A Very generalized Overview of Metabolism.
Note: "Energy source" is better called "source of electrons." Also, the lateral lines (directed to the right) represent the transfer of energy (1) given off when electrons are released during catabolism and (2) given off when ADP goes back to ATP, having provided energy for anabolic reactions.
|Element||% of dry weight||Source||Major Functions|
|Carbon (C)||50||organic compounds or CO2||Essential elemental constituent of cellular material.|
|Oxygen (O)||20||H2O, organic compounds, CO2, and O2||Constituent of cell material and cell water; O2 is electron acceptor in aerobic respiration.|
|Nitrogen (N)||14||NH3+, NO3–, organic compounds, N2||Constituent of amino acids, the bases in nucleic acids and nucleotides, coenzymes, vitamins.|
|Hydrogen (H)||8||H2O, organic compounds, H2||Most numerous elemental constituent of organic compounds and cell water.|
|Phosphorus (P)||3||inorganic phosphates (PO43–)||Constituent of ATP and nucleic acids (the phosphate component); also phospholipids, lipopolysaccharide, teichoic acids, vitamins.|
|Sulfur (S)||1||SO42–, H2S, S0, organic sulfur compounds||Constituent of two amino acids (cysteine, methionine), some coenzymes, vitamins.|
|Potassium (K)||1||Potassium salts||Main cellular inorganic cation and cofactor for certain enzymes|
|Magnesium (Mg)||0.5||Magnesium salts||Inorganic cellular cation, cofactor for certain enzymatic reactions|
|Calcium (Ca)||0.5||Calcium salts||Inorganic cellular cation, cofactor for certain enzymes and a component of endospores|
|Iron (Fe)||0.2||Iron salts||Component of cytochromes and certain nonheme iron-proteins and a cofactor for some enzymatic reactions|
An Attempt to Produce an Ultra-General "Universal Diagram" to Summarize Catabolism.
Note: This diagram does not adequately address the concept of "reducing power," the provision of electrons and ATP to anabolic activities, and the key role of NAD. Click here.
(For fermentation, as will be noted below, ATP formation is not dependant on the "energy" shown in this diagram.)
Examples of electron donors that are oxidized include:
To make a long story short, we can use the example of a typical fermentation pathway (stepwise/sequential chain of chemical reactions) where glucose is oxidized to pyruvate (the oxidation stage of the pathway) and pyruvate is then reduced to fermentation products such as lactic acid (the reduction stage of the pathway). At a certain step in the oxidation stage where compound A loses electrons (which are picked up by co-enzyme NAD+), the electrons can be transferred to compound C in the reduction stage (thus "regenerating" the NAD+).
In the generation of ATP, the "P" (phosphate) can be "free" (inorganic) or attached to an intermediate compound in the pathway (organic). As this phosphate is transferred to ADP in the formation of ATP and does not incorporate energy associated with electron transport, the term "substrate-level phosphorylation" applies.
Above shows AEROBIC RESPIRATION with the use of oxygen. ANAEROBIC RESPIRATION uses an "oxygen substitute" such as nitrate, sulfate, etc.
Phototrophy can be OXYGENIC (evolving O2 when H2O serves as the electron donor) or ANOXYGENIC (non-O2-evolving).
Simplified Representations of Fermentation, Respiration and Phototrophy.
|Nutritional Type||Carbon Source||Energy Source||Examples|
|Photoautotrophs||CO2||Light (These organisms are generally "photolithotrophic" in that electron transfer usually involves the oxidation of inorganic compounds.)||Algae; cyanobacteria; some purple and green bacteria.|
|Photoheterotrophs||Organic compounds||Light (These organisms are generally "photoorganotrophic" in that electron transfer usually involves the oxidation of organic compounds.)||Some purple and green bacteria; a few algae.|
|Chemoautotrophs||CO2||Usually inorganic compounds (chemolithotrophy) – e.g., H2, NH4+, NO2–, H2S||Relatively few bacteria and many archaea.|
|Chemoheterotrophs||Organic compounds||Usually organic compounds (chemoorganotrophy)||Protozoa, fungi, most bacteria, some archaea.|
|+||–||+||–||Animals, plants, algae, molds, protozoa, many prokaryotes.|
|+||+||+||+||Many bacteria, yeasts, some protozoa, few animal cells.|
|+||+||–||+||The "lactic acid bacteria." (Aerotolerant anaerobes would be included in facultative anaerobes in the more general use of these terms as listed above.)|
|–||+||–||+ or –||Many prokaryotes (e.g., Clostridium).|
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