Cycles within ecosystems

Welcome to the World of Ecosystem Cycles!

In an ecosystem, materials like water, carbon, and nitrogen are constantly recycled between living organisms (plants, animals, and microorganisms) and the non-living environment (air, soil, and water).

Understanding these cycles is essential to understanding how ecosystems sustain life and how human activities can disrupt these natural balances.

Section 1: The Role of Decomposers

Decomposers are microorganisms (mainly bacteria and fungi) that break down dead organic matter and waste products.

Why are Decomposers Essential?

1. Nutrient and Mineral Recycling:

• When organisms die or produce waste, vital minerals and nitrogen compounds are locked inside organic matter.
• Decomposers break down these materials, releasing mineral ions (such as ammonium and nitrates) back into the soil so plants can absorb them.

2. Carbon Recycling:

• Decomposers carry out aerobic respiration, breaking down carbon compounds and releasing carbon dioxide ( ext{CO}_2) back into the atmosphere for photosynthesis.

Section 2: The Water Cycle

Water moves continuously between the Earth's surface and the atmosphere, powered by heat energy from the Sun.

Key Processes in the Water Cycle

• Evaporation: Heat from the Sun causes liquid water in lakes, rivers, and oceans to turn into water vapour and rise into the atmosphere.
• Transpiration: Evaporation of water from the surface of a plant's leaves through stomata into the atmosphere.
• Condensation: As warm water vapour rises and cools, it condenses back into liquid water droplets, forming clouds.
• Precipitation: Water falls back to the Earth's surface as rain, snow, sleet, or hail.

Section 3: The Carbon Cycle

Carbon moves through living organisms, the atmosphere, soil, and oceans through specific biological and chemical processes.

1. Removing Carbon Dioxide from the Atmosphere

• Photosynthesis: Green plants and algae take in ext{CO}_2 from the atmosphere and fix it into carbohydrates (glucose), converting light energy into chemical energy. This carbon becomes part of the plant's biomass.

2. Passing Carbon Along the Food Chain

• Feeding: When primary consumers eat plants, carbon compounds are transferred along the food chain to secondary and tertiary consumers.

3. Returning Carbon Dioxide to the Atmosphere

• Respiration: Plants, animals, and decomposers respire aerobically, breaking down glucose and releasing ext{CO}_2 as a waste product:
  ext{glucose} + ext{oxygen} ightarrow ext{carbon dioxide} + ext{water}
• Decomposition: Decomposers break down dead plant and animal remains, respiring and releasing ext{CO}_2.
• Combustion: Burning fossil fuels (coal, oil, gas) or biomass (wood, forests) oxidises carbon, releasing ext{CO}_2 back into the atmosphere.

Key Note: Human activities, such as burning fossil fuels and deforestation, have increased atmospheric ext{CO}_2, enhancing the greenhouse effect and contributing to global warming.

Section 4: The Nitrogen Cycle (Paper 2 Only)

Nitrogen is required by all living organisms to make proteins, amino acids, and DNA. Although nitrogen gas makes up about 78% of the atmosphere, plants and animals cannot absorb it directly in this unreactive form.

The nitrogen cycle relies on specific groups of bacteria to convert nitrogen between different forms:

1. Nitrogen-Fixing Bacteria

• Convert unreactive nitrogen gas ( ext{N}_2) from the air into ammonia or nitrates in the soil.
• Some nitrogen-fixing bacteria live free in the soil, while others live inside root nodules of leguminous plants (such as peas, beans, and clover) in a mutualistic relationship.
• Note: Lightning can also fix nitrogen directly by causing nitrogen and oxygen to react, forming nitrates that wash into soil via rain.

2. Decomposers (Fungi and Bacteria)

• Break down animal waste (urea, faeces) and dead organic matter, converting organic nitrogen compounds (such as proteins) into ammonia/ammonium ions in the soil.

3. Nitrifying Bacteria

• Convert ammonia/ammonium ions into nitrites, and then oxidise nitrites into nitrates ( ext{NO}_3^-).
• Plants can absorb nitrates from the soil via active transport through their root hair cells to synthesise amino acids and proteins.

4. Denitrifying Bacteria

• Convert nitrates in the soil back into nitrogen gas ( ext{N}_2), which is released into the atmosphere.
• Denitrifying bacteria are typically active in anaerobic conditions, such as un-aerated or waterlogged soils.

Syllabus reminder: You do not need to memorise specific genus or species names of bacteria for the nitrogen cycle; knowing the general group names and their roles is sufficient.

Summary Table: Key Bacteria in the Nitrogen Cycle