Conditions for life on Earth Flashcards
Appropriate temperature range
Most areas of Earth have temperatures above 0C, which allows liquid water to be present.
Most enzymes require liquid water as a solvent and denature at higher temperatures, so most living
organisms are found within the range 0C to 40C. Some organisms, such as thermophilic bacteria,
can withstand much higher temperatures, sometimes over 80C.
Suitable ambient gases
Suitable ambient gases for developing and sustaining life are carbon dioxide, for photosynthesis
and climate control, and nitrogen, for protein synthesis.
Light and radiation from the sun
• Sunlight provides energy for photosynthesis.
• Heat production when absorbed - the source of energy for the water cycle and warming of the
Earth.
• Little harmful ultraviolet (UV) and ionising radiation.
The position of the Earth in the solar system, its size, structure, composition and rotation
The distance from the sun controls light levels and temperatures. The daily rotation controls the
duration of day and night, and therefore the range of temperatures. The tilted axis produces seasonal
variations and the molten layers beneath the crust produce the Earth’s magnetic field, which deflects
harmful radiation coming from the Sun.
Atmospheric oxygen
The concentration of oxygen in the atmosphere has declined as it reacted with other elements, for
example by oxidation weathering with elements such as iron on the surface of exposed rocks.
The continued presence of oxygen in the atmosphere relies on processes that continually replace
it as other processes remove it. A small amount of oxygen was released by photolysis of water.
Photosynthetic bacteria, then, much later, photosynthetic plants gradually released much larger
amounts of oxygen.
This oxygen absorbed UV light from the sun, causing the oxygen molecules to split. This produced
monatomic oxygen that reacted with diatomic oxygen to produce ozone. These reactions allowed
the ozone layer to develop, which provided protection from UV light to living organisms on Earth.
Before the ozone layer developed, abundant life was not possible on land so early organisms lived
in oceans where water protected them from UV light.
Atmospheric carbon dioxide
Carbon dioxide is naturally released into the atmosphere by volcanoes. It is an essential greenhouse
gas, helping to retain heat in the atmosphere. Without carbon dioxide, the earth would be too cold
to support life. If there were excessive carbon dioxide, however, levels would cause temperatures to
rise too high for life to survive.
The light output of the sun increases by about 10 per cent every billion years, so it is about 30
per cent brighter now than when life first developed. If the carbon dioxide content had remained
constant, then the Earth would have heated up and life would have died out, but living organisms
have helped maintain suitable atmospheric temperatures by removing carbon dioxide from the
atmosphere and storing the carbon in fossil fuels and carbonate rocks such as limestone and chalk.
The water cycle
Water does not come into or leave planet earth. Water is continuously transferred between the
atmosphere and the oceans. This is known as the water cycle.
Evaporation turns water into a gas and it then becomes part of the atmosphere. Transpiration is
similar to evaporation but is the loss of water as a vapour from plant and tree leaves. The combined
effect of evaporation and transpiration is called evapotranspiration.
Transpiration from leaves is unavoidable as the moist stomata must be open to allow exchange of
gases such as carbon dioxide.
