Food is crucial to organisms’ survival. Being organisms ourselves, we need food. We need food to gain energy from their chemical bond, helping us sustaining life and grow.
All carbohydrates, fats or proteins can become the fuel for a process names Cellular Respiration (CR) to be converted into energy.
CR needs ATP, ADP, existing carbon molecules and food to create more energy. To best show exactly the steps of CR, glucose will be used as the food.

1. Glycolysis:
Glycolysis happens in the cytoplasm. Its main job is to break down glucose into smaller pieces because glucose by itself is too big to fit into the Krebs cycle.
Glucose is 6 carbon molecules combined. Glycolysis breaks it down into two pieces of 3 carbon molecules named Pyruvate.
By using up two ATP to break the bonds, glycolysis created 4 ATP but used up 2 ATP, meaning there is a net total of 2 ATP produced along with 2 NADH molecules (that will be explained laterJ!)

Here is a link to a cartoon about glycolysis

2. Link Reaction (or Pyruvate Decarboxylation):
Even though the glucose has been broken down to pyruvates, they are still not small enough for the next step of CR. 4 oxygen molecules are introduced to draw away one carbon molecule from each 2 pyruvates. This happens in the mitochondria.
This step marks the beginning of needing oxygen. If performing anaerobically then no further energy can be gained. With the completion of this step, 1 Acetyl-CoA, 1 NADH molecule and one CO2 molecule is formed for one pyruvate. Therefore, 2 Acetyl-CoA, 2 NADH per glucose.3

3. Krebs cycle (or Citric acid cycle):
Can be very confusing so be careful when reading. (This steps needs oxygen)
The Krebs cycle adds another 4-carbon structure to 1 Acetyl-CoA produced above, turning it into a 6-carbon structure (it’s not glucose, I know, it’s weird). Using 2 oxygen molecules, another NADH and CO2 molecule is produced, reducing the 6-carbon structure to a 5-carbon structure. (1)
Using 2 oxygen molecules, another NADH and CO2 molecule is produced with an extra ATP, reducing the 5-carbon structure to a 4-carbon structure. (2)
Further oxidization creates 1 NADH molecule and 1 FADH molecule. The remaining is the original 4-carbon structure that will be added on to the next Acetyl-CoA.
The total outcome is 2 ATP, 6 NADH and 1 FADH per one glucose molecule.
Here is a cartoon that details the Krebs Cycle

4. Electron Transport Chain
Remember all those NADH and FADH we got? 10 NADH and 1 FADH to be exact, well, by adding Oxygen and other substances, we can convert them into ATP. 1 NADH can be convert into 3 ATP while 1 FADH can be converted into 2 ATP.
With 10 NADH and 1 FADH, this means about an extra 32 ATP per glucose (not to mention that extra ATP in the Krebs cycle). Without oxygen, one glucose can only provide 2 ATP in the glycolysis.
If anyone wants the formula, here it is:
NADH + H+ + 3 ADP + 3 Pi + 1/2 O2 NAD+ + H2O + 3 ATP

FADH2 + 2 ADP + 2 Pi + 1/2 O2 FAD+ + H2O + 2 ATP
If you want to have an animation here too, look at this

Works Cited
Hodgkinson, Wayne Mark. Cellular Respiration. Science Bio Class, UNIS. Building 6, UNIS Compound. 17 May 2012. Lecture.
McGraw-Hill Higher Education. “Animation: How the Krebs Cycle Works (Quiz 1).” McGraw-Hill Online Learning Center. McGraw-Hill Higher Education, 2006. Web. 21 May 2012. <‌sites/‌0072507470/‌student_view0/‌chapter25/‌animationhow_the_krebs_cycle_worksquiz_1_.html>.
Pearson Education Inc. “Cell Respiration.” Prentice Hall School. Pearson Education Inc, n.d. Web. 21 May 2012. <‌science/‌biology_place/‌biocoach/‌cellresp/‌intro.html>.
Wikipedia contributor. “Cellular Respiration.” Wikipedia the Free Encyclopedia. Wikimedia Foundation, 17 May 2012. Web. 21 May 2012. <‌wiki/‌Cellular_respiration>.