Summary — What This Chapter Covers

• Dead plants and animals don't just disappear — they're actively broken down by bacteria and fungi, known as decomposers.
• Decomposers work by secreting enzymes onto dead material — this is called saprotrophic (extracellular) nutrition.
• These enzymes break big, insoluble molecules (like cellulose) into small, soluble ones (like glucose) that the decomposer can absorb.
• Once absorbed, the small molecules are used in respiration — and this respiration is exactly why decomposition releases CO₂ and methane.
• Decomposition also releases waste nutrients directly into the soil, which growing plants can absorb through their roots.
• This whole process is the engine that keeps the carbon cycle turning — without decomposers, carbon would stay locked inside dead bodies forever.

Topic 1 — The Role of Microorganisms in Decay

What Are Decomposers, and Why Do We Need Them?

Every living thing on Earth is built from a fairly small pool of chemical elements — carbon, nitrogen, hydrogen, oxygen, and so on. Those atoms don't get "used up"; they get borrowed. A carbon atom might spend a decade inside an oak tree, then a week inside a caterpillar that eats a leaf, then the rest of its existence cycling through soil, air, and other organisms.

But there's a problem: when a plant or animal dies, all of that borrowed carbon is now locked up inside a dead body. If nothing broke that body down, the atoms would just sit there, unavailable, forever. Ecosystems would slowly run out of usable carbon, nitrogen, and other nutrients.

This is where decomposers come in — mainly bacteria and fungi. Their entire biological "job" is to break down dead organic matter (dead tissue, waste, egested material) and, in doing so, release those trapped chemicals back into a form that living organisms can use again.

How Decomposers Actually Break Things Down

Here's the part that often gets missed: bacteria and fungi don't have a mouth, stomach, or gut. They can't "eat" a dead leaf the way an animal eats food. So how do they break down something as tough as cellulose in a plant cell wall, or the proteins in dead muscle tissue?

The answer is that decomposers secrete enzymes directly onto the dead organic matter, outside their own cells. These enzymes chemically digest the large, complex, insoluble molecules (like cellulose, starch, and proteins) into small, soluble molecules (like glucose and amino acids) right there in the environment — on the surface of the dead leaf, inside the rotting log, in the soil.

Once those molecules are small enough, the decomposer absorbs them straight through its cell surface membrane (or hyphae, in the case of fungi). This whole strategy — digesting food outside the body and then absorbing the products — is called saprotrophic nutrition (sometimes called extracellular digestion).

What Happens to Those Absorbed Molecules? Respiration & Gas Release

Once the decomposer has absorbed small molecules like glucose, it doesn't just sit on them — it uses them the same way any living cell would: as fuel for respiration, to release energy for growth and reproduction.

This is the crucial link to remember for exams: respiration by decomposers is exactly what produces the gases released during decay. Depending on whether the microorganisms are respiring aerobically or anaerobically (e.g. deep inside a compost heap where oxygen is limited), decomposition releases:

• Carbon dioxide (CO₂) — the normal waste product of aerobic respiration.
• Methane (CH₄) — produced when decomposition happens anaerobically (without oxygen), for example deep within landfill sites, waterlogged soils, or the guts of some animals.

Both gases are released into the atmosphere. This is actually a significant real-world issue — decaying organic waste in landfill sites is one of the major human-related sources of atmospheric methane, which is a much more potent greenhouse gas than CO₂.

How This Fits Into the Carbon Cycle

The individual steps above — secreting enzymes, absorbing molecules, respiring, releasing CO₂ — all combine to make decomposers one of the most important "return paths" in the carbon cycle. Without them, the carbon locked in dead bodies would never get back to the atmosphere, and plants would eventually run out of CO₂ to photosynthesise with.

Here's the full loop, matching the diagram you'll see in your textbook and exam papers:

Trace it through step by step: plants fix atmospheric CO₂ into biomass via photosynthesis. Animals eat plants (feeding), passing that carbon along. Both plants and animals lose some carbon back to the air constantly through their own respiration. When organisms die, or egest waste, that carbon ends up in dead tissue and waste. Decomposers then respire using that dead material as fuel, releasing the carbon straight back to atmospheric CO₂ — completing the cycle. If decomposition is incomplete (e.g. no oxygen, high pressure, over millions of years), the carbon instead gets locked away as fossil fuels, which only re-enters the atmosphere later through combustion.