AQA A-Level Biology Paper 1, June 2022: Question 9

15 marks · Medium difficulty · Extended Answer

Describe the transport of carbohydrates in plants, compare and contrast the structure of starch and cellulose, and describe the complete digestion of starch in mammals.

Practise this question

Question

An exam question split into three parts: part 09.1 asks to describe the transport of carbohydrate in plants for 5 marks; part 09.2 asks to compare and contrast the structure of starch and the structure of cellulose for 6 marks; part 09.3 asks to describe the complete digestion of starch by a mammal for 4 marks, with answer lines provided below each.
Question text

09.1 Describe the transport of carbohydrate in plants.

[5 marks]

09.2 Compare and contrast the structure of starch and the structure of cellulose.

[6 marks]

09.3 Describe the complete digestion of starch by a mammal.

[4 marks]

Mark scheme

Show the mark scheme Mark scheme detailing the acceptable marking points for questions 09.1, 09.2, and 09.3, including guidance on active transport of sucrose in the phloem, structural comparisons between alpha and beta glucose polymers, and the enzymatic hydrolysis of starch into maltose and glucose.

Question Marking Guidance Mark Comments

1. Sucrose actively transported into phloem (cell); 5 max 1. Accept sieve

(element/tube/cell) for

OR (5 x

+ phloem (cell)

Sucrose is co-transported/moved with H into AO1)

phloem (cell);

4. Accept description of

2. (By) companion/transfer cells;

gradient, eg higher WP

3. Lowers water potential (in phloem) and water

enters (from xylem) by osmosis;

09.1 4. ((Produces) high(er) (hydrostatic) pressure;

OR 5. Accept transport OR

movement for flow

(Produces hydrostatic) pressure gradient;

5. Accept buds/young

5. Mass flow to respiring cells leaves/fruit/seeds/shoot

tip/root tip/

meristems/root

OR

Mass flow to storage tissue/organ;

6 Accept facilitated

6. Unloaded/removed (from phloem) by active–A-LEVEL BIOLOGYdiffusion– –

transport;

1. Both polysaccharides 6 max Must include 1, 2 OR

(6 x 3 to achieve 6 marks

OR

AO1) All statements must

Both are glucose polymers be clearly

OR comparative or linked

by the candidate, not

Both are made of glucose monomers; inferred from separate

2. Both contain glycosidic bonds (between statements

monomers); Additional mark point

3. Both contain carbon, hydrogen and oxygen/C, H 8. Starch has 1–6

and O; glycosidic bonds

and cellulose

4. Starch has α-glucose and cellulose has β-

does not

09.2 glucose;

OR

5. Starch (molecule) is helical/coiled and cellulose

Starch contains

(molecule) is straight; two types of

6. Starch (molecule) is branched and cellulose is molecule and

not/unbranched; cellulose contains

one type of

7.Cellulose has (micro/macro) fibrils and starch molecule

does not; OR

Starch is amylose

and amylopectin

and cellulose is

one type of

molecule;

1. Hydrolysis; 4 max Other than 5., do not

(4 x penalise incorrect site

2. (Of) glycosidic bonds;

AO1) for digestion or

09.3 3. (Starch) to maltose by amylase; incorrect site of

enzyme production

4. (Maltose) to glucose by disaccharidase/maltase;

5. Accept microvilli for

5. Membrane-bound (disaccharidase/maltase); membrane

How to answer it

Carbohydrate Transport, Structure, and Digestion

What this question tests

This sequence of questions tests your knowledge of plant transport mechanisms (mass flow hypothesis), comparative polysaccharide biochemistry (starch vs. cellulose structure), and mammalian enzymatic digestion. Examiners expect precise biological terminology, clear comparative links, and sequential ordering of physiological processes.

