Final answer:
In biological systems, oxygen is the substance that acts as the terminal electron acceptor, necessary for sustaining the flow of electrons in the electron transport chain and essential for the production of ATP through oxidative phosphorylation.
Step-by-step explanation:
The substance that picks up electrons to keep the electrons flowing through the electron transport chain in biological systems is oxygen. In the final step of the electron transport chain, electrons are passed through a series of protein complexes to oxygen, which acts as the terminal electron acceptor. Oxygen picks up these electrons along with hydrogen ions (protons) to form water. If oxygen were not present to accept the electrons, the entire electron transport chain would cease to function, which would ultimately lead to the cell's death due to energy deficiency, highlighting the critical role of oxygen in cellular respiration.