Water molecules take part in a great many vital chemical reactions. For example, in green plants, water combines with carbon dioxide to form sugar. In animals, water helps to break down and dissolve food molecules. Blood is made up of cells and a liquid called plasma. This plasma is 92% water and acts as a transport medium for many dissolved substances, such as carbon dioxide, urea, digested food and hormones. Blood cells are carried around the body in the plasma.

Water also acts as a transport medium in plants. Water passes up the plant from the roots to the leaves in xylem vessels and carries with it dissolved mineral ions. Phloem vessels transport sugars and amino acids in solution form the leaves to their places of use or storage.

Water plays an important role in excretion in animals. It acts as a powerful solvent for excretory materials, such as nitrogenous molecules like urea, as well as salts, spent hormones and drugs. The water has a diluting effect, reducing the toxicity of the excretory materials.

The physical and chemical properties of water differ from those of most other liquids but make it uniquely effective in supporting living activities. For example, water has a high capacity for heat (high thermal capacity). This means that it can absorb a lot of heat without its temperature rising to levels that damage the proteins in the cytoplasm. However, because water freezes at 0C most cells are damaged if their temperature fall below this and ice crystals form in the cytoplasm. (Oddly enough, rapid freezing of cells in liquid nitrogen at below – 196C does no harm them).

Role of water in animals

  1. About 70% of the human body consists of water. Water is found in cell cytoplasm, blood, digestive juices, tissue fluid, fluid in joints and contained within organs i.e. spinal cord, the brain, the eyes, gastrointestinal tract, etc.
  2. Water moderates body temperature. It has high specific heat capacity, which means that a lot of energy is required to raise the temperature of water by 1C. Hence , water helps the cell resist changes in temperature.
  3. It plays a role in evaporative cooling. Water is a component of sweat, which removes heat from the body when it evaporates.
  4. Water is a reactant in certain chemical reactions in the body, such as the hydrolysis of food molecules during digestion.
  5. Water is a component of body fluids with lubricative or protective properties such as lubricants in joints, coating the stomach lining, mucus in the oesophagus, and cervical mucus in the female reproductive system.
  6. Water is an extremely versatile solvent. More things dissolve in water than in any other solvent. Because of this property,
    • water is the medium in which chemical reactions take place in living organisms, and
    • water serves as a transportation medium. It transports water-soluble food products from the small intestine to other parts of the body and waste materials from cells to the excretory organs for removal. It transports hormones to the target organs or tissues. Blood is the main transport medium in the body.

Role of water in plants

  1. Water is a key reactant in photosynthetic processes.
  2. It provides physical support to the plant in the form of turgor pressure.
  3. Water is required to transport dissolved mineral salts from the roots to other parts of the plant through xylem vessels.
  4. Water is required to transport sugars made in the leaves to other parts of the plant.

Structure and properties of water

Water is the main component of living things. Most human cells are approximately 80% water. Water provides the environment in which the biochemical reactions of life can occur. It also take part in and is produced by many reactions. Two of its most important properties (its solvent properties and its heat capacity) are due to its molecular structure, which consists of two hydrogen atoms each bonded to an oxygen atom by a covalent bond.

The water molecule is unusual because it has a small positive charge on the two hydrogen atoms and a small negative charge on the oxygen atom. Because of this arrangement, water is said to be a polar molecule. Polar molecules are those that have an unevenly distributed electrical charge so that there is a positive region and a negative region. Sugars and amino acids are also polar molecules.

A weak bond can form between the negative charge of one water molecule and the positive charge of another. This type of bond, known as hydrogen bond, is responsible for many of the properties of water.

Hydrogen bonds between water molecules hold them together in a network, resulting in a phenomenon known as cohesion. Cohesive forces give water, many of its biologically important properties. For example, they enable water to be drawn up inside the xylem of a plant stem in a continuous column. Strong pulling forces, produced as water evaporates from the leaves at the top of tall trees, draw water and dissolved minerals up great distances to the tips of branches high above the ground. Cohesion is also responsible for surface tension, which enables some small organisms to ‘walk on water’, and contributes to the thermal properties of water too.

