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Human Influences on Ecosystems - Pollution (Greenhouse effect)

Grade 12IGCSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The greenhouse effect is a natural process where greenhouse gases in the atmosphere, such as carbon dioxide (CO2CO_2), methane (CH4CH_4), and nitrous oxide (N2ON_2O), trap long-wave infrared radiation reflected from the Earth's surface.

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Short-wave solar radiation passes through the atmosphere and warms the Earth. The Earth then re-emits this energy as long-wave (infrared) radiation.

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The 'Enhanced Greenhouse Effect' refers to the additional warming caused by human activities that increase the concentration of these gases, leading to global warming.

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The main sources of increased CO2CO_2 are the combustion of fossil fuels (coal, oil, and gas) and widespread deforestation, which reduces the amount of CO2CO_2 removed via photosynthesis.

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Methane (CH4CH_4) is produced during anaerobic decomposition in rice paddies, landfill sites, and the digestive systems of livestock (ruminants).

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Consequences include the melting of polar ice caps, thermal expansion of oceans leading to rising sea levels, and ocean acidification caused by the formation of carbonic acid (H2CO3H_2CO_3) when CO2CO_2 dissolves in seawater.

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Global warming shifts biomes and alters migration patterns, potentially leading to the extinction of species that cannot adapt quickly enough to temperature changes.

📐Formulae

6CO2+6H2O→light energyC6H12O6+6O26CO_2 + 6H_2O \xrightarrow{\text{light energy}} C_6H_{12}O_6 + 6O_2

C6H12O6+6O2→6CO2+6H2O+ATPC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}

CO2(g)+H2O(l)⇌H2CO3(aq)CO_{2(g)} + H_2O_{(l)} \rightleftharpoons H_2CO_{3(aq)}

CH4+2O2→CO2+2H2OCH_4 + 2O_2 \rightarrow CO_2 + 2H_2O

💡Examples

Problem 1:

Explain why the concentration of CO2CO_2 in the atmosphere often shows a seasonal fluctuation.

Solution:

During the spring and summer months, the rate of photosynthesis (6CO2+6H2O→C6H12O6+6O26CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2) increases due to more foliage and longer daylight hours, removing more CO2CO_2 from the atmosphere. In winter, photosynthesis decreases while respiration continues, leading to a temporary rise in CO2CO_2 levels.

Explanation:

This demonstrates the role of the global 'carbon sink' provided by deciduous forests.

Problem 2:

Calculate the impact of deforestation on the carbon cycle if an area of forest containing 50,00050,000 kg of stored carbon is burned.

Solution:

Burning the biomass converts the stored carbon into CO2CO_2. Using the molar masses (Carbon ≈12\approx 12, O≈16O \approx 16), the mass of CO2CO_2 produced is 50,000×4412≈183,33350,000 \times \frac{44}{12} \approx 183,333 kg of CO2CO_2.

Explanation:

Combustion (C+O2→CO2C + O_2 \rightarrow CO_2) releases carbon that was previously sequestered in biological molecules like cellulose (C6H10O5)nC_6H_{10}O_5)_n back into the atmosphere.

Problem 3:

How does the increase in atmospheric CO2CO_2 affect marine organisms with calcium carbonate shells?

Solution:

Increased CO2CO_2 dissolves in oceans to form H2CO3H_2CO_3. This dissociates to release H+H^+ ions, lowering the pH. The H+H^+ ions react with carbonate ions (CO32−CO_3^{2-}), making them less available for organisms to form CaCO3CaCO_3 shells.

Explanation:

Ocean acidification interferes with the calcification process, threatening coral reefs and shellfish populations.