The biosphere is the shell of the Earth inhabited by living organisms and transformed by their activity. The term "biosphere" was introduced by E. Suess (1875), and the complete theory of the biosphere was created by V. I. Vernadsky (1926). The exam asks about the boundaries of the biosphere, the types of matter, the functions of living matter, the cycles of substances and environmental problems, including the Aral Sea disaster.
§1. Structure and boundaries of the biosphere
The biosphere covers the adjoining parts of three geospheres:
Atmosphere — up to roughly the ozone layer (20–25 km); above it, short-wave ultraviolet radiation prevents life.
Hydrosphere — all of it, down to the deepest ocean trenches (about 11 km).
Lithosphere — the upper part, down to a few kilometres (bacteria have been found in oilfield waters 3–4 km deep); below that, high temperature limits life.
The most densely populated layer is the "film of life": the land surface, the soil and the upper, sunlit layer of water.
Vernadsky divided the matter of the biosphere into types:
Living matter — all living organisms together. Its mass is a tiny fraction of the biosphere, but it is the most active part.
Biogenic matter — formed by the activity of living organisms: coal, oil, peat, limestone, natural gas.
Inert matter — formed without the participation of life: igneous rocks, volcanic lava.
Bio-inert matter — the product of living and non-living nature acting together: soil, natural waters, silt at the bottom of water bodies.
Worked example
Sort into types of matter: granite, peat, soil, a pigeon, oil, basalt.
Pigeon — a living organism → living matter.
Peat and oil — formed from the remains of past organisms → biogenic matter.
Soil — the joint result of rock, organisms and climate → bio-inert matter.
Granite and basalt — igneous rocks → inert matter.
§2. Functions of living matter
Energy function — photosynthesis captures solar energy, stores it in the chemical bonds of organic substances and passes it along food chains.
Gas function — shaping the composition of the atmosphere: photosynthesis releases oxygen, respiration releases CO₂; nitrogen circulates with the help of bacteria. Today’s oxygen-rich atmosphere is the result of living organisms’ activity.
Concentration function — organisms accumulate certain elements from the environment: horsetails — silicon, seaweeds (kelp) — iodine, molluscs and corals — calcium (shells and reefs).
Redox function — oxidation and reduction of substances: for example, the activity of iron and sulphur bacteria.
Destructive function — decomposers break organic remains down to mineral substances; organisms weather rocks (lichens).
§3. Biogeochemical cycles
Substances in the biosphere circulate many times, but energy does not circulate: it comes from the Sun, passes along food chains in one direction and is eventually lost as heat.
The biosphere exists thanks to a constant inflow of solar energy and an almost closed cycling of substances.
Water cycle — evaporation (including transpiration by plants), precipitation, run-off.
Carbon cycle — plants absorb CO₂ in photosynthesis and turn it into organic matter; respiration, decay and burning of fuel return CO₂ to the atmosphere. Some carbon "leaves" the cycle for a long time in coal, oil and limestone.
Nitrogen cycle — atmospheric nitrogen (N₂) is unavailable to most organisms. It is fixed by nitrogen-fixing bacteria (nodule bacteria of legumes, free-living bacteria, cyanobacteria). Decay bacteria break proteins down to ammonia, nitrifying bacteria convert ammonia into nitrates, and plants absorb the nitrates. Denitrifying bacteria return nitrogen to the atmosphere as N₂.
Oxygen cycle — released in photosynthesis, used in respiration and combustion; some oxygen forms ozone (O₃).
Worked example
By what route does a nitrogen atom from the atmosphere end up in a protein of the human body?
Nodule bacteria fix N₂, turning it into compounds.
A legume (for example, peas) uses this nitrogen to make amino acids and proteins.
A person eats the peas; the proteins are broken down into amino acids.
From these amino acids the person builds their own proteins.
Answer: N₂ → nodule bacteria → plant protein → human protein.
§4. Evolution and productivity of the biosphere
The biosphere did not appear all at once — it has changed for billions of years under the influence of living things. The Earth is about 4.6 billion years old, and the oldest traces of life are remains of bacteria about 3.5 billion years old. The early atmosphere had almost no free oxygen. Oxygen built up gradually through the photosynthesis of cyanobacteria; as it increased, the ozone layer formed, and once it shielded the surface from ultraviolet light, life could move from water onto land. Today’s atmosphere and soil are thus products of living matter.
