Chemical digestion
The Purpose of Chemical Digestion
Chemical digestion is the process where large, insoluble food molecules are broken down into small, soluble molecules. This breakdown is essential because only small soluble molecules can be absorbed through the walls of the alimentary canal (digestive system) into the bloodstream.
Without chemical digestion, large molecules like starch and proteins would remain too big to pass through cell membranes. Therefore, the primary role of chemical digestion is to produce these small soluble units ready for absorption.
Enzymes in Digestion
Enzymes are biological catalysts that speed up chemical reactions without being used up. In digestion, specific enzymes break down specific food molecules (substrates) into their building blocks (products). The key digestive enzymes are amylase, proteases, and lipase.
The Three Main Digestive Enzymes
Each enzyme targets a specific food group. It is crucial to memorize the substrate and product for each.
| Enzyme | Substrate (Food) | Product(s) |
|---|---|---|
| Amylase | Starch | Simple reducing sugars (specifically maltose) |
| Protease (e.g., pepsin, trypsin) | Protein | Amino acids |
| Lipase | Fats and oils (lipids) | Fatty acids and glycerol |
⚠︎ Confusing Enzyme Products
Error: Students often state that amylase breaks starch down into glucose, or that lipase breaks fats down into glucose.
Correction: Amylase breaks starch down into maltose (a disaccharide), not directly to glucose. Glucose is only produced later when maltase acts on maltose. Lipase produces fatty acids and glycerol, never glucose.
Secretion Sites and Action Sites of Enzymes
You must know where each enzyme is secreted (produced) and where it acts (works). Note that some enzymes are secreted by one organ but act in another.
| Enzyme | Secreted By | Acts In |
|---|---|---|
| Amylase | Salivary glands and Pancreas | Mouth (from salivary glands) and Small intestine (from pancreas) |
| Protease (Pepsin) | Stomach | Stomach |
| Protease (Trypsin) | Pancreas | Small intestine |
| Lipase | Pancreas and Small intestine | Small intestine |
Identifying Enzyme Sources in Diagrams
Context: When asked to identify the organ secreting an enzyme from a diagram or list.
Tip: Examiners accept specific gland names. For amylase, you must mention salivary glands (for mouth action) and pancreas (for small intestine action). Do not just say 'mouth' as the source; the mouth is where it acts, but the salivary glands secrete it. Similarly, for proteases, distinguish between the stomach (pepsin) and pancreas (trypsin).
Example: If asked 'Where is amylase secreted?', write: 'Salivary glands and pancreas.' This covers both sources required by the syllabus.
The Role of Hydrochloric Acid (HCl)
Hydrochloric acid is found in gastric juice secreted by the stomach lining. It has two specific functions:
| Function 1 | **Kill harmful microorganisms** (bacteria/pathogens) in food to prevent infection. |
|---|---|
| Function 2 | Provide an acidic pH (low pH) which is optimum for the enzyme pepsin (a protease) to work. |
⚠︎ Misunderstanding the Function of Stomach Acid
Error: Students often state that HCl 'breaks down food' or 'digests proteins' directly.
Correction: HCl does not digest food itself. It creates the acidic environment required for the enzyme pepsin to function. The enzyme pepsin does the digestion; the acid just enables it.
The Role of Bile
Bile is produced by the liver and stored in the gall bladder. It is released into the duodenum (start of the small intestine). Bile is an alkaline mixture.
Its primary chemical role is to neutralise the acidic chyme (food and gastric juices) coming from the stomach. This raises the pH to a suitable/alkaline level for enzymes like trypsin and lipase to work, as they are damaged or inactive in acid.
Explaining the Need for Bile
Context: When asked to explain why bile is necessary in the duodenum.
Tip: You must mention neutralisation. Examiners look for the phrase 'neutralises gastric juices' or 'neutralises stomach acid'. Simply saying 'it makes it alkaline' is often insufficient; you must explain what is being neutralised and why (to provide optimum pH for enzymes).
Example: 'Bile neutralises the acidic gastric juices from the stomach, providing an alkaline pH suitable for trypsin activity.'
⚠︎ Calling Bile an Enzyme
Error: Students frequently state that bile is an enzyme or that it emulsifies fats (which is true, but not its pH role).
Correction: In the context of pH and enzyme activity, bile is not an enzyme. It is a chemical mixture. Do not say 'bile digests fats'; it prepares the environment for lipase to digest fats.
Detailed Pathway: Digestion of Starch (Supplement)
Starch digestion is a two-step process involving different enzymes:
| Step | Process | Enzyme & Location |
|---|---|---|
| 1 | Amylase breaks down starch into maltose. | Occurs in the mouth (salivary amylase) and small intestine (pancreatic amylase). |
| 2 | Maltase breaks down maltose into glucose. | Occurs on the membranes of the epithelium lining the small intestine. |
Note: Glucose is the final product that can be absorbed. Maltose cannot be absorbed directly.
Detailed Pathway: Digestion of Protein (Supplement)
Protein digestion also involves two main proteases in different pH conditions:
| Step | Process | Enzyme & Location |
|---|---|---|
| 1 | Pepsin breaks down protein into smaller peptides/amino acids. | Occurs in the stomach (requires acidic conditions provided by HCl). |
| 2 | Trypsin breaks down remaining protein into amino acids. | Occurs in the small intestine (requires alkaline conditions provided by bile). |
Describe the functions of hydrochloric acid in gastric juice.
Q:
Describe two functions of hydrochloric acid in the stomach. [2]
A:
- It kills harmful microorganisms (bacteria/pathogens) in food. 2. It provides an acidic pH (low pH) which is optimum for the enzyme pepsin.
State where enzymes are secreted and act.
Q:
Complete the table to show where amylase, protease, and lipase are secreted and where they act. [3]
A:
Amylase: Secreted by salivary glands/pancreas; acts in mouth/small intestine. Protease: Secreted by stomach/pancreas; acts in stomach/small intestine. Lipase: Secreted by pancreas/small intestine; acts in small intestine.
Explain the role of bile.
Q:
Explain why bile is needed for the action of lipase in the duodenum. [3]
A:
- Bile is alkaline. 2. It neutralises the acidic gastric juices from the stomach. 3. This provides a suitable/alkaline pH for lipase to work (lipase is inactive in acid).
Describe starch digestion.
Q:
Describe the digestion of starch in the human digestive system. [4]
A:
- Amylase breaks down starch into maltose. 2. This occurs in the mouth and small intestine. 3. Maltase (on the epithelium of the small intestine) breaks down maltose into glucose. 4. Glucose is then absorbed.