The Digestive System and Enzymes

Food travels through the alimentary canal (digestive tract):

Mouth → Oesophagus → Stomach → Small intestine → Large intestine → Rectum → Anus

The Journey of Food

Pancreas

• Produces digestive enzymes that are released into the small intestine:
  • Amylase — digests starch
  • Protease (trypsin) — digests proteins
  • Lipase — digests fats (lipids)

Liver

• Produces bile, which is stored in the gall bladder
• Bile is released into the duodenum (small intestine)
• Bile has two key roles:
  1. Emulsifies fats — breaks large fat droplets into smaller droplets, increasing the surface area for lipase to work on
  2. Neutralises stomach acid — bile is alkaline, creating the optimum pH for enzymes in the small intestine

Gall Bladder

• Stores and concentrates bile produced by the liver
• Releases bile into the duodenum when needed

Enzymes are proteins that act as biological catalysts — they speed up chemical reactions without being used up or changed.

How Enzymes Work

• Each enzyme has an active site with a specific shape
• The substrate (the molecule the enzyme acts on) fits into the active site like a key in a lock — this is the lock-and-key model
• When the substrate binds, an enzyme-substrate complex forms
• The enzyme catalyses the reaction, and the products are released
• The enzyme is then free to work on another substrate molecule

$\text{Enzyme} + \text{Substrate} \rightarrow \text{Enzyme-Substrate Complex} \rightarrow \text{Enzyme} + \text{Products}$

Key Properties of Enzymes

• Specific — each enzyme catalyses only one type of reaction (because the active site has a unique shape)
• Reusable — not used up in the reaction
• Affected by temperature and pH — optimum conditions vary for different enzymes
• Biological molecules — made of protein (chains of amino acids folded into a specific 3D shape)

Digestive enzymes are catabolic — they break down large molecules into smaller ones.

Summary of Digestion

$\text{Starch} \xrightarrow{\text{amylase}} \text{Sugars (maltose)}$ $\text{Proteins} \xrightarrow{\text{protease}} \text{Amino acids}$ $\text{Lipids} \xrightarrow{\text{lipase}} \text{Fatty acids} + \text{Glycerol}$

Temperature

• As temperature increases, enzyme activity increases (molecules move faster, more successful collisions between enzyme and substrate)
• At the optimum temperature (~37°C for most human enzymes), the rate of reaction is at its maximum
• Above the optimum, the enzyme begins to denature — the active site changes shape, and the substrate can no longer fit
• At very high temperatures, the enzyme is completely denatured and activity drops to zero

pH

• Each enzyme has an optimum pH at which it works best
• Moving away from the optimum pH (too acidic or too alkaline) causes denaturation
• Examples:
  • Pepsin (stomach protease): optimum pH ~2 (acidic)
  • Amylase (mouth/small intestine): optimum pH ~7 (neutral)
  • Lipase (small intestine): optimum pH ~8 (slightly alkaline)

Substrate Concentration

• Increasing substrate concentration increases the rate of reaction (more substrates available to bind to active sites)
• Eventually, the rate plateaus — all active sites are occupied (enzyme saturation)
• To increase the rate further, you would need to add more enzyme

Denaturation occurs when an enzyme loses its specific 3D shape due to extreme temperature or pH.

• The bonds holding the enzyme's tertiary structure (shape) break
• The active site changes shape permanently
• The substrate can no longer fit into the active site
• The enzyme is no longer functional

Important: Denaturation is NOT the same as the enzyme being "killed" — enzymes are not alive. Say the enzyme is "denatured" or "the active site has changed shape."

The products of digestion are absorbed in the ileum (part of the small intestine).

The ileum is adapted for efficient absorption:

• Villi (singular: villus) — finger-like projections that increase the surface area enormously
• Microvilli — tiny projections on the surface of villi cells, further increasing surface area
• Thin walls — villi are only one cell thick for a short diffusion distance
• Rich blood supply — a network of capillaries maintains a concentration gradient by carrying absorbed nutrients away

Nutrients absorbed:

• Glucose and amino acids — absorbed into the blood capillaries
• Fatty acids and glycerol — absorbed into the lacteal (lymph vessel) in each villus

Investigating the effect of pH on amylase activity:

Method

1. Place starch solution and amylase in a test tube in a water bath at 37°C
2. Add a buffer solution at a specific pH to the mixture
3. Every 30 seconds, take a drop of the mixture and place it on a spotting tile containing iodine solution
4. Iodine turns blue-black in the presence of starch and stays brown/orange when starch has been fully digested
5. Record the time taken for the iodine to stop turning blue-black (all starch digested)
6. Repeat for different pH values (e.g., pH 2, 4, 6, 7, 8, 10)

Variables

• Independent variable: pH of the buffer solution
• Dependent variable: Time taken for starch to be digested
• Control variables: Temperature (37°C), volume of starch solution, volume of amylase, concentration of amylase

Expected Results

• Amylase works fastest at pH 7 (neutral — its optimum pH)
• At very low or very high pH values, the enzyme is denatured and starch is not digested (or digested very slowly)

Question: Explain why pepsin works best in the stomach but amylase works best in the mouth. (4 marks)

Solution:

Pepsin has an optimum pH of approximately 2 (acidic). The stomach produces hydrochloric acid, creating an acidic environment that matches pepsin's optimum pH.

Amylase has an optimum pH of approximately 7 (neutral). Saliva in the mouth is approximately neutral, providing the right conditions for amylase.

If amylase entered the stomach, the acidic pH would denature it — the active site would change shape, and it would no longer be able to break down starch.

Similarly, pepsin would not function efficiently in the neutral pH of the mouth.

• Use precise language: enzymes are denatured, not "killed" or "destroyed."
• Always link enzyme function to the shape of the active site being complementary to the substrate.
• In required practical questions, mention control variables — examiners love to see you understand fair testing.
• Bile does NOT contain enzymes — a common misconception. Bile emulsifies fats but does not chemically digest them.

• The digestive system breaks down food mechanically (teeth, churning) and chemically (enzymes, acid, bile).
• Amylase breaks down starch, protease breaks down proteins, lipase breaks down fats.
• Enzymes work best at their optimum temperature and pH; extreme conditions cause denaturation.
• Bile emulsifies fats and neutralises acid — it is NOT an enzyme.
• The small intestine is adapted for absorption with villi, a thin wall, and a rich blood supply.