Which of the following is not a component of the endomembrane system? a. mitochondrion b. Golgi apparatus c. en doplasmic reticulum d. lysosome

The endomembrane system is a group of organelles within eukaryotic cells that work together to transport, modify, and synthesize proteins and lipids. These organelles are either directly connected, or they exchange material through vesicle transport. The main components of the endomembrane system include the endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles.

a. Mitochondrion: This organelle is involved in energy production (ATP) through cellular respiration, and it is not directly part of the endomembrane system. b. Golgi apparatus: This organelle is involved in the modification, sorting, and packaging of proteins and lipids received from the endoplasmic reticulum. It is a key component of the endomembrane system. c. Endoplasmic reticulum: This organelle is involved in the synthesis, folding, quality control, and despatch of proteins and lipids. It is a major component of the endomembrane system. d. Lysosome: This organelle is involved in the degradation and recycling of cellular waste material. It is also a component of the endomembrane system as it interacts with endocytic vesicles and the endoplasmic reticulum in a cell.

Based on the comparisons made in Step 2, the mitochondrion (option a) is not a component of the endomembrane system. The other options, Golgi apparatus, endoplasmic reticulum, and lysosome, are all components of the endomembrane system. Therefore, the correct answer is a. mitochondrion.