Vitamins and the diseases of deficiency · Chemistry… | StemPy Lab
Course 30.3·Lesson 1 of 3
Vitamins and the diseases of deficiency
Sailors on long voyages once lost their teeth and bled from the gums, and a few oranges cured them. A vitamin is a small molecule that the body needs in tiny amounts and cannot make for itself. We sort the vitamins by solubility and learn what each one does, and what happens when it is missing.
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The sailors and the oranges
Long ago, sailors on sea voyages lived on dried food. They developed bleeding gums and weak joints, a disease called scurvy. Fresh oranges and lemons cured it. A factor in the fruit was needed, in very small amounts. We now call it vitamin C.
Vitamins are organic compounds that the body needs in small amounts for normal growth and health. The body cannot make most of them, or cannot make enough, so they come from food. A lack of a vitamin causes a deficiency disease. They are not fuel and not building blocks. Many work as parts of enzymes.
Two groups, by solubility
We sort vitamins by what they dissolve in. The molecule decides.
Two structures side by side. Left, vitamin A (retinol, C20H30O): a six-membered ring with three methyl groups on the left, joined to a long zigzag chain of carbon atoms with four double bonds and two methyl groups, ending in OH. The note says that it is a long hydrocarbon part and one OH, so it dissolves in fat and not in water. Right, vitamin C (ascorbic acid, C6H8O6): a five-membered ring containing one oxygen and a C=O group, with an OH on each of the two carbon atoms joined by a double bond, and a side chain CH(OH)CH2OH. The note says that it has four OH groups and a ring oxygen, so it dissolves in water.
Vitamin A is mostly a long hydrocarbon chain with a single -OH group. It dissolves in fat and oil, and not in water. Vitamin C has several -OH groups on a small ring. They form hydrogen bonds with water, so it dissolves well in water.
Fat-soluble vitamins: A, D, E and K. They are stored in the liver and in fatty tissue. The body can build up a large store. This is useful, but too much can be harmful, because the excess is not lost easily.
Water-soluble vitamins: the B group and C. They are not stored in large amounts. Any excess leaves in the urine. So they must be supplied regularly. Vitamin B12 is the exception. The liver can keep it for years.
The fat-soluble vitamins
A (retinol): carrots, milk and fish liver oil. A lack causes xerophthalmia (hardening of the cornea of the eye) and night blindness.
D (calciferol, D3 is cholecalciferol): fish liver oil, egg yolk and sunlight. A lack causes rickets in children and osteomalacia in adults.
E (tocopherol): vegetable oils and nuts. A lack makes red blood cells fragile and the muscles weak.
K (phylloquinone): green leafy vegetables. A lack makes blood take longer to clot.
Vitamin A. In the eye, the aldehyde form, retinal, takes part in the detection of light. Plants make β-carotene, which the body turns into vitamin A.
Vitamin D. The skin makes vitamin D3 from a cholesterol derivative when sunlight falls on it. The vitamin helps the gut to take up calcium. A child without it develops soft, bent bones.
Vitamin K. The liver needs it to make some of the proteins that clot blood.
The water-soluble vitamins
B1 (thiamine): yeast, whole grain and pulses. A lack causes beri-beri, with loss of appetite and weak nerves.
B2 (riboflavin): milk, eggs and liver. A lack causes cracks at the corners of the mouth (cheilosis), digestive disorders and burning skin.
B3 (niacin, or nicotinic acid): meat and groundnuts. A lack causes pellagra.
B6 (pyridoxine): yeast, cereals and milk. A lack causes convulsions.
B7 (biotin): yeast and egg yolk. A lack causes paralysis and body pain.
B9 (folic acid): green vegetables. A lack causes anaemia and, in pregnancy, defects of the spinal cord.
B12 (cyanocobalamin): meat, fish, eggs and milk. A lack causes pernicious anaemia, with too few red blood cells.
C (ascorbic acid): citrus fruits, amla and guava. A lack causes scurvy, with bleeding gums.
Vitamin B12 is special. It is the only vitamin that contains a metal. The cobalt is held in a ring of four nitrogen atoms, like the iron in haemoglobin. It is a coordination compound of the kind we studied earlier. Plants do not make B12, so people who eat no animal products must take a supplement.
The B vitamins mostly make coenzymes, the helpers of the enzymes in the second course. Niacin makes NAD+, riboflavin makes FAD, and pantothenic acid makes coenzyme A. Thiamine makes thiamine pyrophosphate, and pyridoxine makes pyridoxal phosphate.
Vitamin C as a chemist sees it
Ascorbic acid is a lactone, a ring ester, of a sugar acid. It has an ene-diol, two -OH groups on a carbon-carbon double bond. This group gives up electrons easily, so vitamin C is a reducing agent. That has three results:
Air and heat oxidise it. Long cooking destroys much of the vitamin C in vegetables.
Iodine oxidises it, in a 1 : 1 reaction, C6H8O6+I2→C6H6O6+2HI. We use this to measure the vitamin C in juice, by titration.
It is needed to make collagen, the protein of skin and gums. Without it the collagen is weak, and the gums bleed.
Counting with vitamins
Molar masses follow from the formulas: retinol C20H30O is 286.46g/mol, vitamin D3 C27H44O is 384.65g/mol, and ascorbic acid C6H8O6 is 176.12g/mol. Cyanocobalamin has the formula C63H88CoN14O14P and a molar mass of 1355.37g/mol. The cobalt makes up 58.93/1355.37=4.35% of the mass.
Start with the short half-life of 5 days. The store levels off after about three weeks, and it stays small. Now move the half-life to 100 days. The store is much larger, and it is still rising after 60 days. This is why an excess of a fat-soluble vitamin can do harm, and an excess of a water-soluble one is usually passed in the urine.
Practise with new numbers
Try it in Python
The program works out the molar mass of six vitamins from their formulas. It also counts the oxygen atoms for each carbon atom, which is a rough guide to how polar a molecule is.
Common mistakes
Saying that all vitamins dissolve in water. A, D, E and K dissolve in fat.
Mixing up a vitamin and its disease. Scurvy is vitamin C. Rickets is vitamin D. Pernicious anaemia is vitamin B12. Beri-beri is vitamin B1.
Thinking that vitamins give energy. They help enzymes, and they do not burn as fuel.
Thinking an excess of any vitamin is harmless. Too much of a fat-soluble vitamin builds up and can poison.
Remember these
Fat-soluble: A, D, E and K. Water-soluble: B group and C. Water-soluble vitamins are not stored, except B12.