What is the endothermic reaction for aerobic respiration in a living cell?

#color(blue)"Endothermic or exothermic?"#
The difference between endothermic and exothermic in this context:

• endothermic = a reaction that requires energy to occur
• exothermic = a reaction the creates energy

  #color(blue)"Cellular respiration"#
  Cellular respiration can be divided into three steps:

  1. Glycolysis
  2. Krebs Cycle
  3. Electron Transport Chain

     When you look at cellular respiration (aerobic) as a whole, it is an exothermic reaction because it creates chemical energy in the form of ATP.

     There is an endothermic step in glycolysis. Glycolysis is the breakdown of glucose into 2 pyruvate molecules. As a whole glycolysis is exothermic, yielding net 2 ATP. Glycolysis itself can be devided into:

     1. Glycolysis I : #color(red)"endothermic"# process in which glucose is converted into fructose with 2 phosphate groups #-># investment of 2 ATP
     2. Glycolysis II : #color(green)"exothermic"# process in which 2 pyruvate molecules and 4 ATP is formed.

        The image below visualizes this process. Glycolysis I is the only endothermic reaction in cellular respiration, the other processes are exothermic.