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Name: Burhan
Status: student
Grade: 9-12
Location: Outside U.S.
Country: Pakistan
Date: Fall 2013

So the evolution of Mitochondria and Chloroplast suggests that these two respectively were, billions and trillions of years ago, not part of animal and plant cells, but actually uni-cellular organisms that were engulfed by our cells and now have evolved to their current state as organelles. Chloroplast, as we all know is the only food manufacturing factory on the Earth. Mitochondria is responsible for aerobic respiration and is the Energy factory. My question is, HOW, those billion years ago, were plants and animals surviving without these extremely ESSENTIAL organelles in their cells. I looked up the net and nobody had a theory. Maybe somebody could give me an answer here.

Hi Burhan,

Thanks for the question. To the best of my knowledge, there is no evidence of evolution trillions of years ago. According to the theory of evolution--which cannot be proved--there were no plants or animals billions of years ago. So the question of how plants and animals survived without chloroplasts and mitochondria is not applicable. I would recommend rephrasing the question.

I hope this helps. Please let me know if you have more questions. Thanks Jeff

Well, to answer your question-the fossil record shows us that animals and plants WEREN'T around at that time. Photosynthesis began in bacteria and that type of bacteria is still around today. Plants don't show up in the fossil record until later. OUR cells did not engulf mitochondria. Today's plants didn't engulf chloroplasts. The theory predicts that endosymbiosis occurred between two unicellular organisms, neither of which had a nucleus. One may have been larger than the other. If the smaller one could break down glucose in a more efficient manner than the larger one, they would both benefit from the relationship. The smaller one would live inside of another, have food to break down and perhaps have some left over to share with the larger one. This relationship would allow both to survive and reproduce more often than others of their species so the relationship gave them an advantage. If the same cell engulfed a photosynthesizing organism, then there was even more of an advantage. Multicellularity (ie plants and animals) came later. Of course, I wasn't there, and can't prove any of this. However, there is much evidence to support this idea. Mitochondria are the same size and shape as bacteria, they have one circular chromosome like bacteria do, they have their own ribosomes that resemble bacterial ribosomes and the sequence of the DNA in mitochondria matches bacteria DNA most closely.


Consider prokaryotes like bacteria. They lack mitochondria, yet they have still survived for eons. Bacteria still have the ability to respire and generate ATP, except that it's done "in-house" instead of offloading that duty to the endosymbiont (the mitochondria).

Prior to endosymbiosis, there were no green plants. When the milestone event that led to the engulfment of a chloroplast took place, it was found to be value-added and mutually beneficial for both parties. This totally upended life on earth. The atmosphere was now rich with oxygen, a normally toxic material. Organisms had to develop coping strategies to branch out into this uncharted territory. Green plants were one of the evolutionary outcomes.

Dr. Tim Durham Assistant Professor of Agronomy and Agricultural Science Ferrum College


There are a number of misconceptions here about the role of chloroplasts and mitochondria. First of all, you assume that chloroplasts are the only way to convert sunlight into useful energy-containing molecules like glucose. Similarly, you assume that the mitochondria is the only way to break down glucose for energy. Clearly, this cannot be the case - prokaryotes like bacteria do not have internal organelles, but they are able to function just fine. All bacteria can break down glucose to produce ATP, the cell's energy currency, and special bacteria called cyanobacteria are able to photosynthesize as well, all without the use of a mitochondrion or chloroplast!

The obvious question, then, is why eukaryotes go to all the trouble of having these large organelles to carry out photosynthesis and respiration? The answer is that organelles are highly specialized and can perform these tasks with a much greater degree of efficiency. Bacteria, lacking any internal membranes, have to place their electron transport chains in the cell membrane, rather than the inner membrane of the mitochondria. Similarly, cyanobacteria place their photosynthetic machinery in special folds of their cell membrane. For a variety of reasons, this is not as well optimized for getting the most energy out of either sunlight or glucose.

What's more interesting is that some organisms are able to survive without the use of photosynthesis OR aerobic respiration. These are called chemoautotrophs, and they often live in deep-sea vents without access to either sunlight or much in the way of oxygen. They use electron donors in their area as a source of energy to change chemicals (like carbon dioxide) into useful molecules. Some scientists have speculated that chemoautotrophs were the first kind of organism on the planet; they started producing oxygen as a byproduct of their normal chemical reactions, which led to the evolution of organisms that used aerobic respiration.

S. Unterman Ph.D.

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