Our Environment

Why This Matters

Look around you. Grass, insects, a tiger, the soil, the air, the sunlight. They all depend on each other. The grass needs sunlight. The insect eats the grass. The tiger eats animals that eat the grass. If you remove one of them, the others are affected too. All of these living and non-living things together are called the environment. And you are a part of it.

This chapter connects to things you see in the news. Pesticides are sprayed on a field. So why do they end up inside the fish you eat? Years ago there was a worldwide worry about a “hole in the ozone layer”. Why? A plastic wrapper stays in the ground for hundreds of years, but a banana peel rots away in a few weeks. Why the difference? These may look like separate stories. But they all come from the same two ideas: how energy and food move through nature, and what goes wrong when humans disturb that flow.

This is not just for the exam. Once you understand it, you think twice before throwing a wrapper on the ground. Because now you know exactly where it goes and how long it stays.

The Big Idea

An ecosystem means living things and their non-living surroundings working together as one system. The living things are called biotic. The non-living things are called abiotic. Energy moves through this system in one direction. It comes from the Sun, goes to plants (producers), then to animals (consumers). At each step, about 90% of the energy is lost, and only about 10% moves on. But food materials (nutrients) are different. They are recycled again and again by decomposers. When humans disturb this system — by spraying pesticides, releasing CFCs, or dumping plastic and other non-biodegradable waste — the whole system suffers.

There are two flows here. Keep them separate in your mind. First, energy flows in and is lost. It goes one way only: Sun → plants → animals → and finally escapes as heat. It never comes back. Second, nutrients go round and round in a cycle: soil → plants → animals → decomposers → back to soil. Almost everything in this chapter is built on these two ideas.

Let’s Break It Down

What is an ecosystem?

An ecosystem is all the living things in one area, together with the non-living things around them. They all interact with each other and work as one unit. (An organism just means any living thing — a plant, an animal, a tiny germ.)

The parts of an ecosystem are of two types:

• Biotic components — the living parts. For example, plants, animals, and tiny living things like bacteria.
• Abiotic components — the non-living, physical parts. For example, temperature, rainfall, wind, soil, and minerals.

A garden, a forest, a pond, and a lake are all ecosystems. Some ecosystems are natural, like a forest or a pond. They run on their own, without any human help. Others are artificial, like a garden, a crop field, or an aquarium. These are made and looked after by humans.

Inside an ecosystem, living things have three main jobs, based on how they get their food:

• Producers — green plants and some bacteria. They make their own food from sunlight, through a process called photosynthesis. They are the starting point of all food, so they sit at the base of everything.

But how exactly does a plant turn sunlight into food? Here is a quick reminder from Life Processes before we go on.

• Consumers — living things that cannot make their own food, so they eat producers or other consumers. There are different kinds. Herbivores eat only plants (like a cow or deer). Carnivores eat only meat (like a lion). Omnivores eat both plants and meat (like humans). Parasites live on or inside another living thing and take food from it (like a tapeworm).
• Decomposers — bacteria and fungi. When a plant or animal dies, decomposers break down its dead body and waste into simple substances. These simple substances go back into the soil, and plants use them again. So decomposers act like nature’s cleaners and recyclers.

Food chains, food webs and trophic levels

A food chain is a line of living things, where each one eats the one before it. For example: grass → deer → tiger. The grass is eaten by the deer, and the deer is eaten by the tiger. Each feeding step in this line is called a trophic level — it is simply the position of an organism in the food chain, based on what it eats.

Stacked up, these levels form a pyramid — wide at the bottom, narrow at the top. Figure 13.1 below shows why, and names each level.

Tap to zoom

• 1st level — Producers (the green plants). Sunlight falls on their leaves. They catch only about 1% of that sunlight and turn it into food energy.
• 2nd level — Primary consumers. These are the plant-eaters (herbivores).
• 3rd level — Secondary consumers. These are small meat-eaters that eat the herbivores.
• 4th level — Tertiary consumers. These are large meat-eaters that eat the smaller meat-eaters.

