The digestive system is an organ system: several organs work together to digest and absorb food. Digestion turns large food molecules into small soluble molecules that can be absorbed into the blood. Digestive enzymes do this job. Each enzyme catalyses one specific reaction because of the shape of its active site. In the simplified lock and key model, only the substrate that fits the active site can bind to it, like a key fitting one lock. High temperatures or the wrong pH change the shape of the active site, so the enzyme stops working.
There are three main groups of digestive enzyme. Carbohydrases break down carbohydrates into simple sugars. Amylase is a carbohydrase that breaks down starch. Proteases break down proteins into amino acids. Lipases break down lipids (fats) into glycerol and fatty acids. Amylase is made in the salivary glands, the pancreas and the small intestine. Protease is made in the stomach, the pancreas and the small intestine. Lipase is made in the pancreas and the small intestine. For example: starch, with amylase, makes sugars.
Bile is made in the liver and stored in the gall bladder. It is alkaline, so it neutralises the hydrochloric acid that comes from the stomach. Bile also emulsifies fat, which means it breaks large fat droplets into many small droplets. This gives a bigger surface area, so lipase can break the fat down faster. The alkaline conditions also suit the enzymes in the small intestine.
The small soluble products of digestion are absorbed into the blood. They are used to build new carbohydrates, lipids and proteins, and some glucose is used in respiration.
Required practical 4 uses reagents to test food. Benedict's reagent is heated with a sample and turns from blue to green, yellow, orange or brick red if sugar is present. Iodine solution turns from orange to blue-black if starch is present. Biuret reagent turns from blue to purple if protein is present. Required practical 5 investigates the effect of pH on amylase. Samples of the mixture are tested with iodine every 30 seconds, and the time taken for the starch to be completely digested is recorded. The iodine stops turning blue-black when all the starch has gone. The temperature is kept constant using a water bath.
