Single-celled organisms, like bacteria and amoeba, are incredibly simple but incredibly efficient. They absorb oxygen and nutrients directly through their cell membrane, which acts as a gatekeeper. Here's how it works:

Diffusion:

* Oxygen: Oxygen dissolves in the water surrounding the cell. Since there's usually more oxygen outside the cell than inside, oxygen molecules move across the cell membrane from the area of high concentration to the area of low concentration. This process is called diffusion.

* Nutrients: Similarly, nutrients like sugars and amino acids are dissolved in the surrounding water. They also diffuse across the cell membrane, moving from areas of higher concentration (outside the cell) to lower concentration (inside the cell).

Surface Area:

* Single-celled organisms have a very high surface area-to-volume ratio. This means that a lot of their cell membrane is in contact with the surrounding environment. This maximizes the efficiency of diffusion, allowing them to quickly absorb the necessary oxygen and nutrients.

Active Transport:

* In some cases, single-celled organisms may also use active transport to absorb nutrients. This process requires energy to move molecules against their concentration gradient (from low to high concentration). This is helpful when the concentration of a particular nutrient is low in the surrounding environment.

In summary: Single-celled organisms take in oxygen and nutrients through diffusion across their cell membrane, aided by a high surface area-to-volume ratio. They may also employ active transport when necessary to ensure they have the resources they need for survival.