The Digestive System and Enzymes
The digestive system is an organ system that breaks large insoluble food molecules into small soluble ones that can be absorbed into the bloodstream, using enzymes as biological catalysts. Carbohydrases such as amylase break down starch to simple sugars, proteases break down proteins to amino acids, and lipases break down lipids to glycerol and fatty acids, while bile, made in the liver, neutralises stomach acid and emulsifies fat to increase the surface area for lipase.
Method
- Learn the enzyme model in the exam's own words: the substrate fits into the active site of the enzyme, the shape of the active site is complementary to the substrate, and if the shape changes the substrate no longer fits, so the enzyme is denatured and the reaction stops.
- Explain temperature effects in two halves: up to the optimum, raising the temperature increases the rate because molecules move faster and collide more often; above the optimum, the bonds holding the enzyme's shape break, the active site changes shape and the enzyme is denatured, so the rate falls sharply to zero.
- Explain pH the same way: each enzyme has an optimum pH (about pH 2 for stomach protease, about pH 7 to 8 for pancreatic and small intestine enzymes), and a pH far from the optimum denatures the enzyme. Never say an enzyme is 'killed'; it is not alive.
- Learn the three enzyme families with their site of production and their products: amylase (salivary glands, pancreas, small intestine) starch to simple sugars; protease (stomach, pancreas, small intestine) protein to amino acids; lipase (pancreas, small intestine) lipids to glycerol and fatty acids.
- Learn bile separately, because it is the classic exam trap: made in the liver, stored in the gall bladder, released into the small intestine; it is alkaline so it neutralises hydrochloric acid from the stomach, and it emulsifies fats, increasing the surface area so lipase works faster.
- Learn the four food tests as reagent, method and positive result: iodine solution turns from orange-brown to blue-black for starch; Benedict's solution heated in a water bath turns from blue to green, yellow, orange or brick red for sugars; Biuret solution turns from blue to purple for protein; ethanol shaken with the sample and then poured into water gives a cloudy white emulsion for lipids.
Worked example
A student times how long amylase takes to break down all the starch in a solution. At pH 6 it takes 40 seconds; at pH 8 it takes 100 seconds. Calculate the rate of reaction at each pH in units of one over seconds, and state which pH is closer to the optimum.
- Use rate = 1 / time, which is the standard way of turning a 'time taken' result into a rate.
- At pH 6: rate = 1 / 40 = 0.025 per second.
- At pH 8: rate = 1 / 100 = 0.010 per second.
- Compare: 0.025 is greater than 0.010, so the reaction is faster at pH 6.
- Interpret: the faster rate is closer to the enzyme's optimum pH, so of the two, pH 6 is closer to the optimum for this amylase.
- Note the limitation: only two values were tested, so the true optimum could lie either side of pH 6 and more values would be needed to locate it.
Practice questions
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Q1What is an enzyme?Show answer
Answer: A biological catalyst: a protein that speeds up a reaction without being used up.
Q2Name the products of protein digestion.Show answer
Answer: Amino acids.
Q3Where is bile made and where is it stored?Show answer
Answer: Made in the liver, stored in the gall bladder.
Q4State two functions of bile.Show answer
Answer: It neutralises stomach acid, and it emulsifies fats into small droplets to increase the surface area for lipase.
Q5Describe the positive result of the Biuret test.Show answer
Answer: The solution turns from blue to purple, showing protein is present.
Q6Explain what happens to an enzyme above its optimum temperature.Show answer
Answer: The bonds holding its shape break, so the active site changes shape and the substrate no longer fits. The enzyme is denatured and the reaction stops.
Q7Why does a food test for sugars need a water bath?Show answer
Answer: Benedict's solution only reacts with sugars when heated, so the tube is placed in a hot water bath at about 75 degrees Celsius for a few minutes.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Describe how you would test a food sample for starch and for lipids. Give the reagent and the positive result for each.
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Explain why a person whose gall bladder has been removed may find fatty meals difficult to digest.
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A student investigates the effect of temperature on the activity of amylase, testing 10, 20, 30, 40, 50 and 60 degrees Celsius. Predict the shape of the graph of rate against temperature and explain it in terms of enzyme structure.
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