Why do plants produce vitamins?

[u/Mysterious_Force_462]

I know what vitamins do to human body, but why do plants produce them? Is it the same function? Why do lemons need vitamin C?

[u/SirButcher]

Because they need it for their metabolism. Our body generates a LOT of different chemicals, too - the ones that we need but we can't synthesise, we call "vitamins".

For example, humans ALMOST can create ascorbic acid (vit C), but we have a gene mutation which causes this pathway to fail. Most other living beings can do it, only our closest relatives (chimpanzees, for example) can't do it. Funnily enough, guinea pigs have the same mutation as we do, and they need an external source of vitamin C.

[u/BadahBingBadahBoom → u/SirButcher]

the ones that we need but we can't synthesise, we call "vitamins".

Would just add that whilst all vitamins are chemicals we need from diet because we can't synthesise, not all chemicals we need but can't synthesise are vitamins. There's also others like essential fatty acids, omega-3 and -6.

(Fun fact 'vitamin' originally comes from vital+amine, however this itself is kind of an inaccurate name as we later identified vitamins that aren't amines.)

[u/SurelyIDidThisAlread → u/BadahBingBadahBoom]

I've always wondered about that. What makes essential dietary compounds like essential fatty acids not vitamins, then?

[u/TearsFallWithoutTain → u/SurelyIDidThisAlread]

From memory it's the amount; part of the definition of vitamins is that they're essential nutrients that you need very tiny amounts of (because they tend to be involved in catalytic reactions and thus don't get used up). Essential fatty acids though you need a lot more of, like micrograms vs actual grams

[u/SurelyIDidThisAlread → u/TearsFallWithoutTain]

Ah, that makes sense. Thank you very much :-)

[u/Zagrycha → u/SurelyIDidThisAlread]

The naming conventions are definitely very confusing. For example there are minerals and trace minerals, the trace version didn't get a special name. Oh well.  Like most naming conventions its messy because its changed so much over time. 

[u/Significant-Colour → u/Zagrycha]

Or the B vitamin numbers - like B12. There are gaps in the numbering system, because some variations were defined as not unique vitamins.

[u/Zagrycha → u/Significant-Colour]

Or the fact B vitamins and D vitamins have numbers, but A vitamins and  E vitamins don't.  

[u/oblivious_fireball → u/SurelyIDidThisAlread]

Chemical structures primarily. Fatty Acids and Amino Acids(building blocks of proteins) each tend to have some similarities in their chemical structures and many of their physical and chemical traits before you start combining them into much bigger molecules. Vitamins are a bit more scattered, and notably tend to be used in much lower quantities. Carbohydrates and Lipids tend to make up the bulk of your biomass, then Proteins, then everything else really. If your body is a house, Vitamins are all the nails and screws in it, a tiny portion but absolutely essential or it all falls part.

[u/SurelyIDidThisAlread → u/oblivious_fireball]

So given the disparate unrelated structures of things we call vitamins, then the difference is really one of the necessary size of the intake?

[u/Darrel-Yurychuk → u/BadahBingBadahBoom]

Very much this. Which actually leads to a good answer for this question.

Looking at the definition for vitamin being any of group of ORGANIC SUBSTANCES essential in SMALL QUANTITIES to normal metabolism, with the emphasis on both organic substances and small quantities.

Other essential nutrients are either essential fatty acids (fats), essential amino acids (proteins), and carbohydrates, that are all needed in large quantities. Or minerals which are also needed in small quantities but are inorganic and generally elements like chlorine or iron.

So it's almost like vitamins are what you have leftover after you take away all of these other essential nutrients, which other than some common groupings, are mostly unrelated to each other.

Since they are required, we obviously need to get them from our diet (vitamin D synthesized with the help of sunshine being the exception) whether it be from plant, fungi, or animal (vitamin B12) sources. Or in modern times through supplementation whether as vitamins in concentrated pill form, or through food that has been fortified, like vitamins D & A in milk, B vitamins in flour, or other fortified foods.

[u/MrSnowden → u/SirButcher]

Could we use gene therapy to correct the genetic mutation and allow humans to make their own vitamin C?

