Components of the main food groups:

The main food groups are carbohydrates, lipids and proteins. All three groups are made from smaller molecules.

Carbohydrates are large molecules made from one or more sugars (e.g. both Starch and Glycogen are both polymers of Glucose)

Proteins are polymers of Amino Acids

Lipids are made from one glycerol molecule and three fatty acid molecules joined together.

The enzymes used in digestion are produced by cells and then released into the gut to mix with food.

Food GroupFunction
Lipids (fats & oils)Used as a long-term energy store (much easier to store than carbohydrates). Also have a role in protection and insulation
CarbohydratesMade from single sugars or chains of sugars. They are used in respiration to provide energy
ProteinsBroken down into amino acids, which our body absorbs and assembles into new proteins. The proteins are used for growth and repair
FibreRegulates bowel movement. Sloughs off old lining of intestine
WaterEssential as a solvent for chemical reactions ( e.g. cytoplasm), heat loss (e.g. transpiration), transport (e.g. blood) etc
Vitamins and MineralsEssential for the normal function of some enzymes and proteins e.g. Fe2+ is an essential part of Haemoglobin and Mg2+ is part of Chlorophyll

Digestive enzymes break down big molecules

  • Starch, proteins and fats are big molecules. They’re too big to pass through the walls of the digestive system, so digestive enzymes break these big molecules down into smaller ones like sugars (e.g. glucose and maltose), amino acids, glycerol and fatty acids. These smaller, soluble molecules can pass easily through the walls of the digestive system, allowing them to be absorbed into the bloodstream.

Carbohydrases Convert Carbohydrates into Simple Sugars

Amylase is an example of a carbohydrate. It breaks down starch.

Amylase is made in three places:

1) The salivary glands 2) The pancreas 3) The small intestine

Proteases Convert Proteins into Amino Acids

Proteases are made in three places:

1) The stomach (it’s called pepsin there) 2) The pancreas 3) The small intestine

Lipases Convert Lipids into Glycerol and Fatty Acids

Lipases are made in two places

1) The pancreas 2) The small intestine

  • The body makes good use of the products of digestion. They can be used to make new carbohydrates, proteins and lipids. Some of the glucose (a carbohydrate) that’s made is used in respiration.

Bile neutralises the stomach acid and emulsifies fats

  1. Bile is produced in the liver. It’s stored in the gall bladder before it’s released into the small intestine.
  2. The hydrochloric acid in the stomach makes the pH too acidic for enzymes in the small intestine to work properly. Bile is alkaline – it neutralises the acid and makes conditions alkaline. The enzymes in the small intestine work best in these alkaline conditions.
  3. It emulsifies fats. In other words it breaks the fat into tiny droplets. This gives a much bigger surface area of fat for the enzymes lipase to work on – which makes its digestion faster.

Biological molecules


Carbon is an element present in all biological molecules. Carbon atoms can join together to form chains or ring structures, so biological molecules can be very large (macromolecules), often constructed of repeating sub-units (monomers). Other elements always present are oxygen and hydrogen. Nitrogen is sometimes present. When macromolecules are made of long chains of monomers held together by chemical bonds, they are known as polymers (poly means ‘many’). Examples are polysaccharides (chains of single sugar units such as glucose), proteins (chains of amino acids) and nucleic acids (chains of nucleotides). Molecules constructed of lots of small units often have different properties from their sub-units, making them suitable for specific functions in living things. For example, glucose is very soluble and has no strength, but cellulose (a macromolecule made of glucose units) is insoluble and very tough – ideal for the formation of cell walls around plant cells.

Cells need chemical substances to make new cytoplasm and to produce energy. Therefore the organism must take in food to supply the cells with these substances. Of course, it is not quite as simple as this; most cells have specialised functions and so have differing needs. However, all cells need water, oxygen, salts and food substances and all cells consist of water, proteins, lipids, carbohydrates, salts and vitamins or their derivatives.


These may be simple, soluble sugars or complex materials like starch and cellulose, but all carbohydrates contain carbon, hydrogen and oxygen only. A commonly occurring simple sugar is glucose, which has the chemical formula C6H12O6.

The glucose molecule is often in the form of a ring, represented as

Figure 2.1 Glucose molecule showing ring structure

Two molecules of glucose can be combined to form a molecule of maltose C12H22O11

Figure 2.2 Formation of maltose

Sugars with a single carbon ring are called monosaccharides, e.g. glucose and fructose. Those sugars with two carbon rings in their molecules are called disaccharides, e.g. maltose and sucrose. Mono- and disaccharides are readily soluble in water.

When many glucose molecules are joined together, the carbohydrate is called a polysaccharide. Glycogen Figure 2.3 is a polysaccharide that forms a food storage substance in many animal cells. The starch molecule is made up of hundreds of glucose molecules joined together to form long chains. Starch is an important storage substance in the plastids of plant cells. Plastids are important organelles in plant cells. They are the sites where molecules like starch are made and stored. One familiar example of a plastid is the chloroplast.

Cellulose consists of even longer chains of glucose molecules. The chain molecules are grouped together to form microscopic fibres, which are laid down in layers to form the cell wall in plant cell Figures 2.4 and 2.5

Polysaccharides are not really soluble in water

Figure 2.3 Part of a glycogen molecule
Figure 2.4 Cellulose. Plant cell walls are composed of long, interwoven and interconnected cellulose fibres, which are large enough to be seen with the electron microscope. Each fibre is made up of many long-chain cellulose molecules.


