Why do single-celled organisms not require specialized organs for taking in food and exchanging gases?
Because their entire surface is in contact with the environment
Single-celled organisms such as Amoeba or Paramecium consist of just one cell. Because that cell is tiny and floats in the surrounding medium, its plasma membrane is directly exposed to the water or air around it.
Food particles can be taken in by simple engulfment or diffusion, and gases like oxygen and carbon dioxide diffuse in and out across the whole surface. There is no need for a specialised digestive or respiratory organ. They do not have a separate transport system (multicellular animals do), they certainly need food and oxygen to survive, and their body designs are simple, not complex.
Hence, the answer is Because their entire surface is in contact with the environment.
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