(a) What is ozone ? How is it formed in the upper layers of the Earth's atmosphere ? How does ozone affect our ecosystem ? OR (b) (i) List two human-made ecosystems. (ii) "We do not clean a pond in the same manner as we do in an aquarium." Give reason to justify this statement.

(a) Ozone ($O_3$): Ozone is a molecule formed by three atoms of oxygen.

Formation in Upper Atmosphere:

Ozone at the higher levels of the atmosphere is a product of ultraviolet (UV) radiation acting on oxygen ($O_2$) molecules. The high-energy UV radiation splits apart molecular oxygen ($O_2$) into free oxygen ($O$) atoms. These highly reactive free atoms then combine with molecular oxygen to form ozone:

$$O_2 \xrightarrow{\text{UV}} O + O$$

$$O + O_2 \rightarrow O_3$$

Effect on Ecosystem:

• Beneficial effect (Stratosphere): It shields the surface of the Earth from harmful ultraviolet (UV) radiation from the Sun, which is known to cause skin cancer in human beings and damage other organisms.
• Harmful effect (Ground level): At the ground level, ozone is a deadly poison.

OR

(b) (i) Two human-made ecosystems:

1. Gardens
2. Crop-fields (or Aquariums)

(ii) Justification:

A pond is a natural, self-sustaining ecosystem. It contains a complete community of producers, consumers, and decomposers (like bacteria and fungi). The decomposers naturally break down dead organic remains and waste products into simple inorganic substances, which are recycled back into the soil and water.

An aquarium, on the other hand, is a human-made, incomplete ecosystem. It lacks a balanced population of decomposers to clean up organic wastes and dead matter. Consequently, if left uncleaned, toxic waste products accumulate, which can harm or kill the aquatic organisms. Thus, it must be cleaned regularly.