Grade 10-11 (age 15-17)

high school Matter Cycling & Ecosystem Stability

Energy enters from the Sun and leaves as heat, flowing one way, but matter such as carbon and nitrogen goes round and round between living things and their surroundings. Nitrogen is essential for building proteins and nucleic acids, yet the N₂ that makes up nearly 80% of the air is so stable that most organisms cannot use it as it is. So bacteria or lightning fix N₂, nitrifying bacteria turn it into nitrate for plants to take up, and denitrifying bacteria return it to N₂, carrying nitrogen around a full loop. In the diagram you can see which process moves nitrogen along each arrow between the air, the soil and living things.

Energy Flows, Matter Cycles

♻️ Telling the Two Apart

①Energy passes from the Sun to producers and on to consumers, leaving as heat at every step and never being used again

②Matter such as carbon and nitrogen moves endlessly between living bodies and the air, soil and water

③So an ecosystem needs a constant supply of energy from the Sun, but it uses the same atoms of matter over and over

The Nitrogen Cycle

🌱 How Nitrogen Reaches Living Things

①Nitrogen is part of proteins and nucleic acids (DNA and RNA)

②N₂ in the air is very stable, with its two nitrogen atoms held by a triple bond, so most organisms cannot use it directly

③Plants can take it up only after nitrogen-fixing bacteria such as those in root nodules turn N₂ into ammonium, or lightning turns it into nitrate

④Animals get nitrogen by eating plants, and decomposers return the nitrogen in dead bodies and wastes to ammonium

The Four Steps of the Nitrogen Cycle

Nitrogen cycle

Four Processes That Move Nitrogen

Energy Flow and Matter Cycling

Energy vs matter

They Move in Different Directions

Aspect	Energy	Matter (carbon, nitrogen)
Direction	one way (Sun → heat)	cycles (living things ↔ environment)
Used again?	no: must be supplied by the Sun all the time	yes: the same atoms are used over and over
Where it ends up	as heat, leaving the ecosystem	back to the air, soil and water

From concept to problem

Example 1: Identifying a Step of the Nitrogen Cycle

Nitrate (NO₃⁻) in the soil is turned into nitrogen gas (N₂) and escapes into the air. What is this process called, and which organisms carry it out?

Look for the process that returns nitrate to nitrogen in the air.

NO₃^- → N₂

This is denitrification, carried out by denitrifying bacteria.

▸ denitrification, by denitrifying bacteria

It runs exactly opposite to nitrogen fixation (N₂ → NH₄⁺). In the diagram the fixation arrow goes down from the air to the soil and the denitrification arrow goes up from the soil to the air. Fixation brings nitrogen into the living world; denitrification sends it back to the air.

Summary

Flow of the nitrogen cycle

N₂ →(fix)→ NH₄⁺ →(nitrify)→ NO₃^- →(assimilate)→ protein, NO₃^- →(denitrify)→ N₂

fixation, nitrification and denitrification are carried out by bacteria; assimilation is carried out by plants

Ecosystem Balance

the more complex the food web, the more stable the ecosystem

ecosystem balance is a state in which the kinds and numbers of organisms and the amounts of matter stay nearly constant; small disturbances are undone through feeding relationships

CSAT-style problem (Life Science I), adapted

The figure shows part of the nitrogen cycle in an ecosystem. ㉠ is N₂ in the air → NH₄⁺, ㉡ is NH₄⁺ and NO₃⁻ → plant protein, and ㉢ is NO₃⁻ → N₂ in the air. Choose the correct statement.

①㉠ is denitrification and is carried out by plants directly.

②㉡ is nitrogen assimilation, in which plants take up inorganic nitrogen and make organic matter.

③When ㉢ occurs, the nitrogen in the soil increases.

④All three processes occur without bacteria.

⑤㉠ is part of photosynthesis.

▸ ② ㉡ is nitrogen assimilation, in which plants take up inorganic nitrogen and make organic matter.

㉠ (N₂ → NH₄⁺) is nitrogen fixation, done by bacteria such as Rhizobium; plants do not do it directly and it has nothing to do with photosynthesis, so ①, ④ and ⑤ are wrong.

㉡ is nitrogen assimilation, in which plants take up NH₄⁺ and NO₃⁻ and make protein, so ② is correct.

㉢ (NO₃⁻ → N₂) is denitrification, which sends soil nitrogen into the air and lowers the nitrogen in the soil, so ③ is also wrong.

🎯 Exam Points

①Energy flows one way and does not cycle, but matter cycles

②The nitrogen cycle runs through fixation (N₂ → NH₄⁺), nitrification (NH₄⁺ → NO₃⁻), assimilation (plants take it up and make protein) and denitrification (NO₃⁻ → N₂)

③Nitrogen fixation is done by bacteria such as Rhizobium and by lightning; denitrification is done by denitrifying bacteria

④Carbon enters living things from the air through photosynthesis and returns to the air through respiration and combustion

⑤The more complex the food web, the more stable the ecosystem, and small disturbances are undone on their own

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Last updated 2026-10-17

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