Human body systems, Year 10: enzymes, digestion and food tests

The active site

HintThe first word suggests "where the action happens".

The whyAn enzyme is a large protein molecule, but only one small region of it does the work. The molecule the enzyme acts on, the substrate, fits into this region, the reaction happens there, and the products leave.

Only a substrate with the matching shape fits its active site

HintThink why your front-door key will not open next door.

The whyThe enzyme is the lock and the substrate is the key. A molecule of a different shape cannot fit, so the enzyme has no effect on it. That is why the body needs many different enzymes. The model is a simplification, which AQA expects you to know.

Its active site has changed shape (it is denatured), so the substrate no longer fits

HintHeat has altered the one feature the lock and key model depends on.

The whyUp to the optimum, warming speeds an enzyme up, because molecules collide more often. Beyond it, the heat changes the shape of the enzyme molecule. The change is permanent, which is why cooling does not bring the activity back. An extreme pH has the same effect.

The iodine solution stays orange-brown

HintThink what colour that reagent turns when the substance being broken down is still there.

The whyIodine solution turns blue-black with starch. While starch remains, each sample goes dark. When a sample causes no colour change, the amylase has broken down all the starch, and the time taken is recorded. A shorter time means a faster reaction.

Temperature also changes how fast enzymes work, so it must be kept the same

HintAsk which other factor, besides the one being tested, could alter the result.

The whyThe practical tests one variable, pH. Enzyme activity also depends on how warm the mixture is, so if that varied between tubes you could not tell which factor caused a difference. Keeping it constant makes the test fair.

4

HintA shorter time means a faster reaction; compare the two times.

The whyRate is proportional to 1 ÷ time. The same amount of starch was digested in a quarter of the time, so the reaction was 200 ÷ 50 times as fast. You can also compare 1/50 with 1/200.

glycerol; fatty acids

HintThere are two products: one has a name ending in "-ol", and the other is named after what it is made from.

The whyA lipid molecule is too large to be absorbed. Lipase splits it into smaller molecules that can pass into the body. Lipase is made in the pancreas and the small intestine.

amino acids

HintThey are the small building blocks that ribosomes join together.

The whyProteins are long chains. Protease enzymes cut them into their small soluble units, which can be absorbed into the blood. Proteases are made in the stomach, the pancreas and the small intestine.

pancreas

HintThe same organ also makes the hormone insulin.

The whyThe pancreas makes all three types of digestive enzyme and passes them into the small intestine. Amylase is also made in the salivary glands, and protease in the stomach, and the wall of the small intestine makes all three as well.

It neutralises the hydrochloric acid from the stomach

HintFood arriving out of the previous organ is far too sour for the enzymes that work next.

The whyFood leaving the stomach is strongly acidic. The enzymes in the small intestine work best in slightly alkaline conditions. Bile, which the liver makes and the gall bladder stores, neutralises the acid and so lets those enzymes work quickly.

It gives a larger surface area for lipase to work on

HintCompare how fast a sugar cube and a spoonful of loose sugar dissolve.

The whyLipase can only reach the fat molecules on the outside of a drop. Many tiny droplets have far more outside than one large drop of the same volume, so the enzyme meets more fat at once. Emulsifying is a physical change: the fat molecules themselves are not broken down.

Sugars (such as glucose)

HintIt is the type of carbohydrate that tastes sweet.

The whyBenedict's solution is blue. Heated with a sugar such as glucose it changes colour, through green and yellow to brick red. It does not react with every sugar: table sugar (sucrose) gives no change, a detail beyond what AQA asks.

heated

HintThe test tube is stood in a hot water bath for a few minutes.

The whyThe colour change with Benedict's solution is a chemical reaction that is too slow at room temperature. A water bath is used, not a direct flame, so the liquid does not boil over. The other two tests give their result at room temperature.

blue-black; purple

HintOne result is very dark, like ink; the other is the colour of lavender or lilac.

The whyEach reagent has a starting colour and a positive colour, and both must be learned. If the reagent keeps its starting colour, the result is negative. Lilac and mauve are accepted descriptions of the Biuret result.

They show whether a substance is present, not exactly how much

HintCompare "is there any?" with "how many grams?".

The whyA qualitative test gives a yes or no answer from an observation such as a colour. A quantitative test would give a number. To find out how much sugar a food contains, a different, measured method is needed.

The food that turned brick red contains more sugar

HintThe further the colour moves from the starting blue, the stronger the result.

The whyBenedict's solution changes from blue through green, yellow and orange to brick red as the amount of sugar increases. So the final colour gives a rough comparison of amounts, as long as the same volumes are used and both are heated for the same time.

protein

HintIt is the nutrient the body uses for growth and repair.

The whyEach reagent detects one type of molecule: Biuret detects protein, iodine detects starch and Benedict's detects sugars. Only the Biuret test was positive, so only its molecule was found. The practical also covers lipids, which need a further test.