In the human body oxygen is absorbed by the lungs and nutrients are absorbed by the small intestine. In a single-celled organism this absorption directly involves the what?

In single-celled organisms, such as bacteria or protists, absorption occurs directly through the cell membrane. The cell membrane allows for the passage of oxygen, nutrients, and other molecules into the cell through processes like diffusion or active transport. This direct contact with the external environment allows single-celled organisms to efficiently obtain the necessary resources for survival and growth.