A Level H2 Biology Practice Paper 3

TuitionGoWhere Exam Practice (AI) - Biology H2 A-Level

Answer Key & Marking Scheme Paper: Practice Paper 3 (Version 3 of 5)
Subject: Biology H2
Topic: Cells & Biomolecules

Section A: Structured Questions

1. Membrane Structure and Transport (a)

• A: Channel protein / Protein channel [1]
• B: Glycoprotein [1]
• C: Cholesterol [1]

(b)

• Cholesterol fits between phospholipid tails. [1]
• At high temperatures, it restricts the movement of phospholipids, reducing membrane fluidity/preventing it from becoming too fluid. [1]
• At low temperatures, it prevents phospholipids from packing too closely, maintaining fluidity/preventing crystallisation. [1] (Note: Award marks for either high or low temp explanation, but max 3 marks requires both aspects or detailed mechanism of one).

(c)

• Substance X (non-polar): Diffuses directly through the phospholipid bilayer. [1]
• It moves down its concentration gradient. [1]
• Substance Y (polar ion): Cannot pass through the hydrophobic core. [1]
• It requires a carrier protein or channel protein (facilitated diffusion) or active transport. [1]

2. Enzymes (a)

• A region on the enzyme surface. [1]
• With a specific shape/complementary to the substrate. [1]

(b)

• Non-competitive inhibition. [1]
• Vmax is reduced (lower maximum rate). [1]
• The inhibitor binds to an allosteric site, changing the shape of the active site so substrate cannot bind effectively, regardless of substrate concentration. [1]

(c)

• All active sites are saturated with substrate. [1]
• The enzyme is working at its maximum rate; adding more substrate cannot increase the rate further as there are no free active sites. [1]

3. Mitochondria (a)

• Oxidative phosphorylation / Electron Transport Chain / Chemiosmosis. [1]

(b)

• Electrons move down the electron transport chain, releasing energy. [1]
• This energy is used to pump protons (H+) from the matrix into the intermembrane space. [1]
• This creates an electrochemical/proton gradient. [1]
• Protons flow back into the matrix through ATP synthase, driving the synthesis of ATP from ADP and Pi. [1]

(c)

• Oxygen consumption will stop / decrease to zero. [1]
• Oxygen is the final electron acceptor in the electron transport chain. [1]
• If the chain is blocked, electrons cannot flow, so oxygen cannot accept electrons to form water. [1]

4. Nucleic Acids (a)

• Sugar: DNA = Deoxyribose; RNA = Ribose. [1]
• Bases: DNA = A, T, C, G; RNA = A, U, C, G. [1]
• Strands: DNA = Double-stranded (helix); RNA = Single-stranded. [1]

(b)

• DNA helicase breaks hydrogen bonds between bases, unzipping the helix. [1]
• Each original strand acts as a template. [1]
• Free nucleotides pair with complementary bases on the template strands (A-T, C-G). [1]
• DNA polymerase joins nucleotides via phosphodiester bonds to form new strands. [1] (Note: Mentioning "semi-conservative" means each new molecule has one old and one new strand is implied by the process description).

5. Proteins (a)

• Peptide bond. [1]

(b)

• The primary structure is the sequence of amino acids. [1]
• The R-groups (side chains) of the amino acids interact (hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions). [1]
• These interactions cause the polypeptide to fold into a specific 3D shape (tertiary structure). [1]

(c)

• High temperature breaks hydrogen bonds and other weak interactions holding the tertiary structure. [1]
• The enzyme denatures / loses its specific 3D shape. [1]
• The active site changes shape and is no longer complementary to the substrate, so no enzyme-substrate complexes form. [1]

Section B: Data Interpretation and Application

6. Gel Electrophoresis (a)

• DNA is negatively charged. [1]
• An electric field/potential difference is applied. [1]
• DNA fragments move towards the positive electrode (anode). Smaller fragments move faster/further through the gel matrix than larger fragments. [1]

(b)

• Child 1: Aa (Heterozygous) [1]
• Child 2: AA or aa (Homozygous - depending on which band is dominant, but usually the single band matches one parent. If Father is AA and Mother is aa, Child 2 is AA if it matches Father. Let's assume standard dominant/recessive logic: Child 1 has both, Child 2 has one. If disorder is recessive, and parents are carriers, this pattern is different. Based on prompt: Father 1 band, Mother 1 band (different), Child 1 both. This implies Codominance or simple allele tracking. Child 1 is Heterozygous. Child 2 is Homozygous for Father's allele). [1] (Accept: Homozygous for the allele represented by the single band).

