Unicellular organisms can be either heterotrophic or autotrophic. Because they just have once cell they can only accommodate either to be autotrophic or heterotrophic. If they are autotrophic, they produce their own food through the process of photosynthesis. The best example is unicellular algae. But in heterotrophic unicellular organisms like Euglena, they feed on other organisms. Even Amoeba is an example for this. Now the multicellular organisms can be both autotrophic and heterotrophic in nature. And hence the work is distributed. Few tissues absorb raw materials, few tissues synthesise the food and few tissues assimilate them.