AQA A-Level Biology Paper 1, June 2024: Question 10

✅ Correct Marking Points

• Homogenisation: Blend/grind the tissue to break open cells and release organelles.
• Filtration: Filter the homogenate to remove large, unbroken tissue/cells and debris.
• Cold temperature: Keep the solution ice-cold to slow down enzyme activity and prevent self-digestion of organelles.
• Isotonic water potential: Ensure the solution has an equivalent water potential to prevent osmosis, which would otherwise cause organelles to shrink or burst.
• Buffered solution: Add a buffer to maintain constant pH and prevent denaturation of enzymes and proteins.
• Initial centrifugation: Centrifuge at a slow/low speed so that the heaviest organelle (nuclei) forms a pellet at the bottom.

💡 Key Knowledge

Cell fractionation isolates specific organelles based on density. The sequence of pelleting typically starts with nuclei (densest), followed by mitochondria/chloroplasts, and finally ribosomes/vesicles. Remember the golden trio of solution conditions: Cold, Buffered, Isotonic.

🧠 Exam Technique

Always state why a condition is used, not just what it is. Examiners look for the paired descriptive and explanatory statement (e.g., "cold to prevent enzyme activity"). Saying "isotonic to cells" is penalised because the cells are already broken; phrase it as "isotonic to prevent organelles bursting/shrinking."

❌ Common Errors

• Stating "cold to kill enzymes" (enzymes are not living organisms; use "slow down enzyme activity").
• Confusing water potential with concentration terms.
• Forgetting to mention filtration before ultracentrifugation.

✅ Correct Marking Points

• DNA role: DNA in the nucleus contains the genetic code / base sequence for the enzyme/protein.
• Ribosomes / RER: Ribosomes or rough endoplasmic reticulum translate the code to produce the protein/enzyme polypeptide chain.
• RER processing: The rough endoplasmic reticulum transports and modifies/processes the newly synthesized enzymes.
• Mitochondria: Mitochondria supply ATP required for protein synthesis, transport, and vesicle movement.
• Golgi apparatus: Golgi apparatus further modifies, packages, and sorts the enzymes into transport vesicles (or forms glycoproteins).
• Exocytosis: Vesicles fuse with the cell-surface membrane to release the enzymes out of the cell.

💡 Key Knowledge

Trace the secretory pathway sequentially: Nucleus ➔ RER ➔ Golgi ➔ Vesicle ➔ Cell Membrane. Never state that mitochondria "produce energy" (violates conservation of energy rules); instead, state they "produce ATP" or "release energy via aerobic respiration."

🧠 Exam Technique

The question explicitly states: "Do not include details of transcription in your answer." Avoid wasting time writing about mRNA synthesis or RNA polymerase; start directly from the genetic code instructing translation or ribosome function to secure immediate credit.

❌ Common Errors

• Including detailed transcription steps which gain zero credit and risk contradictions.
• Writing "RER makes energy" instead of producing proteins.
• Forgetting the final fusion of vesicles with the cell membrane (exocytosis).

✅ Correct Marking Points

• Chemical components: ATP is composed of a ribose sugar, an adenine organic base, and 3 phosphate groups.
• Breakdown: ATP is broken down into ADP and an inorganic phosphate ( Pi ) via a hydrolysis reaction catalysed by ATP hydrolase (or ATPase).
• Resynthesis: ADP and an inorganic phosphate ( Pi ) are joined to resynthesise ATP catalysed by ATP synthase .
• Reaction type: Resynthesis of ATP occurs via a condensation reaction.

💡 Key Knowledge

ATP stands for Adenosine Triphosphate. The bond between the final phosphate groups is unstable and easily broken, releasing a manageable parcel of energy. The enzyme names directly reflect their actions: hydrolase breaks bonds using water, synthase synthesises molecules.

🧠 Exam Technique

When describing ATP structure, make sure to name all three constituent molecules explicitly (ribose, adenine, 3 phosphates). Do not confuse ribose with deoxyribose. Always link breakdown and synthesis to their specific enzymes and reaction types to guarantee full marks.

❌ Common Errors

• Writing "deoxyribose" instead of "ribose" (DNA vs RNA sugar confusion).
• Omitting the word "inorganic" when referring to phosphate ( Pi ).
• Mixing up the enzymes (attributing condensation to ATP hydrolase).