8.1 Energy, matter, and enzymes  (Page 2/10)

Organisms can also be identified by the energy source they use. All energy is derived from the transfer of electrons, but the source of electrons differs between various types of organisms. The prefixes photo- (“light”) and chemo- (“chemical”) refer to the energy sources that various organisms use. Those that get their energy for electron transfer from light are phototroph s , whereas chemotroph s obtain energy for electron transfer by breaking chemical bonds. There are two types of chemotrophs: organotroph s and lithotroph s . Organotrophs, including humans, fungi, and many prokaryotes, are chemotrophs that obtain energy from organic compounds. Lithotrophs (“litho” means “rock”) are chemotrophs that get energy from inorganic compounds, including hydrogen sulfide (H ₂ S) and reduced iron. Lithotrophy is unique to the microbial world.

The strategies used to obtain both carbon and energy can be combined for the classification of organisms according to nutritional type. Most organisms are chemoheterotroph s because they use organic molecules as both their electron and carbon sources. [link] summarizes this and the other classifications.

Oxidation and reduction in metabolism

The transfer of electrons between molecules is important because most of the energy stored in atoms and used to fuel cell functions is in the form of high-energy electrons. The transfer of energy in the form of electrons allows the cell to transfer and use energy incrementally; that is, in small packages rather than a single, destructive burst. Reactions that remove electrons from donor molecules, leaving them oxidized, are oxidation reaction s ; those that add electrons to acceptor molecules, leaving them reduced, are reduction reaction s . Because electrons can move from one molecule to another, oxidation and reduction occur in tandem. These pairs of reactions are called oxidation-reduction reactions, or redox reaction s .

Energy carriers: nad ⁺ , nadp ⁺ , fad, and atp

The energy released from the breakdown of the chemical bonds within nutrients can be stored either through the reduction of electron carriers or in the bonds of adenosine triphosphate (ATP) . In living systems, a small class of compounds functions as mobile electron carrier s , molecules that bind to and shuttle high-energy electrons between compounds in pathways. The principal electron carriers we will consider originate from the B vitamin group and are derivatives of nucleotides; they are nicotinamide adenine dinucleotide , nicotine adenine dinucleotide phosphate , and flavin adenine dinucleotide . These compounds can be easily reduced or oxidized. Nicotinamide adenine dinucleotide ( NAD ⁺ /NADH ) is the most common mobile electron carrier used in catabolism. NAD ⁺ is the oxidized form of the molecule; NADH is the reduced form of the molecule. Nicotine adenine dinucleotide phosphate ( NADP ⁺ ), the oxidized form of an NAD ⁺ variant that contains an extra phosphate group, is another important electron carrier; it forms NADPH when reduced. The oxidized form of flavin adenine dinucleotide is FAD , and its reduced form is FADH ₂ . Both NAD ⁺ /NADH and FAD/FADH ₂ are extensively used in energy extraction from sugars during catabolism in chemoheterotroph s, whereas NADP ⁺ /NADPH plays an important role in anabolic reactions and photosynthesis . Collectively, FADH ₂ , NADH, and NADPH are often referred to as having reducing power due to their ability to donate electrons to various chemical reactions.