Nutrients and food testsIB MYP Biology: Revision notes
Section 1
Carbohydrates, proteins and lipids
- Carbohydrates (starch and sugars): the main source of energy. Sources: bread, rice, pasta, potatoes, fruit.
- Proteins: for growth and repair; made of amino acids. Sources: meat, fish, eggs, beans, lentils.
- Lipids (fats and oils): an energy store, insulation and part of cell membranes. Sources: butter, oil, nuts, oily fish.
Section 2
Vitamins, minerals, water and fibre
- Vitamin A: healthy eyes and skin (carrots, liver).
- Vitamin C: healthy skin and gums (oranges, peppers).
- Vitamin D: helps the body absorb calcium (oily fish, eggs; also made in the skin in sunlight).
- Iron: needed to make haemoglobin in red blood cells (red meat, spinach).
- Calcium: strong bones and teeth (milk, cheese).
- Water: the solvent in which reactions happen; transports substances in the blood.
- Fibre: keeps food moving through the gut, preventing constipation (wholegrain bread, vegetables).
Vitamins and minerals are not the same. Iron and calcium are minerals; vitamins A, C and D are vitamins.
Section 3
Food tests
Test a food by mixing it with water first (for lipids, shake it with ethanol).
- Starch: add iodine solution. Orange-brown turns blue-black.
- Reducing sugar: add Benedict's solution and heat in a hot water bath. Blue turns green, yellow, orange or brick-red, depending on how much sugar there is.
- Protein: add biuret solution. Blue turns purple (lilac).
- Lipid: shake the food with ethanol, then pour the liquid into water. A cloudy white emulsion means lipid is present; if it stays clear there is no lipid.
Wear eye protection: some of the solutions are irritants. Use a water bath, not a flame, for heating Benedict's solution.
Name the starting and finishing colours of every test: for example 'blue to brick-red', not just 'it changes colour'.
Section 4
Energy content of food
Foods contain stored chemical energy. You can measure it by burning a known mass of food under a boiling tube containing a known mass of water and measuring the temperature rise.
Energy transferred = mass of water × 4.2 × temperature rise (J).
Worked example: 1.0 g of a food heats 20 g of water by 30 °C. Energy = 20 × 4.2 × 30 = 2520 J (2.52 kJ) per gram.
The result is lower than the true value because some heat is lost to the surroundings, and the food may not burn completely. Shielding the tube and repeating the test make it more accurate.
Section 5
A balanced diet
A balanced diet contains all the nutrients in the right amounts: carbohydrates, proteins, lipids, vitamins, minerals, water and fibre. The energy needed varies with age, activity and body size. For example, a teenage athlete needs more energy than an office worker of the same size, and growing children need extra protein and calcium.
Section 6
Deficiency diseases and obesity
A deficiency disease is caused by a lack of a nutrient:
- Scurvy: lack of vitamin C. Bleeding gums and slow-healing wounds.
- Rickets: lack of vitamin D (or calcium). Soft, bent bones in children.
- Anaemia: lack of iron. Tiredness and pale skin, because less haemoglobin is made.
- Kwashiorkor: lack of protein. Poor growth and a swollen belly.
Obesity is being very overweight, caused by taking in more energy than the body uses. It increases the risk of type 2 diabetes and heart disease.
Must know
- Functions: carbohydrates (energy), proteins (growth and repair), lipids (energy store), iron (haemoglobin), calcium (bones), vitamin C (gums), vitamin D (absorbs calcium)
- Tests: iodine (blue-black), Benedict's (brick-red), biuret (purple), ethanol emulsion (cloudy white)
- Energy is measured by burning food under water
- Scurvy (C), rickets (D), anaemia (iron), kwashiorkor (protein)
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Nutrients and food tests
- A 15-year-old girl in Doha eats very little red meat or green vegetables. She feels tired all the time, and her doctor finds that she has anaemia.Explain why a lack of iron makes the girl feel tired.2 marks
- Sailors on long sea voyages in the 1700s, who ate no fresh fruit, often developed bleeding gums and wounds that would not heal. Today, some children whose diets contain too little vitamin D develop soft, bent leg bones.Explain why a diet low in vitamin D leads to soft bones.2 marks
- A student has four unlabelled food samples, W, X, Y and Z, each mixed with water. She tests each one with iodine solution, with Benedict's solution (heated in a water bath), with biuret solution, and with the ethanol emulsion test. Sample W turns blue-black with iodine and gives no other change. Sample X turns brick-red with Benedict's solution and purple with biuret solution, and gives no other change. Sample Y gives a cloudy white layer in the ethanol emulsion test and no other change. Sample Z gives no change in any test.State the nutrient or nutrients present in samples W, X and Y.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).