Questions on Autotrophs

  1. I understand that plant cells have both Chloroplasts for Photosynthesis and Mitochondria for cellular respiration (breaking down the glucose that photosynthesis creates?). Does this mean that plants use both oxygen and carbon dioxide? Most non-science people tend to think that we use oxygen, and plants use CO2. However, plants are really using both for those two processes, right?

    And because autotrophs create their own food from the sun's energy, does that mean that autotrophs were the first living things? It seems as if heterotrophs consume autotrophs and so on, so wouldn't there have to first be autotrophs before heterotrophs could consume them?
  2. jcsd
  3. jim mcnamara

    jim mcnamara 1,563
    Science Advisor
    Gold Member

    Short answer: first life on Earth lived in completely anaerobic conditions. Example: Methanogenic bacteria cannot live in the presence of oxygen. Respiration using oxygen evolved when free oxygen became available in the atmosphere. Oxygen arose from photosynthetic bluegreen cyanobacteria, which also do not use oxygen. They give it off more like a waste product.

    So, your assumptions are backwards: anaerobic processes gave rise to oxygen which later gave rise to aerobic processes.

    Generally aerobic processes are confined to eukaryotes - those cells with organelles like mitochdondria. The mitochondria as contained organelles looong ago arose as the result of two separate species "merging" into one more complex cell. Mitochondria still retain a tiny amount of DNA. Mitochondria are central to aerboic respiration.

    There is lots more to this. Example: see
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  4. Ygggdrasil

    Ygggdrasil 1,820
    Science Advisor

    This is incorrect. There are plenty of prokaryotes capable of aerobic respiration such as the E. coli commonly used in laboratory research. In fact, mitochondria are thought to have evolved from endosymbiosis of aerobic bacteria, so the evolution of aerobic respiration must necessarily predate the evolution of mitochondria and eukaryotes.
  5. phyzguy

    phyzguy 2,489
    Science Advisor

    You are assuming that heterotrophs consume autotrophs, which is not necessarily correct. There are heterotrophs that consume inorganic energy sources, such as the bacteria that live around "black smokers" under the sea, and the thermophlic bacteria that live in hot springs. Bacteria like this are thought to have dominated life on Earth before the autotrophs evolved. Try looking up archaebacteria.
  6. Ygggdrasil

    Ygggdrasil 1,820
    Science Advisor

    There seems to be a bit of confusion here regarding autotrophs vs heterotrophs and phototrophs vs chemotrophs. Whether an organism is an autotroph or a heterotroph depends on the carbon source of the organism: autotrophs fix carbon dioxide from their environment in order to obtain carbon for building cellular materials while heterotrophs obtain their carbon from organic sources.

    How an organism obtains its main source of energy classifies it as either a phototroph (the organism uses energy from the sun) or a chemotroph (the organism obtains energy from the oxidation of organic or inorganic molecules). Thus, the bacteria you describe that derive their energy from inorganic materials are better classified as chemotrophs rather than heterotrophs.

    Although most extant autotrophs are phototropic (using solar energy and water to create reduced carbon compounds from carbon dioxide) and most extant heterotrophs are chemotrophic (obtaining both their energy and carbon from organic molecules), there can be heterotrophs that are phototrophic (photoheterotrophs) and autotrophs that are chemotrophic. Indeed, the bacteria and archaea you describe, the chemoautotrophs that fix carbon dioxide by using inorganic compounds as the reducing agents, are thought to be some of the first organisms on Earth.

    Semantics aside, your general point that chemotrophs evolved prior to phototrophs is correct.
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  7. phyzguy

    phyzguy 2,489
    Science Advisor


    You're right, I was confused on the nomenclature. Thanks for setting me straight.
  8. Thanks for the replies, guys.

    I'm trying to decide what area of Biology to pursue, what does this fall under? Evolutionary and Ecology?
  9. I'm interested in this stuff too. I met a scientist this summer that studies methanogenic archea. It looked cool he had to keep his cultures in no oxygen air. I would classify this as microbial ecology, ecological theory, with a little of evolution thrown in.
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