tRNA anticodons pair with mRNA codons
  6. Amino acids join by peptide bonds
  7. Polypeptide chain is formed
Answer: Ribosomes read mRNA codons, facilitate tRNA binding, catalyze peptide bond formation between amino acids during translation
Question 3
Carbohydrates AS Level

What type of bond joins two monosaccharides together?

Solution:
  1. Two monosaccharides join in a condensation reaction
  2. A water molecule is removed (H from one, OH from other)
  3. An oxygen bridge forms between the two sugars
  4. This creates a disaccharide
  5. Example: Glucose + Glucose → Maltose + Water
Answer: Glycosidic bond
Question 4
DNA & RNA A2 Level

State three differences between DNA and RNA.

Solution:
  1. Structure:
    • DNA: Double-stranded (double helix)
    • RNA: Single-stranded
  2. Sugar:
    • DNA: Deoxyribose sugar
    • RNA: Ribose sugar
  3. Bases:
    • DNA: A, T, C, G (thymine)
    • RNA: A, U, C, G (uracil instead of thymine)
  4. Also: DNA is longer, RNA is shorter
Answer: 1) DNA double-stranded, RNA single-stranded 2) DNA has deoxyribose, RNA has ribose 3) DNA has thymine, RNA has uracil
Question 5
Cell Membranes Challenge

Explain how temperature affects membrane permeability.

Solution:
  1. Low temperature (below 0°C):
    • Phospholipids have little energy, don't move much
    • Membrane rigid, channel proteins denature
    • Permeability low
  2. Moderate temperature (0-45°C):
    • Phospholipids can move
    • Membrane partially permeable
    • Permeability increases with temperature
  3. High temperature (above 45°C):
    • Phospholipid bilayer breaks down
    • Proteins denature
    • Membrane becomes very permeable/loses structure
Answer: Low temp = rigid/low permeability; Moderate = partial permeability increases with temp; High temp = membrane breaks down/very permeable
Question 6
Gene Expression A2 Level

Define a gene mutation.

Solution:
  1. A gene mutation is a change in the base sequence of DNA
  2. Types of mutations:
    • Substitution: One base replaced by another
    • Deletion: One or more bases removed
    • Insertion: One or more bases added
  3. Can result in different amino acid sequence
  4. May affect protein structure and function
Answer: A change in the base sequence of DNA (substitution, deletion, or insertion)
Question 7
Ecosystems Challenge

Describe the role of decomposers in the nitrogen cycle.

Solution:
  1. Decomposition:
    • Decomposers (bacteria, fungi) break down dead organisms
    • Also break down urea and other waste products
  2. Ammonification:
    • Proteins and urea converted to ammonia (NH₃)
    • Ammonia forms ammonium ions (NH₄⁺) in soil
  3. Nitrification:
    • Nitrifying bacteria convert NH₄⁺ to NO₂⁻ (nitrite)
    • Then NO₂⁻ to NO₃⁻ (nitrate)
    • Nitrates can be absorbed by plant roots
Answer: Decomposers break down proteins/urea into ammonia (ammonification), which is then converted to nitrates by nitrifying bacteria
Question 8
Respiration Challenge

Calculate the net ATP yield from one molecule of glucose in aerobic respiration.

Solution:
  1. Glycolysis (cytoplasm):
    • Produces: 4 ATP
    • Uses: 2 ATP
    • Net: 2 ATP
  2. Krebs cycle (mitochondrial matrix):
    • 2 turns per glucose
    • Produces: 2 ATP
  3. Oxidative phosphorylation (cristae):
    • Reduced NAD: 10 × 2.5 ATP = 25 ATP
    • Reduced FAD: 2 × 1.5 ATP = 3 ATP
    • Subtotal: 28 ATP
  4. Total: 2 + 2 + 28 = 32 ATP (approximately)
  5. Note: Some sources give 30-32 ATP depending on shuttle system
Answer: Approximately 32 ATP (or 30-32 ATP)
Question 9
Evolution Challenge

Explain how natural selection leads to evolution.

Solution:
  1. Variation: Random mutations create genetic variation in population
  2. Selection pressure: Environmental factors (predators, disease, competition)
  3. Differential survival: Individuals with advantageous alleles more likely to survive
  4. Reproduction: Survivors reproduce and pass on advantageous alleles
  5. Increased frequency: Over generations, frequency of advantageous alleles increases
  6. Evolution: Change in allele frequency = evolution of population
Answer: Variation from mutations → selection pressure → differential survival → advantageous alleles passed on → allele frequency changes over time
Question 10
Enzymes A2 Level

Explain how competitive inhibitors affect enzyme activity.

Solution:
  1. Competitive inhibitors have similar shape to substrate
  2. They compete with substrate for active site
  3. When inhibitor binds to active site, substrate cannot bind
  4. Fewer enzyme-substrate complexes form
  5. Rate of reaction decreases
  6. Effect can be overcome by increasing substrate concentration
  7. Does NOT permanently damage enzyme
Answer: Inhibitor competes with substrate for active site, reduces enzyme-substrate complex formation, decreases reaction rate (can be overcome by ↑ substrate)