Year 8 Science: Nutrition

Key ideas:

•  A balanced diet is necessary for humans to remain healthy and minimise the risk of debilitating conditions
•  Recap the Y7 work on the 7 functions of living things
•  Practical on food testing
•  Most animal foods contain protein and fats
•  Most plant foods contain carbohydrates, particularly in the form of starch
•  Starch and similar large molecules cannot enter the bloodstream so the process of digestion breaks them into smaller particles

Key words:

•  carbohydrate
•  protein
•  fats
•  fibre
•  vitamins
•  minerals
•  malnutrition
•  starch
•  test

Useful notes:

Diet

A balanced diet consists of appropriate amounts of carbohydrate, protein, fats, fibre, vitamins, minerals and water. If any one of these is in short supply, then the person will suffer from one of several possible malnutrition conditions. Kwashiorkor, for example, results from a shortage of protein in the diet. People use a certain amount of energy in their daily activities, plus some for body maintenance. A person taking in too much energy in their food will store it as fat. Too little food results in poor body maintenance and some of the materials of the body will be broken down for energy. This will result in loss of weight, but if it is taken too far, vital organs of the body start to fail, leading to heart or liver failure and then death. Men use more energy than women, because on average they are heavier. People in energetic activities (footballer, runner) will usually use up more energy than people in passive activities (sewing, cooking).

Food tests

Fats can be recognised by rubbing a food onto paper. If it becomes translucent (lets light through, looks like tracing paper) then it is fatty. Also the food could be crushed with a little alcohol, then add some water. If the mixture goes cloudy, then fats are present. This is occasionally seen in alcoholic drinks like Cointreau® which contain oil of orange - the (neat) 40% alcohol solution is clear, but adding more water makes the oil appear as a cloud in the mixture. We do not recommend that pupils try this one! Proteins require a more sophisticated test - crush the food, add Biuret solution A, then Biuret solution B. If the mixture goes a nice violet colour, then protein is present. Most sugars are identified by the Benedict's test - Crush food, add some Benedict's solution and then leave in a hot water bath (60ºC) for about 10 minutes. A brick-red colour develops if the food contains sugars. This will not recognise sucrose sugar, the one we put in our tea, but does recognise reducing sugars like fructose from fruit. Starch is a simple test - add iodine to the food. The iodine colour turns from brown to black (really really dark blue) if starch is present.

Vitamins are fairly simple compounds of natural origin. Some are used as starting materials for more complicated compounds by the body, e.g. carrots contain b -carotene which is a starting material for a protein used in the eye. Very few proteins can be synthesised (made from parts) within the body. Shortage of any vitamins can cause symptoms from tiredness (lack of B-vitamins) to death (scurvy caused by lack of vitamin C). The wall of the intestine is only one cell thick, so small molecules of food chemicals can easily diffuse (soak through) into the bloodstream. We do an experiment to simulate this using a mixture of sugar and starch sealed inside a “Visking®” porous tube (like sausage skin), which is placed in water. After a few minutes, the water around the tube is sampled for stach and sugars. Starch molecules are too big to go through the tube material, so only sugars are found in the water.