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Biology 20

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The biosphere

Ten questions of mixed difficulty, covering The biosphere. Print it, or work through it on screen — the answer key starts on its own page.

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The biosphere

Biology 20 · maddyhelps.com

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  1. Carbon enters the living part of an ecosystem mainly by

    1. a) respiration
    2. b) combustion
    3. c) photosynthesis
    4. d) decomposition

  2. Roughly how much of the energy at one trophic level is passed to the next?

    1. a) About 50%
    2. b) About 90%
    3. c) About 10%
    4. d) Nearly all of it

  3. The two abiotic factors that most determine which biome develops in a region are

    1. a) day length and oxygen level
    2. b) temperature and precipitation
    3. c) soil pH and wind
    4. d) latitude and altitude

  4. The ultimate source of energy for almost every ecosystem on Earth is

    1. a) sunlight captured by producers
    2. b) heat from Earth's interior
    3. c) oxygen in the atmosphere
    4. d) organic matter in soil

  5. An organism that builds its own organic molecules from inorganic sources is

    1. a) a decomposer
    2. b) an autotroph
    3. c) a consumer
    4. d) a heterotroph

  6. The biosphere is

    1. a) all the water on Earth
    2. b) the zone of Earth where living things are found
    3. c) the solid outer layer of Earth
    4. d) the layer of gases surrounding Earth

  7. Denitrification differs from the other steps of the nitrogen cycle because it

    1. a) converts ammonium into nitrite
    2. b) releases ammonia from dead tissue
    3. c) fixes nitrogen into ammonia
    4. d) returns nitrogen to the atmosphere as N₂ gas

  8. Nitrogen fixation is the conversion of

    1. a) protein into ammonia by decomposers
    2. b) ammonium into nitrate by plants
    3. c) nitrate back into N₂ gas
    4. d) atmospheric N₂ into ammonia or ammonium, largely by bacteria

  9. Decomposers are essential to nutrient cycles because they

    1. a) fix nitrogen from the atmosphere
    2. b) produce oxygen for consumers
    3. c) store carbon permanently in the soil
    4. d) break organic matter down into inorganic forms producers can take up again

  10. Burning fossil fuels raises atmospheric CO₂ because it

    1. a) stops plants from photosynthesizing
    2. b) prevents carbon from dissolving in the oceans
    3. c) releases carbon from a long-term store faster than natural processes can remove it
    4. d) creates new carbon atoms

Answer key · The biosphere

Biology 20 · maddyhelps.com

  1. c) photosynthesis — Photosynthesis takes inorganic CO₂ from the air and fixes it into organic molecules. Respiration, combustion and decomposition all send carbon the other way, back into the atmosphere.
  2. c) About 10% — Most of the energy at any level is spent on the organism's own respiration or lost as heat, and much of the rest is in parts that are never eaten. The surviving fraction averages around a tenth, which is why biomass pyramids narrow so sharply.
  3. b) temperature and precipitation — Latitude and altitude matter because of what they do to temperature and rainfall. Plotting average temperature against annual precipitation predicts the biome remarkably well.
  4. a) sunlight captured by producers — Producers convert light energy into chemical energy, and everything else in the food web is living off that conversion. The rare exceptions are deep-sea vent communities, which run on chemical energy instead.
  5. b) an autotroph — Auto- means self and -troph means feeding. Plants, algae and some bacteria do this. Everything that has to eat something already organic is a heterotroph, including decomposers.
  6. b) the zone of Earth where living things are found — The biosphere is the thin shell where the atmosphere, hydrosphere and lithosphere overlap enough to support life. It is defined by the presence of life, not by a particular substance.
  7. d) returns nitrogen to the atmosphere as N₂ gas — Nitrification, fixation and ammonification all keep nitrogen inside the ecosystem in usable forms. Denitrifying bacteria, working in oxygen-poor soils, remove it — which is why waterlogged soils lose fertility.
  8. d) atmospheric N₂ into ammonia or ammonium, largely by bacteria — Atmospheric nitrogen is nearly 80% of the air but is locked in a triple bond that most organisms cannot break. Nitrogen-fixing bacteria, many of them living in legume root nodules, do the breaking.
  9. d) break organic matter down into inorganic forms producers can take up again — Without decomposition, nutrients would stay locked in dead tissue and the cycles would stop. Decomposers are heterotrophs, but they are the step that returns matter to the abiotic pool.
  10. c) releases carbon from a long-term store faster than natural processes can remove it — Fossil fuels are carbon that was taken out of circulation over millions of years. Burning them returns it in a couple of centuries, and photosynthesis and ocean uptake cannot absorb it at that rate.