8.3 cellular respiration

In aerobic respiration, the final electron acceptor

(i.e., the one having the most positive redox potential) at the end of the ETS is an oxygen molecule (O2) that becomes reduced to water (H2O) by the final ETS carrier.

In reality, the total ATP yield is usually less, ranging from

from one to 34 ATP molecules, depending on whether the cell is using aerobic respiration or anaerobic respiration; in eukaryotic cells, some energy is expended to transport intermediates from the cytoplasm into the mitochondria, affecting ATP yield.

The potential energy of this electrochemical gradient

generated by the ETS causes the H+ to diffuse across a membrane (the plasma membrane in prokaryotic cells and the inner membrane in mitochondria in eukaryotic cells).

Smaller electrochemical gradients are

generated from these electron transfer systems, so less ATP is formed through anaerobic respiration.

ATP synthase (like a combination of the intake and generator of a

hydroelectric dam) is a complex protein that acts as a tiny generator, turning by the force of the H+ diffusing through the enzyme, down their electrochemical gradient from where there are many mutually repelling H+ to where there are fewer H+.

The tendency for movement in this way is much

like water accumulated on one side of a dam, moving through the dam when opened.

Thus, the 10 NADH molecules

made per glucose during glycolysis, the transition reaction, and the Krebs cycle carry enough energy to make 30 ATP molecules, whereas the two FADH2 molecules made per glucose during these processes provide enough energy to make four ATP molecules.

In prokaryotic cells, H+ flows from the outside of the cytoplasmic

membrane into the cytoplasm, whereas in eukaryotic mitochondria, H+ flows from the intermembrane space to the mitochondrial matrix.

These electron transfers take place on the inner part of the cell

membrane of prokaryotic cells or in specialized protein complexes in the inner membrane of the mitochondria of eukaryotic cells.

The number of ATP molecules generated from the catabolism of glucose varies. For example, the

number of hydrogen ions that the electron transport system complexes can pump through the membrane varies between different species of organisms.

Most ATP, however, is generated during a separate process called

oxidative phosphorylation, which occurs during cellular respiration.

The turning of the

parts of this molecular machine regenerates ATP from ADP and inorganic phosphate (Pi) by oxidative phosphorylation, a second mechanism for making ATP that harvests the potential energy stored within an electrochemical gradient.

These carriers can pass electrons along in the ETS because of their

redox potential.

Electron transport is a series of chemical reactions that

resembles a bucket brigade in that electrons from NADH and FADH2 are passed rapidly from one ETS electron carrier to the next.

This electrochemical gradient formed by

the accumulation of H+ (also known as a proton) on one side of the membrane compared with the other is referred to as the proton motive force (PMF).

Many aerobically respiring bacteria, including E. coli, switch to

using nitrate as a final electron acceptor and producing nitrite when oxygen levels have been depleted.

The bacterial electron transport chain is a series of protein

complexes, electron carriers, and ion pumps that is used to pump H+ out of the bacterial cytoplasm into the extracellular space. H+ flows back down the electrochemical gradient into the bacterial cytoplasm through ATP synthase, providing the energy for ATP production by oxidative phosphorylation.

The electron transport system (ETS) is the last

component involved in the process of cellular respiration; it comprises a series of membrane-associated protein complexes and associated mobile accessory electron carriers

membrane. In prokaryotic cells, H+ is pumped to the outside of the

cytoplasmic membrane (called the periplasmic space in gram-negative and gram-positive bacteria), and in eukaryotic cells, they are pumped from the mitochondrial matrix across the inner mitochondrial membrane into the intermembrane space.

This electron carrier, cytochrome oxidase,

differs between bacterial types and can be used to differentiate closely related bacteria for diagnoses.

Beyond the use of the PMF to make ATP, as

discussed in this chapter, the PMF can also be used to drive other energetically unfavorable processes, including nutrient transport and flagella rotation for motility.

Overall, the theoretical maximum yield of ATP made

during the complete aerobic respiration of glucose is 38 molecules, with four being made by substrate-level phosphorylation and 34 being made by oxidative phosphorylation

In each transfer of an electron through the ETS, the

electron loses energy, but with some transfers, the energy is stored as potential energy by using it to pump hydrogen ions (H+) across a membrane.

Cellular respiration begins when

electrons are transferred from NADH and FADH2—made in glycolysis, the transition reaction, and the Krebs cycle—through a series of chemical reactions to a final inorganic electron acceptor (either oxygen in aerobic respiration or non-oxygen inorganic molecules in anaerobic respiration).

The energy of the

electrons is harvested to generate an electrochemical gradient across the membrane, which is used to make ATP by oxidative phosphorylation.

For a protein or chemical to accept

electrons, it must have a more positive redox potential than the electron donor.

