The human digestive system — AQA GCSE Combined Science

An organ system is a group of organs that cooperate to carry out a major function. In the digestive system, each organ has a specialised role: the mouth chews food and starts starch digestion; the oesophagus moves food by peristalsis; the stomach churns food and digests protein; the small intestine completes digestion and absorbs soluble products; the large intestine absorbs water; the rectum stores faeces before egestion. The liver produces bile, which neutralises stomach acid and emulsifies fats, and the pancreas releases digestive enzymes. Digestion converts large insoluble molecules into small soluble ones that can cross the gut lining, while absorption moves those products into the blood. The system therefore works as an integrated whole, not as isolated organs.

Metabolism is the sum of all the chemical reactions occurring in the cells of an organism. Enzymes are biological catalysts: globular proteins that lower the activation energy of a reaction and speed it up without being used up. Each enzyme has a specific active site complementary in shape to its substrate, so it catalyses only one reaction. Anabolic reactions build larger molecules from smaller ones, for example joining glucose molecules to form glycogen in liver and muscle cells, or amino acids to form proteins. Catabolic reactions break larger molecules into smaller ones, for example respiration breaking down glucose to release energy, or digestion hydrolysing starch to maltose. Because every metabolic reaction is controlled by a particular enzyme, the total rate of metabolism depends on enzyme activity, which in turn depends on temperature and pH.

Enzymes are globular proteins with a specific three-dimensional shape. Part of the molecule forms an active site, which is complementary in shape to its substrate, so the substrate binds and the reaction is catalysed. Enzyme activity is affected by temperature and pH. As temperature rises, molecules and substrates gain kinetic energy and collide more frequently, so the rate of reaction increases up to an optimum temperature. Above the optimum, bonds holding the tertiary structure break, the active site changes shape and the enzyme is denatured, so substrate can no longer bind and activity falls sharply. Each enzyme also has an optimum pH. Away from this pH, ionic and hydrogen bonds in the enzyme break, altering the active site shape and reducing activity; extreme pH denatures the enzyme. For example, pepsin works best at acidic pH in the stomach, while amylase works best near neutral pH.

Rate calculations quantify how quickly a reactant is used up or a product is formed. In the digestive system context, they can be applied to enzyme-controlled reactions, for example measuring the volume of gas produced or the loss in mass of a reaction mixture over time. The rate is found by dividing the amount changed by the time taken, giving units such as cm³/s or g/s. For a graph of amount against time, the rate at any point is the gradient of the tangent at that point; the average rate over a period is the total change divided by the total time. Students should choose sensible measurements, record them accurately, and interpret the calculated value in terms of how fast the reaction proceeds.

Enzymes are biological catalysts made of protein. Each enzyme has a region called the active site, whose shape is complementary to the shape of its substrate. The substrate binds to the active site, forming an enzyme-substrate complex, and the reaction is catalysed to form products. Because the active site shape is specific, each enzyme catalyses only one reaction or a small group of similar reactions. This explains why digestive enzymes such as amylase, protease and lipase act on different nutrients. If the enzyme is denatured, for example by high temperature or extreme pH, the active site shape changes and the substrate can no longer bind, so the rate of reaction falls.

Enzymes are biological catalysts made of protein. Each enzyme has a specific active site whose shape is complementary to one substrate. In the lock and key model, the substrate is the key and the enzyme is the lock: only the correctly shaped key fits, forming an enzyme–substrate complex. The substrate is then broken down (or joined) and the products are released, leaving the enzyme unchanged and free to act again. For example, amylase has an active site complementary to starch, so it catalyses starch breakdown, but it cannot catalyse protein digestion. The model explains enzyme specificity and why enzymes are denatured by high temperature or extreme pH: the active site shape changes, so the substrate no longer fits. It is a simplified model because the active site is flexible and can adjust slightly as the substrate binds.

Digestive enzymes are produced in specific organs and act on specific substrates. Amylase is produced in the salivary glands and the pancreas; it catalyses the breakdown of starch to maltose. Proteases are produced in the stomach, the pancreas and the small intestine; they catalyse the breakdown of proteins to amino acids. Lipases are produced in the pancreas and the small intestine; they catalyse the breakdown of lipids (fats) to fatty acids and glycerol. The products are small enough to be absorbed through the wall of the small intestine into the blood. For example, starch is digested by amylase in the mouth and small intestine, while proteins begin digestion in the stomach and continue in the small intestine. Bile, produced by the liver and stored in the gall bladder, emulsifies lipids to increase the surface area for lipase action.

In digestion, enzymes break large insoluble food molecules into smaller soluble products. A word equation names the reactant on the left and the product or products on the right, joined by an arrow, for example starch → glucose, or protein → amino acids. The arrow means 'is converted into'. You should be able to read, complete and write such equations for the three main nutrient groups, naming the substrate, the enzyme type and the products. For example, lipase acts on lipids to produce fatty acids and glycerol, written as lipids → fatty acids + glycerol. You do not need to write or balance chemical symbol equations such as C₆H₁₂O₆, and no formulae are required. Always check that every substance named in the statement appears in your equation and that the arrow points from substrate to product.

Digestion uses enzymes to break large insoluble food molecules into small soluble molecules. Carbohydrases such as amylase convert starch into sugars such as glucose; proteases convert proteins into amino acids; lipases convert lipids into fatty acids and glycerol. These products are small enough to pass through the wall of the small intestine into the blood. The small intestine has a large surface area, a thin wall and a good blood supply, which makes absorption efficient. The blood then carries the soluble products to cells for respiration, growth and repair. Without digestion, large molecules such as starch could not cross the gut wall, so absorption would not occur. You should be able to name the enzyme, its substrate and its products, and explain why the products must be soluble and small.

