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caederemus asked:

Why aren't people/animals photosynthetic, even if just supplementarily? Or rather, since I know evolution is random, would it be advantageous for animals to be capable of photosynthesis? (Like, if instead of just having melatonin absorb light it acted in a fashion similar to - or was simply replaced by - chlorophyll)

This is a fascinating question. Thank you very much for asking it. I was a bit unsure of how to answer it until I really began thinking about it. I am not an evolutionary biologist, so I didn’t think I would be able to answer this without help from the literature, but the development of photosynthesis actually has very little to do with normal evolution we usually think of.

Chloroplasts are the endomembrane organelle capable of turning solar energy into glucose via photosynthesis by splitting water molecules and consuming atmospheric carbon dioxide through a chain of proteins, starting with the photoaystems. The rest of the enzymes aren’t really necessary to discuss to answer this question, but what is necessary to point out is that unlike most endomembrane organelles, chloroplasts contain their own genetic material that is separate from the DNA inside the planet cell’s nucleus. Mitochondria also have their own genetic material. This one of the convincing pieces of evidence for the biological theory of endosymbiosis.

Endosymbiotic theory is strongly supported and widely accepted by biologists. It states that chloroplasts and mitochondria were most likely prokaryotic organisms that were engulfed by early eukaryotic ancestors that then co-evolved to create the most primative eukaruotic cells that then evolved into plant and animal cells. After billions of years of evolution, endomembrane organelles are no longer capable of living outside of a eukaryote.

So basically, the answer to your question of why animals don’t photosynthesize is because the eukaryotic ancestors of modern animal cells did not engulf chloroplasts, or if they did, they did not keep them and co-evolve through a symbiotic relationship. It has nothing to do with random evolution because genomic DNA (the stuff inside the DNA of every eukaryotic cell) does not contain information about photosynthesis. Most of that information is stored in the individual chloroplast, which will transcribe mRNA transcripts on its own. It is rather interesting and noteworthy, however, that some of the mitochondria DNA (mDNA) has been discovered in genomic DNA. This is a separate phenomenon, though.

ALSO, since mDNA (and chloroplast DNA, but this isn’t too relevant since some plants are asexual, anyways, but I’m sure it’s important to plant phylogenists) are not completely incorporated into the eukarytoic genome, your mDNA is not 50% from each parent like your genomic DNA is! mDNA is actually maternally inherited because mitochondria are in the oocyte. Yes, sperm need mitochondria to propel their flagella through the vas deferns, urethra, vagina, uterus, and finally, fallopian tubes to reach the oocyte, but when the membrane around the oocyte is penetrated by a sperm cell, the mitochondria in the sperm are not incorporated into the oocyte, so your mother’s mitochondria are the only ones left to remain in the developing embryo. For this reason we can track maternal lineage using mDNA!

Now, onto the final part of your question, which I am really excited you asked about. Melatonin is a molecule produced by both the pineal gland; melanin is actually the pigment synthesized by your skin cells in response to sunlight to protect you from harmful UV radiation. Both melanin and melatonin are produced in a light-dependent manner, which means they are produced when photosensitive cells are stimulated by sunlight, just as photosynthesis is initiated by sunlight (in most plants - there are exceptions). Chlorophyll, carotene, etc. are pigments that capture solar energy for photosynthesis. Melanin is also a pigment (the more you have, the darker you are, e.g. Europeans have been exposed to far less sunlight than Africans, so there is a difference in skin color, which is why racism is stupid…the amount of melanin in your skin doesn’t make you more or less of a human being…), but it does not store energy to be used for photosynthesis; it just protects your DNA from mutagenic UV rays.

Now, back to melatonin really quick. The pineal gland secretes melatonin when it is day light. Throughout the day, melatonin builds up, which is one of the mechanisms that make you tired, hence why most people sleep at night, after melatonin has reached its peak. This is an example of a negative feedback loop (which I encourage you to send me an ask about!). Melatonin supplements are used to help people fall asleep for this reason. (Side note: another mechanism that makes us tired is the break down of ATP, the major energy source for the cell. Caffeine is an adenosine antagonist, which means it stops adenosine from making you tired.)

Onto what if animals were photosynthetic? Well, it would certainly solve the problem of world hunger, but not eating actual food would deprive our body of vital vitamins, minerals, etc. that are necessary for body functions. The other issue would be that we would all become diabetic. Why? Because we would still have the urge to eat even though our glucose supply is being satisfied by photosynthesis. Ghrelin and orexin are two hormones that regulated food consumption. The acts of putting food in your mouth, chewing, food traveling down the esophagus, and food entering your stomach are mechanisms by which hunger is regulated.

By simply relying on photosynthesis to provide our energy needs, we would not only become malnourished, but we would still always be hungry, which would result in overeating, obesity, etc. The excessive glucose intake from photosynthesis and eating would overwork your pancreas to create insulin, which would eventually create an insulin tolerance (aka diabetes mellitus type II). Not to mention, only the exterior cells of the body would be able to photosynthesize. Leaves are thin enough that light can penetrate all the cells. Any internal cells of the animal’s body wouldn’t be exposed to light, and, consequently, would not be able to photosynthesize.

I think I hit all the points of your question (and then some, hah). If I missed anything or you would like me to clarify, please send me another ask! ^_^

Xoxo, molecularbiologistproblems

While working as a lab assistant in the chemistry department’s chemical storage room, I found a bottle of a reagent that had no received date, so I didn’t know how old it was and if I should toss it or use it. I asked me supervisor and she said, “Well, the manufacturer’s address on the label doesn’t have a zip code, so this has to be before zip codes were created in the 1960s.” It was that day that I realized that zip codes didn’t always exist. Mind blown.

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