One Parent Makes a Copy, Two Parents Make Something New

Because they are produced by different processes. A plant grown from a cutting has one parent, so it is genetically identical to it. A seed formed from two parents mixes features from both, so the offspring resembles neither exactly.

That difference decides how a species adapts. This page covers everything in the CBSE Class 8 Science chapter's second part: how air, water and sunlight sustain life, soil and rocks, interdependence, and reproduction.

Each supplies something no organism can do without, and all three work together.

Sunlight is the source of nearly all energy. Green plants use it in photosynthesis to make food from carbon dioxide and water, and every animal eats either those plants or something that did. Sunlight also drives the water cycle and warms the planet.

Air supplies two essential gases. Oxygen is used in respiration by animals and plants alike to release energy from food; carbon dioxide is the raw material of photosynthesis. Air also carries water vapour, moderates temperature and shields us from radiation and meteors.

Water is needed by every cell. It makes up most of a living body, dissolves and transports nutrients, carries wastes away, and provides a habitat for aquatic organisms. Plants absorb it through roots and use it in photosynthesis.

And they are linked in cycles — the water cycle moves water between the sea, air and land; the oxygen and carbon cycles exchange gases between living things and the atmosphere.

A paddy field uses all three visibly: standing water, sunlight for the crop and air for its roots and leaves.

The reason these are treated together, rather than as three separate needs, is that each depends on the others. Photosynthesis needs sunlight and water and carbon dioxide — remove any one and the process stops, which is why life needs the whole set rather than the most important one.

Soil anchors plants, holds the water and minerals their roots absorb, and is the habitat of earthworms, insects, bacteria and fungi. Rocks are the source of the minerals, and minerals are the nutrients that plants, and through them animals, need.

How soil is formed. Soil comes from the slow breaking down of rock, called weathering:

- Physical weathering — rock is cracked by heating and cooling, by water freezing in cracks, and worn away by wind, flowing water and moving ice
- Chemical weathering — rainwater and dissolved gases react with the minerals in rock and soften it
- Biological weathering — roots grow into cracks and widen them, and lichens and microorganisms break the surface down

The rock fragments then mix with humus, the dark matter left by decomposers from dead plants and animals, and that mixture is soil.

How soil is renewed. Decomposers continuously return nutrients from dead matter to the soil as humus, and weathering slowly adds fresh mineral particles.

Minerals such as iron, calcium, phosphorus and potassium pass from rock to soil to plant to animal, and decomposers send them back — a cycle rather than a one-way flow.

The fact that matters for farming is how slowly soil forms. Weathering takes a very long time to produce even a thin layer, while erosion by wind and rain can strip it away in a single season once the plant cover is removed. That is why soil is treated as a resource to be protected — through tree planting, contour ploughing and avoiding overgrazing — rather than one that simply replaces itself.

Each supplies something the others cannot make for themselves, so the Earth's systems stay in balance.

Plants provide for animals:

- Food, since only plants can make it from sunlight
- Oxygen, released during photosynthesis
- Shelter, shade and nesting places

Animals provide for plants:

- Carbon dioxide, released in respiration, which plants need for photosynthesis
- Pollination, carrying pollen between flowers
- Seed dispersal, carrying seeds to new places in fur or droppings
- Manure, which enriches the soil

Microorganisms provide for both:

- Decomposition — breaking dead matter into humus and returning nutrients to the soil
- Nitrogen fixation — Rhizobium in the root nodules of legumes converting nitrogen from the air into a usable form
- Helping digestion inside animals, including in our own intestines

The gases make the neatest case: plants release the oxygen animals breathe, and animals release the carbon dioxide plants use — so the two groups keep each other supplied, and the atmosphere stays near 21 percent oxygen and 0.03 percent carbon dioxide.

None of these three groups could survive alone, and that is the conclusion worth stating. Plants would eventually exhaust the soil's nutrients without decomposers; animals would starve and suffocate without plants; and decomposers would have nothing to work on without either — which is why the phrase is interdependence rather than cooperation.

Asexual reproduction needs one parent and produces offspring identical to it. Sexual reproduction needs two parents and produces offspring that differ from both.

Asexual reproduction:

- Only one parent; no gametes involved
- Offspring are exact copies, so there is no variation
- Usually faster and produces many offspring
- Examples: binary fission in amoeba and bacteria; budding in yeast and hydra; spore formation in fungi such as bread mould; vegetative propagation in plants — a potato tuber sprouting, a rose or sugarcane grown from a cutting, a banana from a sucker, a bryophyllum leaf growing plantlets from its margins

Sexual reproduction:

- Two parents, each contributing a gamete — a male gamete and a female gamete
- The gametes fuse, a process called fertilisation
- Offspring show variation, resembling both parents but identical to neither
- Slower and produces fewer offspring
- Examples: most animals, including human beings, and flowering plants producing seeds

How a new individual develops. The fusion of the two gametes forms a single cell called the zygote.

- The zygote divides repeatedly to form a ball of cells
- This develops into an embryo, in which the cells begin to differentiate into different tissues
- The embryo grows into a young one — inside the mother's body in mammals, inside an egg in birds, and inside a seed in plants
- Given the right conditions it grows into a new individual resembling its parents

So a seed sown in soil germinates because the embryo inside it resumes growing.

The reason both methods survive in nature is that each has an advantage. Asexual reproduction is quick and reliable when conditions are good, which is why farmers use cuttings to get identical plants. Variation from sexual reproduction is what lets a species adapt when conditions change — and a field of identical cloned plants has none of it, which is exactly why one disease can destroy the whole crop.