When an onion is sliced, its strong smell and tear-inducing effects raise a curious question: is it still alive? These sensory experiences are direct results of complex biological processes within the onion. These reactions connect to the biological definition of life that persists even after the onion is separated from its plant.
What “Alive” Means for a Plant
Defining “life” for a plant differs from that of an animal, yet shares fundamental biological criteria. A plant, or its component parts, is considered alive if its cells exhibit organized structure, metabolic activity, and the ability to respond to stimuli. Plant cells contain organelles such as mitochondria, which are involved in respiration and energy production through the breakdown of glucose. This metabolic activity is essential for maintaining cellular processes.
Plant cells are distinguished by features not found in animal cells, including cell walls made of cellulose, chloroplasts for photosynthesis, and a large central vacuole that regulates turgor pressure. Even when harvested, individual plant cells within tissues can remain metabolically active for a period, consuming stored energy and maintaining cellular integrity. This continued activity demonstrates that plant cells, even post-harvest, retain signs of life.
The Chemistry of a Cut Onion
The effects experienced when cutting an onion stem from a chemical cascade triggered by cellular damage. Onions contain sulfur compounds, such as S-alk(en)yl-L-cysteine sulfoxides, which are stored separately from enzymes like alliinase within the intact cells. When an onion is cut, the cellular compartments are ruptured, allowing these compounds and enzymes to mix.
Upon mixing, alliinase rapidly converts the S-alk(en)yl-L-cysteine sulfoxides into unstable sulfenic acids, which quickly rearrange. Another enzyme, lachrymatory factor synthase (LFS), then acts upon these rearranged compounds, specifically 1-propenesulfenic acid, to form syn-propanethial S-oxide. This volatile compound rapidly vaporizes and diffuses into the air. When it comes into contact with the moisture in the eyes, it reacts to form a mild sulfuric acid, stimulating sensory neurons and triggering the familiar tearing response.
Life After the Cut
While a whole onion bulb exhibits sustained cellular activity, the longevity of “life” in a cut onion depends on the extent of damage and environmental conditions. The individual cells within a cut onion piece continue to respire and perform basic metabolic functions for some time, drawing on stored sugars and nutrients. This metabolic activity allows the cells to maintain their structure and some level of responsiveness, illustrating that they are still technically alive. However, the onion as a whole organism is no longer growing or undergoing the complex processes of an intact plant.
A significant distinction exists between a living cell and a living organism. Although diced or sliced onion pieces will eventually decay as cellular integrity is lost and nutrient supply ceases, the root end of an onion bulb retains enough latent life to potentially regrow. If provided with moisture and light, the cells in the basal plate of an onion can sprout new roots and shoots, demonstrating a remarkable capacity for regeneration. This highlights the viability of specialized plant cells even after being separated from the main plant body.