Water has unusual thermal properties. A large amount of energy is needed to break the many weak hydrogen bonds between the water molecules. This gives water a high specific heat capacity – it can absorb or give of a great deal of heat energy without its temperature changing very much. A stable temperature is important to living things because the range of temperatures in which biological reactions can occur is quite narrow. The thermal properties of water allow it to keep an organism’s temperature fairly constant. Within the body, water can act as a temperature regulator – for example, blood carries heat from warmer parts of the body, such as the liver, to cooler parts such as the feet.

When liquid water evaporates and becomes vapour, many hydrogen bonds between the molecules must be broken, so evaporation requires a lot of energy. As a result, water is a liquid at most temperatures found on Earth, and it has a high boiling point. When it evaporates – for example, when an animal sweats – it carries a great deal of heat with it and thus acts as a coolant for the body.

Water is sometimes known as a universal solvent. It’s polarity makes it an excellent solvent for other polar molecules. Most inorganic icons, such as sodium, potassium and chloride ions, dissolve well as their positive or negative charges are attracted to the charges of water molecules. Polar organic molecules, such as amino acids and sugars, are also soluble in water. Water is the medium in which most biochemical reactions take place since almost all the substances involved dissolve well in it. Protein synthesis and most of the reactions of photosynthesis and respiration take place in an aqueous (water) solution.

The solvent properties of water also make it an excellent medium for transporting substances around the bodies of all organisms. In plants, the xylem carries dissolved minerals from the roots to the leaves, while the phloem transports soluble sugars up and down the plant. Many animals have blood as their transport medium. Blood is predominantly water, and the blood plasma carries dissolved sugars, amino acids and carbon dioxide as well as many other solutes.

The properties of water are summarised in Table below

The positive and negative charges of water molecules attract ions with negative or positive charges so that they dissolve.
cohesionHydrogen bonds hold water molecules togetherWater can travel in continuous columns, for example in the stems of plants, and act as a transport medium
solventThe polar molecules of water can interact with other polar moleculesIons dissolve easily. Large molecules with polar side groups, such as carbohydrates and proteins, can also dissolve. So water acts as an excellent transport medium and as a medium for metabolic reactions
thermalWater has a high heat capacity. Large amounts of energy are needed to break hydrogen bonds and change its temperatureThe temperature of organisms tends to change slowly. Fluids such as blood can transport heat around their bodies
Water has a high boiling point compared with other solvents because hydrogen bonds need large amounts of energy to break themWater is liquid at most temperatures at which life exists, so is a useful medium for metabolic reactions
Water evaporates as hydrogen bonds are broken and heat from water is usedSweating and transpiration enable animals and plants to lose heat. Water acts as a coolant.
Table: Summary of the properties of water

Simple carbohydrates

  1. Carbohydrates are organic molecules made up of carbon, hydrogen, and oxygen with the general formula for most carbohydrates being Cn H2n On.
  2. Carbohydrates are classified into 3 main groups: monosaccharides, disaccharides and polysaccharides depending on the number of basic sugar units they have.
  3. Monosaccharides are the most basic unit of carbohydrates and are the simplest form of sugars. Common examples are glucose, fructose and galactose.
  4. Disaccharides are formed when two monosaccharides undergo a condensation reaction. Common examples are maltose (formed by 2 glucose units), sucrose (1 glucose, 1 fructose) and lactose (1 galactose, 1 glucose).
  5. A condensation reaction is a chemical reaction when two molecules combine together to form a single molecule with the elimination of a water molecule.
  6. A disaccharide can be split into its component monosaccharides by undergoing hydrolysis in which a water molecule is added to the disaccharide to break it down into its component monosaccharides. Enzymes are usually required for this process.

