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

In a single-celled organism, the absorption directly involves the cell membrane. The cell membrane is responsible for keeping the shape of the cell and for deciding what goes inside the cell and what stays out. It lets the substances that help and sustain the cell in and keeps the substances that would harm the cell outside and thus it is directly involved in the absorption of nutrients.