AQA AS Level Biology Paper 1, June 2023: Question 9

✅ Correct Answer Structure

• Point 1: Sucrose is actively transported (or co-transported) into the phloem at the source (leaf cells).
• Point 2: This lowers the water potential inside the phloem (makes it more negative).
• Point 3: Water enters the phloem from surrounding xylem/tissues down a water potential gradient by osmosis, increasing hydrostatic pressure.

💡 Key Knowledge

• The mass flow hypothesis relies on generating high hydrostatic pressure at the source and low hydrostatic pressure at the sink.
• Active loading requires carrier proteins and ATP to move sucrose against its concentration gradient.

🧠 Exam Technique

Ensure you link transport processes sequentially. Do not skip steps: solute accumulation must lower water potential before water can enter by osmosis to build pressure.

❌ Common Errors

Students frequently lose marks by stating sucrose enters via "facilitated diffusion" instead of active transport/co-transport. Examiners also penalize vague answers like "water enters, making it heavier" instead of referencing water potential and osmosis.

✅ Correct Answer Structure

• Point 1: Carbohydrates are made by photosynthesis in the leaf.
• Point 2: Fewer sucrose-transport proteins mean less sucrose is transported into the phloem (at the source) OR less sucrose reaches the sink (potato/root) to be stored as starch.

💡 Key Knowledge

• Source-to-sink relationships: leaves act as sources producing sugars, while tubers (potatoes) act as sinks storing carbohydrates (typically as insoluble starch).
• Rate of translocation depends directly on the abundance of functional transport proteins in source membranes.

❌ Common Errors

Candidates often forget to connect the lack of transport proteins back to the passage information, failing to state that less sugar is loaded into the phloem or removed from the leaf.

✅ Correct Answer Structure

Give any two distinct reasons explaining the increased uptake:

• Reason 1 (Respiration/ATP): Increased respiration rate in muscle cells to provide more ATP for muscle contraction.
• Reason 2 (Concentration Gradient / Transporters): More glucose is used up internally, maintaining/steepening the concentration gradient for glucose OR increased number of glucose-transporter proteins fused into the cell-surface membrane.
• Mechanism detail: Glucose enters via facilitated diffusion (using carrier/channel proteins).

💡 Key Knowledge

• Mammalian muscle cells store glucose-transporter proteins in internal membranes, mobilizing them to the surface membrane upon stimulation.
• Facilitated diffusion relies on a pre-existing concentration gradient, which is kept steep by rapid consumption of glucose in respiration.

🧠 Exam Technique

Read the question carefully: it asks for two reasons why uptake increases, and to explain your answers. Make sure each point pairs a physiological cause (e.g., muscle contraction needing ATP) with its transport consequence.

❌ Common Errors

Stating that glucose enters by active transport during exercise. The passage explicitly states mammalian cells use facilitated diffusion via transport proteins.

✅ Correct Answer Answer

The correct box to tick is the fourth box:

• The secondary structure is held by hydrogen bonds.
• The tertiary structure allows the protein to be positioned spanning the whole cell membrane.

💡 Key Knowledge

• Secondary structure: Formed by alpha-helices and beta-pleated sheets, stabilized entirely by hydrogen bonds between amino acids in the peptide backbone.
• Tertiary structure: Folding into a 3D shape containing hydrophobic regions on the exterior that interact with the phospholipid bilayer, allowing transmembrane (spanning) carrier or channel proteins to function.

❌ Common Errors

Confusing hydrogen bonds in secondary structure with disulfide bridges, ionic bonds, or hydrophobic interactions which typically stabilize tertiary structure.