8.3 Cellular respiration  (Page 2/8)

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 (H ₂ O ₂ ) or superoxide $\left({{\text{O}}_2}^{–}\right).$
• The cell lacks a sufficient amount of oxygen to carry out aerobic respiration.

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 $\left({{\text{NO}}_3}^{–}\right)$ and nitrite $\left({{\text{NO}}_2}^{–}\right)$ as final electron acceptors, producing nitrogen gas (N ₂ ). 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.

Microbes using anaerobic respiration commonly have an intact Krebs cycle, so these organisms can access the energy of the NADH and FADH ₂ molecules formed. However, anaerobic respirers use altered ETS carriers encoded by their genomes, including distinct complexes for electron transfer to their final electron acceptors. Smaller electrochemical gradients are generated from these electron transfer systems, so less ATP is formed through anaerobic respiration.

Chemiosmosis, proton motive force, and 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. 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. 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. 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). 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. 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.

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). 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 ( [link] ). The tendency for movement in this way is much like water accumulated on one side of a dam, moving through the dam when opened. 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 ⁺ . 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. The turning of the parts of this molecular machine regenerates ATP from ADP and inorganic phosphate (P _i ) by oxidative phosphorylation , a second mechanism for making ATP that harvests the potential energy stored within an electrochemical gradient.