The structure of a bile duct is well-adapted to its function of transporting bile from the liver and gallbladder to the small intestine. It exhibits the following adaptations:

• Muscular walls: The bile duct walls contain smooth muscle tissue. This allows for peristalsis – rhythmic contractions that propel bile forward.
• Connects to both the liver and gallbladder: The bile duct receives bile from the hepatic ducts (which carry bile from the liver) and the cystic duct (which carries bile from the gallbladder). This ensures a continuous supply of bile.
• Relatively large diameter: The bile duct has a relatively large diameter to accommodate the volume of bile produced by the liver and stored in the gallbladder.
• Lined with epithelial cells: The inner lining of the bile duct is made up of epithelial cells that secrete mucus, which helps to protect the duct from damage by bile.

Enzymes are biological catalysts that speed up the rate of chemical reactions, including the digestion of food. They work by lowering the activation energy required for the reaction to occur. Each enzyme has a specific active site that binds to a particular substrate (the molecule it acts upon).

Carbohydrate Digestion: Amylase is the key enzyme involved in carbohydrate digestion. Salivary amylase, produced in the salivary glands, begins the breakdown of starch into maltose in the mouth. Pancreatic amylase, produced by the pancreas, continues this process in the small intestine. Maltase, sucrase, and lactase are enzymes located on the intestinal lining that break down disaccharides (maltose, sucrose, lactose) into monosaccharides (glucose, fructose, galactose) for absorption.

Protein Digestion: Proteases are enzymes that break down proteins. Pepsin, produced in the stomach by chief cells, begins the breakdown of proteins into peptides. Trypsin and chymotrypsin, produced by the pancreas, continue the breakdown of peptides into smaller peptides and amino acids in the small intestine. Peptidases, located on the intestinal lining, break down small peptides into individual amino acids.

Fat Digestion: Lipase is the enzyme that breaks down fats (triglycerides) into fatty acids and glycerol. Gastric lipase, produced in the stomach, plays a minor role in fat digestion. Pancreatic lipase, produced by the pancreas, is the main enzyme responsible for fat digestion in the small intestine. Bile, produced by the liver and stored in the gallbladder, emulsifies fats, increasing the surface area for lipase to act upon.

Controlled Variables (Variables kept constant):

• Temperature: The temperature of the reaction mixture must be kept constant throughout the experiment. This can be achieved by using a water bath.
• Amylase Concentration: The concentration of amylase used in each test should be the same.
• Substrate Concentration (Starch): The concentration of starch used as the substrate should be the same in each test.
• Volume of Reaction Mixture: The total volume of the reaction mixture (amylase + starch + buffer) should be the same in each test.
• pH of the Buffer Solution: The pH of the buffer solution used to maintain a constant pH should be the same in all tests.

Variables to be Investigated (Independent Variable):

• pH of the Buffer Solution: The pH of the buffer solution should be varied systematically (e.g., pH 4, pH 6, pH 8, pH 10).

Ensuring Reliable Results:

• Replicates: The experiment should be repeated multiple times (at least three replicates) for each pH value. This helps to reduce the impact of random errors.
• Accurate Measurement: Accurate measurements of pH and volumes of solutions are essential.
• Consistent Technique: The same procedure should be followed for each replicate to minimize experimental error.
• Use of a Buffer Solution: A buffer solution should be used to maintain a constant pH throughout the experiment. This prevents changes in pH from affecting enzyme activity.
• Control Group: A control group (e.g., starch + buffer at a specific pH with no amylase) should be included to provide a baseline for comparison. This helps to determine if the amylase is actually having an effect.