In aerobic respiration in mitochondria, the passage of electrons from one molecule of NADH generates

enough proton motive force to make three ATP molecules by oxidative phosphorylation, whereas the passage of electrons from one molecule of FADH2 generates enough proton motive force to make only two ATP molecules.

Chemiosmosis, Proton Motive Force, and Oxidative Phosphorylation

Chemiosmosis, Proton Motive Force, and Oxidative Phosphorylation

Electron Transport System

Electron Transport System

For example, the gram-negative opportunist

Pseudomonas aeruginosa and the gram-negative cholera-causing Vibrio cholerae use cytochrome c oxidase, which can be detected by the oxidase test, whereas other gram-negative Enterobacteriaceae, like E. coli, are negative for this test because they produce different cytochrome oxidase types.

There is an uneven distribution of H+ across the membrane that

establishes an electrochemical gradient because H+ ions are positively charged (electrical) and there is a higher concentration (chemical) on one side of the membrane.

Because the ions involved are H+, a pH gradient is

also established, with the side of the membrane having the higher concentration of H+ being more acidic.

Microbes using anaerobic respiration commonly have

an intact Krebs cycle, so these organisms can access the energy of the NADH and FADH2 molecules formed.

One possible alternative to aerobic respiration is

anaerobic respiration, using an inorganic molecule other than oxygen as a final electron acceptor.

There are many types of anaerobic respiration found in

bacteria and archaea.

Denitrifiers are important soil

bacteria that use nitrate (NO3-)(NO3-) and nitrite (NO2-)(NO2-) as final electron acceptors, producing nitrogen gas (N2).

The four major classes of electron carriers involved in

both eukaryotic and prokaryotic electron transport systems are the cytochromes, flavoproteins, iron-sulfur proteins, and the quinones.

source transiction reaction carbon flow molecules of reduced coenzymes produced net atp molecules made by substrate level phosphorylation net atp molecules made by oxidative phosphorylation theoretical maximum yield of atp molecules

carbon flow 2 pyruvates ( 3c) = 2 acetyl (2c) + 2CO2 molecules of reduced coenzymes produced 2 NADH net atp molecules made by substrate level phosphorylation net atp molecules made by oxidative phosphorylation 6 ATP from 2 NADH theoretical maximum yield of atp molecules 6

source krebs cycle carbon flow molecules of reduced coenzymes produced net atp molecules made by substrate level phosphorylation net atp molecules made by oxidative phosphorylation theoretical maximum yield of atp molecules

carbon flow 2 acetyl ( 2c) = 4 CO2 molecules of reduced coenzymes produced 6 NADH 2 FADH2 net atp molecules made by substrate level phosphorylation 2 ATP net atp molecules made by oxidative phosphorylation 18 ATP from NADH 4 ATP from 2 FADH2 theoretical maximum yield of atp molecules 24

total carbon flow molecules of reduced coenzymes produced net atp molecules made by substrate level phosphorylation net atp molecules made by oxidative phosphorylation theoretical maximum yield of atp molecules

carbon flow glucose ( 6c) = 6 CO2 molecules of reduced coenzymes produced 10 NADH 2 FADH2 net atp molecules made by substrate level phosphorylation 4 ATP net atp molecules made by oxidative phosphorylation 34 ATP theoretical maximum yield of atp molecules 38 ATP

source glycolysis ( emp) carbon flow molecules of reduced coenzymes produced net atp molecules made by substrate level phosphorylation net atp molecules made by oxidative phosphorylation theoretical maximum yield of atp molecules

carbon flow glucose (6c) = 2 pyruvates (3c) molecules of reduced coenzymes produced 2 nadh net atp molecules made by substrate level phosphorylation 2 atp net atp molecules made by oxidative phosphorylation 6 atp from 2 nadh theoretical maximum yield of atp molecules 8

However, anaerobic respirers use altered ETS

carriers encoded by their genomes, including distinct complexes for electron transfer to their final electron acceptors.

We have just discussed two pathways in glucose

catabolism—glycolysis and the Krebs cycle—that generate ATP by substrate-level phosphorylation.

This flow of hydrogen ions across the membrane, called

chemiosmosis, must occur through a channel in the membrane via a membrane-bound enzyme complex called ATP synthase

Therefore, electrons move

from electron carriers with more negative redox potential to those with more positive redox potential.

There are many circumstances under which aerobic respiration is not possible, including any one or more of the following:

The cell lacks genes encoding an appropriate cytochrome oxidase for transferring electrons to oxygen at the end of the electron transport system. The cell lacks genes encoding enzymes to minimize the severely damaging effects of dangerous oxygen radicals produced during aerobic respiration, such as hydrogen peroxide (H2O2) or superoxide (O2-).(O2-). The cell lacks a sufficient amount of oxygen to carry out aerobic respiration.