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Mitochondria and chloroplasts display similarities with bacteria that led to the endosymbiont theory. This theory states that an early ancestor of eukaryotic cells engulfed an oxygen using non-photosynthetic prokaryotic cell. Eventually, the engulfed cell formed a relationship with the host cell 1n which it was enclosed, becoming an endosymbiont (a cell living within another cell). Indeed, over the course of evolution, the host cell and its endosymbiont merged into a single organism, a eukaryotic cell with a mitochondrion. At least one of these cells may have then taken up a photosynthetic prokaryote, becoming the ancestor of eukaryotic cells that contain chloroplasts.

So my question is, why is it that all the cells that took in the chloroplasts became plants or other simple animals but not large mammalian or reptilian animals? Is there a limiting factor that restricted chloroplasts from being beneficial to large animals? Why is it that large plants can have both chloroplasts and mitochondria but not relatively larger animals?

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    $\begingroup$ "cells that took in the chloroplasts became plants or other simple animals" -- what does this refer to? I am not aware of any animal (metazoa) species whose cells contain chloroplasts? There are symbionts like Elysia chlorotica that ingest algae and have adapted to make use of their chloroplasts for photosynthesis, but that seems quite different. Perhaps by "simple animals" you mean protozoa? $\endgroup$
    – Roland
    Nov 5, 2017 at 18:01
  • $\begingroup$ I'm no expert in evolutionary biology...but I suppose that it could be because of the higher energy needs in plants than animals.The plants have several more biochemical pathways than many larger animals so to maintain so many enzymes there must be more energy needs.And altogether its disadvatageous,perhaps it saved them from the intense competition during biogeny. $\endgroup$
    – user 33690
    Nov 6, 2017 at 6:55
  • $\begingroup$ There are no animals with chloroplasts. $\endgroup$
    – John
    Jul 22, 2019 at 16:32

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I believe the gist of it is that chloroplasts simply cannot create enough sugar from photosynthesis to significantly power the energy-intense needs of any but the simplest of animals. Consider that plants maximize their surface area to the extent possible for a living organism, and still only have enough energy to grow and reproduce - movement is essentially impossible.

Here's a pretty good article I stumbled across a while back: http://blogs.discovermagazine.com/notrocketscience/2012/09/18/will-we-ever-photosynthesise-like-plants/

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  • $\begingroup$ Venus fly traps manage to move pretty well, though I imagine you mean move from place to place. However, you haven't given any evidence that plants' lack of movement is because of a lack of energy. One might instead posit that plants don't move because they "chose" to get their water supply by fixing themselves into the ground. And perhaps plants could make a trade-off and grow a little less to free up energy for moving. For example, small trees seem to do just fine until they get surrounded by taller trees that block out their light. $\endgroup$
    – David Richerby
    Nov 5, 2017 at 19:44
  • $\begingroup$ Venus fly traps can only move a few times before they wear out, it is useless for movement. $\endgroup$
    – John
    Jul 22, 2019 at 16:33

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