1.3 Inorganic compounds

The role of water in the maintenance of life (ESG46)

As mentioned in Table 1.1, up to \(\text{65}\%\) of our bodies are made up of water. Water is an inorganic compound made up of two hydrogen atoms and one oxygen atom. Its molecular formula is \(\text{H}_{2}\text{O}\). Water plays an important role in the maintenance of biological systems.

Temperature regulation: in humans, the sweat glands produce sweat which cools the body as it evaporates from the body surface in a process called perspiration. In a similar way, plants are cooled by the loss of water vapour from their leaves, in a process called transpiration.

Form and support: water is an important constituent of the body and plays an important role in providing form and support in animals and plants. Animals, such as worms and jellyfish, use water in special chambers in their body to give their bodies support. This use of water pressure to provide body form, and enable movement is called a hydrostatic skeleton . Plants grow upright and keep their shape due to the pressure of water ( turgor pressure) inside the cells.

Transport medium: water transports substances around the body. For example, water is the main constituent of blood and enables blood cells, hormones and dissolved gases, electrolytes and nutrients to be transported around the body.

Lubricating agent: water is the main constituent of saliva which helps chewing and swallowing and also allows food to pass easily along the alimentary canal. Water is also the main constituent of tears which help keep the eyes lubricated.

Solvent for biological chemicals: the liquid in which substances dissolve is called a solvent. Water is known as the universal solvent as more substances dissolve in water than in any other liquid.

Medium in which chemical reactions occur: all chemical reactions in living organisms take place in water.

Reactant: water takes place in several classes of chemical reactions. During hydrolysis reactions, water is added to the reaction to break down large molecules into smaller molecules. Water can also be split into hydrogen and oxygen atoms to provide energy for complex chemical reactions such as photosynthesis.

Temperature	Structure and support		Lubrication
Sweating helps human bodies cool down.	Jellyfish and worms use a hydrostatic (water pressure) skeleton to keep their body shape.	Water helps maintain the upright structure of plants.	Water is an important lubricant in the eye.

Table 1.2: The role of water in living organisms.

Minerals (ESG47)

Dietary minerals are the chemical elements that living organisms require to maintain health. In humans, essential minerals include calcium, phosphorous, potassium, sulfur, sodium, chlorine and magnesium.

Macro-elements (macro-nutrients) are nutrients that are required in large quantities by living organisms (e.g carbon, hydrogen, oxygen, nitrogen, potassium, sodium, calcium, chloride, magnesium, phosphorus and sulfur).

Micro-elements (micro-nutrients) are nutrients that are required in very small quantities for development and growth and include iron, cobalt, chromium, copper, iodine, manganese, selenium, zinc and molybdenum.

Nutrients required for human health

Table 1.3 below summarises some important minerals required for proper functioning of the human body. Proper nutrition involves a diet in which the daily requirements of the listed mineral nutrients are met.

Nutrients required for plant growth

The previous section examined the key nutrients important for animal growth. In Table 1.4 we will now look at the key nutrients required for plant growth.

Fertilisers (ESG48)

Use of fertilisers

When crops are regularly grown and harvested on the same piece of land, the soil becomes depleted of one or more nutrients. Fertilisers are natural or non-natural mixtures of chemical substances that are used to return depleted nutrients to the soil, improve the nutrient content of the soil and promote plant growth. Inorganic nutrients (such as nitrates and phosphates) are added to the soil in the form of inorganic fertilisers.

Effect of fertilisers on the environment

Using large amounts of fertilisers can be harmful to the environment. Fertilisers wash off into rivers where they are poisonous to plant and animal life. The accumulation of fertilisers in rivers can lead to a process known as eutrophication. This process occurs when excessive nutrients (nitrates and phosphates) from the land (typically from fertilisers) run off into rivers and lakes. This leads to high growth of water plants. Plants grow and produce food by photosynthesis which requires high quantities of oxygen. The high oxygen demand of the rapidly growing water plants removes oxygen available to other organisms in the rivers and lakes. The overgrowth of water plants also blocks sunlight from entering the water, so that plants underwater can no longer photosynthesise and stop producing oxygen. These two processes combine to deplete the water of oxygen and cause aquatic organisms to suffocate and die. The biodegradation of the dead organisms results in a massive increase in bacteria, fungi and algae degrading the dead organic matter, which also require oxygen. This further depletes the available oxygen, and further contributes to the death of fish and other aquatic species.

Natural fertilisers: an application of indigenous knowledge systems

The fertilisers discussed above are non-natural inorganic compounds such as nitrates, phosphates etc. However, as a means of reducing the negative impact of the inorganic fertilisers discussed earlier, organic fertilisers that occur naturally can be used. Natural fertilisers consist of organic compounds derived from manure, slurry, worm castings, peat, seaweed etc.

Natural fertilisers supply nutrients to the soil through natural processes such as composting. This means that the nutrients are released back to the soil slowly, and excessive nutrients do not wash off into rivers causing over-fertilisation and eutrophication. However, the use of organic fertilisers is more labour-intensive and the nutrient composition tends to be more variable than the inorganic fertilisers. As a result it is difficult to know for sure whether the particular nutrient required by the plant is actually being supplied by the natural fertiliser.