← Back to 1. Evolution and the theme of Biology and Scientific Inquiry

📝 Chemical and nutrient cycles in ecosystems (7 MCQs)

📖 From Campbell Biology • 1. Evolution and the theme of Biology and Scientific Inquiry • 7 questions available

←Previous Topic:Photosynthesis process explained📝 Chemical and nutrient cycles in ecosystemsProducers vs consumers in ecosystem:Next Topic→

What is Chemical and nutrient cycles in ecosystems?

Definition:
Chemical and nutrient cycles, also known as biogeochemical cycles, are the natural pathways by which essential elements such as carbon, nitrogen, phosphorus, and water move through the biotic (living) and abiotic (non-living) components of an ecosystem, ensuring that these nutrients are continuously recycled and made available for organisms, thereby sustaining life and maintaining ecological balance.

Working:
These cycles work through a combination of geological, biological, and chemical processes; for example, in the carbon cycle, photosynthesis fixes atmospheric $CO_2$ into organic matter, respiration releases it back, and decomposition returns it to the soil, with the overall exchange represented by the equation $\text{Photosynthesis: } 6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$ and $\text{Respiration: } C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP$ , while the nitrogen cycle involves fixation, nitrification, assimilation, ammonification, and denitrification.

Example:
A simple example is the carbon cycle in a forest ecosystem, where trees absorb $CO_2$ during photosynthesis to produce glucose, animals consume the trees and release $CO_2$ through respiration, decomposers break down dead organic matter returning carbon to the soil, and some carbon is stored as fossil fuels or released through combustion, completing the cycle.

Reason:
Understanding nutrient cycles is essential because they are the foundation of ecosystem productivity and health, and human activities like burning fossil fuels and deforestation disrupt these cycles, leading to climate change, eutrophication, and loss of biodiversity, making it crucial for environmental conservation and sustainable resource management.

Easy

Medium

Hard

📝 All Chemical and nutrient cycles in ecosystems MCQs

Q1. Which process most directly returns carbon from living organisms to the atmosphere as CO2CO_2?

A.Photosynthesis

B.Cellular respiration ✅

C.Nitrogen fixation

D.Mineral weathering

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: Cellular respiration breaks down organic molecules and releases CO2CO_2 as a metabolic product. This directly transfers carbon from biological tissues back into the atmosphere, whereas photosynthesis removes atmospheric CO2CO_2 and incorporates it into organic molecules.

Q2. A forest receives abundant rainfall, but its soil contains little available nitrogen. Which explanation best accounts for this observation?

A.Nitrogen is continuously removed from the atmosphere by photosynthesis

B.Heavy rainfall can increase leaching of soluble nitrogen compounds from soil ✅

C.Plants convert all soil nitrogen directly into atmospheric nitrogen

D.Carbon dioxide prevents nitrogen compounds from entering soil

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Soluble nitrogen compounds can be carried downward or away by water, reducing their availability to plants. Thus, high rainfall can contribute to nutrient loss even when biological productivity and water availability are otherwise high.

Q3. A lake receives excess fertilizer containing nitrogen and phosphorus. Algal biomass rises rapidly, followed by a sharp decline in dissolved oxygen. Which sequence best explains the change?

A.Nutrient enrichment → algal growth → decomposition → oxygen consumption ✅

B.Nutrient enrichment → photosynthesis stops → oxygen production increases

C.Fertilizer directly removes oxygen → algae decompose less → oxygen falls

D.Nutrient enrichment → nitrogen fixation stops → oxygen immediately disappears

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Excess nitrogen and phosphorus can stimulate algal growth. When much of this biomass dies, decomposers consume organic matter and use dissolved oxygen during respiration, potentially producing oxygen-poor conditions in the water.

Q4. A student claims, Because carbon is recycled, ecosystems cannot lose carbon. What is the strongest correction to this reasoning?

A.Recycling means carbon is never chemically transformed

B.Carbon can move between ecosystem pools and may also leave an ecosystem through processes such as gas exchange or export of biomass ✅

C.Carbon exists only in living organisms, so it cannot leave ecosystems

D.Carbon cycling prevents organisms from releasing CO2CO_2

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Cycling does not mean that a particular ecosystem retains every atom of an element indefinitely. Carbon can move among organisms, soil, water, and atmosphere, while carbon-containing material can also be exported or exchanged with surrounding systems.

Q5. A model predicts the following soil nitrogen concentration after four years: Year 1 = 20 units, Year 2 = 17 units, Year 3 = 14 units, Year 4 = 11 units. If the trend continues, which conclusion is most justified?

A.Nitrogen concentration is increasing by 3 units each year

B.Nitrogen concentration is decreasing by approximately 3 units per year ✅

C.Nitrogen concentration will remain constant because nitrogen cycles

D.Nitrogen concentration must reach zero after exactly four more years

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The values decline from 20 to 17 to 14 to 11, representing a consistent decrease of 3 units per year. The pattern supports a declining trend, although extending the model indefinitely would require additional ecological evidence.

Q6. An ecosystem receives nutrients from outside, stores some in organisms and soil, and loses some through runoff. Which change would most likely increase the ecosystem's nutrient retention?

A.Increasing runoff

B.Reducing uptake by plants

C.Increasing the rate at which nutrients are incorporated into biomass and soil ✅

D.Removing decomposers from the ecosystem

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: Greater biological and soil uptake can retain nutrients within the ecosystem rather than allowing them to leave through runoff. Decomposers also help return nutrients to usable forms, so simply removing them would generally disrupt cycling.

Q7. Two ecosystems receive the same amount of atmospheric carbon input. Ecosystem X rapidly incorporates carbon into plant biomass, while Ecosystem Y has slower plant uptake and greater carbon loss through respiration and export. Which inference is most reasonable?

A.Ecosystem Y necessarily contains more carbon because respiration increases carbon storage

B.Ecosystem X may retain more carbon temporarily because a larger fraction of incoming carbon enters biomass ✅

C.Both ecosystems must store identical amounts because their carbon input is equal

D.Ecosystem X cannot release carbon because plants store it

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: Equal inputs do not guarantee equal storage. Ecosystem X can temporarily retain more carbon if uptake into biomass exceeds losses, whereas Ecosystem Y may have a lower net retention because respiration and export transfer carbon out of the ecosystem more rapidly.

🔗 Related Topics (MCQs)

📝 The Study of Life Reveals Unifying Themes MCQs 📝 Hierarchical organization of life MCQs 📝 Reductionism in biology MCQs 📝 Emergent properties biology MCQs 📝 Systems biology MCQs

←Previous Topic:Photosynthesis process explained📝 Chemical and nutrient cycles in ecosystemsProducers vs consumers in ecosystem:Next Topic→