What cellular structures are responsible for energy production and metabolism in a cell?

Mitochondria are often referred to as the "powerhouses" of the cell because they are the primary sites where energy production occurs. They generate adenosine triphosphate (ATP), the main energy currency of the cell, through a process called cellular respiration. This process involves breaking down nutrients, such as glucose and fatty acids, using oxygen to produce ATP, carbon dioxide, and water.

Additionally, mitochondria play a critical role in metabolic processes, including the Krebs cycle and oxidative phosphorylation, which are essential for converting food into usable energy. The structure of mitochondria, which includes an inner and outer membrane, facilitates these processes by creating separate compartments for chemical reactions.

Other cellular structures mentioned, such as ribosomes, are involved in protein synthesis, while the nucleus houses genetic material and controls cell activities. The cytoplasm is the fluid that fills the cell and contains various organelles, but it is not the site of energy production. Therefore, among the choices provided, mitochondria exclusively handle energy production and metabolic activities, making them the correct answer.