Biogeochemical Cycles — Practice · Grade 10 Science

Chapter: Ecosystems & Conservation Decisions

What this practice covers

By the end of Biogeochemical Cycles you should be able to:

10 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: biogeochemical cycle, carbon cycle, nitrogen fixation, transpiration, photosynthesis, cellular respiration, nitrification, denitrification.

Problem 1

Which process removes carbon dioxide from the atmosphere and stores its carbon in sugars?
  1. Photosynthesis
  2. Nitrogen fixation
  3. Combustion
  4. Cellular respiration
Answer and explanation

Photosynthesis

Photosynthesis absorbs carbon dioxide and builds it into sugars inside producers.

Problem 2

Nitrogen fixation is important because it
  1. breaks nitrates down into nitrogen gas
  2. converts nitrogen gas into a form plants can absorb
  3. releases carbon dioxide into the atmosphere
  4. turns liquid water into vapor
Answer and explanation

converts nitrogen gas into a form plants can absorb

Nitrogen fixation, done mainly by bacteria, converts unusable N₂ gas into ammonia and nitrates that plants can take up.

Problem 3

Air is about 78% nitrogen gas by volume. In a 500 mL sample of air, about how many milliliters are nitrogen gas?
Answer and explanation

390 mL

500 × 0.78 = 390 mL of nitrogen gas.

Problem 4

In the water cycle, water vapor released into the air from the leaves of plants is called
  1. condensation
  2. precipitation
  3. transpiration
  4. runoff
Answer and explanation

transpiration

Transpiration is water evaporating from plant leaves; it is a major source of atmospheric water vapor.

Problem 5

Explain how fertilizer washing off farm fields into a lake can end up killing the fish there.
Answer and explanation

The fertilizer adds extra nitrogen and phosphorus to the lake, which fuels an explosive bloom of algae. When the algae die, decomposer bacteria break them down and use up much of the dissolved oxygen in the water. With the oxygen depleted, fish and other aquatic animals suffocate. This process is called eutrophication.

Problem 6

Explain how excess nutrients can lead to low dissolved oxygen in a water body. Explain the reasoning in your own words.
Answer and explanation

Nutrient enrichment can promote blooms followed by oxygen depletion, depending on conditions.

Added nutrients can stimulate producer growth under limiting conditions.

Decomposition of added organic matter consumes oxygen faster than it is replenished.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 7

Distinguish a carbon reservoir of 1000 units from an input of 20 units/year. Show the reasoning, not just the conclusion.
Hints

Nutrients do not directly become oxygen depletion without a mechanism.

Answer and explanation

The reservoir is a stored amount; the input is a rate. They cannot be compared as the same quantity.

Problem 8

Why can matter cycle while energy needs ongoing input?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Atoms are reused among stores, while usable energy disperses and must be replenished.

Problem 9

Abundant atmospheric nitrogen needs suitable transformationsAbundant atmospheric nitrogen needs suitable transformationsAir N₂Usable compoundsBiomassMicrobial pathways: fixation, nitrification and denitrification differ.
Original selected nitrogen-route model, omitting several compounds and microbial pathways; it is not a full balanced nitrogen-cycle chemistry chart.
Interpret the supplied visual. Why does 78% nitrogen in air not mean a plant can directly build proteins from that N₂?
Answer and explanation

The appropriate chemical transformation and uptake pathway are needed; atmospheric abundance and usable form differ.

Nitrogen fixation converts atmospheric N₂ into forms that can enter biological nitrogen use. Nitrification and denitrification are different microbial transformations; denitrification can return nitrogen to gaseous forms under suitable conditions. Many plants take up nitrogen compounds rather than directly using N₂, with some relying on microbial associations. Availability depends on pathways, soil/water conditions and transport, not only the total nitrogen nearby. A diagram should trace compound forms and mechanisms rather than assign every microbial process the word fixation.

The appropriate chemical transformation and uptake pathway are needed; atmospheric abundance and usable form differ.

Response criteria

  • Read the structures, axes, directions and units correctly.
  • Connect the evidence with the biological mechanism.
  • Respect stated model assumptions and distinguish an observation from a further inference.

Problem 10

A nitrogen store begins at 50 units and receives 12 while losing 9 each year for four years. Find its final amount and distinguish that result from the annual input.
Answer and explanation

The final store is 50+4×(12−9)=62 units. Twelve units/year is an input flux, not the stock or net annual gain; the net gain is three units/year.

The final store is 50+4×(12−9)=62 units. Twelve units/year is an input flux, not the stock or net annual gain; the net gain is three units/year.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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