The photosynthesis equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (powered by light energy, with chlorophyll).
Biomass and productivity. Biomass is the total mass of organisms in a given area; primary productivity is the organic matter made by producers per unit of time. In the biosphere:
land biomass is far greater than ocean biomass and consists mainly of plants (above all forests);
in the ocean the producers are tiny single-celled algae (phytoplankton); they multiply fast and are eaten fast, so although ocean biomass is small, its productivity is considerable;
on land the most productive ecosystems are tropical rainforests, the least productive deserts and tundra; in the ocean shallow coastal waters, coral reefs and estuaries are productive, while the centre of the open ocean is poor (an “ocean desert”).
Productivity is limited by light, water, temperature and mineral nutrients (especially nitrogen and phosphorus): deserts lack water, tundra lacks warmth, the open ocean lacks minerals.
Worked example
A tree makes 900 g of glucose by photosynthesis in one season. How much CO₂ is absorbed and how much O₂ released? (M(C₆H₁₂O₆) = 180, M(CO₂) = 44, M(O₂) = 32 g/mol)
Glucose: 900 / 180 = 5 mol.
From the equation, CO₂ and O₂: 5 × 6 = 30 mol each.
CO₂: 30 × 44 = 1320 g; O₂: 30 × 32 = 960 g.
Answer: about 1.32 kg of CO₂ absorbed and 0.96 kg of O₂ released.
Worked example
Why is most of the open ocean unproductive while coastal waters are productive?
Phytoplankton needs light and mineral nutrients (nitrogen, phosphorus).
In the open ocean the sunlit upper layer is poor in minerals — they sink to the depths.
Near coasts, rivers bring in minerals and deep water rises to the surface.
Answer: the limiting factor is mineral nutrients; where they are plentiful, productivity is high.
§5. The noosphere and global environmental problems
According to Vernadsky, human activity has become a geological force, and the biosphere is turning into the noosphere — the sphere of reason. In this state the development of the biosphere should be guided by conscious, rational human activity.
Major global problems:
The greenhouse effect — build-up of CO₂, methane and other gases in the atmosphere warms the climate; glaciers melt and weather becomes more extreme.
Ozone depletion — caused by freons (chlorofluorocarbons); as a result more ultraviolet radiation reaches the Earth.
Acid rain — sulphur and nitrogen oxides dissolve in water to form acids, damaging forests and water bodies.
Deforestation, desertification, soil erosion and salinisation, water and air pollution, and loss of biodiversity.
The Aral Sea disaster — in the second half of the 20th century so much water was taken from the Amu Darya and Syr Darya for irrigation that little reached the sea. Its level fell sharply, its area shrank several times over, salinity rose and fishing disappeared. A new sandy, salty desert — the Aralkum — formed on the dry seabed; salt-and-dust storms harm people’s health and farming in the surrounding areas. Today saxaul forests are being planted in the Aral region and water-saving irrigation is being introduced.
Nature conservation:
The Red Book — a list of species threatened with extinction (international and the Red Book of Uzbekistan).
Nature reserves — areas under strict protection: Zaamin, Chatkal, Hissar, Zarafshan, Kyzylkum, Nurata and others.
National parks — protection combined with recreation and tourism (for example, Zaamin and Ugam-Chatkal).
Breeding centres for rare animals — the "Jeyran" eco-centre in Bukhara region.
Check yourself
What sets the upper boundary of the biosphere in the atmosphere?
Which type of matter is soil in Vernadsky’s classification?
Which function of living matter is shown when kelp accumulates iodine?
Why does energy not cycle the way substances do?
Which bacteria return nitrogen to the atmosphere?
What is the main reason the Aral Sea dried up?
Which organisms first enriched the atmosphere with oxygen?
A plant absorbs 264 g of CO₂ in photosynthesis. How many grams of glucose form?
Common mistakes
Calling soil inert matter — it is bio-inert matter.
Saying energy cycles like substances — the energy flow is one-way.
Classing oil and coal as inert matter — they are biogenic matter.
Confusing nitrogen fixation with denitrification: fixers bind N₂, denitrifiers return it to the atmosphere.
Equating ozone depletion with the greenhouse effect — their causes and consequences differ.
Taking the noosphere to mean "nature without people" — it is a stage of the biosphere guided by human reason.