I think all of the above( or maybe I’m wrong) because producers commonly are plants, but also can be bacteria or protists. Plants are always multicellular, while bacteria and protists are unicellular. Primary consumers that consume producers, like us(humans) are multicellular. But, since organisms that obtain their energy from other organisms are called consumers. ... All animals are consumers, and they eat other organisms. Fungi and many protists and bacteria are also consumers. [ If you don’t count in the bacteria, protist, or fungi (all unicellular) then I guess it could also be only producers?]
Answer: "To move substances against a concentration or electrochemical gradient, a cell must use energy. Active transport mechanisms do just this, expending energy (often in the form of ATP) to maintain the right concentrations of ions and molecules in living cells."
