Digestive System

Question 1 [12 marks]

Award 1 mark per correctly named organ (6) + 1 mark per correct event (6).

Why: The path follows the continuous alimentary canal; each region has a specialised chemical/mechanical role reflecting the food's changing state (solid → bolus → chyme → absorbed nutrients → faeces).

Question 2 [10 marks]

(a) [4]

• Mechanical digestion (2): the physical breakdown of food into smaller pieces without changing its chemical composition (e.g. chewing, churning). Increases surface area.
• Chemical digestion (2): the breakdown of large molecules into smaller absorbable ones by enzymes/acids, involving the breaking of chemical bonds (hydrolysis). The chemical nature of the molecule changes.

(b) [4] (½ mark each, must state location)

• Mechanical: chewing by teeth (mouth); churning by stomach muscles; segmentation in small intestine.
• Chemical: amylase on starch (mouth); pepsin/HCl on protein (stomach); lipase on fat (small intestine).

(c) [2] Mechanical digestion breaks food into smaller particles, increasing the total surface area exposed (1). Enzymes act only on exposed surfaces, so more surface area = more sites for enzyme–substrate contact = faster chemical digestion (1).

Question 3 [12 marks]

Award 2 marks per fully-correct row (1 for enzyme+substrate, 1 for site+product). Any 6 valid rows:

(Peptidase, sucrase, lactase also acceptable.)

Why: Each macronutrient class (carbohydrate, protein, fat) needs specific enzymes; digestion is sequential (e.g. starch → maltose → glucose requires amylase then maltase).

Question 4 [10 marks]

(a) [4] Two functions (2 each):

1. Kills pathogens/bacteria ingested with food, providing chemical defence.
2. Provides the optimum acidic pH (~2) for pepsin to function / denatures proteins by unfolding them, exposing peptide bonds to pepsin. (Also: activates pepsinogen → pepsin.)

(b) [3] At pH 7, pepsin is far from its optimum pH (1). Its active site shape changes / it is much less active (denatured), so protein digestion in the stomach slows dramatically or stops (1). Protein digestion would then rely almost entirely on pancreatic proteases in the small intestine (1).

(c) [3] The stomach lining secretes a thick layer of alkaline mucus (1) that coats and protects the epithelial cells from acid and pepsin (1). Additionally, pepsin is secreted as an inactive precursor (pepsinogen), only activated by HCl once in the lumen, preventing self-digestion within the cells (1).

Question 5 [10 marks]

(a) [6] (2 marks each — feature + how it helps)

1. Villi (finger-like projections): massively increase surface area for absorption.
2. Microvilli (brush border on villi cells): further increase surface area.
3. Thin (single-cell) epithelium / rich capillary + lacteal network: short diffusion distance and maintains steep concentration gradient by removing absorbed nutrients quickly. (Length of intestine also acceptable.)

(b) [4] Mechanism: active transport (1). Glucose is being moved against its concentration gradient (from low gut → high blood concentration) (1). Simple diffusion only moves substances down a concentration gradient (high → low) (1), so it cannot move glucose uphill; active transport uses ATP/energy and carrier proteins to do so (1).

Question 6 [6 marks]

(a) [3] Peristalsis is a wave of muscular contraction (1). Circular muscles contract behind the bolus (and longitudinal relax) narrowing the tube, while ahead longitudinal muscles contract/circular relax (1); the alternating contraction/relaxation of these antagonistic muscle layers pushes food along the gut (1).

(b) [3] The large intestine's main role is reabsorption of water from undigested material (1). In diarrhoea, material passes through too quickly for adequate water reabsorption (1), so large volumes of water are lost in watery faeces, leading to loss of body water/salts → dehydration (1).

[
  {"claim":"Q1 requires 6 organs + 6 events = 12 marks total",
   "code":"organs=6; events=6; result = (organs*1 + events*1 == 12)"},
  {"claim":"Q3 awards 2 marks per row for 6 rows = 12 marks",
   "code":"result = (2*6 == 12)"},
  {"claim":"Paper total marks sum to 60",
   "code":"q=[12,10,12,10,10,6]; result = (sum(q) == 60)"},
  {"claim":"Q5 part a (3 features x 2) + part b (4) = 10 marks",
   "code":"result = (3*2 + 4 == 10)"}
]