Chapter 13 Solutions — Our Environment

Two ways in which biodegradable substances affect the environment:

1. Nutrient recycling: When biodegradable substances (like dead leaves, animal waste, food scraps) decompose, they are broken down by microorganisms into simpler inorganic substances such as CO2, water, and mineral salts. These nutrients are returned to the soil and atmosphere, making them available for plants and other organisms. This maintains the fertility of the soil and keeps biogeochemical cycles running.

2. Pollution from excessive accumulation: When large quantities of biodegradable waste accumulate faster than decomposers can break them down (e.g., sewage discharge into water bodies, organic waste in landfills), they cause problems:

- Decomposition of excess organic matter in water bodies uses up dissolved oxygen, leading to the death of aquatic organisms.

- Accumulation of rotting waste produces foul smell and breeds disease-causing organisms.

- Excessive organic matter can lead to eutrophication of water bodies.

Two ways in which non-biodegradable substances affect the environment:

1. Biological magnification (biomagnification): Non-biodegradable chemicals like pesticides (DDT) and heavy metals enter food chains and accumulate at progressively higher concentrations at each trophic level. For example, DDT sprayed on crops enters water bodies, is absorbed by aquatic plants, and then accumulates in increasing concentrations through fish, birds, and other predators. Top-level consumers (including humans) end up with dangerously high concentrations, causing health problems such as kidney damage, reproductive failure, and cancer.

2. Soil and water pollution: Non-biodegradable substances like plastics, glass, and e-waste persist in the environment for hundreds or thousands of years. Plastics clog drains, contaminate soil, and break into microplastics that enter water supplies and food chains. Heavy metals from industrial waste leach into groundwater, making it unfit for drinking and irrigation. These substances cannot be broken down naturally, so they keep accumulating in the environment.