The biosphere refers to the part of the Earth where life exists. It is a highly integrated and interacting zone that includes the atmosphere (air), hydrosphere (water), and lithosphere (land).

• The biosphere is a narrow layer surrounding the Earth’s surface. To visualize, if the Earth were the size of an apple, the biosphere would be as thin as the apple’s skin. Life within the biosphere is abundant between about 200 metres below the ocean surface and up to 6,000 metres above sea level.
• Life is largely absent at the extremes — such as the North and South Poles, the highest mountains, and the deepest parts of the oceans, due to hostile conditions. While some spores of fungi and bacteria have been detected even beyond 8,000 metres, they remain dormant and metabolically inactive.
• The energy for life in the biosphere comes primarily from the Sun, while essential nutrients are obtained from air, water, and soil. These nutrients are continuously recycled to sustain life processes.
• Organisms in the biosphere are not uniformly distributed. Few species are found in the harsh polar regions, while the tropical rainforests support the richest diversity of plants and animals, holding about 50% of global biodiversity.