Tree of life
All Organisms Share Common Characteristics
As I explained in April’s column, all life forms share at least six common properties (variations of this list that include additional properties):
Respiration is the three-step (note: here I am using step to indicate a group of biochemical reactions, not a single reaction) each metabolic process by which non-photosynthetic cells obtain (conserve) energy. In oxic (oxic – an environment in which oxygen is present) environments aerobic respiration is the primary energy producing process. In anoxic (anoxic – in environment in which oxygen is absent or at a concentration too low to support aerobic metabolism) environments anaerobic respiration or fermentation occur. Fermentation is a form of energy metabolism that does not involve the electron transport chain (ETC). Figure 1 illustrates these three forms of energy metabolism. For aerobic respiration, oxygen (O2) serves as the electron transport chain’s (ETC’s) terminal electron acceptor. For anaerobic respiration, alternative inorganic molecules (for example, carbon dioxide (CO2), nitrate (NO3–), nitrite (NO2–), iron (Fe3+), manganese (Mn4+), sulfate (SO42-), sulfur (S0), etc.) serve as the ETC’s terminal electron acceptor.
Fig 1. Respriation – all respiration pathways begin with glycolysis.
Step 1 – Glycolysis is the metabolism of glucose to pyruvate (Figure 2). Each 6-carbon (C6) glucose molecule is catabolized (broken down) to two C3 pyruvate molecules. This pathway generates 8 adenosine triphosphate (ATP) molecules (remember that ATP is the primary energy molecule in all cells).
Fig 2. Glycolysis – Note the role of ATP and the net generation of 2 ATP molecules per glucose molecule.
Nicotinamide adenine dinucleotide (NAD+ and NADH) plays a key role in electron transfer. The reversable electron transfer between NAD+ and NADH (NAD+ + e– ↔ NADH) drives oxidative phosphorylation (ADP + PO4– → ATP). As we will see below, flavin adenosine dinucleotide (FAD+ and FADH2) play a similar role to NAD+ and NADH in the ETC.
Step 2 – As illustrated in Figure 3, pyruvate is metabolized via the Krebs Cycle (also known as the Citric Acid Cycle or Tricarboxylic Acid Cycle). The Krebs Cycle can theoretically generate an additional 24 ATP molecules.
Fig 3. The Krebs Cycle.
Step 3 – The final sequence of respiration reactions occurs in the ETC (Figure 4). The ETC is a membrane-bound system through which electrons flow down a redox gradient. As noted above, O2 is the terminal electron acceptor for aerobic respiration and other inorganic molecules can function as terminal electron acceptors for non-fermenting anaerobic bacteria. Sulfate reducing bacteria (SRB) use SO4=, nitrate reducers use NO3–, and other, specific metabolic groups use the other anions listed above. Note that organisms assigned to a group based on their terminal electron acceptor can be genetically diverse.
Fig 4. Electron Transport Chain – ETC.
Aerobic and anaerobic respiration generate a net of 30 to 32 ATP molecules per molecule of glucose. In contrast, fermentation yields only two ATP molecules per molecule of glucose.
Figure 5 shows two fermentation pathways – homolactic and heterolactic. Homolactic fermentation produces two molecules of lactate per glucose molecule, Heterolactic fermentation produces one lactate and one ethanol molecule (note – 6 carbon atoms in and 6 carbon atoms out). As shown in Figure 6, alcohol fermentation produces two ethanol and two CO2 molecules (again 1 C6 sugar &rarr 2 C2 alcohol + 2 CO2 molecules – six carbon atoms in and six caron atoms out).
Fig 5. Homolactic and heterolactic acid fermentation pathways.
Fig 6. Ethanol and acetic acid fermentation pathways.
From an energy generation perspective, fermentation generates only 2 ATP per glucose molecule. Thus, respiration produces energy (i.e., ATP) much more effectively than fermentation does.
As with the other characteristics common to all living organisms, energy generation is universal. The two primary energy production processes used are respiration and fermentation. Anerobic or anaerobic respiration produces 32 to 38 ATP molecules per glucose molecule catabolized. Fermentation produces only two. The figures I’ve presented in this overview are simplified versions of the actual pathways. I invite those who are interested in a more detailed explanation of any of these metabolic pathways to search the internet or pick up a biochemistry textbook.
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