Chapter 14, Problem 1RQ

Summary Introduction

To review:

The source and sink of electrons in respiration, lithotrophy, and photolysis.

Introduction:

An electron transport chain is a series of reactions in which the electron carrier transfers the electron to the carrier of the next- higher reduction potential (E) in a sequential manner. The electron flow is initiated with an initial electron donor from the outside of the cell that transfers the electrons to a terminal electron acceptor leaving the cell.

Explanation of Solution

Lithotrophy is an energy-yielding form of metabolism in which many reduced minerals and single-carbon compounds can serve as electron donors. Many kinds of electron donors like metals and anions to single-carbon groups are included in lithotrophy. A specialized electron-accepting oxidoreductase is required by the electron donors like H2 (hydrogen), NH4+ (ammonium) or Fe2+ (ferrous). Compared to the organic donors such as glucose, inorganic substrates other than H2 are relatively poor electron donors due to which the terminal electron acceptor is usually a strong oxidant, such as O2 (oxygen), NO3- (nitrate) or Fe3+ (ferric ions).

In the process of photosynthesis, energy is derived from the photoexcitation of the light-absorbing pigment. The process of photolysis which is a light-driven separation of an electron from a molecule coupled to an electron transport chain is a consequence of photoexcitation. Photolysis is termed as “ light-reactions� that is coupled to “light-independent reactions� of carbon dioxide fixation in the plant chloroplast and cyanobacteria. The process of photoexcitation in ETS (electron transport system)-based photosynthesis causes the separation of the electron from a donor molecule like H2O (water) or H2S (hydrogen sulfide). The electrons are then transferred to an electron transport system which produces a proton potential and the reduced cofactor NADPH (nicotinamide adenine dinucleotide phosphate hydrogen). Proton potential is responsible to drive ATP (adenosine triphsophate) synthesis through an F1 and F0 ATP synthase.

Bacteria that are obligate aerobes grow only by using oxygen as the terminal electron acceptor like Sinorhizobiummeliloti. Although in anaerobic respiration, some bacteria and Archae use terminal electron acceptors like metals, oxidized ions of nitrogen and sulphur, and chlorinated organic molecules. In Escherichiacoli,different terminal oxidoreductases to reduce alternative electron acceptors are present which are termed as “reductases�. Some of the inorganic electron acceptors are nitrate (NO3) reduced to nitrite (NO2-) or NO2- reduced to NO (nitric oxide). The organic electron acceptors also play an important role in the process of food decomposition. In aerobic bacteria, the respiratory electron transport system leads to the formation of water by receiving electrons from NADH and FADH2 (flavin adenine dinucleotide dihydrogen) and transferring them to oxygen.