In 1937, scientist Vanden Honert introduced the ion carrier theory. According to this theory, there is outer space outside the cell membrane and inner space inside the cell membrane. Ions can move freely between the extracellular and intracellular compartments, but not within the cell membrane. Ions from the outside environment come directly to the outside. On the outside, ions and carriers join to form ‘ion complexes’. The ‘ion complex’ compound passes through the cell membrane and reaches the endoplasmic reticulum. On reaching the bottom, the ‘ion complex’ breaks down and the ions and carriers are released. The free ion accumulates inside the cell. The carrier returns to its previous location and participates in another ion transport. In this case the metabolic energy to create, move and break down compounds comes from respiration.