Enzymes

• To catalyse (speed up) a reaction, the reacting chemical (substrate) must bind to the enzyme's active site.
• The active site will only fit specific substrates.

• The active site is like a lock and the substrate is like a key.
• In the same way, there is usually only one enzyme for every substrate (or one key for each lock).

pH

• Enzymes have an optimum pH (measure of acidity).
• If the pH changes away from the optimum pH, then the enzyme activity decreases.
• If the pH is too low or too high, then the enzyme is denatured and will not function.

Temperature

• Increasing the temperature of a working enzyme initially increases the reacting activity.
• Enzymes have an optimum temperature. Once this temperature is reached, the activity decreases.
• Past a certain temperature, the active site changes shape, and the enzyme is denatured (loses its catalytic activity).

Calculating the rate of reaction

• The rate of reaction is equal to the gradient of the graph, which shows the mass of the product plotted against the reaction time.
• The gradient is calculated by picking two points on the graph, then dividing the change in the value on the y-axis (mass) by the change in the x-axis value (time), between those two points.
• In the example above, the rate of reaction = 10g ÷ 60s = 0.17 g/s.

Purpose of digestive enzymes

• The digestive enzymes catalyse (speed up) the breakdown of large, insoluble food molecules into smaller, soluble (can be dissolved) molecules that are then small enough to be absorbed into the bloodstream.
• Digested molecules can be used to construct new carbohydrates, proteins and lipids in the body.
• Some glucose produced by digestion is used for respiration.

The 3 main digestive enzymes are:

• Amylase
• Protease
• Lipase

The active sites of amylase

• The small intestine
• The Mouth

• Amylase breaks down starch into its constituent simple sugars (predominantly maltose).
• Starch → Maltose (+ other sugars).
• Amylase is produced in:
  • Small intestine
  • Pancreas
  • Salivary glands

Proteases are produced in the:

• small intestine
• pancreas
• stomach
• The sites of action of proteases are the:
  • small intestine
  • stomach
• Proteases break down proteins into amino acids.
• Protein → Amino acids.

• Lipase breaks down lipids into a molecule called glycerol and fatty acids.
• Lipid → Glycerol + Fatty Acids.
• Lipase is produced in the:
  • Small intestine
  • Pancreas

Bile is an alkaline substance produced in the liver and stored in the gall bladder. Bile is important because:

• Bile breaks up fats into tiny droplets, through a process called emulsification.
• The tiny droplets have a higher surface area than the original fat drop.
• This increases the rate of the lipase-catalysed reactions that break fats down.
• Enzymes in the small intestine operate best in alkaline conditions.
• Bile neutralises the acid from the stomach to stop these enzymes becoming denatured (lose their activity).

Reagents can be used to test for the presence of various food substances. The first step is to grind up the food and add distilled water to dissolve some of the food. You can then test for the food substances:

Sugars

• To test for sugar, add Benedict’s reagent and heat for about two minutes.
• It will turn any of green, yellow or red if sugar is present.
• The colour depends on the concentration.

Starch

• To test for starch, add iodine solution.
• It will turn blue-black if starch is present.

Proteins

• To test for proteins, add Biuret solution.
• It will turn mauve or purple if proteins are present.

Lipase

• To test for lipids, add Sudan III.
• If lipids are present, a red-stained oil layer will float on the water surface.