Digestion converts large, insoluble food molecules into small, soluble molecules that can be absorbed into the blood. Carbohydrases are enzymes that catalyse the hydrolysis of carbohydrates into simple sugars. For example, starch is a polysaccharide made of many glucose units joined by glycosidic bonds; the carbohydrase amylase catalyses its breakdown into maltose and eventually glucose. Amylase is produced in the salivary glands and pancreas and acts in the mouth and small intestine. In the laboratory, amylase activity can be followed by testing starch with iodine solution, which turns from blue-black to brown as starch disappears, or by testing for reducing sugar with Benedict's solution.

Proteins are large polymers made of amino acid monomers joined by peptide bonds. They are too large and insoluble to be absorbed directly, so they must be hydrolysed during digestion. Proteases are enzymes that catalyse the breakdown of proteins into amino acids. Proteases are produced in the stomach, pancreas and small intestine. In the stomach, hydrochloric acid provides a low pH that is optimum for stomach protease; in the small intestine, pancreatic and intestinal proteases work at a higher pH. The amino acids produced are small, soluble molecules that can be absorbed through the wall of the small intestine into the blood and used to build new proteins in cells.

Lipids are large, insoluble food molecules that must be chemically digested before absorption. Lipases are enzymes that catalyse the hydrolysis of lipids, splitting each triglyceride molecule into glycerol and three fatty acid molecules. In the human digestive system, lipase is produced by the pancreas and secreted into the small intestine, and also by the small intestine itself; bile from the liver emulsifies lipids into small droplets, increasing surface area so lipase acts faster. The products are small, soluble enough to cross the intestinal wall and be absorbed into the blood, then reassembled or used for energy and storage. A useful check is that lipase is specific to lipids, so it does not digest starch or protein.

Digestion converts large food molecules into small soluble products that are absorbed into the blood. These products are not simply excreted; they are used by cells to synthesise new biological molecules. Amino acids are joined by condensation reactions to build new proteins, including enzymes and structural proteins. Fatty acids and glycerol are reassembled into new lipids for membranes and energy storage. Glucose and other simple sugars are used to build new carbohydrates such as glycogen, and some glucose is oxidised in respiration to release energy for cell processes. This links digestion to metabolism, growth and repair. A useful example is a muscle cell using amino acids to build contractile proteins and glucose in respiration to power contraction.

Bile is a digestive fluid produced by liver cells and stored in the gall bladder before release into the small intestine. It is alkaline, so it neutralises the hydrochloric acid delivered with chyme from the stomach, creating the slightly alkaline conditions that lipase needs. Bile salts emulsify large fat globules into many small droplets, greatly increasing the surface area of fat exposed to lipase. Because lipase acts on the surface of fat droplets, this larger surface area, together with the alkaline pH, raises the rate of fat breakdown into fatty acids and glycerol. Bile itself contains no lipase; it is not an enzyme. For example, without emulsification a single large fat droplet would be digested slowly, whereas many small droplets allow faster enzyme action.

This required practical uses qualitative reagents to identify food molecules. Benedict's solution tests for reducing sugars such as glucose: add it to the food sample and heat in a water bath, and a blue to green, yellow, orange or brick-red colour indicates a reducing sugar. Iodine solution tests for starch: add a few drops to the sample and a blue-black colour is a positive result. Biuret reagent tests for protein: add it to the sample and a purple or violet colour indicates protein. Ethanol emulsion tests for lipids: add ethanol to the sample, shake, then pour into water, and a white emulsion indicates lipid. A colour change shows the food molecule is present, but these tests are qualitative and do not give an exact concentration.

Food samples are tested chemically to identify reducing sugars, starch and protein. Benedict’s test: add Benedict’s solution to a food solution and heat in a water bath; a blue-to-brick-red colour change shows reducing sugar, with green, yellow or orange indicating intermediate amounts. Iodine test: add iodine solution to the sample; a yellow-brown to blue-black change shows starch. Biuret test: add Biuret reagent; a blue to purple or lilac change shows protein. Each test detects a different nutrient, so a full analysis uses all three. Controls, such as water, show the negative colour. These qualitative tests are assessed in written papers and may be performed in practical work.

Amylase breaks down starch to maltose. In this practical, starch solution, amylase and buffer solutions of different pH are mixed at a controlled temperature. At intervals, a drop of the mixture is placed on a spotting tile with iodine solution. While starch remains, iodine turns blue-black; when starch is fully digested, iodine stays yellow-brown. The time to reach this end point is measured for each pH, and rate can be compared as 1 ÷ time. A pH close to the enzyme’s optimum gives the shortest time and fastest rate; extremes of pH slow the reaction because the enzyme is denatured. Temperature, starch and amylase volumes and concentrations must be controlled.

This required practical investigates how pH affects the rate at which amylase digests starch. You set up a series of tubes, each containing starch solution, amylase and buffer solution of a chosen pH, and keep them all at the same temperature using a water bath or electric heater. At 30-second intervals you remove a small sample from each tube, add it to iodine reagent on a spotting tile, and record the colour. Starch gives a blue-black colour with iodine; as digestion proceeds the colour becomes progressively less intense, and the time at which no blue-black colour remains is the time taken for starch to be completely digested. Repeating across a range of pH values lets you compare digestion times and identify an optimum pH.