Most people associate respiration with the process of breathing to take in oxygen and remove carbon dioxide from the body. Cellular respiration is the process of producing ATP (adenosine triphosphate) and NADH to fuel cell processes. There are two types of cellular respiration:
- Aerobic respiration uses oxygen to make energy.
- Anaerobic respiration does not use oxygen to make energy.
Any of the macronutrients – proteins, carbohydrates, and fats – can be used for aerobic respiration, but the carbohydrate glucose is involved in all four important metabolic pathways: glycolysis, the breakdown of pyruvate, the citric acid cycle (Krebs cycle), and oxidative phosphorylation. The other macronutrients are broken down into components that can enter into the cellular respiration process at various points.
Aerobic Respiration: Glycolysis, Breakdown of Pyruvate, and Citric Acid Cycle
Glycolysis occurs in the cytosol, which is the area within the cell that surrounds the organelles. During glycolysis, glucose is broken down into:
- two pyruvate molecules
- two ATP molecules
- two NADH molecules
The two pyruvate molecules enter a mitochondrion where they are each broken down into one acetyl group and one carbon dioxide molecule. NAD+ is part of this process and receives two electrons from the pyruvate and a hydrogen atom resulting in NADH. The products of this reaction are
- two acetyl groups
- two carbon dioxide molecules
- two NADH
Each acetyl group is further broken down into two carbon dioxide molecules during the citric acid cycle. The byproducts of the process for each acetyl group are one ATP, three NADH, and one FADH2. The products of this reaction are:
- four carbon dioxide molecules
- two ATP
- six NADH
- two FADH2
Aerobic Respiration: Oxidative Phosphorylation
Oxidative phosphorylation consists of the electron transport chain and ATP synthase. The NADH and the FADH2 produced in the previous reactions (10 NADH + 2 FADH2) are used to supply electrons for the production of ATP. The removal of electrons is called oxidation.
The electron transport chain is a series of protein structures, or proton pumps, embedded in the inner mitochondrial membrane. “The electron transport chain is also called the respiratory chain because the oxygen we breathe is used in this process,” explain Robert J. Brooker, et al. “The electron transport chain removes electrons from NADH or FADH2 and pumps H+ across the inner mitochondrial membrane.” Oxygen is the final electron acceptor in the chain. The protons (H+) are pumped from the mitochondrial matrix into the intermembrane space (between the inner- and outer-membranes).
ATP synthesis is the second part of oxidative phosphorylation that uses the enzyme ATP synthase. ATP synthase is complex molecular machinery that uses the built-up proton electrochemical gradient produced in the intermembrane space by the electron transport chain. It binds ADP (adenosine diphosphate) to another phosphate (Pi) to form ATP. Note: adenosine diphosphate contains two phosphate molecules, and adenosine triphosphate contains three phosphate molecules.
The entire process of oxidative phosphorylation produces 30 to 34 ATP.
Anaerobic Respiration and Fermentation
In some cases, cellular respiration must occur in the absence of oxygen. Some organisms live in environments where oxygen is absent. Muscle cells will work in an anaerobic environment during periods of intense activity. One solution is to replace oxygen as the final electron acceptor in the electron transport change.
In the case of the muscle cells, NADH converts the pyruvate from the first step of glycolysis to lactic acid. The lactic acid creates fatigue or a burning sensation that triggers the organism to slow down.
According to Brooker, et al., fermentation is the breakdown of organic molecules without net oxidation. Electrons are removed, but are later returned. It produces far fewer ATP molecules – only 2 – compared with the maximum of 38 ATP produced by aerobic cellular respiration during the complete breakdown of glucose.
Cellular respiration encompasses the metabolic pathways that break down molecules: catabolism. It is a component of metabolism that releases energy from organic molecules to fuel other important cell processes, such as allowing muscles to contract and relax.