Answer:
4. transforming the energy in glucose and related molecules in a chemical form that cells can use for work
Explanation:
Glycolysis breaks down glucose into two molecules of pyruvate which is transformed into acetyl CoA to enter the Kreb's cycle. Kreb's cycle breakdown the acetyl CoA into CO2 and H2O. The energy stored in the glucose molecule is released during glycolysis and Kreb's cycle. The released energy is stored in the form of NADH and FADH2 as well as in few molecules of ATP.
The NADH and FADH2 enter the final step of cellular respiration, the oxidative phosphorylation. Here, NADH and FADH2 are oxidized with the help of electron transport chain (ETC). During the transfer of electrons through ETC, the proton motive force is generated which then helps in ATP synthesis.
Hence, the three steps of cellular respiration (glycolysis + Kreb's cycle + oxidative phosphorylation) retrieve the energy from nutrients such as glucose and store it in the form of ATP. ATP is used by cells as an energy source for various other functions.
