Chemistry · Organic chemistry · Grade 7

Chemistry in our body

Which elements is our body made of, and how much energy does a plate of plov provide?

The situation

On the festive table there is a dish of plov, hot flatbread (non), yoghurt (qatiq) and a basket of apples. Grandmother says: “Eat an apple, it will build up your blood — it is rich in iron.” An unripe apple is sour, a ripe one sweet; and a cut apple quickly turns brown. How much energy does plov give us, is there really a lot of iron in an apple, and which chemical elements is our body actually built from?

Topic

Chemistry in our body: elements, nutrients and vitamins. Plov and an apple: protein, fat, carbohydrate, vitamins and minerals — which elements our body is made of

Key ideas

Biogenic elements
Protein, fat, carbohydrate
Vitamins and minerals

Formulas

E = 17 m_{protein} + 37 m_{fat} + 17 m_{carb}

Energy of food, kJ ( $m$ in grams): protein and carbohydrate ≈ 17 kJ/g (4 kcal/g), fat ≈ 37 kJ/g (9 kcal/g); 1 kcal = 4.184 kJ.

C_{6} H_{12} O_{6} + 6 O_{2} \overset{}{\to} 6 CO_{2} + 6 H_{2} O

In the cell glucose “burns”: ≈ 2 800 kJ/mol, that is ≈ 15.6 kJ/g of energy is released.

m (element) = ω \cdot m (body)

The mass of an element in the body: for example, the calcium fraction is ≈ 1.4 % — about 0.64 kg in a 45 kg student.

C_{6} H_{8} O_{6} + I_{2} \overset{}{\to} C_{6} H_{6} O_{6} + 2 HI

Vitamin C (ascorbic acid) decolourises iodine — this is how its amount is determined.

m (vit. C) = n (I_{2}) \cdot M (C_{6} H_{8} O_{6})

The 0.004 mmol of iodine in the test tube “takes up” 0.004 · 176.1 = 0.70 mg of vitamin C: we measure the volume of juice that contains that much vitamin.

Theory

What elements our body is made of

≈ 96 % of an adult's body mass is four elements: oxygen (≈ 64 %), carbon (≈ 19 %), hydrogen (≈ 10 %) and nitrogen (≈ 2.6 %). Most of the oxygen and hydrogen is in water — ≈ 60 % of body mass. Next come calcium (≈ 1.4 %), phosphorus (≈ 1.1 %), potassium, sulfur, sodium, chlorine and magnesium — these are the macroelements. Iron, zinc, fluorine, copper, iodine, selenium and the like are present in very small amounts — trace elements (in a 70 kg person iron is ≈ 4 g, iodine ≈ 20 mg) — but life is impossible without them. C, H, O, N, P and S are the biogenic elements (organogens): proteins, fats, carbohydrates and nucleic acids are built from them. (The figures are approximate averages for a 70 kg adult; sources differ slightly, for example oxygen 61–65 %, carbon 18–23 %; and every person differs too.)

Proteins, fats, carbohydrates

Proteins are long molecules built from amino acids; they always contain nitrogen (on average ≈ 16 %). Muscle, enzymes, hormones and antibodies are protein. Fats are esters of glycerol and fatty acids; they are an energy store: 37 kJ/g. Carbohydrates: glucose $C_{6} H_{12} O_{6}$ , sucrose (sugar), starch (rice, bread, potatoes) and fibre (cellulose). Protein and carbohydrate give 17 kJ/g. These values come from the European Union food-labelling rule (1169/2011). Plov: calculated from the recipe, 100 g contains ≈ 5.6 g of protein, 10 g of fat and 21 g of carbohydrate — ≈ 820 kJ (≈ 200 kcal); nearly half of the energy comes from fat. WHO recommendation: fat should not exceed 30 % of daily energy, and salt should be under 5 g a day.

Vitamins

Vitamins are organic substances needed in small amounts; the body cannot make enough of them itself. Some are water-soluble (C, the B group) and some fat-soluble (A, D, E, K). Vitamin C is ascorbic acid $C_{6} H_{8} O_{6}$ : it is needed for connective tissue (collagen), it is an antioxidant, and it increases the absorption of plant iron; a shortage causes scurvy (the gums bleed). Children aged 9–13 need ≈ 45 mg a day (US recommendations), and adults 75–90 mg. It breaks down with heat and in air — little is left in food that has been boiled for a long time. Rich sources (per 100 g): blackcurrant ≈ 180 mg, red bell pepper ≈ 128 mg, lemon and orange ≈ 53 mg, cabbage ≈ 37 mg; an apple has only ≈ 4.6 mg. β-carotene (provitamin A) in carrots is fat-soluble — in plov it is absorbed well with the fat.

Minerals and “the iron in apples”

Calcium is in bones and teeth ( $Ca_{10} (PO_{4})_{6} (OH)_{2}$ ), 1300 mg a day at ages 9–13: milk, yoghurt, cheese. Iron is part of haemoglobin, which carries oxygen in the blood; 8 mg a day at ages 9–13; a shortage causes anaemia. “Haem” iron in meat and liver is absorbed well, iron in plants less well (the oxalates in spinach interfere), and vitamin C helps. An apple has very little iron: ≈ 0.12 mg per 100 g — you would need 6–7 kg of apples for the daily requirement! The browning of a cut apple is not caused by iron either (see below). Iodine is for thyroid hormones; iodised salt is used (WHO: 20–40 mg of iodine per kg of salt). Fluorine is for tooth enamel.

Practical: finding the composition of an apple

Starch: iodine solution gives a blue-black colour with starch. An unripe apple has a lot of starch; as it ripens the starch turns into sugar — and the iodine stays yellow-brown; a potato has a great deal of starch. Acid: the pH of apple juice is ≈ 3.3–4 (mainly malic acid), of lemon ≈ 2–2.6 — universal indicator shows this. Vitamin C: we drip juice into an iodine–starch solution (blue); the vitamin reduces the iodine and the blue colour disappears. From the volume used: $c = n (I_{2}) \cdot 176.1 / V$ . Browning: in a cut apple the enzyme polyphenol oxidase oxidises polyphenols with oxygen from the air — brown substances form. Lemon juice (vitamin C and acid) slows this down. Safety: iodine solution stains clothes and skin; do not taste anything in the laboratory; be careful with the knife.

Model and sources

Nutrients and vitamins are rounded from the US USDA FoodData Central (SR Legacy) tables; the iron and calcium of non (bread) are approximate, for unfortified flour. The composition of plov was calculated from the recipe (rice 1 kg, lamb 1 kg, carrots 1 kg, onion 0.3 kg, cottonseed oil 0.23 kg → ≈ 4.5 kg of plov), and vitamin C is taken to be lost in long cooking. The daily requirements are from the US National Academies (ages 9–13); 8 400 kJ for energy is a typical value from an adult label; every person's needs depend on age, sex and activity — this is not medical advice. The pH of the juices are typical measured ranges (US FDA). In the iodine titration, other reducing agents in the juice are not taken into account; the rate of browning and the colours of starch are schematic; in the apple laboratory time is sped up 60 times.

Did you know?

Four elements — oxygen, carbon, hydrogen and nitrogen — make up about 96 % of the mass of the human body; most of the oxygen is in water.
Scurvy, caused by a lack of vitamin C, killed many sailors on long sea voyages; in 1747 the Scottish doctor James Lind showed by experiment that oranges and lemons cure the disease.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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