The question of whether all organisms “breathe” receives a complex answer in biology: yes and no. The common understanding of breathing involves the physical action of inhaling and exhaling air using lungs, which is a process known as ventilation. This macroscopic action is only one solution to a universal biological requirement: the exchange of gases with the environment. Every living thing, from the smallest bacterium to the largest whale, must constantly manage the intake of certain gases and the expulsion of others to power life. The vast diversity of life has led to an equally diverse array of methods for achieving this necessary gas exchange.
Breathing Versus Cellular Respiration
The confusion surrounding “breathing” stems from blending two distinct biological concepts: the mechanical process and the chemical one. Breathing, or ventilation, is the physical movement of air or water across a specialized surface, such as lungs or gills, solely to facilitate gas exchange. This is a macroscopic, external process that moves gases between the environment and the organism’s body. Cellular respiration, by contrast, is a chemical process that occurs inside individual cells to generate energy. This internal process typically uses oxygen to break down fuel molecules, releasing carbon dioxide as a waste product. All organisms must perform some form of cellular respiration to stay alive, making the physical act of moving air merely a delivery system for the gases required at the cellular level.
The Universal Need for Energy Production
The core driver for all respiratory processes is the need to produce Adenosine Triphosphate (ATP), the universal energy currency of life. ATP is a nucleotide molecule that stores and releases energy by breaking and reforming a high-energy phosphate bond. The energy released from breaking ATP is used to power nearly every cellular function, including muscle contraction, nerve impulse transmission, and the construction of complex molecules. In the most common form, aerobic respiration, glucose is broken down in the presence of oxygen to yield a significant amount of ATP. This energy production is a requirement for maintaining cellular homeostasis and sustaining life in all known organisms.
Diverse Mechanisms of Gas Exchange
For organisms that rely on oxygen, the challenge is how to move oxygen from the environment to the cells deep inside the body, and simultaneously move carbon dioxide out. The simplest organisms, such as single-celled life and tiny invertebrates, rely on simple diffusion across their outer cell membrane. Gases passively move across the surface from areas of high concentration to low concentration. Larger, more complex organisms developed specialized structures to overcome the limitations of their surface-area-to-volume ratio. These diverse solutions all accomplish the same goal: moving the necessary gases across a permeable barrier to sustain cellular function.
Specialized Structures for Gas Exchange
Fish and other aquatic life use gills, which are highly branched filaments that efficiently extract dissolved oxygen from water. Insects have a unique system called the tracheal system, a network of air-filled tubes that delivers oxygen directly to the tissues through small pores called spiracles. Amphibians and earthworms utilize cutaneous respiration, exchanging gases directly through their moist skin supported by a dense network of capillaries. Even plants manage gas exchange through tiny pores on their leaves called stomata, which regulate the intake of carbon dioxide for photosynthesis and the release of oxygen.
The Organisms That Do Not Require Oxygen
While the majority of complex life is classified as aerobic, meaning it uses oxygen, a subset of life demonstrates that oxygen is not a universal requirement for energy production. These organisms, known as anaerobes, thrive in environments lacking oxygen, such as deep-sea sediments or intestinal tracts. Obligate anaerobes are poisoned by oxygen and must use alternative chemical pathways to generate ATP.
Instead of using oxygen as the final electron acceptor in the cellular respiration chain, these organisms utilize other molecules like sulfate, nitrate, or carbon dioxide. Other microbes rely on fermentation, a less efficient process that breaks down glucose without oxygen, yielding byproducts like lactic acid or alcohol. The discovery of the parasite Henneguya salminicola in 2020 challenged previous assumptions by being the first multicellular animal found to have completely lost the genes for aerobic respiration.