(c)

• Child 1 has two bands. [1]
• This indicates the presence of two different alleles (one from each parent) which produce DNA fragments of different sizes/migration rates. [1]

7. Lipids (a)

• X: Glycerol [1]
• Y: Fatty acid [1]

(b)

• High energy content per gram (more than carbohydrates) due to many C-H bonds. [1]
• Insoluble in water, so they do not affect the water potential of cells/osmotic balance. [1]
• Compact storage / lightweight. [1]
• (Any two points, well explained).

(c)

• Phospholipids have a hydrophilic phosphate head and hydrophobic fatty acid tails. [1]
• In an aqueous environment, they arrange themselves into a bilayer. [1]
• Heads face outward towards water, tails face inward away from water, forming a barrier. [1]

8. Gene Regulation (a)

• A group of genes. [1]
• Controlled by a single promoter/operator region (expressed together). [1]

(b)

• The repressor protein binds to the operator region. [1]
• This prevents RNA polymerase from binding to the promoter. [1]
• Transcription of the structural genes (lacZ, lacY, lacA) is blocked/prevented. [1]

(c)

• Saves energy/resources. [1]
• Enzymes are only produced when the substrate (lactose) is available, preventing waste of amino acids/ATP. [1]

9. Osmosis (a)

• 0.3 mol dm⁻³. [1]
• At this concentration, there is no net change in mass, meaning the water potential of the solution is equal to the water potential of the potato cells (isotonic). [1]

(b)

• The sucrose solution has a lower water potential (more negative) than the potato cells. [1]
• Water moves out of the potato cells by osmosis. [1]
• This causes the cells to lose mass (and become flaccid/plasmolysed). [1]

10. Haemoglobin (a)

• It consists of four polypeptide chains (subunits). [1]
• Each chain is associated with a haem group. [1]

(b)

• Binding of the first oxygen molecule changes the shape of the haemoglobin (conformational change). [1]
• This makes it easier for subsequent oxygen molecules to bind to the remaining subunits. [1]
• This allows efficient loading of oxygen in the lungs (high pO2) and unloading in tissues (low pO2). [1]

Section C: Extended Response

11. Importance of Water

• Solvent: Polar nature allows it to dissolve ions and polar molecules, facilitating metabolic reactions and transport in blood/xylem. [2]
• High Specific Heat Capacity: Buffers temperature changes, providing a stable environment for enzymes and organisms. [2]
• High Latent Heat of Vaporisation: Evaporation removes large amounts of heat, enabling cooling mechanisms (sweating/transpiration). [2]
• Cohesion/Tension: Hydrogen bonding allows water columns to be pulled up xylem vessels (transpiration stream). [2]
• Metabolite: Used in hydrolysis reactions and photosynthesis. [1]
• Incompressible: Provides turgor pressure in plants and hydrostatic skeleton in some animals. [1] (Max 10 marks. Quality of explanation and linking property to function is key).

12. Structure and Function of Organelles

• Mitochondria:
  • Inner membrane folded into cristae: Increases surface area for electron transport chain enzymes/ATP synthase. [2]
  • Matrix contains enzymes for Krebs cycle and mitochondrial DNA/ribosomes for protein synthesis. [2]
  • Function: Aerobic respiration/ATP production. [1]
• Chloroplasts:
  • Thylakoids stacked into grana: Increases surface area for light-dependent reactions/photosystems. [2]
  • Stroma contains enzymes for Calvin cycle. [2]
  • Function: Photosynthesis. [1]
• (Alternative: Rough ER - ribosomes for protein synthesis; Golgi - cisternae for processing/packaging).
• Discussion: Link specific structural features (membranes, compartments, surface area) directly to the metabolic efficiency of the organelle. [2] (Max 10 marks. 5 marks per organelle if balanced, or 6/4. Must include specific structural details and functional links).