AQA A-Level Biology Paper 1, November 2021: Question 10

✅ Mark Scheme Points

• Polymer of nucleotides (accept 'polynucleotide').
• Each nucleotide consists of deoxyribose, a phosphate group, and an organic/nitrogenous base.
• Phosphodiester bonds link adjacent nucleotides.
• Double helix structure featuring two strands held together by hydrogen bonds.
• Complementary base pairing rules: adenine pairs with thymine, and cytosine pairs with guanine.

💡 Key Knowledge

Ensure you distinguish between the monomer (nucleotide) and the polymer (polynucleotide/DNA). Memorise the three distinct components of a DNA nucleotide and the specific type of covalent bond linking them along the sugar-phosphate backbone (phosphodiester bond).

🧠 Exam Technique

A 5-mark list question requires 5 distinct, valid marking points. Write in concise, bullet-like sentences. Avoid vague phrasing like "made of bases"—always specify organic or nitrogenous bases and name the complementary pairs explicitly.

❌ Common Errors

• Confusing deoxyribose (in DNA) with ribose (in RNA).
• Stating that hydrogen bonds hold the backbone together instead of phosphodiester bonds.
• Failing to link specific bases together (adenine with thymine, cytosine with guanine).

✅ Mark Scheme Points

1. Simple diffusion: Small/non-polar molecules move down a concentration gradient directly through the phospholipid bilayer.
2. Facilitated diffusion: Polar/charged molecules move down a concentration gradient via protein channels or carriers (passive process).
3. Osmosis: Movement of water molecules down a water potential gradient across a partially permeable membrane.
4. Active transport: Movement of substances against a concentration gradient using carrier proteins and ATP.
5. Co-transport: Simultaneous transport of two different substances using a carrier protein (e.g. symport).

💡 Key Knowledge

Transport mechanisms depend heavily on the nature of the molecule (size, polarity, charge) and concentration gradients. Remember that active transport and co-transport require metabolic energy in the form of ATP, whereas diffusion and osmosis are passive.

🧠 Exam Technique

The command word is "Name and describe". If you only name the process (e.g. "osmosis"), you will receive zero marks for that point. You must include the mechanism and gradient direction for every single entry.

❌ Common Errors

• Omitting the direction of the gradient (e.g. forgetting to say "down" a concentration gradient or "against" it).
• Describing active transport as passive or failing to mention ATP.
• Confusing simple diffusion with facilitated diffusion by omitting reference to carrier/channel proteins.

✅ Mark Scheme Points (Any 5)

• Magnification figures show Cell A (animal/eukaryote) is bigger than Cell B (bacterium/prokaryote).
• Cell A has a distinct nucleus, whereas Cell B has free/unbound DNA.
• Cell A contains mitochondria, whereas Cell B does not.
• Cell A has Golgi apparatus / endoplasmic reticulum, whereas Cell B does not.
• Cell A lacks a cell wall, whereas Cell B has a murein/peptidoglycan/glycoprotein cell wall.
• Cell A lacks a capsule, whereas Cell B has a capsule.
• Cell A's DNA is bound to histones / linear, whereas Cell B's DNA is not associated with histones / circular.
• Cell A has larger ribosomes than Cell B (80S vs 70S).

💡 Key Knowledge

Figure 14 compares a eukaryotic animal cell (Cell A) with a prokaryotic bacterial cell (Cell B). Examiners look for direct, comparative contrasts. Use comparative terms ("whereas", "in contrast") rather than listing features of each cell in isolation.

🧠 Exam Technique

Because the command word is "Contrast", every mark point must explicitly mention both Cell A and Cell B. Writing "Cell A has mitochondria" scores 0 unless you add "whereas Cell B does not".

❌ Common Errors

• Writing descriptive statements about each cell separately instead of direct contrasts.
• Incorrectly claiming animal cells (Cell A) possess a cellulose cell wall.
• Confusing prokaryotic features (such as 70S ribosomes or circular DNA) as being present in the eukaryotic animal cell.