Digestion, enzymes and practicals overview
Organisation • Animal tissues, organs and systems
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Key concepts
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The digestive system as an organ system
The digestive system comprises organs that produce secretions, move food and absorb products. Food moves from mouth → oesophagus → stomach → small intestine → large intestine → anus; associated organs (pancreas, liver, gall bladder, salivary glands) secrete enzymes or bile but do not form part of the food pathway . Villi line the small intestine and provide a large surface area and rich blood capillaries to absorb small, soluble products of digestion into the bloodstream for transport to cells .
Types of digestive enzymes and sites of production
Carbohydrases (including amylase) break carbohydrates to simple sugars. Amylase is produced in the salivary glands and pancreas and acts in the mouth and small intestine to begin and continue starch digestion . Proteases are produced by the stomach lining and pancreas; stomach proteases start protein digestion in an acidic environment and pancreatic proteases finish protein digestion in the small intestine . Lipases are produced by the pancreas and act in the small intestine to break lipids into glycerol and fatty acids .
How lipases break down lipids
Lipase enzymes catalyse the hydrolysis of triglyceride molecules (lipids) into glycerol and fatty acids. Emulsification of large fat globules by bile increases surface area so lipase molecules access more lipid surface and digest fats faster . The net chemical change converts complex, insoluble lipid molecules into small, soluble glycerol and fatty acid molecules that can be absorbed through the intestinal wall into the blood.
Simple word equations for digestion
Starch → (amylase) → sugars (glucose or maltose) is the carbohydrate breakdown. Protein → (protease) → amino acids is the protein breakdown. Lipid (fat) → (lipase) → glycerol + fatty acids is the lipid breakdown. These word equations summarise reactants, enzyme type and products and are used to link enzyme location to function in the digestive tract .
Enzymes as biological catalysts and the lock-and-key model
Enzymes are protein molecules that act as biological catalysts to speed up chemical reactions without being used up; they lower the activation energy required for a reaction and allow metabolism to proceed at physiologically useful rates . The lock-and-key model explains specificity: an enzyme’s active site has a precise shape complementary to its substrate so only matching substrates bind and undergo reaction; changes to shape (denaturing) prevent binding and stop activity .
Enzyme activity: effects of temperature and pH
Enzyme activity increases with temperature due to higher kinetic energy and more collisions, up to an optimum; above the optimum heat denatures the enzyme and activity falls. pH affects ionisation of amino acids in the active site; extremes of pH denature the enzyme and reduce activity. Practical investigation of pH effects uses buffers and timed observations to determine the pH giving fastest reaction for amylase or protease .
Bile: production, storage and functions
Bile is produced by the liver and stored in the gall bladder before release into the small intestine. Bile acts as an emulsifier: it breaks large fat droplets into many small droplets, increasing surface area for lipase action and speeding lipid digestion. Bile is alkaline and neutralises acidic chyme from the stomach, raising pH toward neutral so pancreatic and intestinal enzymes operate near their optimum .
Products of digestion and metabolism
Small, soluble products of digestion (glucose, amino acids, glycerol and fatty acids) pass through intestinal villi into capillaries and enter the blood. Cells use glucose for respiration and amino acids and sugars as building blocks: enzymes promote synthesis to assemble new carbohydrates, lipids and proteins required for growth, repair and metabolic functions . Excess nutrients store as glycogen or fat and contribute to metabolic regulation.
Required practicals: food tests and amylase pH
A practical for testing foods uses iodine for starch (blue-black positive), Benedict’s reagent for reducing sugars (brick-red precipitate after heating for strong positive), Biuret (copper sulfate + NaOH) for protein (lilac positive) and an emulsification test for lipids (cloudy emulsion) . A practical investigating pH effect on amylase uses starch + iodine to time disappearance of the starch indicator at different buffered pH values; the shortest time indicates optimum pH for amylase activity .
Rate calculations and handling practical data
Rate calculations for reactions measured by time use either rate = change in amount / time or rate = 1 / time when time-to-complete is recorded (for example, time until iodine cross becomes visible). Calculating means and plotting graphs with error bars helps identify optimum conditions and anomalies; control of variables ensures valid comparisons between pH or temperature treatments .
Key notes
Important points to keep in mind