4c Cycles within Ecosystems¶

Part of 4 Ecology and the Environment.

Matter is continually recycled through ecosystems rather than used once and lost. The important cycles here are the carbon cycle and the nitrogen cycle.

Learning Objectives¶

The Carbon Cycle¶

Carbon moves between the atmosphere, living organisms and the earth in a continuous cycle:

• Photosynthesis: plants and algae absorb carbon dioxide from the atmosphere and use it to make organic molecules (glucose, then proteins, fats, etc.). This removes carbon from the atmosphere.
• Respiration: plants, animals and decomposers all respire. Aerobic respiration releases carbon dioxide back into the atmosphere.
• Decomposition: bacteria and fungi break down dead plant and animal bodies and waste products, releasing carbon dioxide through their own respiration.
• Combustion: burning of plant material or fossil fuels (the compressed remains of ancient dead organisms) releases stored carbon as carbon dioxide into the atmosphere.

The cycle links the atmosphere, living organisms and dead organic material in one continuous exchange of carbon compounds.

Biology-Only Content: The Nitrogen Cycle¶

This content is required for Biology-only students and is not required for Combined Science students.

Nitrogen makes up most of the atmosphere (about 78%), but it is chemically unreactive in this form and cannot be used directly by most living things. Plants need nitrogen compounds (particularly nitrates) to make amino acids and therefore proteins. Several groups of microorganism are essential in making this possible:

• Nitrogen-fixing bacteria convert atmospheric nitrogen (N₂) into nitrogen-containing compounds (ammonium ions and nitrates) that plants can absorb. Some live freely in soil; others live in root nodules of legume plants such as peas and clover. Lightning can also fix small amounts of nitrogen from the atmosphere into nitrate compounds.
• Decomposers (bacteria and fungi) break down proteins in dead organisms and waste products (such as urea), releasing ammonia (ammonium ions) into the soil.
• Nitrifying bacteria convert ammonia into nitrites and then into nitrates in a process called nitrification. This process requires oxygen and produces the nitrate ions that plants can absorb through their roots by active transport.
• Denitrifying bacteria convert nitrates back into nitrogen gas (N₂) and return it to the atmosphere. They work anaerobically and thrive in waterlogged, oxygen-poor soils.
• Plants absorb nitrates from the soil through their roots and use them to build amino acids and proteins.
• Animals obtain nitrogen compounds by eating plants or other animals.

The Nitrogen Cycle¶

Plants need nitrogen compounds to make amino acids and proteins, but they cannot use atmospheric nitrogen directly. Nitrogen-fixing bacteria convert atmospheric nitrogen into forms plants can use. Decomposers return nitrogen compounds to the soil from dead matter and waste. Nitrifying bacteria convert ammonia into nitrates, while denitrifying bacteria return nitrogen to the atmosphere.

Why Cycles Matter¶

• Without these cycles, essential atoms would become locked away in forms that organisms could not reuse.
• Cycles connect ecology to nutrition, photosynthesis and respiration across the rest of the course.
• Human activities that disrupt these cycles — such as deforestation and the addition of excess fertilisers — can have serious consequences for ecosystems.

Key Terms¶

• Carbon cycle: the cycling of carbon atoms through living organisms, the atmosphere, soil and sediments via photosynthesis, respiration, decomposition and combustion.
• Nitrogen cycle: the cycling of nitrogen through the atmosphere, soil and living organisms, mediated by bacteria.
• Decomposition: the breakdown of dead organic material by bacteria and fungi.
• Combustion: the burning of organic material or fossil fuels, releasing carbon dioxide.
• Nitrogen fixation: the conversion of atmospheric nitrogen gas into nitrogen compounds usable by plants.
• Nitrogen-fixing bacteria: bacteria that convert atmospheric nitrogen into ammonia or nitrates; found in soil and legume root nodules.
• Nitrifying bacteria: bacteria that convert ammonia into nitrites and then nitrates in the soil (nitrification).
• Denitrifying bacteria: bacteria that convert nitrates back into nitrogen gas, returning it to the atmosphere; work anaerobically in waterlogged soil.
• Decomposer: an organism (bacterium or fungus) that breaks down dead organic matter, releasing nutrients.

Connected Pages¶

• 4b Feeding Relationships
• 4d Human Influences on the Environment
• 2e Nutrition
• IGCSE Biology Key Terms Glossary