The part of the yeast cell that converts nutrients like sugar into energy is the mitochondria.

Here's why:

* Mitochondria are often referred to as the "powerhouses" of the cell. They are responsible for cellular respiration, a process that breaks down glucose (sugar) in the presence of oxygen to produce ATP (adenosine triphosphate), the cell's primary energy currency.

* While yeast can also ferment sugar in the absence of oxygen, this process occurs primarily in the cytoplasm. Fermentation is less efficient than respiration, but it allows yeast to survive in anaerobic conditions.

So, while both the cytoplasm and mitochondria play a role in energy production, the mitochondria are the primary site for the conversion of sugar into energy through respiration.