The human digestive system — AQA GCSE Biology

This statement asks you to connect digestive enzymes to metabolism as a whole. Metabolism is the sum of all the chemical reactions in the body, and almost all of them are controlled by enzymes. Digestive enzymes are extracellular: carbohydrases, proteases and lipases are released into the gut, where they hydrolyse large food molecules into small soluble products. Those products are then absorbed and used in metabolism. Some are built into new molecules in anabolic reactions, such as amino acids joining to form proteins or glucose joining to form glycogen. Others are broken down in catabolic reactions, such as glucose being oxidised in respiration to release energy. So digestion supplies the substrates for metabolism, and enzymes control both the digestive reactions and the metabolic reactions that follow.

Enzymes are proteins folded into a precise three-dimensional shape. This shape creates a small region called the active site, which is complementary to one specific substrate. The substrate binds, forming an enzyme–substrate complex, and the reaction is catalysed. Because activity depends on shape, temperature and pH matter. Raising temperature increases kinetic energy and collision frequency, so rate rises until the optimum; beyond it, bonds holding the tertiary structure break, the active site denatures and substrate no longer fits, so rate falls sharply. pH changes alter charges and bonding within the protein, again distorting the active site. Each enzyme has an optimum pH; pepsin works in acidic stomach conditions while intestinal enzymes prefer alkaline conditions. Describing means stating what enzymes are and explaining how these two variables change activity.

Rate calculations quantify how quickly a reaction proceeds. In enzyme practicals, rate can be found by measuring the amount of product formed or reactant used over a measured time. A common method mixes enzyme and substrate, then records the volume of gas produced or the loss in mass at intervals. Rate is calculated by dividing the change in amount by the time taken, for example rate = volume of gas ÷ time, giving units such as cm³ s⁻¹. Alternatively, rate can be found from the gradient of a tangent drawn to a curve of amount against time. Choosing the initial, steepest part of the curve gives the initial rate. Students should select sensible intervals, repeat readings, control variables such as temperature and pH, and calculate a mean where appropriate.

Enzymes are proteins that act as biological catalysts. Each enzyme has an active site, a region with a shape complementary to one particular substrate. Because shapes are specific, only that substrate binds, forming an enzyme–substrate complex. The enzyme lowers the activation energy needed for the reaction, so the reaction happens faster at body temperature. After the reaction, products are released and the enzyme is unchanged, ready to catalyse further reactions. This specificity explains why amylase acts on starch, protease acts on proteins and lipase acts on lipids. The lock-and-key idea helps describe how shape determines which reaction is catalysed, and it links directly to denaturation when shape is altered.

The lock and key theory models an enzyme as a lock and its substrate as the matching key. The substrate binds to a region called the active site, whose shape is complementary to the substrate. This binding is specific: only a substrate with the correct complementary shape fits, so each enzyme catalyses one type of reaction. Once bound, the enzyme helps the substrate break down or join, forming products. The products are released because their shapes no longer match the active site, leaving the enzyme unchanged and free to act again. For example, the enzyme amylase has an active site complementary to starch, not to protein, so amylase does not digest protein. The model is simplified: it does not show how temperature or pH change the active site's shape, which is why denaturation is described separately.

Digestive enzymes are produced in specific organs and act on specific substrates. Amylase is produced in the salivary glands and the pancreas; it acts on starch, breaking it down to sugars such as maltose. Proteases are produced in the stomach, the pancreas and the small intestine; they act on proteins, breaking them down to amino acids. Lipases are produced in the pancreas and the small intestine; they act on lipids, breaking them down to fatty acids and glycerol. The products are small soluble molecules that can be absorbed into the bloodstream. For example, starch eaten in bread is partly digested by salivary amylase in the mouth, then further digested by pancreatic amylase in the small intestine.

A word equation names the reactants on the left and the products on the right, separated by an arrow that means changes into. For digestion, the general pattern is substrate → products, for example starch → sugars, protein → amino acids and lipid → fatty acids + glycerol. The arrow is not an equals sign; it shows the direction of the change. Word equations summarise the action of a named enzyme, so you may write amylase above the arrow to show which enzyme catalyses the reaction. You do not need to write chemical formulae or balance symbol equations for this specification point. For example, writing lipid → fatty acids + glycerol correctly shows that one lipid molecule is broken into two types of product, and the plus sign separates products rather than showing addition of reactants.

Large food molecules cannot pass through the wall of the small intestine, so digestive enzymes break them into small soluble molecules. Amylase converts starch into sugars, proteases convert proteins into amino acids, and lipases convert lipids into fatty acids and glycerol. These products dissolve and are small enough to be absorbed through the lining of the small intestine into the bloodstream. The blood then carries them to cells for use in respiration and growth. For example, amino acids absorbed from digested protein are used to build new proteins in cells. Absorption happens mainly in the small intestine, whose lining has a large surface area and a good blood supply, features that increase the rate of absorption.

