A-Level Biology Key Concepts

Question 1

Which organelle is the site of aerobic respiration in eukaryotic cells?

Accepted Answer

Mitochondrion

Answer

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?

Accepted Answer

Splitting water molecules to release electrons, protons, and oxygen

Answer

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?

Accepted Answer

Unwinding the double helix by breaking hydrogen bonds between base pairs

Answer

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?

Accepted Answer

Co-transport (secondary active transport) with sodium ions

Answer

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?

Accepted Answer

It carries specific amino acids to the ribosome and has an anticodon that pairs with mRNA codons

Answer

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.

A-Level Revision Practice Test

A-Level Revision Practice Test

A-Level Biology Key Concepts