Our Environment Class 10: Notes and Important Questions

Our Environment Class 10 explains how living things and their surroundings work together as ecosystems, how energy moves through food chains, and how human activity affects the ozone layer and creates waste. This guide covers each topic in the chapter's order, works through the ten percent law with numbers, and ends with important questions, model answers and common mistakes.

Ecosystems: biotic and abiotic parts

An ecosystem is a functional unit of nature in which living organisms interact with each other and with their physical surroundings.

Ecosystems can be natural (forests, ponds, lakes) or artificial, that is human-made (gardens, crop fields, aquariums). An aquarium is artificial because people must keep it going by cleaning it and providing food or oxygen.

Organisms are grouped by how they get food:

• Producers: green plants and certain bacteria that use the Sun's energy to make organic compounds (sugar and starch) by photosynthesis.
• Consumers: depend on producers directly or indirectly.
• Decomposers: bacteria and fungi that break down dead remains into simple inorganic substances, replenishing the soil.

Without decomposers, dead remains and waste would pile up, the soil would not get its nutrients back, and in time the producers and the whole ecosystem would suffer.

Food chains, trophic levels and food webs

A food chain is a series of organisms in which each one eats the one below it and is eaten by the one above. Each step is a trophic level.

In nature, one organism is usually eaten by several others, so food chains branch and link into a food web. If one food source fails, a consumer may have alternatives, which keeps the ecosystem stable.

Energy flow and the ten percent law

• Green plants capture only about 1% of the sunlight falling on their leaves.
• At each level, much energy is lost as heat and used for digestion, growth and reproduction.
• On average, only 10% of the food eaten becomes the organism's own body and is available to the next level.

So energy flow is unidirectional: it never returns from herbivores to plants or from plants to the Sun. With about 90% lost at every step, food chains rarely go beyond four steps, and there are generally more individuals at the lower levels, especially producers.

Worked example 1: Producers have 10,000 J.

• Primary consumers: 10,000 × 0.10 = 1,000 J
• Secondary consumers: 1,000 × 0.10 = 100 J
• Tertiary consumers: 100 × 0.10 = 10 J

Worked example 2: Producers have 2000 J. Then 200 J reaches primary consumers, 20 J reaches secondary consumers, and only 2 J reaches tertiary consumers.

Biological magnification

Pesticides and other chemicals used on crops wash into soil and water. Plants absorb them and they enter the food chain. Because these chemicals are non-degradable, they are not broken down or excreted, so they build up in organisms. At each higher trophic level the concentration rises. This is biological magnification.

Humans usually sit at the top of food chains, so the highest concentration can build up in our bodies. That is why even washed fruits and vegetables may carry pesticide residues.

Example: In Wheat → Rat → Snake, the snake has the highest pesticide concentration because it is at the highest trophic level.

The ozone layer and its depletion

Ozone (O₃) is a molecule of three oxygen atoms. At ground level it is a deadly poison, but in the upper atmosphere it shields the Earth from harmful ultraviolet (UV) radiation, which can cause skin cancer and harm other organisms.

How ozone forms:

• O₂ —(UV)→ O + O (UV splits oxygen molecules into free atoms)
• O + O₂ → O₃ (a free atom joins an oxygen molecule)

In the 1980s, the ozone layer began to thin sharply because of synthetic chemicals called chlorofluorocarbons (CFCs), used in refrigeration and fire extinguishers. In 1987, the UNEP forged an agreement to freeze CFC production. Most refrigerators are now CFC-free, helping the ozone layer recover.

O₂ versus O₃: O₂ is needed for respiration by aerobic organisms; O₃ is poisonous at ground level but protective in the upper atmosphere.

Biodegradable and non-biodegradable waste

Enzymes are specific in their action. Decomposer enzymes work on natural organic matter, not on synthetic materials like plastic.

Disposable items and non-biodegradable packaging have increased the amount of waste. Good waste management means separating the two types and using recycling, composting and proper sewage treatment.

The cup example: the textbook compares plastic cups, clay kulhads and paper cups on trains. Plastic persists; making very large numbers of clay cups removes fertile topsoil; paper can biodegrade. The lesson is to judge an alternative by its whole environmental cost, not only by whether it breaks down.

Remember this

• Biotic = living; abiotic = non-living.
• Producers are the 1st trophic level; energy flows one way only.
• Ten percent law: only about 10% passes to the next level.
• Non-degradable chemicals become more concentrated higher up the chain.
• UV forms ozone from oxygen; CFCs deplete ozone.

Important questions with answers

1. Why is an aquarium an artificial ecosystem? It is made by people and needs regular cleaning and supply of food or oxygen to stay balanced.

2. What would happen if all decomposers were removed? Dead remains would pile up and the soil would not be replenished, so producers and then the whole ecosystem would suffer.

3. Name the trophic levels in Grass → Insects → Frog → Snake. Grass: producer (1st). Insects: primary consumer (2nd). Frog: secondary consumer (3rd). Snake: tertiary consumer (4th).

4. If producers have 50,000 J, how much reaches the secondary consumers? Primary: 50,000 × 0.10 = 5,000 J. Secondary: 5,000 × 0.10 = 500 J.

5. Why do food chains rarely have more than four steps? About 90% of energy is lost at each level, so very little is left to support another level.

6. Why do humans often have the highest pesticide levels? Humans are usually at the top of food chains, and non-degradable chemicals become more concentrated at each higher level.

7. Write the reactions for ozone formation. O₂ —(UV)→ O + O, then O + O₂ → O₃.

8. Why can enzymes not break down plastic? Enzymes are specific. Decomposer enzymes act on natural organic matter, not synthetic materials.

Common mistakes to avoid

• Saying energy cycles back to the producers. It flows one way only.
• Applying the 10% rule only once when the question asks about a level two or three steps up.
• Calling ozone harmless everywhere; it is a poison at ground level.
• Assuming any material that breaks down is automatically harmless.

For guided revision of food chains and energy flow, study this chapter with Joy on Learnijoy.