Question

assignment questions:
please answer all questions completely so that you earn full credit
part i – the carbon cycle

1. pick one step in the carbon cycle and briefly research it. what step did you choose? give a real-world example of this step and explain what happens to the carbon atoms as they move.
2. how does burning fossil fuels (e.g. coal, oil, natural gas) disrupt the natural carbon cycle? why is this a problem?

part ii – the nitrogen cycle

1. pick one step in the nitrogen cycle and briefly research it. what step did you choose? give a real-world example of this step and explain what happens to the nitrogen atoms as they move.
2. how are human activities disrupting the natural nitrogen cycle? why is this a problem?

Answer:

Part I - The Carbon Cycle

1. Step Selection and Explanation

Let’s choose photosynthesis (a key step in the carbon cycle).

• Real - world example: Plants (e.g., a maple tree) performing photosynthesis.
• What happens to carbon atoms: During photosynthesis, plants take in carbon dioxide ($\ce{CO_2}$) from the atmosphere. Using energy from sunlight, along with water ($\ce{H_2O}$), they convert the carbon in $\ce{CO_2}$ into glucose ($\ce{C_6H_{12}O_6}$) (a carbohydrate) and release oxygen ($\ce{O_2}$). The chemical equation is: $$6\ce{CO_2} + 6\ce{H_2O} \xrightarrow[\text{Chlorophyll}]{\text{Sunlight}} \ce{C_6H_{12}O_6} + 6\ce{O_2}$$ So, carbon atoms move from the gaseous $\ce{CO_2}$ in the air into the solid (or dissolved) glucose molecules within the plant’s cells. These glucose molecules can then be used for the plant’s growth (e.g., to build cellulose for cell walls) or stored as starch.

2. Disruption by Burning Fossil Fuels

• How it disrupts: The natural carbon cycle has a balance between carbon uptake (e.g., photosynthesis) and carbon release (e.g., respiration, decomposition). Fossil fuels (coal, oil, natural gas) are long - term carbon stores (they formed from the remains of ancient organisms over millions of years). When we burn fossil fuels, the reaction is (using methane, $\ce{CH_4}$, as a representative fossil fuel component):

$$\ce{CH_4} + 2\ce{O_2} ightarrow \ce{CO_2} + 2\ce{H_2O}$$

This process releases a large amount of $\ce{CO_2}$ into the atmosphere in a very short time. The rate of $\ce{CO_2}$ release from fossil fuel burning is much faster than the rate at which natural processes (like photosynthesis and ocean absorption) can remove $\ce{CO_2}$ from the atmosphere.

• Why it’s a problem: The excess $\ce{CO_2}$ in the atmosphere acts as a greenhouse gas. It traps more heat from the Sun, leading to global warming. This causes climate change, which can result in rising sea levels (due to melting ice caps), more extreme weather events (like heatwaves, droughts, and intense storms), and disruptions to ecosystems (e.g., changing the habitats of many species, affecting their survival and reproduction).

Part II - The Nitrogen Cycle

3. Step Selection and Explanation

Let’s choose nitrogen fixation (the process of converting atmospheric nitrogen, $\ce{N_2}$, into forms usable by living organisms).

• Step chosen: Biological nitrogen fixation (done by nitrogen - fixing bacteria, e.g., Rhizobium in the roots of legumes like soybeans).
• Real - world example: Soybean plants and Rhizobium bacteria.
• What happens to nitrogen atoms: Atmospheric nitrogen ($\ce{N_2}$) is a very stable molecule. Rhizobium bacteria, living in nodules on the roots of soybean plants, use an enzyme called nitrogenase to break the triple bond in $\ce{N_2}$. They then combine the nitrogen atoms with hydrogen to form ammonia ($\ce{NH_3}$), and then ammonium ions ($\ce{NH_4^+}$). The chemical reaction for nitrogen fixation by Rhizobium can be summarized as: $$\ce{N_2} + 8\ce{H^+} + 8\ce{e^-} + 16\ce{ATP} \xrightarrow{\text{Nitrogenase}} 2\ce{NH_3} + \ce{H_2} + 16\ce{ADP} + 16\ce{Pi}$$ The ammonium ions are then used by the soybean plant to make organic nitrogen - containing compounds like amino acids (the building blocks of proteins) and nucleic acids.

4. Disruption by Human Activities

• How human activities disrupt: One major way is through the use of synthetic nitrogen fertilizers. When farmers apply large amounts of nitrogen fertilizers (e.g., ammonium nitrate, $\ce{NH_4NO_3}$) to crops, it introduces a large quantity of reactive nitrogen (in the form of $\ce{NH_4^+}$ or $\ce{NO_3^-}$) into the soil and water systems. Another way is through the burning of fossil fuels and biomass, which releases nitrogen oxides ($\ce{NO_x}$) into the atmosphere. Also, deforestation and land - use changes can disrupt the natural nitrogen - cycling processes in ecosystems.
• Why it’s a problem:
• Water pollution: Excess nitrogen from fertilizers can leach into groundwater or run off into rivers, lakes, and oceans. This causes eutrophication. In water bodies, the excess nitrogen promotes the rapid growth of algae (algal blooms). When the algae die, decomposers (like bacteria) break them down, using up large amounts of oxygen in the water. This leads to hypoxia (low oxygen levels) in the water, which can kill fish and other aquatic organisms.
• Atmospheric pollution: Nitrogen oxides ($\ce{NO_x}$) released into the atmosphere contribute to the formation of smog (by reacting with volatile organic compounds in the presence of sunlight) and acid rain (when $\ce{NO_x}$ reacts with water vapor to form nitric acid, $\ce{HNO_3}$).
• Soil degradation: Over - use of nitrogen fertilizers can reduce the soil’s ability to retain nutrients naturally and can also change the soil’s microbial community, affecting the natural nitrogen - cycling processes in the soil.