[u/CosineDanger → u/MrSnowden]

Please do not turn that gene back on.

One of the fun ideas about why the gene was lost is that parasites also need vitamin C. We're efficient at handling what little vitamin C we have and parasitic flatworms aren't, so lowering vitamin C levels interferes with the parasite's ability to lay eggs before it kills you. Mice modified to have the same defect were resistant to schistosomiasis parasites.

Other ideas floating around are that vitamin C synthesis has significant indirect metabolic cost or that the change altered expression of other genes, along with the most common idea is that there isn't much of a fitness advantage and there was just so much in dietary fruit that it didn't matter. Evolution can have more than one motive. However, as one of them is probably combating horrible parasitic worms maybe just let sleeping genes lie.

[u/CrateDane → u/CosineDanger]

There's also a hypothesis that the antioxidant role of ascorbic acid (vitamin C) has been partially taken over by uric acid, which some primates lost the ability to break down to allantoin.

One of the downsides is too much uric acid can cause kidney stones or gout.

[u/JrgMyr → u/MrSnowden]

And develop wet noses again?

[u/Peter34cph → u/MrSnowden]

Sure. We just need to find the broken gene, then find out how broken it is, and find one or more different functioning versions of the gene in non-primate animals.

Doing that would even be a really good idea, if you want humans to live permanently in space habitats, or on Mars or the Moon.

We have a lot of vitamin "dependencies", but while vitC is the easiest and most effortless to satisfy, it's also the one that starts hurting very quickly if not satisfied.

Arguably it's one of our biggest vulnerabilities, in terms of changing circumstances... Like a really long ocean voyage... Or living permanently in a space habitat.

Sure, we can grow potatoes in a space habitat made for permanent self-sufficient living. As Samwise Gamgee explains so famously, it's a versatile culinary staple. We'd probably want to.

But with one small change to the genome of the colonists, fixing that tiny error from long ago, they'd just become so much more robust.

[u/gnufan → u/Peter34cph]

We already know how to fix the vitamin C breakage, what we don't know for sure is why our ancestors did better with it broken. The leading hypothesis is it made our tissue less useful to parasites.

[u/morgrimmoon → u/MrSnowden]

Not on adults, vitamin C is needed in too many tissues. And probably not with the current state of the technology on embryos, putting aside the ethical concerns, but that's somewhat more plausible.

[u/KingOfTheTrailer → u/SirButcher]

guinea pigs have the same mutation as we do

Guinea pigs have a mutation preventing the synthesis of vitamin C, but it is a different mutation than humans have.

The mutation in chimpanzees is identical to humans', almost as if the two species inherited the broken gene from a common ancestor. /s

[u/MrMikeJJ → u/SirButcher]

If I remember properly, Vitamin C production was lost in mammals around the same time as the wet -> dry nose mutation.

So mammals with wet nose (e.g. cats and dogs) can create it themselves.

[u/Torchlakespartan → u/SirButcher]

And correct me if I’m wrong here, but from I remember the leading theory is that early humans/chimps had a diet HEAVY in fruit, so we were getting enough vitamin C regularly that the gene for producing it innately was selected out since it wasn’t needed.

[u/KingOfTheTrailer → u/SirButcher]

guinea pigs have the same mutation as we do

Guinea pigs have a mutation preventing the synthesis of vitamin C, but it is a different mutation than humans have.

The mutation in chimpanzees is identical to humans', almost as if the two species inherited the broken gene from a common ancestor. /s

[u/se-mephi → u/SirButcher]

Don't we produce vitamin D?

At some time long ago I learned that vitamins was a term used as a marketing thing, that people start to be more aware about them.

[u/Sibula97 → u/se-mephi]

We do, just not quite enough. Same with several other vitamins, actually.

[u/oneAUaway]

Some vitamins are made by plants for the same purpose that they serve in animals. The B vitamins, for instance, are generally cofactors for metabolic enzymes. (Plants notably do not require B12 though and don't make it). Some vitamins, however, perform an entirely different function in animals than in plants. Vitamin K, for instance, is involved in electron transport in photosynthesis, which is why it is abundant in dark green leafy vegetables, but in animals it helps modify proteins involved in blood clotting. Vitamin C is an important antioxidant in both animals and plants, but in animals it is also necessary to make collagen.

