Cellular respiration is the set of metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy in the process as weak so-called “high-energy” bonds are replaced by stronger bonds in the products. Respiration is one of the key ways a cell gains useful energy to fuel cellular activity. Cellular respiration is considered an exothermic redox reaction which releases heat. The overall reaction occurs in a series of biochemical steps, most of which are redox reactions themselves. Although technically, cellular respiration is a combustion reaction, it clearly does not resemble one when it occurs in a living cell due to slow release of energy from the series of reactions.
The electron transport chain consists of 3 complexes of integral membrane proteins
- the NADH dehydrogenase complex (I)
- the cytochrome c reductase complex (III)
- the cytochrome c oxidase complex (IV)
and two freely-diffusible molecules:
- Cytochrome c
That shuttle electrons from one complex to the next.
The electron transport chain accomplishes:
- the stepwise transfer of electrons from NADH (and FADH2) to oxygen molecules to form (with the aid of protons) water molecules (H2O);(Cytochrome c can only transfer one electron at a time, so cytochrome c oxidase must wait until it has accumulated 4 of them before it can react with oxygen.)
- harnessing the energy released by this transfer to the pumping of protons (H+) from the matrix to the intermembrane space.
- Approximately 20 protons are pumped into the intermembrane space as the 4 electrons needed to reduce oxygen to water pass through the respiratory chain.
- The gradient of protons formed across the inner membrane by this process of active transport forms a miniature battery.
- The protons can flow back down this gradient only by reentering the matrix through ATP synthase, another complex (complex V) of 16 integral membrane proteins in the inner membrane. The process is called chemiosmosis.
Chemiosmosis in mitochondria
The energy released as electrons pass down the gradient from NADH to oxygen is harnessed by three enzyme complexes of the respiratory chain (I, III, and IV) to pump protons (H+) against their concentration gradient from the matrix of the mitochondrion into the intermembrane space (an example of active transport).
As their concentration increases there (which is the same as saying that the pH decreases), a strong diffusion gradient is set up. The only exit for these protons is through the ATP synthase complex. As in chloroplasts, the energy released as these protons flow down their gradient is harnessed to the synthesis of ATP. The process is called chemiosmosis and is an example of facilitated diffusion.
How many ATPs?
It is tempting to try to view the synthesis of ATP as a simple matter of stoichiometry (the fixed ratios of reactants to products in a chemical reaction). But (with 3 exceptions) it is not.
Most of the ATP is generated by the proton gradient that develops across the inner mitochondrial membrane. The number of protons pumped out as electrons drop from NADH through the respiratory chain to oxygen is theoretically large enough to generate, as they return through ATP synthase, 3 ATPs per electron pair (but only 2 ATPs for each pair donated by FADH2).
With 12 pairs of electrons removed from each glucose molecule,
- 10 by NAD+ (so 10×3=30); and
- 2 by FADH2 (so 2×2=4),
this could generate 34 ATPs.
Add to this the 4 ATPs that are generated by the 3 exceptions and one arrives at 38. But,
- The energy stored in the proton gradient is also used for the active transport of several molecules and ions through the inner mitochondrial membrane into the matrix.
- NADH is also used as reducing agent for many cellular reactions.
So the actual yield of ATP as mitochondria respire varies with conditions. It probably seldom exceeds 30.
The three exceptions
A stoichiometric production of ATP does occur at:
- one step in the citric acid cycle yielding 2 ATPs for each glucose molecule. This step is the conversion of alpha-ketoglutaric acid to succinic acid.
- at two steps in glycolysis yielding 2 ATPs for each glucose molecule.
References for Cellular Respiration: