In which part of the cell do the Krebs cycle and electron transport chain occur?

The Krebs cycle and the electron transport chain are both critical processes in cellular respiration responsible for producing ATP, the energy currency of the cell. These processes occur specifically in the mitochondria, which are often referred to as the "powerhouses" of the cell.

The Krebs cycle, also known as the citric acid cycle, takes place in the mitochondrial matrix, where it processes acetyl-CoA to produce electron carriers that are essential for later stages of energy production. Following this, the electron transport chain is located in the inner mitochondrial membrane, where the electrons derived from the Krebs cycle are transferred through a series of proteins, ultimately leading to the synthesis of ATP.

The nucleus contains the genetic material and is involved in processes such as transcription and replication, but it does not play a direct role in energy production. The cytoplasm is where glycolysis occurs, the first step of cellular respiration, but it does not host the Krebs cycle or the electron transport chain. The endoplasmic reticulum is involved in protein and lipid synthesis, rather than energy production processes.

Thus, the mitochondria are the correct answer because they are the organelles specifically designed for the Krebs cycle and electron transport chain, facilitating the energy production that is vital for cellular function.