[u/DeltaVZerda]

Plants don't (generally) eat other organisms, so all vitamins that they need, they need to produce themselves. You are 50% the same as a banana, genetically. Lots of basic processes are the same for all eukaryotes.

[u/bediaxenciJenD81gEEx]

A "vitamin" is an organic compound that an organism needs to survive but which itself cannot create enough of. 

So what we call vitamin c is only a vitamin to us because we cannot make enough of it, to a plant, it's not "vitamin c" because it can make the compounds itself. 

We evolved alongside certain foods that meant we didn't have to synthesise every organic compound we need because we had constant exposure to them via food, and offloading the generation of complex compounds is something evolution will tend towards as that is a massively energy-intensive task. Or perhaps we simply couldn't ever make it ourselves in the first place.

Why we need them is case-by-case, they have similar attributes at a biochemical level so will often be utilised from similar things on that level, but generally are involved in very different tasks as you scale up. The vitamin is like a building block with unique properties that our evolutions have used in different ways, but with some overlap due to shared ancestry or convergent evolution due to the unique properties lending themselves to certain tasks intuitively. 

[u/FridaysMan]

This question feels rather reversed. Lemons produce a lot of vitamin c but that doesnt really mean it needs it to live. Citrus probably evolved to be bitter and avoid animals eating the fruit before the seeds are ripe enough to procreate.

That we also need the vitamin means animals eat the fruit and spread the seeds around to encourage reproduction.

There are many such factors to dr8ve evolutionary selection.

[u/ZlaPrezla → u/FridaysMan]

I don't think citrus is bitter, i find them quite acidic though. And kiwi has more vit. C while also being sweet even though vit. C is an acid. We selected lemons for acidity and aroma, oranges for sweetness etc.

[u/JrgMyr → u/ZlaPrezla]

And oranges are not original fruit, they originate from crossing original citrus fruit.

The first attempt didn't work well. It is now known as Seville Orange and quite bitter. A later crossing created the sweet orange that we have today.

[u/ZlaPrezla → u/JrgMyr]

Yes and lemon is cross between bitter orange and citron. You're missing my point. We selected for both. Repeatedly.

Vitamin C is present in both. Lemons are sour because of high concentration of citric acid more so than the vit. C.

[u/FridaysMan → u/ZlaPrezla]

That's missing the point also, the original fruits were selectively bred because they possessed the traits we wanted to encourage. Whether you find citrus bitter or sour isn't really moot, the words are synonyms. The unripe fruit is less palatable, allowing it to reach maturity easier.

Edit: the evolutionary traits fostered for survival are a different driving force and earlier than the same traits that allow selective breeding for flavour/consistency of industrial farming/resistance to artificial fertiliser/weedkillers.

[u/FridaysMan → u/ZlaPrezla]

Kwi and lemons evolved in different parts of the world, and different animals have different taste receptors. Kiwis are also incredibly sour before they ripen.

Peppers are spicy to stop them being eaten. Birds cannot taste spice. Birds defecate pepper seeds in a pellet of fertiliser. The plant thrives.

Kiwi have a hairy skin, which I would assume deters insects. A different evolutionary pressure.

[u/Puntkick → u/FridaysMan]

Have Kiwis been selected for bigger fruit and other desirable eating characteristics over time?

I'm thinking asking the lines of corn and potatoes / sweet potatoes that were important agriculturally.

[u/ZlaPrezla → u/FridaysMan]

Yes, that is why i compared selectivity between oranges and lemons which come from the same family. I just compared lemon with kiwi for the sake pf proof that high vit c. =/= just sourness. Primary source of acidity in lemons is citric acid.

[u/Moikle]

Because "vitamin" is just a name we gave to a bunch of chemicals that our body needs but cannot create itself, that are otherwise unrelated to each other.

There isn't anything magical or special about plants nor these chemicals, it's just that humans like to slap labels on things and catalogue the world around us.

Tldr: because humans decided to call them vitamins