Key Processes of Exchange
Decomposition is the natural breakdown of complex organic compounds (such as carbon chained polymers) into simpler substances that can be more readily incorporated into living systems. The tissue of dead plants and animals is broken down by a combination of physical and chemical processes. Physical breakdown occurs when scavengers such as vultures, rats, and ants eat the flesh of dead animals, then excrete as feces all material not digested completely. Beetles, termites, and their larvae eat the bark of fallen trees and the stems of other plants, and flatworms consume smaller bits of organic matter. Other organisms that participate in the physical decomposition process include mice, moles, earthworms, mites, maggots (fly larvae) and other larvae, slugs, and snails.
The most important chemical decomposers are usually the smallest, and the least appreciated—the bacteria and fungi. These microorganisms secrete digestive enzymes onto the organic compounds to break them down. They then absorb molecules directly through their body or cell membranes. A million bacteria can live in 1 gram of soil, where they break down organic compounds from dead tissue and use them for food. Fungi also break down organic compounds, including cellulose, gums, and waxes that other microorganisms avoid.
As microorganisms complete the process of decay, they also renew the cycle of life by releasing organic matter, composed primarily of carbon, hydrogen, oxygen, and nitrogen, into the atmosphere. Chemical compounds released include carbon dioxide, ammonia, and methane. These substances are absorbed by green plants for nutrition, beginning a new food chain. Without decay, the essential building blocks of life would remain locked inside dead tissue.