For example:

  • human cells (eukaryotes) can utilize the Krebs cycle pathway to generate more ATP after glycolysis, but most bacteria cannot utilise the Krebs cycle
  • plant cells can utilise the Calvin's cycle for photosynthesis but human cells cannot.
  • yeast can utilise the ethanol fermentation pathway but human cells can only utilise the lactic acid fermentation pathway.

Why can't humans use the Calvin's cycle? Or why can't yeast use the lactic acid fermentation pathway instead? What determines the metabolic pathways that a biological cell is able to utilise?

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    $\begingroup$ Metabolic pathway is a sequence of enzyme-catalyzed reactions . . . $\endgroup$ – user37894 Sep 7 '18 at 11:26
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    $\begingroup$ It is fortunate that many bacteria can utilize the Krebs cycle and oxidative phosphorylation or eukaryotes would never have acquired this capacity through their endosymbiotic capture to produce of mitochondria. And the answer is trivial — evolution. Metabolic pathways are no different in this respect from the elephant's trunk, the giraffe's neck, the birds wings or the fishes' fins. All are a result of evolution of the genome under environmental pressure. $\endgroup$ – David Sep 7 '18 at 12:12
  • $\begingroup$ Is it possible to genetically engineer new pathways in bacteria cells the $\endgroup$ – user60513 Sep 7 '18 at 13:49
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    $\begingroup$ Yes, it is possible to genetically engineer new pathways in bacteria — this is called Synthetic Biology and you can search for articles about it. Putting in the genes for a pathway is not enough, though, you need to consider how they will be induced and regulated etc. $\endgroup$ – David Sep 7 '18 at 17:14

The determining factor is the hereditary traits that are passed on to the organism. Metabolic pathways require protein catalysts of which the instructions to make are stored in DNA. In other words humans can't use the Calvin cycle because our parents can't/couldn't, but plants can because their parents can/could.

In the long run though, very slowly organisms can gain or lose the ability to use pathways that their ancestors had.

  • $\begingroup$ By protein catalysts, do you mean enzymes? $\endgroup$ – user45869 Sep 8 '18 at 2:10
  • $\begingroup$ That's right. Enzymes are catalytic proteins. $\endgroup$ – Cell Sep 8 '18 at 2:11
  • $\begingroup$ In that case, if the enzymes were manually added to the cells in a bioreactor environment, would the reaction be possible? For example, human cells cannot utilise the carboxylation pathway that converts PEP into oxaloacetate since the instructions to make the enzyme PEP carboxylase is not stored in our DNA. However if we isolate and manually add PEP carboxylase to a human cell, would the carboxylation reaction be possible? $\endgroup$ – user45869 Sep 8 '18 at 2:23
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    $\begingroup$ If by "manually" you mean to purify the enzyme and literally add the powder in the reactor then probably no. Because the cells will likely not transport it in and the enzyme will likely get denatured. If you add the gene for that enzyme then probably yes. There are other factors too like if the gene is expressed properly and the enzyme goes to where it needs to go. Modifying metabolic pathways is something people do right now. For example researchers are genetically modifying fungi to ferment all the sugars in plant biomass and inhibitors in the medium. This involves adding new genes. $\endgroup$ – Cell Sep 8 '18 at 11:17
  • $\begingroup$ Thank you. So the only way to make new metabolic pathways is to genetically modify the cell, and more specifically, isolate the desired gene and insert it into a cell plasmid? (making recombinant DNA) $\endgroup$ – user45869 Sep 9 '18 at 8:32

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