Cellular Respiration - The Process

The reason human and organisms digest food is to break it down and to make it smaller to pass through the gut wall, allow it to enter into the small intestine, and get absorbed through the villi and micro-villi. Otherwise, our body would not be able to take in the nutrients to then use for our body to function properly.
Cellular respiration occurs in both animal and plant cells, and in cells capable of either/both aerobic and anaerobic respiration. There are 4 stages in aerobic respiration, and 2 in anaerobic respiration.
Aerobic Respiration
Organelle it occurs in
Anaerobic Respiration
Organelle it occurs in
    • Transition reaction/link reaction/pyruvate decarboxylation
Matrix of mitochondria
    • Krebs cycle/citric acid cycle/tricarboxylic acid cycle
Matrix of mitochondria

    • Electron transport chain
Inner membrane of mitochondria


Link to see a more detailed diagram of the mitochondria:

The 4 steps visually in a graph:

This is an interactive animation about the different steps in aerobic and anaerobic respiration:

Cellular respiration starts by using glucose(mainly), which is the product of photosynthesis:

6CO2 + 6H2O (+ light energy) -> C6H12O6 + 6O2

For cellular respiration to occur in animal cells, animals (like humans) would have to eat food and have it digested to gain access to organic materials such as glucose.
The outcomes of cellular respiration are:
  • Energy ready to be used, in the form of ATP (adenosine triphosphate) is released at the end of the process of cellular respiration.
  • CO2 as metabolic waste - but then, it binds with water to form carbonic acid to help maintain blood pH. Not too much CO2 is kept though or it would lower pH too greatly, so it gets released by the respiratory system (exhaling).

Glycolysis is the first step in both aerobic and anaerobic respiration that occurs in the cytosol. Glycolysis is the first phase of the 'chain-reaction' in breaking down carbohydrates through endothermic(drawing in heat) and exothermic reactions(giving off heat) reactions and catabolism. Using an example of 1 glucose molecule, first 2 ATP molecules have to be hydrolysed (use of water to split something into small pieces) into ADP and energy. The carbon backbone of glucose (ring structure) gets energized by a high energy phosphate from ATP, forming glucose-6-phosphate. The molecules then rearrange to form fructose-6-phosphate. Another phosphate is then added onto the fructose; turning it to fructose-1,6-biphosphate . The energy from the hydrolysis reaction is then used to "split" the fructose-1,6-biphosphate so that it forms two 3-carbon compounds with a phosphate group attached to each; they are then called PGAL molecules (phosphoglyceraldehyde). The 2 PGAL molecules then are oxidized (broken down), and in doing so, each are reconfigured into a 3-carbon group called pyruvate. During the oxidation process, PGAL molecules turn 2 molecules of ADP into ATP each and add a hydrogen to NAD+ to turn it into NADH.

Concluding glycolysis, you would need:
1 glucose molecule + 2NAD+ + 2ATP + 4ADP + 2 Phosphate groups ----glycolysis---> 2 pyruvates + 2 NADH + 2ADP + 4ATP

Closer look at Glycolysis:

Image of Pyruvate:

The pyruvate (negatively charged pyruvate acid that is crucial in biochemistry) then stands at a 'cross-road' in the cytosol after glycolysis. If the cell this process in can respire aerobically, then the pyruvate would go through the transition reaction / link reaction / pyruvate decarboxylation. If there is no oxygen (anaerobic respiration), then it go through the fermentation process.

Going down the aerobic respiration road, the next step is the pyruvate decarboxylation/link reaction, which is like preparation for the pyruvate to undergo the Krebs cycle. In this oxidation step, one of the carbon on the 3-carbon pyruvate is sliced off, leaving you with two molecules of the 2-carbon acetyl Coenzyme A (acetyl CoA), releasing CO2, and NADH.

Krebs and citric acid cycle or the tricarboxylic acid cycle. It occurs in the space inside of the inner membrane of the mitochondria, the matrix, and is basically a series of chemical reactions. Here the Acetyl CoA joins with a chemical to form a 6-carbon compound molecule. The first step in the Krebs cycle produces 2 molecules of ATP and 2 CO2 molecules. The role of the cycle is to give energy to the reduced NADH for the electron transport chain afterwards.

The electron transport chain is where the bulk of ATP's are produced. There are oxidation steps in the electron transport chain that turn ADP into ATP molecules. High-energy electrons from NADH (created in previous steps) and FADH2 lose energy n the electron transport chain. The energy is then used to work to pump protons across the inner membrane from the matrix to the intermediate space of the outer and inner membrane, creating a strong hydrogen concentration gradient (similar to osmosis). This process is called chemiosmosis, and the difference in the proton concentrations creates an electrical potential and pH potential across the membrane. As protons diffuse, carrier proteins use energy to add a phosphate group to ADP, turning it into ATP, with the help of the ATP synthase complex. Then, it moves through the electron transport chain so that the excess energy gets combined with excess hydrogen ions and oxygen to form water, which then turns to waste which has to be removed from our body; through urination.

This is to learn more about the ATP Synthase complex:

The electron transport chain produces 34 ATP molecules. This means that the net ATP produced by 1 glucose molecule in aerobic respiration is 38 ATP molecules, but because of there sometimes being "leaky membranes", the number is somewhere closer to 29/30 ATP molecules.

When respiration can be done aerobically, with O2(oxygen), and anaerobically, without O2. Anaerobic respiration create a considerable amount of ATP less than aerobic respiration. In aerobic respiration, up to 38 ATP molecules can be produced from a single glucose molecule; it can be a 1:38 ratio. In anaerobic respiration, only up to 2 ATP molecules can be produced for every glucose molecule; it is a 1:2 ratio. From this, it is easier to see which process creates more energy.
There is a large difference in the number of ATP molecules produced in aerobic and anaerobic respiration, because the bulk of ATP formation occurs in the electron transport chain in aerobic respiration. This stage of cellular requires oxygen, and because there is none in anaerobic cells, those cells would only produce 2 extra ATP molecules from glycolysis before the pyruvate molecules go to the fermentation stage.

Websites Used:

This website contains videos describing each process of Cellular Respiration with subtitiles for note taking.

"Cellular Respiration." sumanasinc. N.p., n.d. Web. 17 Feb. 2012.
This video explains the processes that occur during cellular respiration and what is being used in these processes.

Cellular Respiration
. N.p., 11 May 2008. Web. 17 Feb. 2012.