Now here is the most important rule of this chapter, called the 10% rule. When one trophic level is eaten by the next, only about 10% of its energy is passed on. The other 90% is lost. It is used up as heat, and in digesting food, moving about, and other life activities. So very little energy is left at each step. Because of this, a food chain almost never has more than three or four steps. There simply isn’t enough energy left to support a fifth level.

But why exactly 10%? Where does the missing 90% actually go? It does not vanish — it slips away in four ordinary ways. Think of a deer that eats 100 units of energy from grass. A big chunk is lost as body heat: the deer’s body stays warm, and that warmth keeps glowing away into the cool air. Another chunk is burned just to stay alive — walking, breathing, digesting, and pumping blood all cost energy. A third chunk simply passes out as dung and waste, because the deer cannot digest every part of the grass. After all that leaks away, only about 10 units are left stored inside the deer’s body as new flesh. And new flesh is the only part a tiger can eat. So when the tiger eats the deer, it can collect at most those 10 units. That is the whole reason only about 10% moves up. The next animal can never eat the heat that has already escaped, or the energy already burned, or the dung.

Figure 13.2 below follows those 100 units and shows each leak, so you can see the 10% appear on its own.

Tap to zoom

This also explains why chains stay short. Start the chain with 10,000 units and you reach 10 units by the fourth level. A fifth level would have only about 1 unit to share — far too little to feed a whole population of animals.

In real life, things are not so neat. One animal is usually eaten by several different animals, and it also eats several different things. So these straight food chains cross and join with each other. Many food chains linked together like this form a food web. Figure 13.3 below shows one.

Tap to zoom

Let’s put the 10% rule to work and watch how fast the energy shrinks across just a few steps.

Energy flow is one-way

The flow of energy has two main features:

• It is one-way (the science word is unidirectional). Energy goes from the Sun to producers to consumers, and it never comes back. Once energy leaves a level, it is gone from that level forever.
• It gets smaller at every level, because about 90% is lost at each step. This is the exact reason the energy pyramid gets narrower as you go up, and why food chains are short.

Biological magnification

Here is a worrying part of the story. Farmers spray pesticides and other chemicals on their crops to kill pests. These chemicals wash into the soil and water. Plants take them in, and so the chemicals enter the food chain. The problem is that many of these chemicals do not break down. They stay inside the body of any animal that eats them.

Now think about what happens up the food chain. A small fish eats many tiny plants, so it collects all their chemicals. A big fish eats many small fish, so it collects even more. So the chemical builds up more and more at each higher trophic level. And humans sit at the top of most food chains. This means the largest amount of chemical ends up in our bodies. This step-by-step build-up of chemicals up the food chain is called biological magnification (also written as biomagnification). It is the reason why grains, vegetables, fruit, and even meat often carry small amounts of pesticide, and why simply washing them does not remove it all.

You can actually see this build-up in Figure 13.4 below — watch the toxin dots grow more crowded at each step up the chain.

Tap to zoom

Ozone layer and its depletion

Ozone (O₃) is a gas. Its molecule is made of three oxygen atoms joined together. (Normal oxygen, the kind we breathe, is O₂, with only two atoms.) Down here near the ground, ozone is harmful to breathe. But high up in the atmosphere, ozone does a very important job. It acts like a shield. It stops most of the Sun’s harmful ultraviolet (UV) rays from reaching Earth. These UV rays can cause skin cancer and other damage to living things.

How does this ozone form high up? UV light hits a normal oxygen molecule (O₂) and splits it into two separate oxygen atoms. Each free oxygen atom then joins another O₂ molecule to make ozone (O₃):

O₂ → (UV) → O + O, then O + O₂ → O₃

In the 1980s, scientists found that the amount of ozone was dropping fast. The cause was a group of man-made chemicals called chlorofluorocarbons (CFCs). CFCs were used in refrigerators and fire extinguishers.

