A-Level Biology Key Concepts — Questions and Answers
Question 1: Which organelle is the site of aerobic respiration in eukaryotic cells?
- Mitochondrion (Correct answer)
- Chloroplast
- Ribosome
- Golgi apparatus
Correct answer: Mitochondrion
The mitochondrion is where the Krebs cycle and oxidative phosphorylation occur, producing most ATP during aerobic respiration. The matrix and inner membrane (cristae) provide the necessary compartments.
Question 2: During the light-dependent reactions of photosynthesis, what is the role of photolysis?
- Splitting water molecules to release electrons, protons, and oxygen (Correct answer)
- Breaking down glucose into pyruvate
- Fixing carbon dioxide into organic molecules
- Reducing NADP to form NADPH without water
Correct answer: Splitting water molecules to release electrons, protons, and oxygen
Photolysis is the light-driven splitting of water (2H₂O → 4H⁺ + 4e⁻ + O₂). The electrons replace those lost from photosystem II, the protons reduce NADP⁺, and oxygen is released as a by-product.
Question 3: In DNA replication, what is the function of DNA helicase?
- Unwinding the double helix by breaking hydrogen bonds between base pairs (Correct answer)
- Joining Okazaki fragments on the lagging strand
- Adding complementary nucleotides to the template strand
- Proofreading newly synthesised DNA for errors
Correct answer: Unwinding the double helix by breaking hydrogen bonds between base pairs
DNA helicase unwinds the double helix by breaking the hydrogen bonds between complementary base pairs, creating two single-stranded templates at the replication fork.
Question 4: What type of transport moves glucose into epithelial cells of the small intestine against its concentration gradient?
- Co-transport (secondary active transport) with sodium ions (Correct answer)
- Simple diffusion
- Facilitated diffusion through channel proteins
- Endocytosis
Correct answer: Co-transport (secondary active transport) with sodium ions
Glucose is co-transported with Na⁺ ions via a sodium-glucose co-transporter (SGLT1). The sodium gradient, maintained by the Na⁺/K⁺ ATPase pump on the basolateral membrane, drives glucose uptake.
Question 5: Which statement correctly describes the role of tRNA in translation?
- It carries specific amino acids to the ribosome and has an anticodon that pairs with mRNA codons (Correct answer)
- It carries the genetic code from the nucleus to the cytoplasm
- It forms the structural component of ribosomes
- It catalyses the formation of peptide bonds between amino acids
Correct answer: It carries specific amino acids to the ribosome and has an anticodon that pairs with mRNA codons
Transfer RNA (tRNA) has a cloverleaf structure with an anticodon loop that base-pairs with mRNA codons, and an amino acid attachment site at its 3' end. Each tRNA is specific to one amino acid.
Question 6: In a food chain, approximately what percentage of energy is typically transferred from one trophic level to the next?
- 10% (Correct answer)
- 50%
- 90%
- 1%
Correct answer: 10%
Approximately 10% of energy transfers between trophic levels. The remaining ~90% is lost through respiration, heat, excretion, and egestion, limiting food chains to typically 4-5 levels.
A-Level Biology Key Concepts
Which organelle is the site of aerobic respiration in eukaryotic cells?