Biology Study Notes: Functions of the Digestive System and Enzymes

Physical Digestion is defined as the breakdown of food into smaller pieces without any chemical change occurring to the food molecules.
The primary function of physical digestion is to increase the surface area of food, which facilitates the action of enzymes during chemical digestion.
Chemical Digestion is defined as the breakdown of large, insoluble molecules into small, soluble molecules.
The role of chemical digestion is to produce small soluble molecules that are capable of being absorbed across the gut wall into the blood.
A critical biological constraint is that only small molecules can pass through the gut wall.

Enzymes function as biological catalysts that break down specific substrates into smaller products: - Amylase: Responsible for breaking down starch into simple reducing sugars (e.g., maltose). - Proteases: Responsible for breaking down protein into amino acids. Specific examples include pepsin and trypsin. - Lipase: Responsible for breaking down fats and oils into fatty acids and glycerol.
Secretion and Action Sites: - Amylase is secreted by salivary glands and the pancreas; it acts in the mouth and small intestine. - Proteases (such as pepsin) are secreted in the stomach; others are secreted by the pancreas and act in the small intestine. - Lipase is produced by the pancreas and secreted into the small intestine where it performs its function.

Saliva is a fundamental secretion containing multiple components: - Water: Acts as a solvent and is required for hydrolysis reactions. - Mucus: Lubricates food and sticks particles together. - Salivary Amylase: Initiates the breakdown of starch into maltose.
Functions of Saliva: - To moisten and soften food for easier swallowing. - To stick food together to form a structure known as a bolus.
The tongue plays a role in pushing the food to the back of the mouth and rolling it into a bolus.
After leaving the mouth, the bolus travels down the oesophagus to arrive in the stomach.

The stomach is a muscular bag, approximately the size of a tennis ball when empty.
Hydrochloric Acid ( $HCl$ ) in gastric juice has two primary roles: - Killing harmful microorganisms present in the food. - Providing an acidic pH (approximately $pH\,2$ ) which is the optimum level for protease enzyme activity.
Pepsin is the primary protease in the stomach used for the digestion of protein. It only becomes activated and functions correctly when $HCl$ provides a low pH environment.
Mucus is produced in the stomach to protect the stomach wall from the acidic conditions and enzymes.
Churning is the physical movement of the stomach muscles that aids in mechanical digestion.
Chyme is the term for the semi-liquid mixture of food, acid, and enzymes produced in the stomach.

Bile is an alkaline mixture produced in the liver and stored/released into the duodenum (the first part of the small intestine).
Functions of Bile: - It neutralises the acidic mixture of food and gastric juices (chyme) entering from the stomach. - It provides a suitable alkaline pH ( $pH\,8/9$ ) for enzyme action in the small intestine. - It performs emulsification, which breaks down large fat and oil droplets into smaller ones, thereby increasing the surface area for lipase to act during chemical digestion.
Sodium hydrogen carbonate is produced by the pancreas to further assist in neutralising the acidic chyme.

Saliva: Maintains approximately $pH\,7$ , providing the optimum environment for salivary amylase.
Stomach Juice: Contains $HCl$ , maintaining approximately $pH\,2$ for the optimum activity of pepsin.
Small Intestine (Duodenum): Facilitated by bile and pancreatic secretions, it maintains an alkaline environment ( $pH\,8/9$ ) for further enzymatic breakdown.
Mention of Dr. William Beaumont: A historical figure associated with early observations of stomach functions.

Recap Activity on Pepsin Digestive Action: Analyze two test tubes over $5\,\text{mins}$ : - Tube A: Contains $5\,cm^3$ pepsin, $3\,cm^3$ acid, and a cube of egg white. Prediction: Digestion will occur because the acidic pH ( $HCl$ ) provides the optimum condition for the pepsin to break down the protein in the egg white. - Tube B: Contains $5\,cm^3$ pepsin, $3\,cm^3$ water, and a cube of egg white. Prediction: Digestion will be significantly slower or absent because pepsin lacks the acidic environment required for activation and optimum function.
Flow Diagram Task: Create a flow diagram tracing food through the alimentary canal starting from the mouth, through the oesophagus, then to the stomach.
The "Why" and "How" of Digestion: - Understanding Respiration is necessary to comprehend the "why" (obtaining nutrients for energy). - Understanding Enzymes is necessary to comprehend the "how" (the mechanism of breakdown).
Structure and Function Identification: The digestive system includes the mouth, salivary glands, epiglottis, oesophagus, stomach, small intestine, liver, pancreas, large intestine, rectum, and anus.