Matter Cycling & Ecosystem Stability

Energy flows one way from the sun to heat, but matter such as carbon and nitrogen cycles between organisms and the environment. Nitrogen is essential for proteins and nucleic acids, yet most plants cannot use atmospheric N₂ without bacterial fixation. Through fixation, nitrification, assimilation, and denitrification, nitrogen returns to the air. Follow the nitrogen-cycle canvas and the classification tables to see each step.

Energy flows, matter cycles

♻️ Do not confuse the two

①Energy: sun → producer → consumer → escapes as heat (one-way, never cycles)

②Matter (carbon, nitrogen): cycles endlessly between organisms and the environment

③So energy must be supplied continuously, but matter is recycled

Nitrogen cycle — air nitrogen into life

🌱 Why does nitrogen matter?

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

②But atmospheric N₂ is stable, so most organisms cannot use it directly

③Bacteria (nitrogen fixation) must convert it into soil forms before plants absorb it

The four steps of the nitrogen cycle

Nitrogen cycle

Four key processes

How energy flow differs from matter cycling

Energy vs matter

ChartDifferent directions of flow

Aspect	Energy	Matter (C, N)
Direction	one-way (sun → heat)	cyclic (organisms ↔ environment)
Recycled?	no — must be resupplied	yes — reused repeatedly
Final form	leaves to space as heat	returns to the environment

From concept to problem

Example 1 — identify the nitrogen process

Nitrate (NO₃⁻) in the soil turned into atmospheric nitrogen (N₂) and escaped into the air. What is this process called, and which organism carries it out?

Find the process that returns nitrate to atmospheric nitrogen.

NO₃^- → N₂

This process is denitrification, carried out by denitrifying bacteria.

▸ denitrification; organism = denitrifying bacteria

It is exactly opposite to nitrogen fixation (N₂→NH₄⁺). Fixation brings nitrogen into the biosphere; denitrification returns it to the air.

Summary

Flow of the nitrogen cycle

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

fixation, nitrification, assimilation, denitrification — bacteria are central to all

Ecosystem stability

the more complex the food web, the higher the stability

stability = species makeup and numbers kept roughly constant; small disturbances recover on their own

CSAT-style (adapted, Biology I)

A figure shows part of the nitrogen cycle. ㉠ is atmospheric N₂ → NH₄⁺, ㉡ is NH₄⁺/NO₃⁻ → plant protein, ㉢ is NO₃⁻ → atmospheric N₂. Choose the correct statement.

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

②㉡ is nitrogen assimilation: plants take up inorganic nitrogen and make organic matter.

③When ㉢ occurs, soil nitrogen increases.

④All three processes occur without bacteria.

⑤㉠ is part of photosynthesis.

▸ ② ㉡ is nitrogen assimilation: plants take up inorganic nitrogen and make organic matter.

㉠ (N₂→NH₄⁺) is nitrogen fixation done by bacteria such as root-nodule bacteria → ①, ④, ⑤ wrong.

㉡ is assimilation, where plants absorb NH₄⁺/NO₃⁻ and synthesize protein → ② correct.

㉢ (NO₃⁻→N₂) is denitrification, so soil nitrogen leaves to the air and decreases → ③ wrong.

🎯 Exam Points

①Energy = one-way (no cycle), matter = cyclic

②Nitrogen cycle: fixation (N₂→NH₄⁺), nitrification (NH₄⁺→NO₃⁻), assimilation, denitrification (NO₃⁻→N₂)

③Fixation by root-nodule bacteria and lightning; denitrification by denitrifying bacteria

④Carbon cycle: photosynthesis (takes CO₂) ↔ respiration/combustion (releases CO₂)

⑤Stability: a more complex food web is more stable, small disturbances self-recover

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