Matter Cycling & Ecosystem Stability

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

📊Different 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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