The course of envents made it necessary for Joseph to start working

Why both a source and sink are necessary for converting heat into work ?

When force is exerted on a body what is necessary for work done ?

Why a leaf must be destarched before starting the experiment showing that light is necessary for photosynthesis ?

Human body is made up to trillons of cells. Why is it necessary for every cell to have the entir set of chromosomes pairs?