Carbon, hydrogen, oxygen and nitrogen are the four most common elements found in living organisms.

Carbon, hydrogen and oxygen are found in all the key organic molecules – proteins, carbohydrates, nucleic acids and lipids. Proteins and nucleic acids also contain nitrogen.

Any compound that does not contain carbon is said to be inorganic. A variety of inorganic substances are found in living things and are vital to both the structure and functioning of different organisms. Some important roles of inorganic elements are shown in Table below

ElementExample of role in
Examples of role in plantsExample of role in animals
sulfur (S)a component of two amino acids a component of two amino acidsa component of two amino acids, needed to make some antibodies
calcium (Ca)co-factor in some enzyme reactionsco-factor in some enzyme reactionsimportant constituent of bones, needed for muscle contraction
phosphorus (P)a component of ATP and DNAa component of ATP and DNAa component of ATP and DNA
iron (Fe)a component of cytochrome pigmentsa component of cytochrome pigmentsa component of hemoglobin and cytochrome pigments
sodium (Na)important in membranes, changes solute concentration and affects osmosisimportant in membranes, changes solute concentration and affects osmosisimportant in membranes, changes solute concentration and affects osmosis; also important in transmission of nerve impulses
Table: Roles of inorganic elements in living things

Organic and inorganic compounds

Chemical compounds are divided into two groups: organic and inorganic. Organic compounds include all the complex compounds of carbon found in living organisms, but not simple carbon-containing compounds such as carbon dioxide, carbonates and hydrogencarbonates. These and all other compounds are inorganic. Both groups are found in living organisms.

Building blocks of organic molecules

Many organic molecules are very large and complex but they are built up of small subunits, which can be relatively simple. Figure 3.1 shows some of these building blocks. Subunits called monomers are built into complex polymers.

Figure 3.1 The basic structures of glucose, amino acids, fatty acids and ribose – the building blocks of organic molecules


Carbohydrates contain only carbon, hydrogen and oxygen and they are the most abundant category of molecule in living things. In both plants and animals they have an important role as a source of energy, and in plants they also have a structural function. Carbohydrates occur in different forms: monosaccharides, which contain only one subunit; disaccharides, which have two; and polysaccharides, which are formed from long chains of monosaccharides. Table below shows examples of carbohydrates and their uses.

Form of
ExamplesExamples of use in plantsExamples of use in animals
monosaccharideglucose, galactose, fructosefructose is a component of fruits, making them taste sweet and attracting animals to eat them, thereby dispersing the seeds insideglucose is the source of energy for cell respiration – it is obtained from the digestion of carbohydrate foods
disaccharidemaltose, lactose, sucrosesucrose is transported from leaves to storage tissues and other parts of the plant to provide an energy sourcelactose is found in milk and provides energy for young mammals
polysaccharidestarch, glycogen, cellulosecellulose is a structural component of plant cell walls
starch is used as a food store
glycogen is the storage carbohydrate of animals, found in the liver and muscles
Table: Examples and roles of carbohydrates

Condensation and hydrolysis

In a condensation reaction, two molecules can be joined to form a larger molecule, held together by strong covalent bonds. Each condensation reaction requires an enzyme to catalyse the process and it produces one molecule of water. The condensation of two monosaccharides products a disaccharide. For example:

If further monosaccharides are added to a disaccharide, a polysaccharide is formed, as you can see in Figure 3.2

Figure 3.2 Monosaccharide subunits (glucose in this case) are joined in a condensation reaction, forming a disaccharide (maltose) and water. Glycogen is a polysaccharide, formed from long chains of glucose subunits
Figure 3.3 Two amino acids combine to form a dipeptide

In a similar way, two amino acids can be linked to form a dipeptide Figure 3.3:

amino acid + amino acid ⟶ dipeptide + water

When more than two amino acids are joined in this way, a polypeptide is formed. Polypeptide chains form protein molecules.

In another condensation reaction, glycerol links to fatty acids to produce triglyceride lipid molecules Figure 3.4

Figure 3.4 How a triglyceride lipid is formed from glycerol and three fatty acids in a condensation reaction

Hydrolysis reactions occur every time food is digested. These reactions involve breaking down polysaccharides, polypeptides and triglycerides into the smaller units of which they are made. Water molecules are used in hydrolysis reactions – they are the reverse of condensation reactions. Once again enzymes are required to catalyse the reactions.

