Chemical Basis of Life
Senior 4 · Biology
Chemical Basis of Life
Lesson 2 of 5
25 min
Look at the young orange seedling in the photo further down this lesson. It has water, light and carbon dioxide, yet one new leaf has turned pale yellow while its veins stay green. The plant is short of iron, and it needs only a few milligrams of it. People can be short of iron in the same way: they eat enough food, but not enough of the right food. The Rwanda Demographic and Health Survey 2019–20 found that 37 % of children aged 6–59 months and 13 % of women aged 15–49 were anaemic.
Mineral salts and vitamins are needed in tiny amounts and give no energy, but without them the large molecules of life cannot be made or used. This lesson builds on what you learned in S1 about food nutrients: here you will link each one to the molecule or process it serves, and measure one of them.
Definition
Mineral salts (mineral ions): inorganic ions that organisms need, such as , and . Plants absorb them from the soil solution; animals get them from food and water.
Macronutrients (major elements): mineral elements needed in relatively large amounts, for example calcium, phosphorus, potassium and magnesium. Micronutrients (trace elements): those needed in very small amounts, for example iron, iodine, zinc and copper.
Vitamin: an organic compound needed in small amounts for normal metabolism, which the body cannot make, or cannot make in large enough amounts, so it must be in the diet. Vitamins A, D, E and K are fat-soluble; the B group and vitamin C are water-soluble.
Deficiency disease: a disease caused by a lack of one nutrient, for example scurvy (vitamin C) or goitre (iodine).
| Ion | Main roles | Good sources | Effect of deficiency |
|---|---|---|---|
| Calcium, | Calcium phosphate hardens bones and teeth; needed for blood clotting, muscle contraction and the release of transmitter at synapses | Milk, ikivuguto, small dried fish eaten whole, beans, dark green leaves | Rickets in children (soft, bent bones), osteomalacia and osteoporosis in adults |
| Phosphate, | Bones and teeth; part of ATP, DNA, RNA and phospholipids | Milk, meat, eggs, beans, groundnuts | Weak bones (deficiency is rare) |
| Iron, | The centre of haem in haemoglobin and myoglobin, which carry oxygen; part of cytochromes in respiration | Meat, liver, beans, dark green leaves such as isombe (cassava leaves) and dodo (amaranth) | Iron-deficiency anaemia: tiredness, pale skin, breathlessness |
| Iodide, | Used by the thyroid gland to make the hormone thyroxine, which controls metabolic rate | Iodised salt, fish | Goitre (a swollen thyroid); in pregnancy, poor brain development of the child |
| Sodium and potassium | Nerve impulses, water balance of body fluids; sodium is needed for glucose absorption in the gut | Salt (sodium); bananas, potatoes, beans (potassium) | Muscle cramps; disturbed heartbeat |
| Fluoride, | Hardens tooth enamel | Drinking water, tea, fluoride toothpaste | More tooth decay |
| Zinc, | Cofactor of many enzymes; needed for growth and immunity | Meat, beans, groundnuts, whole cereals | Poor growth, slow wound healing, more infections |
A plant makes all its own organic molecules, but it must take every mineral element from the soil. Some ions can be moved out of old leaves to young ones (mobile ions); others, once built into a leaf, stay there (immobile ions). That is why the position of the symptom is a clue to the missing ion.
| Ion | Main roles in the plant | Deficiency symptom |
|---|---|---|
| Nitrate, (mobile) | Nitrogen for amino acids and proteins, nucleic acids and chlorophyll | Stunted growth; older leaves turn yellow first |
| Phosphate (mobile) | ATP, nucleic acids, phospholipids; root growth | Poor root growth; dull, sometimes purplish leaves |
| Potassium, (mobile) | Opening of stomata; activates many enzymes | Yellow and brown leaf edges on older leaves; weak stems |
| Magnesium, (mobile) | The central atom of every chlorophyll molecule | Yellowing between the veins (chlorosis) of older leaves |
| Iron (immobile) | Needed to make chlorophyll, though not part of it; part of cytochromes | Chlorosis between the veins of young leaves |
| Calcium, (immobile) | Calcium pectate in the middle lamella that holds plant cells together | Growing points and young leaves die back |
Figure 1. Same ring, different metal (simplified). Haem and chlorophyll are both built on a porphyrin ring whose four nitrogen atoms hold a metal ion: iron in haem, magnesium in chlorophyll. Without iron, a person cannot make haemoglobin; without magnesium, a plant cannot make chlorophyll.
