Here's why:

* Heterotrophs are organisms that cannot produce their own food and must consume other organisms to obtain energy. They are consumers in the food chain.

* The energy currency of cells is ATP (adenosine triphosphate). ATP is a molecule that stores and releases energy within cells, making it essential for all life processes.

Here's a clearer breakdown:

* Autotrophs (like plants) are the producers in the food chain. They convert sunlight into chemical energy through photosynthesis, creating sugars (their food source) and releasing oxygen as a byproduct.

* Heterotrophs consume autotrophs or other heterotrophs to obtain their energy. They break down the food they consume to obtain the chemical energy stored within it. This energy is then used to create ATP, the energy currency that powers their cells.

In summary: Heterotrophs are important in the food chain but they are not the energy currency of cells. ATP is the energy currency, and it's produced by both autotrophs and heterotrophs through different processes.