Decomposition & Decay

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.

absorbs saprotrophic nutrition