Carbohydrases are enzymes that catalyse the hydrolysis of carbohydrates into simple sugars. Digestion converts large, insoluble food molecules into small, soluble molecules that can be absorbed across the gut wall. Amylase is a specific carbohydrase that breaks down starch, a polysaccharide, into simpler sugars such as maltose and eventually glucose. Starch is a polymer of glucose joined by glycosidic bonds; hydrolysis uses water to split these bonds. Amylase is produced in the salivary glands and pancreas, and works in the mouth and small intestine. Its active site is complementary to starch, so it is specific. Students should link enzyme specificity to the lock-and-key model and explain why starch must be hydrolysed before absorption.

Proteases are enzymes that catalyse the hydrolysis of proteins into amino acids. Proteins are polymers of amino acids joined by peptide bonds. Hydrolysis uses water to break these bonds, releasing smaller peptides and eventually individual amino acids, which are small enough to be absorbed across the wall of the small intestine into the blood. Proteases are produced in the stomach, pancreas and small intestine. Stomach protease works in acidic conditions, while pancreatic and intestinal proteases work in alkaline conditions. Students should connect the breakdown of proteins to the need for amino acids to build new proteins in cells, and explain specificity using the complementary active site of the enzyme.

Lipases are enzymes that catalyse the hydrolysis of lipids into glycerol and fatty acids. Lipids are large, insoluble molecules made of glycerol joined to fatty acids by ester bonds. Hydrolysis uses water to split these bonds, producing glycerol and fatty acids, which are small enough to be absorbed across the small intestine wall. Lipase is produced in the pancreas and small intestine, and bile from the liver emulsifies lipids into small droplets to increase the surface area for lipase action. Students should link emulsification to a faster rate of digestion and explain enzyme specificity using the complementary active site of lipase.

The products of digestion are absorbed into the blood and used by cells. Simple sugars such as glucose are used to build new carbohydrates, including glycogen, and some glucose is used in respiration to release energy. Amino acids are used to build new proteins, such as enzymes, hormones and structural proteins. Glycerol and fatty acids are used to build new lipids, including cell membranes and energy stores. These processes are examples of synthesis, and they require energy. Students should connect absorption in the small intestine to transport by the blood and then to cellular use, and explain that not all products are used in the same way.

Bile is a digestive fluid produced by liver cells and then held in the gall bladder before release. Liver cells continuously secrete bile, which travels along bile ducts to the gall bladder, a small muscular sac under the liver. During digestion, especially when fatty food enters the small intestine, the gall bladder contracts and bile flows through the bile duct into the duodenum. Bile is not an enzyme; it does not chemically digest nutrients. Instead it creates alkaline conditions and physically disperses fat. Knowing the site of production and the site of storage prevents confusion with enzymes such as lipase, which are made in the pancreas and small intestine. A useful memory link is that the liver makes bile and the gall bladder stores it, ready for release into the small intestine.

Bile is alkaline, so when it enters the small intestine it neutralises the acidic chyme arriving from the stomach. Stomach cells secrete hydrochloric acid, producing a low pH that helps activate stomach enzymes and kill microbes. That acidity would reduce the activity of pancreatic and intestinal enzymes, which work best at around neutral to slightly alkaline pH. Bile contains sodium hydrogencarbonate, which reacts with hydrochloric acid to raise the pH. This neutralisation protects the lining of the small intestine and provides suitable conditions for enzymes such as lipase, amylase and protease. The alkaline conditions are therefore not a digestive action by bile itself but a necessary preparation for efficient chemical digestion in the duodenum.

Bile emulsifies fat, meaning it physically breaks large fat globules into many tiny droplets. This is a physical process, not chemical digestion, because no covalent bonds in fat molecules are broken. The droplets remain fat, but they are spread through the watery contents of the small intestine, forming an emulsion. The key benefit is a much larger total surface area of fat exposed to the enzyme lipase. Lipase molecules can then attach to more fat surface at the same time, so fat is broken down faster. A useful model is shaking oil with water: large drops become many small droplets, increasing the area available for reaction. Emulsification therefore speeds up digestion without itself producing fatty acids and glycerol.

Lipase is the enzyme that chemically digests fat into fatty acids and glycerol. Its activity in the small intestine depends on suitable conditions. Bile provides two of these: it neutralises stomach acid so the pH is close to the optimum for lipase, and it emulsifies fat so the total surface area available to lipase is greatly increased. A larger surface area means more lipase molecules can contact fat at the same time, so the rate of breakdown rises. The alkaline pH also prevents denaturation of lipase by acid. Together, these effects mean fat is digested faster than it would be by lipase alone in acidic, unemulsified conditions.

