Single-celled organisms, also known as unicellular organisms, are living entities made up of just one cell. They can be found in various environments, including soil, water, and even inside other living organisms. Examples of single-celled organisms include bacteria, amoebas, and yeasts. These organisms perform all necessary life functions within their single cell, such as reproduction, metabolism, and response to stimuli. Despite their simplicity, single-celled organisms play crucial roles in ecosystems. They contribute to nutrient cycling, help decompose organic matter, and can even be involved in processes like fermentation. Some single-celled organisms, like certain bacteria, are essential for human health, while others can cause diseases.