Question 09.1

Describe the transport of carbohydrate in plants. [5 marks]

✅ Mark Scheme Points (Max 5)

  • Sucrose is actively transported into the phloem (sieve tube elements) / co-transported with H⁺ ions.
  • Transport involves companion cells or transfer cells.
  • Water potential in the phloem decreases, causing water to enter from the xylem by osmosis.
  • This produces a higher hydrostatic pressure / generates a hydrostatic pressure gradient.
  • Mass flow moves the sap towards respiring cells or storage tissues/organs.
  • Sucrose is unloaded/removed from the phloem (often by active transport).

💡 Key Knowledge

This describes the Mass Flow Hypothesis (Münch hypothesis). Memorise the sequence as a strict chain reaction: Active loading lowers water potential → Osmosis increases pressure → Mass flow down pressure gradient → Unloading at sinks.

🧠 Exam Technique

Causation words are vital here. Do not just list random facts. Use connective phrases like "this leads to", "resulting in", and "causes" to show the examiner you understand the sequence of physical events.

❌ Common Errors

Students frequently lose marks by mixing up xylem and phloem roles (e.g., claiming water enters the phloem due to active transport of water, rather than osmosis driven by sucrose loading). Ensure you specify osmosis for water movement.

Examiner Tip: Any mention of sieve tubes/elements for phloem cells is accepted, but generic terms like "vessels" are penalised because vessels strictly refer to xylem.
Question 09.2

Compare and contrast the structure of starch and the structure of cellulose. [6 marks]

✅ Mark Scheme Points (Max 6)

  • Similarities: Both are polysaccharides / glucose polymers / made of glucose monomers, both contain glycosidic bonds, and both contain carbon, hydrogen, and oxygen.
  • Differences (Starch): Made of alpha-glucose, helical/coiled shape, branched (contains 1-6 glycosidic bonds alongside 1-4), consists of two molecules (amylose and amylopectin), and does not form microfibrils.
  • Differences (Cellulose): Made of beta-glucose, straight/unbranched chains, consists of one type of molecule, and forms microfibrils/macrofibrils (hydrogen bonded together).

🧠 Exam Technique

The "Compare and Contrast" Trap: The mark scheme explicitly states that all statements must be clearly comparative or linked by the candidate, not inferred from separate bullet points. Always structure sentences using comparative words like "whereas", "in contrast", or "both share".

❌ Common Errors

Writing isolated lists of facts about starch followed by facts about cellulose. If you don't explicitly link them (e.g., "Starch is branched, whereas cellulose is straight and unbranched"), you will cap your marks.

Examiner Tip: Make sure you state alpha-glucose vs beta-glucose clearly, and link beta-glucose to the inverted adjacent monomers required to form straight cellulose chains.
Question 09.3

Describe the complete digestion of starch by a mammal. [4 marks]

✅ Mark Scheme Points (Max 4)

  • Involves hydrolysis reactions.
  • Breaks down glycosidic bonds.
  • Starch is broken down into maltose by amylase (produced in salivary glands/pancreas).
  • Maltose is broken down into glucose by disaccharidases / maltase.
  • Maltase/disaccharidases are membrane-bound (attached to the cell surface membrane of epithelial cells in the ileum / microvilli).

💡 Key Knowledge

Never say enzymes "destroy" or "kill" starch. Always use scientific terminology: starch is hydrolysed by breaking glycosidic bonds. Note the distinction between soluble luminal enzymes (amylase) and membrane-bound brush-border enzymes (maltase).

🧠 Exam Technique

To hit all 4 marks cleanly, follow the chronological pathway: Mouth/Duodenum (Starch → Maltose via amylase) → Ileum epithelium (Maltose → Glucose via membrane-bound maltase). Explicitly naming "membrane-bound" is a frequent discriminating marking point.

Examiner Tip: Examiners allow microvilli as an acceptable location for membrane-bound disaccharidases. Ensure you do not confuse maltase with amylase location!

Topics

Biology · 3.1 Biological molecules · 3.3 Organisms exchange substances with their environment

Question and mark scheme from the AQA A-Level Biology examination, Paper 1, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.