This practical develops the skill of identifying biological molecules using qualitative reagents. You prepare a food sample, often by grinding with water and filtering for most tests. Benedict’s solution is added and heated in a water bath; a colour change from blue to brick-red indicates reducing sugar. Iodine solution indicates starch with a colour change from orange-brown to blue-black. For lipids, add ethanol to an unfiltered sample, shake, then pour into water; a cloudy white emulsion indicates lipid. Biuret reagent indicates protein with a colour change from blue to purple. You must use clean apparatus and suitable controls to make comparisons valid.

This statement specifies the three named qualitative tests you must know: Benedict’s test for sugars, the iodine test for starch and the Biuret test for protein. Benedict’s solution is blue and, when heated with a reducing sugar such as glucose, changes through green and yellow to brick-red; the colour reflects how much reducing sugar is present. Iodine solution is yellow-brown and turns blue-black in the presence of starch. Biuret reagent is blue and turns purple when protein is present. You should be able to state the reagent, the conditions and the positive result for each test, and use these tests to identify unknown food samples. The tests are qualitative, so they show whether a substance is present rather than giving an exact concentration.

This statement identifies the apparatus and technique skills developed by the food tests practical. AT 2 covers the safe and correct use of appropriate apparatus and techniques, including using a Bunsen burner, water bath, test tubes, measuring cylinders and dropping pipettes, and handling reagents safely. AT 8 covers making and recording observations, and presenting results appropriately, such as in a results table with the food sample, test carried out and observed colour. When you carry out the practical, you should choose clean apparatus, measure volumes consistently, heat Benedict’s mixtures in a water bath rather than directly, and record what you see accurately. You should also recognise hazards such as ethanol being flammable and iodine staining, and take suitable precautions.

In this practical you investigate how pH affects the rate at which amylase breaks down starch. You set up a series of test tubes containing amylase solution and buffer solutions of different pH values, then add starch solution and start timing. At intervals, you remove a sample and test it with iodine solution on a spotting tile. While starch remains, iodine turns blue-black; when starch has been fully digested, iodine stays yellow-brown. The time taken for starch to disappear is a measure of the rate of reaction, so a shorter time means a faster rate. You should control variables such as temperature, volumes and concentrations, and repeat the investigation to improve reliability. The results typically show an optimum pH at which amylase works fastest, with the rate decreasing at pH values further from the optimum.

This required practical investigates how pH affects the rate at which amylase digests starch. You set up a series of buffered starch–amylase mixtures at different pH values, keep temperature constant, and remove a small sample every 30 seconds. Each sample is tested with iodine reagent on a spotting tile. While starch remains, iodine turns blue-black; once digestion is complete, the iodine stays orange-brown. Continuous sampling means testing repeatedly at fixed intervals rather than testing only at the start and end, so you can identify the time when starch first disappears. That time is the end point for that pH. Comparing end points across pH values shows the optimum pH for amylase. A control without enzyme confirms that starch does not break down on its own.

This required practical investigates how starch is broken down by amylase. Iodine solution is the reagent that tests for starch: a few drops added to a sample turn the solution blue-black if starch is present, and remain orange-brown if starch is absent. Samples of the reaction mixture are taken at 30-second intervals and tested with iodine so that the time taken for starch to disappear can be followed. Testing every 30 seconds gives a series of timed observations rather than a single end point. A control sample of starch solution alone should stay blue-black, showing the test is working. The time when iodine no longer turns blue-black indicates that starch has been digested.

Enzymes are sensitive to temperature, so temperature must be kept constant during the starch–amylase practical. A water bath or electric heater maintains a steady temperature, such as 37 °C, which is close to human body temperature. If temperature changes during the investigation, enzyme activity changes too, so the time taken for starch to be digested would not be a fair comparison between pH values. You should allow the starch, amylase and buffer solutions to reach the set temperature before mixing them, and keep the reaction tube in the water bath throughout. Temperature is therefore a control variable, while pH is the independent variable. A thermometer can be used to check that the water bath is at the required temperature.

This practical develops several apparatus and techniques skills. AT 1 covers using appropriate apparatus to make measurements, such as measuring volumes of starch, amylase and buffer solutions with a measuring cylinder or syringe. AT 2 covers using a water bath or electric heater to control temperature. AT 5 covers using qualitative reagents to test for starch, which here means iodine reagent and observing the blue-black or orange-brown result. AT 8 covers safely using relevant apparatus and techniques, including handling glassware, iodine reagent and hot water. Together these skills support the investigation of how pH affects the time taken for amylase to digest starch. You should be able to describe how each skill is used and why it matters for valid, safe results.