Construct an explanation for how matter cycles through ecosystems, including the carbon, nitrogen, and water cycles, and the role of photosynthesis, respiration, and decomposers (Tennessee Academic Standards for Science, Biology I, BIO1.LS2).

What this topic is asking

The Tennessee LS2 standards ask you to explain how matter cycles through ecosystems, focusing on the carbon, nitrogen, and water cycles. For the Biology I EOC that means knowing how each cycle moves atoms between living things and the environment, the central role of photosynthesis and respiration in the carbon cycle, and the role of decomposers and bacteria. The big contrast with the previous topic is that matter cycles (is reused), while energy flows one way (is lost).

Matter cycles, energy flows

The carbon cycle

Carbon moves between the atmosphere (as carbon dioxide), living things, and the soil and oceans:

• Photosynthesis removes $\text{CO}_2$ from the air, fixing carbon into glucose and other molecules in producers.
• Cellular respiration breaks those molecules down and returns $\text{CO}_2$ to the air. Both plants and animals respire.
• Carbon moves through food as organisms eat one another.
• Decomposers release carbon from dead organisms and waste.
• Combustion (burning fossil fuels and wood) also returns carbon to the atmosphere, which links to human impact and climate change.

So photosynthesis and respiration are the two processes that cycle carbon between organisms and the atmosphere in opposite directions, the most-tested fact in the carbon cycle.

The nitrogen cycle

Nitrogen is needed to build proteins and nucleic acids, but the abundant nitrogen gas ($\text{N}_2$) in the air is unusable by most organisms directly. The nitrogen cycle solves this:

• Nitrogen fixation. Nitrogen-fixing bacteria (many living in soil or on the roots of legumes) convert $\text{N}_2$ into compounds (such as ammonia and then nitrates) that plants can absorb.
• Uptake and feeding. Plants absorb the nitrogen compounds to make proteins; animals get nitrogen by eating plants or other animals.
• Decomposition. Decomposers break down dead organisms and waste, returning nitrogen to the soil.
• Other bacteria can return nitrogen to the air, completing the cycle.

The key EOC point is the role of bacteria: nitrogen-fixing bacteria make atmospheric nitrogen usable, and decomposers recycle it.

The water cycle

Water cycles through evaporation (liquid to vapor, driven by the Sun), condensation (vapor to cloud droplets), precipitation (rain and snow), and transpiration (water evaporating from plant leaves). Water also moves as runoff and groundwater. The water cycle distributes the water that all life depends on and connects the land, oceans, and atmosphere.

Why decomposers are essential

If decomposers were removed, dead matter would pile up and the nutrients in it would stay locked away, so producers would run short and the cycles of matter would stall. This is a favorite EOC scenario: predict the effect of removing the decomposers.

Try this

Q1. Name the two processes that move carbon between living things and the atmosphere in opposite directions, and state the direction of each. [2]

• Cue. Photosynthesis (removes carbon dioxide from the air into producers) and cellular respiration (returns carbon dioxide to the air).

Q2. Explain the role of nitrogen-fixing bacteria in the nitrogen cycle. [2]

• Cue. They convert nitrogen gas from the air into compounds (such as ammonia and nitrates) that plants can absorb and use to build proteins, because most organisms cannot use nitrogen gas directly.