  • Hydrolysis of starch (a polysaccharide) uses water and produces many molecules of glucose.
  • Hydrolysis of protein (made of polypeptide chains) uses water and produces many amino acids.
  • Hydrolysis of a triglyceride (a lipid) uses water and produces fatty acids and glycerol molecules.


Lipids are used as energy molecules in plants and animals. Triglyceride lipids that are solid are generally referred to as fats, while in liquid form triglycerides are known as oils. Animals store energy as fat whereas plant store oils – for example, linseed oil and olive oil. Lipid contains about twice as much energy per gram as carbohydrate but each type of storage molecule has its own advantages.

  • Lipids contain more energy per gram than carbohydrates, so lipid stores are lighter than carbohydrates storing an equivalent amount of energy.
  • Lipids are also less dense than water, so fat stores help large aquatic animals to float.
  • Lipids are non-polar, insoluble molecules so they do not affect the movement of water in and out of the cells by osmosis.
  • Lipids are also important in providing heat insulation. Fat stored under the skin reduces heat loss and is vital for animals, such as seals, polar bears and whales, which live in cold conditions.
  • However, carbohydrates can be digested more easily than lipids, making carbohydrates stores more readily available sources of energy.
MoleculeApproximate energy
content per gram / kj

Organic molecules

Biological molecules are often called organic molecules, since many of them were discovered in living organisms. Chemists have found that these compounds all contain carbon atoms, along with other elements. Carbon atoms bond strongly to other carbon atoms, so organic molecules can be large and show a wide variety of chain and ring structures, with many carbon atoms bonded together. Organisms need organic molecules to:

  • provide energy to drive life processes
  • provide raw materials for the growth and repair of tissues

Nutrition supplies living organisms with the molecules that they need. There are four main groups of organic chemicals used by living things:

  • carbohydrates
  • lipids
  • proteins
  • nucleic acids

Basic biochemistry

Living organisms also contain inorganic molecules (such as water) and a number of ions. The study of the organic and inorganic molecules that make up living organisms is called biochemistry. The sum of all the chemical reactions in living organisms is sometimes called metabolism.

Large organic molecules are usually made up of lots of similar smaller molecules called subunits. The subunits can be split apart by a reaction called hydrolysis, which uses water. They can be joined together again, perhaps in new combinations, by a reaction called condensation, which produces water.

In this way living organisms can take molecules from their environment and rearrange them into shapes that suit their own particular requirements, as illustrated below.

▲ The carbohydrates starch in the maize is hydrolysed in the rat’s cell into subunits called glucose. These are then built up into the carbohydrate glycogen by condensation reactions

Proteins are made up of long chains of subunits called amino acids, joined together in particular sequences which are coded for by genes. The 20 different amino acids can be joined together in a vast number of different orders, and some proteins are thousands of amino acids long. The sequence of amino acids determines the shape of the protein molecule – some are long and thin (such as keratin, the protein in hair and nails), whilst others are more egg shaped or spherical (such as haemoglobin, the oxygen-carrying protein in red blood cells).

Amino acids are soluble so they are easily transported in living organisms, and can take part in reactions in the watery cytoplasm of the cell.

Nucleic acids are made up of long chains of subunits called nucleotides. Each nucleotide is made up of a base, a sugar and a phosphate group. In deoxyribonucleic acid (DNA), there are four different nucleotides, each containing a different base.

In DNA these chains are coiled around one another. The sequence of bases forms a code which carries the genetic information. This code is passed from one generation to the next and instructs a cell or an organism to carry out a particular task.

Carbohydrates, lipids and proteins: three food types


Good sourceFunctions in humansComments
Rice, potatoes, wheat (e.g. pasta) and other cereals provides starch. Food sweetenings, such as those in desserts, sweets and soft drinks, and preservatives provide refined sugars, such as sucrose (cane sugar) and glucose A source of energy. Glucose is oxidised in respiration to release energy for active transport, cell division, muscle contraction and the manufacture of large biological molecules. Excess carbohydrates can be stored as glycogen and as fatCarbohydrates are digested in the mouth and small intestine and absorbed as glucose. Refined sugars are absorbed very rapidly, giving a sudden boost of ‘energy source’. Starch is digested and absorbed more slowly, giving a steady supply of energy source: starches are called slow release carbohydrates


