Lessons 27–28 · 2 lessons · A. G‘afurov, A. Abdukarimov, J. Tolipova, O. Ishankulov, M. Umaraliyeva, I. Abdurahmonova. Biology Grade 11, 1st edition. Sharq Publishing and Printing Joint-Stock Company, Tashkent, 2018
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The limits of the biosphere
Textbook: pp. 136–138
GoalExplain the limits of the biosphere (atmosphere, hydrosphere, lithosphere) and the factors limiting life; describe the importance of the ozone layer.
The biosphere lies where three shells meet: the lower atmosphere, the whole hydrosphere and the upper lithosphere. In the atmosphere life is found mainly in the troposphere (up to 8–10 km at the poles and 18–20 km at the equator); with increasing altitude temperature falls by about 0.6 °C every 100 m. The ozone layer in the stratosphere (peak density at about 20–25 km) absorbs most of the Sun’s harmful ultraviolet rays, which made life on land possible; above it there is no life. In the oceans organisms occur down to the deepest places (the Mariana Trench ≈ 11 km), but photosynthesizers live in the upper layers reached by light (down to about 200–300 m). In the lithosphere temperature rises about 3 °C every 100 m of depth; some bacteria have been found several km down, yet most life is in the soil. The limits of life are set by light, temperature, water, oxygen and pressure.
Worked examples
On the summit of Everest (about 8.8 km) temperature, oxygen and water are so low that no plants grow; yet jumping spiders live up to about 6.7 km, and above that only wind-borne microbes occur temporarily.
At deep-sea hydrothermal vents there is no sunlight, but chemosynthetic bacteria obtain energy from the oxidation of hydrogen sulfide and support a whole community.
Class activity
On paper at 1 cm = 2 km draw a vertical section of the biosphere (from −4 km to +20 km) and mark the limits. Safety: drawing only.
Practice
1
How high is the troposphere at the poles and at the equator?
8–10 km at the poles, 18–20 km at the equator.
2
Surface temperature is 24 °C. If it falls 0.6 °C per 100 m, find the temperature at 3000 m (°C).
6
3
Why is the ozone layer vital for life?
It absorbs most harmful ultraviolet rays and so protects cell DNA.
4
Surface temperature is 15 °C; if it rises 3 °C per 100 m of depth, what is it at 2000 m?