But here is the puzzle that worried scientists most: only a tiny amount of CFC was floating up there, so how could it destroy so much ozone? The answer is that one CFC does not just break one ozone molecule and stop. It sets off a chain reaction that repeats again and again. Here is how. High up, sunlight (UV) hits a CFC and knocks loose a single chlorine atom (Cl). This free chlorine then smashes into an ozone molecule (O₃), steals one of its oxygen atoms, and leaves behind ordinary oxygen (O₂). The ozone is now destroyed. The chlorine is now stuck to that oxygen as ClO. So far that is one ozone gone. But then the ClO bumps into a loose oxygen atom, hands over its oxygen — and the chlorine atom is set completely free again, exactly as it started. Now it is ready to attack the next ozone. And the next. And the next. Because the chlorine keeps coming back unharmed, a single chlorine atom can go on to destroy thousands of ozone molecules before it finally settles. That is why such a small amount of CFC did such huge damage.

Follow the chlorine atom in Figure 13.5 below — notice how the dashed loop sends it back to attack ozone over and over.

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To stop the damage, the countries of the world came together. In 1987 the United Nations made an agreement called the Montreal Protocol. It aimed to slowly stop the making of CFCs. Today, fridges must be made without CFCs everywhere in the world.

Biodegradable and non-biodegradable waste

To understand this, you need one idea: enzymes are specific. An enzyme is a chemical inside living things that breaks down substances. But each enzyme can break down only one particular kind of substance, not just anything. This is why we cannot digest coal, and why bacteria cannot break down many man-made materials — there is no enzyme that fits them.

This gives us two kinds of waste:

• Biodegradable substances can be broken down by living things (bacteria and other decomposers). For example: vegetable peels, paper, leftover food, leather, and wood. (“Bio” means life, so biodegradable means “broken down by living things”.)
• Non-biodegradable substances cannot be broken down by living things. For example: plastics, glass, and metals. They stay in the environment for a very long time and can harm ecosystems.

Figure 13.6 below puts the two side by side, so you can see how differently they end up — one recycled in weeks, the other lingering for centuries.

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Now try sorting a real bin. Watch how one simple question tells you which pile each item belongs in.

Common Mistakes

A few ideas in this chapter trip up almost everyone. Here are the four that catch students most often, starting with the easiest mix-up of all.

Quick Check

Before moving to practice, test yourself on the four big ideas — energy, decomposers, the ozone layer, and waste.

Now put two ideas together in your own words. Try to explain this one fully before reading the answer.

Practice Problems

Try each question on your own first, then tap to check your working. They run from easy to challenge.

Summary

• An ecosystem = biotic (living) parts + abiotic (non-living) parts, all working together as one. It can be natural (forest, pond) or artificial (garden, aquarium).
• Living things have three jobs: producers make their own food, consumers eat others, and decomposers break down dead matter and return it to the soil.
• A food chain is a line of feeding steps (each step is a trophic level). Many food chains joined together form a food web.
• Energy flows one way and keeps getting smaller. Only about 10% passes to the next level (the 10% rule), so chains are limited to 3 or 4 steps. Matter, though, is recycled by decomposers.
• Biological magnification: chemicals that do not break down (like pesticides) become more concentrated up the food chain. Humans, at the top, get the most.
• The ozone layer shields Earth from UV rays. CFCs made it thin. This led to the 1987 UN agreement to stop making CFCs.
• Waste is either biodegradable (broken down by decomposers) or non-biodegradable (stays for a long time and harms the environment). Both need to be managed responsibly.

What’s Next

This is the end of your Class 10 Science journey. You started with the chemistry of carbon and reactions. Then you learned about life processes and how living things reproduce and pass on features. After that came light, electricity, and magnetism. And now you have reached the whole living planet that you are part of.

See how this last chapter ties everything together. Photosynthesis, which you learned in Life Processes, is what builds the producers. Energy, from the Electricity chapter, flows and slowly turns into heat. And chemistry explains why CFCs and plastics behave the way they do. So science is not a set of separate boxes. It is one connected way of understanding the world. Keep asking why, keep noticing how things link up, and you will keep learning long after the syllabus is over.