Definition
Bury an apple peel in the ground and it turns back into soil within weeks or months, but an ordinary plastic bag can last hundreds of years. Biodegradable plastic is designed to work like that apple peel: bacteria, fungi, and other microbes in nature eat it and break it down completely into water and carbon dioxide.
Plastic that microbes can digest
The ordinary plastic we use every day is made from processed oil. Its carbon atoms hold hands tightly in a huge chain. To microbes in nature, this chain is so tough and unfamiliar that it's like a giant rock they can't even bite into. That's why it can last for hundreds of years and linger as microplastics that pollute nature.
Biodegradable plastic, on the other hand, has a weak bond structure that microbes can cut with the enzymes they release. Like soft white bread that's easy to digest, it's built so that microbes can chop the plastic chain into small pieces and take it in as food.
When microbes eat plastic and get energy from it, what's left is mostly water and carbon dioxide, and some of it becomes part of the microbes' own bodies. Where there's no oxygen, methane can form too. That's how plastic waste avoids becoming permanent trash and gets taken back into nature's cycle.
Will it just vanish if you toss it on the street?
With a name like "biodegradable," you might think it disappears like magic within days, even on the road or in the ocean. But it doesn't. Microbes also need the right temperature and moisture to get to work.
The corn-starch-based plastic (PLA) that's widely used today usually breaks down actively only in hot, humid conditions, typically above 58 degrees Celsius (136°F). Thrown into cold seawater at around 15 degrees Celsius (59°F) or onto dry ground, it can sit there for years, just like ordinary plastic.
That's why these products only break down as designed when they're processed at a professional composting facility with steady heat and moisture.
To be more precise: the source material and the breakdown are separate things
Many people think plastic made from corn or sugarcane always rots in nature, but that mixes up the source material and how it breaks down. There are two different ways to sort plastics: "what is it made from?" and "how does it break down?"
Even if it's made from plants, if its chemical structure is synthesized the same as an ordinary PET bottle, it won't rot for a long time, just like ordinary plastic. On the other hand, even if it's made from raw material extracted from oil, if it's synthesized into a structure that microbes can easily break down, it becomes a perfectly good biodegradable plastic.
In the end, what matters for eco-friendly plastic is less whether the source is plants or oil, and more whether the product can return fully to nature when its useful life is over.
Common misconceptions
Biodegradable plastic quickly disappears on its own, even if you throw it on the street or in the ocean.
Most biodegradable plastics break down quickly only under professional composting conditions, with high heat above 50 degrees Celsius (122°F) and the right moisture. In cold seawater or ordinary outdoor settings, breaking down can take years or more.
Where you meet it
PLA drink cups and straws made from corn starch are designed to break down at industrial composting facilities.
Biodegradable mulch film: after farming, you plow it into the soil instead of removing it, and microbes break it down into carbon dioxide and water over time.
In one sentence
Biodegradable plastic is a material made so that microbes can eat it, but it needs the right temperature and moisture to break down properly.