Chemical Digestion
- Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules.
Role of chemical digestion in absorption
- Large food molecules (e.g. starch, proteins, lipids) are polymers that are physically too big to cross the cell membranes of the epithelium lining the alimentary canal.
- Chemical digestion breaks the covalent bonds within these polymers, turning them into tiny, soluble monomers (e.g. glucose, amino acids).
- Once they are small and soluble, they can dissolve in the watery fluids of the digestive tract, pass through the walls of the small intestine, and enter the blood capillaries or lacteals to be transported around the body.
Functions of digestive enzymes
| Enzyme group | Substrate (insoluble) | Product(s) (insoluble) | Secreted by | Site of action |
|---|---|---|---|---|
| Amylase | Starch | Simple reducing sugars (maltose) | Salivary glands, pancreas | Mouth, duodenum (small intestine) |
| Protease | Protein | Amino acids | Stomach wall, pancreas | Stomach, duodenum (small intestine) |
| Lipase | Fats and oils | Fatty acids and glycerol | Pancreas | Duodenum (small intestine) |
Role of hydrochloric acid
- Killing harmful microorganisms. The extreme acidity denatures the enzymes and structural proteins of bacteria and other pathogens present in swallowed food, protecting the body from infection.
- Providing an optimum acidic pH. The main protease enzyme in the stomach requires a highly acidic environment to change into its active shape and catalyze the breakdown of proteins efficiently.
Digestion of starch
- Starch cannot be broken down into glucose all at once. It requires a precise two-step biochemical pathway.
- Amylase breaks down starch into maltose. This occurs via salivary amylase in the mouth and pancreatic amylase in the duodenum.
- Maltase breaks down maltose into glucose. This enzyme is located on the membranes of the epithelium lining the small intestine, ensuring glucose is produced directly at the surface where it can be immediately be absorbed.
- $\text{Starch} \xrightarrow{\text{Amylase}} \text{Maltose} \xrightarrow{\text{Maltase (on epithelium)}} \text{Glucose}$
Digestion of protein
- Proteins are digested by two different types of protease enzymes that are structurally adapted to function at opposite ends of the pH scale.
- Pepsin, site of action is the stomach.
- Requires acidic conditions (provided by HCl) to function safely without denaturing. It breaks long protein chains down into smaller polypeptide fragments.
- Trypsin, site of action is the duodenum (small intestine).
- Secreted by pancreas as part of pancreatic juice. It breaks down the polypeptides into individual, fully soluble amino acids, but it requires alkaline conditions to function.
Neutralizing role of bile
- Bile is an alkaline mixture that neutralizes the acidic mixture of food and gastric juices entering the duodenum from the stomach, to provide a suitable pH for enzyme action.