Complex carbohydrates

  • Polysaccharides include starch, glycogen and cellulose. They are long chains of glucose molecules linked together in condensation reactions. Each chain may contain thousands of glucose molecules.
  • In starch, the glucose molecules are linked together in long straight chains or branched chains. It is a storage molecule in plants.
A starch molecule
  • In glycogen, the glucose molecules are linked together in highly branched chains. It is a storage molecule in animals and fungi.
A glycogen molecule
  • In cellulose, the glucose molecules are linked in long straight chains. The linkage between the glucose molecules is not the same as that in starch. Cellulose is the tough material found in cell walls of plants. Cellulose is the fibre necessary in a health diet.
A cellulose molecule
  • Glycogen and starch are the storage forms of glucose in animal and plant cells respectively. This is because
    • they are insoluble in water and do not affect water potential in cells,
    • they are too large to diffuse out of the cells and thus remain within the cells,
    • they have compact shapes, and
    • they can be easily hydrolysed into glucose for cellular respiration.


  • Fats (lipids) are organic molecules made up of carbon, hydrogen and oxygen. There is no general formula for fats. The ratio of hydrogen to oxygen is much higher in fats than in carbohydrates, where the ratio of hydrogen to oxygen is 2 : 1.
  • Fats are made from two types of smaller molecules: glycerol and fatty acids. Each fat molecule contains a glycerol molecule and 3 fatty acids. Each fatty acid is linked to the glycerol backbone in a condensation reaction.
A fat molecule
  • When 3 water molecules are added to a fat molecule with the help of enzymes in a hydrolysis reaction, the fat molecule breaks down into fatty acids and glycerol.
  • Fats are storage molecules that can store a large amount of energy.
  • They are also an important component of cell membranes.
  • Fats are used to make steroids and certain hormones.
  • Fats are also used as insulating material to prevent the loss of body heat.
  • Fat is also a solvent for fat-soluble vitamins.


  • Proteins are complex organic molecules made up of carbon, hydrogen, oxygen and nitrogen. They may also contain sulfur.
  • In the form of enzymes, proteins participate in all cellular processes and are responsible for almost everything living organisms do.
  • There are tens of thousands of different proteins, each serving a different function and having a unique structure.
  • Proteins are made up of amino acids.
  • An amino acid is a molecule with the general structure:
An amino acid
  • There are about 20 different naturally-concurring amino acids which have different side chains (also known as R groups).
  • Amino acids are combined in many different ways to form different protein molecules.
  • Amino acids link up in a condensation reaction to form a polypeptide chain. The bonds between the amino acids are known as peptide bonds.
  • Proteins are made of one or more polypeptide chains twisted, folded and coiled into a unique 3-dimensional structure.
  • The bonds between the amino acids, peptide bonds, are strong but the bonds that hold the 3-dimensional coiled structures together are weak and can easily be broken by heat or by changes in pH. Examples of such bonds are hydrogen bonds, ionic interactions and van der Waals interactions.
A protein molecule
  • When these bonds are broken, the protein loses its 3-dimensional conformation. This process is called denaturation. Proteins can be denatured if they are heated or placed in an environment with unsuitable pH. Denaturation usually leads to loss of function as proteins require their 3-dimensional shape to function. Denaturation can also cause proteins to lose their solubility and precipitate out of the solution.
  • Many proteins are enzymes, which catalyse chemical reactions within our body.
  • Structural proteins found in muscle cells play a role in movement.
  • Other proteins take part in cell growth, repair and reproduction.
  • Antibodies are proteins in our body that help us fight diseases.

Summary of the main nutrients

NutrientElements presentExamplesSub-units
carbohydratecarbon, hydrogen, oxygenstarch,glycogen, cellulose, sucroseglucose
(oils are liquid at room
temperature, but
fats are solid)
carbon, hydrogen, oxygen
(but lower oxygen content
than carbohydrates)
vegetable oils, e.g. olive
oil; animal fats, e.g. cod
liver oil, waxes
fatty acids and glycerol
proteincarbon, hydrogen, oxygen, nitrogen, sometimes sulfur
or phosphorus
enzymes, muscle, haemoglobin, cell
amino acids (about 20
different forms)