Complete Grade 7 Natural Sciences study guide in English, aligned to the CAPS curriculum. 18 chapters with explanations, worked examples, exercises with solutions, and original practice papers with memos.

Chapters

Chapter 1. The Biosphere FREE

Stand still anywhere in South Africa — on the beach at Hermanus, high up in the Drakensberg, or in the middle of the Karoo — and look around you. You will find life. Birds in the air, ants in the soil, grass between the rocks, perhaps a hadeda screeching. Even in places that look dead like the Namib Desert, billions of organisms live: bacteria in the sand, gemsbok that walk far between water sources, and plants that can wait years for the next rain.

This thin layer around the Earth where life occurs everywhere is what we call the biosphere. It is one of four great spheres that together make up the whole planet — and the biosphere is unique because it overlaps with the other three.

The four spheres of the Earth

Scientists divide the Earth into four great spheres, each with its own character:

The atmosphere is the air around the Earth — a mixture of nitrogen (78%), oxygen (21%) and small amounts of other gases. It stretches about 100 km into space.
The hydrosphere is all the water on the planet: the oceans, rivers, lakes, glaciers, and even the water under the ground. About 71% of the Earth's surface is water.
The lithosphere is the solid rock and soil — the crust we live on, plus the enormous layer of rock that lies beneath it.
The biosphere is all the living things together with the places where they live.

Here it gets interesting: the biosphere is not a separate place that stands apart from the others. It is the overlap zone — the part where all three other spheres meet and make life possible. A tree is a perfect example. Its roots stand in the lithosphere (soil), its trunk draws water from the hydrosphere (groundwater), and its leaves take carbon dioxide from the atmosphere (air). Remove any one of the three and the tree dies.

How thin is the biosphere really?

When you think of the Earth, the biosphere feels enormous — all the forests, oceans, towns and cities. But in proportion to the whole planet it is actually a surprisingly thin layer. Life stretches from about 8 km deep in the oceans to about 12 km high in the atmosphere. If you were to shrink the Earth to the size of an apple, the whole biosphere would be thinner than the apple's skin.

Why is life on Earth possible?

Three features make our planet especially suited to life:

Liquid water. All known life needs water as a solvent in which chemical reactions can take place. Mars is too cold, so water freezes; Venus is too hot, so water evaporates. The Earth sits in the Goldilocks zone — just right.
A moderate temperature. Most of the Earth's surface stays between −40 °C and +40 °C, a range in which cellular processes work well.
An atmosphere with oxygen and an ozone layer. Oxygen makes respiration possible for most life. The ozone layer, high in the atmosphere, blocks most of the harmful ultraviolet rays from the Sun.

Remove any one of these and the whole biosphere would collapse. This is why scientists searching for life in space are actually searching for water — it is a reliable marker that life may be possible.

An example closer to home

A wetland near Stellenbosch is a perfect miniature illustration of all four spheres working together. The water (hydrosphere) lies on the ground (lithosphere). Reeds grow up out of the water (biosphere). Above it all the air (atmosphere) is full of midges, and frogs leap out of the water to catch them. Reduce the water — let the wetland dry up as in a drought — and all the other parts collapse too. No frogs, because no midges, because no water. The biosphere depends entirely on the other three spheres.