Class 10 Science Chapter 13 Our Environment NCERT Solutions 2026 PDF Download

Ans: Decomposers (bacteria/fungi) have specific enzymes that can break down specific materials.
1. Biodegradable: Substances (like wood, paper) that can be broken down by these enzymes are called biodegradable.
2. Non-biodegradable: Substances (like plastic, glass) for which decomposers do not have specific enzymes cannot be broken down and persist in the environment.

Ans: 1. Biomagnification: They enter the food chain and accumulate in higher organisms, causing health hazards (e.g., Pesticides).
2. Pollution: They clog drains, block soil porosity affecting plant growth, and release toxic fumes if burnt.

Ans: The various steps in a food chain where the transfer of food (or energy) takes place are called trophic levels.
Example: Grass → Grasshopper → Frog → Snake.
1. Grass: First Trophic Level (Producer).
2. Grasshopper: Second Trophic Level (Primary Consumer).
3. Frog: Third Trophic Level (Secondary Consumer).
4. Snake: Fourth Trophic Level (Tertiary Consumer).

Ans: Decomposers (bacteria and fungi) break down complex organic remains of dead plants and animals into simple inorganic substances. These substances go back into the soil and are reused by plants. Thus, they act as cleaning agents of the environment and recycle nutrients.

Ans: Ozone ($O_3$) is a molecule formed by three atoms of oxygen.
Function: It forms a protective layer in the stratosphere that absorbs harmful Ultraviolet (UV) radiation from the Sun.
Effect: Without it, UV rays would reach Earth, causing skin cancer in humans, cataracts, and damage to crops/immune systems.

Ans: 1. Segregation: Separate biodegradable (wet) and non-biodegradable (dry) waste at the source. Composting wet waste can create manure.
2. Reduce & Reuse: Minimize the use of single-use plastics and reuse items like glass jars and plastic bags instead of throwing them away.

Ans: Proposed by Lindeman, it states that only 10% of the energy available at one trophic level is transferred to the next trophic level. The remaining 90% is lost as heat to the environment or used in life processes (digestion, respiration, etc.).

Ans: The energy captured by autotrophs from the sun cannot revert to the sun. Similarly, energy passed to herbivores cannot come back to autotrophs. As energy moves progressively through trophic levels, it is lost as heat and is no longer available to the previous level.

Ans: It will cause an ecological imbalance.
- The organisms of the previous trophic level will increase in number (no predators).
- The organisms of the next trophic level will die of starvation (no food).

Ans:

• Natural: Operates naturally without human interference (e.g., Forest, Pond, Lake).
• Artificial: Created and maintained by humans (e.g., Aquarium, Crop field, Garden).

Ans: Chlorofluorocarbons (CFCs), used in refrigerators, ACs, and fire extinguishers.

Ans: 1. High-energy UV radiation splits molecular oxygen ($O_2$) into free oxygen atoms ($O$).
$O_2 \xrightarrow{UV} O + O$
2. These highly reactive free atoms combine with molecular oxygen to form Ozone.
$O + O_2 \rightarrow O_3$ (Ozone).

Ans: It is the phenomenon where the concentration of harmful non-biodegradable chemicals (like DDT) increases at each successive trophic level.
Example: Water (0.01 ppm) → Plankton (1 ppm) → Fish (10 ppm) → Bird (100 ppm).
Since humans occupy the top position in most food chains, these chemicals accumulate in maximum concentration in human bodies.

Ans: Using the 10% law (Energy increases by 10 times as we go backwards):
1. Lion: 100 J.
2. Deer: $100 \times 10 = 1,000$ J.
3. Plant: $1,000 \times 10 = 10,000$ J.
So, 10,000 J was available to the producer.

Ans: 1. An aquarium is an incomplete ecosystem often lacking adequate decomposers to break down waste (fish excreta, uneaten food). This creates toxic ammonia.
2. A natural pond has a natural balance of decomposers and producers to recycle waste naturally. Hence, aquariums need manual cleaning/water changes.

Ans: According to the 10% law, energy decreases significantly at each step. By the 4th trophic level, the energy available is so small that it cannot sustain a viable population of predators at a 5th level.

Ans: In nature, food chains are not isolated straight lines. An organism can be eaten by multiple predators, and a predator can eat multiple types of prey. This network of interconnected food chains is called a Food Web.

Ans: Excessive generation of waste, especially single-use plastics and e-waste (electronic waste), which are difficult to dispose of and pollute the soil and water.

Ans: The Sun is the ultimate source of energy. Plants capture about 1% of the solar energy falling on leaves to convert it into food energy.

Ans: No. While some bacteria cause diseases (pathogens), many are essential as decomposers (recycling nutrients) and nitrogen fixers (Rhizobium) in the ecosystem.

Ans: UNEP stands for United Nations Environment Programme. In 1987, it succeeded in forging an agreement (Montreal Protocol) to freeze CFC production at 1986 levels to protect the Ozone layer.