| Vitamin | Soluble in | Main roles | Good sources | Deficiency disease |
|---|---|---|---|---|
| A (retinol) | Fat | Makes the pigment rhodopsin in the rod cells of the retina; keeps epithelia healthy; immunity | Liver, milk, egg yolk; carotene in orange-fleshed sweet potato, carrots, dark green leaves | Night blindness; dry, damaged cornea; more infections |
| B1 (thiamine) | Water | Coenzyme in aerobic respiration | Whole cereals, beans, groundnuts | Beriberi: weak muscles, nerve and heart damage |
| B3 (niacin) | Water | Part of the coenzyme NAD, which carries hydrogen in respiration | Groundnuts, meat, fish, beans | Pellagra: rough skin, diarrhoea, confusion |
| B9 (folate, folic acid) | Water | Making DNA, so needed for cell division and red blood cell formation | Dark green leaves, beans, liver | Anaemia with large red cells; in early pregnancy, neural tube defects in the baby |
| B12 (cobalamin) | Water | Red blood cell formation; the myelin sheath of nerves | Animal foods only: meat, milk, eggs, fish | Anaemia with large red cells; nerve damage |
| C (ascorbic acid) | Water | Making collagen; helps the gut absorb iron from plant foods; antioxidant | Oranges, guavas, passion fruit, tomatoes, fresh leaves | Scurvy: bleeding gums, slow wound healing |
| D (calciferol) | Fat | Absorption of calcium and phosphate in the gut, so bone formation | Made in the skin in sunlight; eggs, oily fish | Rickets in children; osteomalacia in adults |
| E (tocopherol) | Fat | Antioxidant that protects the lipids of membranes | Vegetable oils, nuts, avocado | Rare; nerve and muscle damage |
| K | Fat | Making clotting factors in the liver | Green leaves; made by gut bacteria | Slow clotting and bleeding |
The blue dye DCPIP (dichlorophenolindophenol) is decolourised by vitamin C, which is a reducing agent. Put a fixed volume of DCPIP solution, say , in a test tube. Add the juice from a graduated pipette or burette, a drop at a time, shaking after each drop, until the blue colour just disappears. Record the volume added. The less juice needed, the more vitamin C it contains. To turn this into a concentration, first do the same with a standard solution of known vitamin C concentration. Because the result depends on judging a colour end point, it is semi-quantitative: repeat it three times and take the mean.
Worked example 1: how much vitamin C is in the juice?
A standard solution containing of vitamin C per took a mean of to decolourise of DCPIP. Fresh orange juice took a mean of ; the same juice, boiled for ten minutes and cooled, took . (a) Find the vitamin C concentration of the fresh juice in mg per . (b) What does the boiled juice show?
Worked example 2: why eat fruit with beans?
The figures here are illustrative. A 15-year-old girl needs to absorb about of iron a day. Cooked beans contain of iron per . (a) If she absorbs 10 % of the iron in her meal, how much iron must she eat, and how many grams of beans is that? (b) Eaten with an orange, absorption rises to 15 %. Repeat the calculation.
The World Health Organization (fact sheet "Anaemia", February 2025) estimates that 40 % of children aged 6–59 months, 37 % of pregnant women and 30 % of women aged 15–49 worldwide are anaemic. It names iron deficiency and malaria as the most commonly recognised causes in low-income settings; infections and worms that cause blood loss add to it. In Rwanda, the Demographic and Health Survey 2019–20 found 37 % of children aged 6–59 months and 13 % of women aged 15–49 anaemic. So preventing anaemia means more than eating iron: varied meals with iron-rich foods and fruit, iodised salt, deworming, sleeping under a treated mosquito net and, when a health worker prescribes them, iron and folic acid tablets in pregnancy.
CrashCourse: "Metabolism & Nutrition, Part 1" (Crash Course Anatomy & Physiology #36). For this lesson, watch from 3:47 to 4:44, on water, vitamins and minerals, and note which vitamin helps iron absorption.
Warning
Common mistakes
Tip
Memory hook. The fat-soluble vitamins are "A DEcK" (A, D, E, K) kept in your fat. For plant symptoms: "old leaves first, the ion has left": if old leaves show the symptom, the ion is mobile and has been moved to the young leaves (N, P, K, Mg); if young leaves show it, the ion is immobile (Fe, Ca).
Practice: mineral salts and vitamins
Key points
Sources and credits
Quick check
On a citrus seedling, the newest leaves turn yellow between green veins while the older leaves stay dark green. Which ion is most likely lacking?
