Nutrient Cycles
Understanding how water, oxygen, carbon, and nitrogen are recycled in ecosystems
The water, oxygen, carbon, and nitrogen cycles show how nutrients are reused in ecosystems. Learners should remember that nutrients are recycled, while energy only flows in one direction.
1. The Water Cycle
The Water Cycle illustrates the movement of water through the biosphere, encompassing various processes that facilitate this continuous flow.
Liquid water from oceans and lakes is heated by the sun, transforming it into water vapor.
Water is lost from plant leaves through small openings called stomata, contributing to the moisture in the atmosphere.
As water vapor rises, it cools and condenses to form clouds.
Eventually, the accumulated water in clouds falls back to Earth as rain, snow, or hail.
Water either seeps into the soil (infiltration) or flows into rivers and oceans (runoff), completing the cycle.
[Ocean/Lake] → Evaporation → [Water Vapor] → Transpiration → [Clouds] → Condensation → Precipitation → [Soil/Rivers/Oceans]
2. The Oxygen Cycle
The Oxygen Cycle is a straightforward process primarily driven by two biological activities: photosynthesis and respiration.
During this process, plants absorb carbon dioxide (CO2) from the atmosphere and, using sunlight, convert it into glucose while releasing oxygen (O2) as a byproduct.
Both animals and plants consume oxygen to break down food for energy, returning carbon dioxide to the atmosphere.
[Plants] → Photosynthesis → [Oxygen (O2)] → [Animals] → Respiration → [Carbon Dioxide (CO2)] → [Plants]
3. The Carbon Cycle
The carbon cycle is important because it helps learners explain climate change and the movement of carbon through living things, the atmosphere, and the soil.
This is the only process that actively removes carbon dioxide (CO2) from the atmosphere, as plants convert it into organic matter.
Both processes release carbon back into the atmosphere. Animals respire, and decomposers break down dead organic matter, returning CO2 to the air.
The burning of fossil fuels or wood releases stored carbon as carbon dioxide, significantly impacting atmospheric levels.
[Atmosphere (CO2)] ← Photosynthesis → [Plants] → Respiration → [Animals] → Decomposition → [Atmosphere (CO2)]
[Combustion] → [Atmosphere (CO2)]
Human Impact: Deforestation reduces the number of trees available to absorb carbon dioxide, leading to increased levels of CO2 in the atmosphere and contributing to climate change.
Quick Check: Cycle Processes
Question 1/51. Which process involves water being lost from plant leaves through stomata?
4. The Nitrogen Cycle
The Nitrogen Cycle is the most complex of the nutrient cycles, as most organisms cannot directly utilize atmospheric nitrogen (N2).
Certain bacteria, found in soil or in the root nodules of legumes, convert nitrogen gas (N2) into nitrates (NO3-), making it accessible to plants.
Plants absorb nitrates to synthesize proteins, which are then consumed by animals.
Decomposers break down dead organic matter, releasing ammonia (NH3) into the soil.
Nitrifying bacteria convert ammonia into nitrates, further facilitating plant uptake.
Denitrifying bacteria convert nitrates back into nitrogen gas (N2), releasing it into the atmosphere and completing the cycle.
[Atmosphere (N2)] → Nitrogen Fixation → [Nitrates (NO3-)] → Absorption → [Plants] → [Animals]
→ Decomposition → [Ammonia (NH3)] → Nitrification → [Nitrates (NO3-)] → Denitrification → [Atmosphere (N2)]
Role of Bacteria in the Nitrogen Cycle
Convert nitrogen gas (N2) into nitrates (NO3-) that plants can use. Found in soil and root nodules of legumes.
Convert ammonia (NH3) into nitrates (NO3-), making nitrogen available for plants.
Convert nitrates (NO3-) back into nitrogen gas (N2), releasing it into the atmosphere.
Break down dead organic matter, releasing ammonia (NH3) through ammonification.
Matching Game: Bacteria in the Nitrogen Cycle
Match each type of bacteria with its correct function.
Nutrient Cycles Comparison
| Cycle | Key Processes | Important Molecules | Human Impact |
|---|---|---|---|
| Water Cycle | Evaporation, Transpiration, Condensation, Precipitation, Infiltration | H2O (water) | Pollution, over-extraction |
| Oxygen Cycle | Photosynthesis, Respiration | O2, CO2 | Deforestation reduces O2 production |
| Carbon Cycle | Photosynthesis, Respiration, Decomposition, Combustion | CO2, organic carbon | Burning fossil fuels, deforestation |
| Nitrogen Cycle | Fixation, Nitrification, Assimilation, Ammonification, Denitrification | N2, NO3-, NH3 | Fertilizer use, pollution |
True or False Quiz
1. Unlike energy, nutrients are continuously cycled between biotic and abiotic components.
2. Most organisms can directly utilize atmospheric nitrogen (N2).
3. Combustion releases stored carbon as carbon dioxide into the atmosphere.
4. Denitrifying bacteria convert nitrates into nitrogen gas, releasing it into the atmosphere.
5. Transpiration is the process where water evaporates from oceans and lakes.
Fill in the Blanks
1. The process where water vapor cools and forms clouds is called _______________.
2. The only process that removes carbon dioxide from the atmosphere is _______________.
3. Bacteria found in root nodules of _______________ help fix nitrogen.
4. The process where decomposers release ammonia from dead matter is called _______________.
Exam Tips
When answering exam questions, make sure you name the process correctly and show where the substance moves next.
- Role of Bacteria in the Nitrogen Cycle: Be able to identify how different types of bacteria contribute to nitrogen fixation, nitrification, and denitrification.
- Impact of Deforestation on the Carbon Cycle: Understand that deforestation reduces the number of trees available to absorb carbon dioxide, leading to increased levels of CO2 in the atmosphere.
Key Terms
What To Remember
Nutrient cycles keep substances moving through ecosystems. Learners should know the main steps in each cycle and be able to explain the role of bacteria in the nitrogen cycle.
- Nutrients are recycled; energy flows one way
- Water moves through evaporation, transpiration, condensation, precipitation
- Oxygen cycle links photosynthesis and respiration
- Carbon is removed by photosynthesis and released by respiration, decomposition, combustion
- Nitrogen-fixing bacteria convert N2 to nitrates for plants
- Deforestation increases atmospheric CO2