Meat and animal foods (eggs, milk, cheese) are rich in saturated fats and cholesterol. Plant sources such as sunflower seeds and peanuts are rich in unsaturated fatsFats and oils are an important source of energy. They are especially valuable as an energy store because they are insoluble in water. They also provide insulation – electrical insulation around nerve cells and thermal insulation beneath the skin – and form part of cell membranes. Steroid hormones, including sex hormones, are made from cholesterolFats and oils are digested in the small intestine and absorbed as fatty acids and glycerol. Some lipids contain saturated fatty acids and others contain unsaturated fatty acids (with at least one carbon-carbon double bond). The body can store unlimited amounts of fat, contributing to obesity. The incorrect balance of saturated and unsaturated fatty acids, or an excess of cholesterol, can cause diseases of the circulation


Meat, fish, eggs from animals, and legumes (peas and beans) and pulses from plants. One of the best sources of proteins is the soya bean. This contains very little fat (unlike most animal sources) and so is suitable for people with health problems caused by fat. Soya beans can be flavoured and textured to make them taste and feel like meat – this textured vegetable protein is used as ‘artificial’ meat. Mycoprotein is also low-fat substitute for meat.Many functions, including
∎ catalysts (enzymes) ∎transport molecules, e.g. haemoglobin
∎structural materials, as in muscles
∎hormones, such as insulin
∎in defence against disease, as antibodies
Digested in the stomach and small intestine, and absorbed as amino acids. 20 different amino acids are needed to make up all of the different proteins in the human body. Some of these must be supplied in the diet as the body cannot make them – these are the essential amino acids. Proteins from animal sources usually contain all 20 amino acids, but plant proteins, often lack one or two of the essential amino acids. Deficiency of protein causes poor growth – in extreme cases may cause marasmus or kwashiorkor

Essential amino acids – cry baby

One factor that determines how comfortable babies feel is the supply of a ‘pleasure chemical’, called serotonin, in the brain. An essential amino acid called lysine is needed to make serotonin. Lysine is in short supply in some artificial milks. Babies fed on these milk substitutes can’t manufacture enough serotonin, they feel uncomfortable and they CCRRYYY ! Natural mother’s milk contains an adequate supply of lysine.

Vegetarians must ensure that their diet contains a wide range of protein sources since very few plants contain all the essential amino acids. An ideal vegetarian meal containing all the essential amino acids baked beans on toast (which also contains a high proportion of dietary fibre).

▲ The protein values of some common foods

Vitamins and minerals

Vitamins and minerals are essential for the body to be able to use the other nutrients efficiently. They are needed in only very small amounts. There are many different vitamins and minerals, and they are usually provided in the foods of a balanced diet.


Water-solubleFood sourceSymptoms of deficiencyComments
C (ascorbic acid)Cherries, citrus fruits e.g. limes, lemons, oranges and fresh green leafy vegetablesScurvy – production of fibres in the body is affectedVitamin C also seems to protect cells from ageing
Fat-solubleFood source Symptoms of deficiencyComments
D (calciferol)Liver, dairy products, eggs, fish liver oilRickets – bones are soft and may bend, because vitamin D is needed for the absorption of calciumCan be made by the body, just under the skin, but only if there is plenty of sunlight


Food sourceSymptoms of deficiencyComments
IronRed meat, liver, some leafy vegetables, e.g. spinachAnaemia – iron is needed to produce haemoglobin for red blood cells. A shortage causes weakness as oxygen needed for respiration cannot be transported efficientlyIron is added to foods when metal utensils are used in cooking – the amount of iron in a piece of beef is doubled when the meat is minced in an iron mincer ready for making burgers!
CalciumMilk, cheese and fishSeveral problems
∎ weak bones and teeth
∎ poor clotting of blood
∎ uncontrolled muscle contractions (‘spasms’)
Calcium shortage causes rickets, the same deficiency disease caused by insufficient vitamin D


Water forms about 70% of the human body. Two-thirds of this water is in the cytoplasm of cells, and the other third is in the tissue fluid and blood plasma. Humans lose about 1.5 litres of water each day, in urine, faeces, exhaled air and sweat – this must be replaced by water in the diet. It is obtained in three main ways:

  • as a drink
  • in food, especially salad foods such as tomatoes and lettuce
  • from metabolic processes (think back to the equation for aerobic respiration – water is one of the products).

A loss of only 5% of body’s water can lead to unconsciousness, and a loss of 10% would be fatal.


Dietary fibre is the indigestible part of food, largely cellulose from plant cell walls, which provides bulk for the faeces. Plenty of fibre in the diet stretches the muscles of the gut wall and helps push the food along by peristalsis. A shortage of fibre can cause constipation, and may be a factor in the development of bowel cancer.