Why are some substances biodegradable and some non-biodegradable?
Substances that are broken down by biological processes are said to be biodegradable. Microorganisms like bacteria and fungi act as decomposers and break down these substances into simpler forms.
Some substances are non-biodegradable because enzymes are specific in their action. Specific enzymes are needed for the breakdown of a particular substance. Many human-made materials like plastics cannot be broken down by bacteria or saprophytes because the required enzymes are not available. These materials may be acted upon by physical processes like heat and pressure, but under ambient environmental conditions, they persist for a long time and may harm the ecosystem.
Explanation
The textbook explains that biodegradable substances are those broken down by biological processes, while non-biodegradable substances are not broken down this way. The key reason for this difference is enzyme specificity - enzymes are specific in their action and particular enzymes are needed for breakdown of specific substances. Human-made materials like plastics lack the corresponding enzymes that can break them down, so they persist in the environment.