Fats are a solid form of a group of molecules called lipids. When lipids are liquid they are known as oils. Fats and oils are formed from carbon, hydrogen and oxygen only. A molecule of fat (or oil) is made up of three molecules of an organic acid, called a fatty acid, combined with one molecule of glycerol.

Figure 2.5

Drawn simply, fat molecule can be represented as in Figure 2.6

Figure 2.6 Fat molecule

Lipids form part of the cell membrane and the internal membranes of the cell such as the nuclear membrane. Droplets of fat or oil form a source of energy when stored in the cytoplasm.


Some proteins contribute to the structures of the cell e.g. to the cell membranes, the mitochondria, ribosomes and chromosomes. These proteins are called structural proteins.

There is another group of proteins called enzymes. Enzymes are present in the membrane systems, in the mitochondria, in special vacuoles and in the fluid part of the cytoplasm. Enzymes control the chemical reactions that keep the cell alive.

Although there are many different types of protein, all contain carbon, hydrogen, oxygen and nitrogen, and many contain sulfur. Their molecules are made up of long chains of simpler chemicals called amino acids Figure 2.7

Figure 2.7 Protein molecule (part of)


This is a category of substances which, in their chemical structure at least, have little in common. Plants can make their own vitamins. Animals have to obtain many of their vitamins ready-made. Vitamins, or substances derived from them, play a part in chemical reactions in cells – for example those which involve a transfer of energy from one compound to another. If cells are not supplied with vitamins or the substances needed to make them, the cell physiology is thrown out of order and the whole organism suffers. One example of vitamin is ascorbic acid (vitamin C).


Most cells contain about 75% water and will die if their water content falls much below this. Water is a good solvent and many substances move about the cells in a watery solution.

Synthesis and conversion in cells

Cells are able to build up (synthesise) or break down their proteins, lipids and carbohydrates, or change one to another. For example, animal cells synthesise glycogen from glucose by joining glucose molecules together Figure 2.3; plant cells synthesise starch and cellulose from glucose. All cells can make proteins from amino acids and they can build up fats from glycerol and fatty acids. Animal cells can change carbohydrates to lipids, and lipids to carbohydrates; they can also change proteins to carbohydrates but they cannot make proteins unless they are supplied with amino acids. Plant cells, on the other hand, can make their own amino acids starting from sugars and salts. The cells in the green parts of plants can even make glucose starting from only carbon dioxide and water.


There are about 20 different amino acids in animal proteins, including alanine, leucine, valine, glutamine, cysteine, glycine and lysine. A small protein molecule might be made up from a chain consisting of a hundred or so amino acids, e.g. glycine-valine-cysteine-leucine-glutamine-, etc. Each type of protein has its amino acids arranged in a particular sequence.

The chain of amino acids in a protein take up a particular shape as a result of cross-linkages. Cross-linkages form between amino acids that are not neighbors, as shown in Figure 2.8. The shape of a protein molecule has a very important effect on its reaction with substances. For example, the shape of an enzyme molecule creates an active site, which has a complementary shape to the substrate molecule on which it acts. This makes enzymes very specific in their action (they usually only work on one substrate).

Antibodies are proteins produced by white blood cells called lymphocytes. Each antibody has a binding site, which can lock onto pathogens such as bacteria. This destroys the pathogen directly, or marks it so that it can be detected by other white blood cells called phagocytes. Each pathogen has antigens on its surface that are a particular shape, so specific antibodies with complementary shapes to the antigen are needed.

When a protein is heated to temperatures over 50C, the cross-linkages in its molecules break down; the protein molecules lose their shape and will not usually regain it even when cooled. The protein is said to have been denatured. Because the shape of the molecules has been altered the protein will have lost its original properties.

Figure 2.8 A small imaginary protein made from only five different kinds of amino acid. Note that cross-linkage occurs between cysteine molecules with the aid of sulfur atoms

Egg-white is a protein. When it is heated, its molecules change shape and the egg-white goes from a clear, runny liquid to a white solid and cannot be changed back again. The egg-white protein, albumen, has been denatured by heat.

Proteins from enzymes and many of the structures in the cell, so if they are denatured the enzymes and the cell structures will stop working and the cell will die. Whole organisms may survive for a time above 50C depending on the temperature, the period of exposure and the proportion of the cells that are damaged.



  1. What do the chemical structures of carbohydrates and fats have in common ?
  2. How do their chemical structures differ ?
  3. Suggest why there are many more different proteins than there are carbohydrates.


After studying the chapter you should know and understand the following:

  • Living matters is made up of a number of important types of molecules, including proteins, lipids and carbohydrates.
  • All three types of molecule contain carbon, hydrogen and oxygen atoms; proteins also contain nitrogen and sometimes phosphorus or sulfur.
  • Carbohydrates are made from monosaccharide units, often glucose.
  • Carbohydrates are used as an energy source; glycogen and starch make good storage molecules. Cellulose gives plant cell walls their strength.
  • Proteins are built up from amino acids joined together by chemical bonds.
  • Lipids include fats, fatty acids and oils.
  • Fats are made from fatty acids and glycerol.
  • Proteins and lipids form the membranes outside and inside the cell.
  • Food test are used to identify the main biological molecules.
  • Water is important in living things as a solvent.

  • In different proteins the 20 or so amino acids are in different proportions and arranged in different sequences.
  • The structure of a protein molecule enables it to carry out specific roles as enzymes and antibodies.
  • DNA is another important biological molecule. It has a very distinctive shape, made up of nucleotides containing bases.
  • Water has an important role as a solvent in organisms.