There’s another issue that one individual cell, at any one time, can only have one membrane potential, and can only have one concentration of calcium ions in its cytoplasm, and can only have one concentration of any given protein in its cytoplasm, and can only have one areal density of any given integral membrane protein on its outer membrane, and so on. (If I understand correctly.) If the optimal values of any of these parameters (say, potassium ion concentration in the cytoplasm) is different for muscle-type-functionality versus neuron-type-functionality versus immune-system-type-functionality etc. (which seems extremely likely to me), then there would be an obvious benefit to splitting those functions into different cells, each with its own separate cytoplasm and so on.
But why is that so? Why are there no parameter combinations here that let you do well simultaneously on all of these tasks, unless you split your system into parts? That is what we are asking.
Could it be that such optima just do not exist? Maybe. It’s certainly not how it seems to work out in small neural networks, but perhaps for some classes of tasks, they really don’t, or are infrequent enough to not be finable by the optimiser. That’s the direct selection for modularity hypothesis, basically.
I don’t currently favour that one though. The tendency of our optimisers to connect everything to everything else even when it seems to us like it should be actively counterproductive to do so, but still end up with a good loss, suggests to me that our intuition that you can’t do good while trying to do everything at once is mistaken. At least as long as “doing good” is defined as scoring well on the loss function. If you add in things like robustness, you might have a very different story. Thus, MVG.
For example, gastric parietal cells excrete hydrochloric acid into the stomach. The cell is a little chemical factory. For the chemistry steps to work properly, the potassium ion concentration in the cytoplasm needs to be maintained at a certain level.
Meanwhile, neurons communicate information by firing action potentials. These involve the potassium concentration in the cytoplasm dropping rapidly at certain times, before slowly returning to a higher level.
So, now suppose the same cell is tasked with both excreting hydrochloric acid and communicating information by firing action potentials. I claim that it wouldn’t be able to do both well. Either the potassium concentration in the cytoplasm is maintained at the level which best facilitates the acid-producing chemical reactions, or else the potassium level in the cytoplasm is periodically dropping rapidly and climbing back up during action potentials. It can’t be both. Right?
(I’m not an expert on cell biology and could be messing up some details here, but I feel confident that there are lots and lots of true stories in this general category.)
It e.g. wouldn’t use potassium to send signals, I’d imagine. If a design like this exists, I’d expect it to involve totally different parts and steps that do not conflict like this. Something like a novel (to us) kind of ion channel, maybe, or something even stranger.
Does it seem to you that the constraints put on cell design are such that the ways of sending signals and digesting things we currently know of are the only ones that seem physically possible?
This is not a rhetorical question. My knowledge of cell biology is severely lacking, so I don’t have deep intuitions telling me which things seem uniquely nailed down by the laws of physics. I just had a look at the action potential wikipedia page, and didn’t immediately see why using potassium ions was the only thing evolution could’ve possibly done to make a signalling thing. Or why using hydrochloric acid would be the only way to do digestion.
You’re a chemical engineer. Your task is to manufacture chemicals A and B. But you only get one well-mixed tank: all the precursors of chemical A have to mix with each other and with the precursors of chemical B without undesired cross-reactions, and the A and B production process need to take place in the same salinity environment and the same pH environment, etc.
Maybe that’s doable.
Oh hey, now also please manufacture chemicals C,D,E,F,G,H,I,J and K. You still have one well-mixed tank in which to do all those things simultaneously.
Maybe that’s doable too. Or maybe not. And if it is doable at all, presumably the efficiency would be terrible.
At some point, you’re going to throw up your hands and go to your boss and say “This is insane, we need more than one well-mixed tank to get all this stuff done. At least let me have a high-pH tank for manufacturing A,B,E,G,H and a low-pH tank for manufacturing C,D,F,I,J,K. Pretty please, boss?”
Anyway, you can say “Design space is huge, I bet there’s a way to manufacture all these chemicals in the same well-mixed tank.” Maybe that’s true, I don’t know. But my response is: there is a best possible way to manufacture A, and there is a best possible way to manufacture B, etc., and these do not involve the exact same salinity, pH, etc. Therefore, when we put everything into one tank, that’s only possible by making tradeoffs. A chemical plant that has multiple tanks will be way better.
Back to biology. (I’m not an expert on cell biology either, be warned.)
A cell cytoplasm is a well-mixed chemical reactor tank, as far as I understand. It’s not compartmentalized (with certain exceptions).
My strong impression is that there are no one-size-fits-all jack-of-all-trades cells. Not in multicellular life, and not in single-cell life either. Different single-cell species specialize in performing different chemical reactions.
Thus, we can have single-cell organism Q that eats food X and spits out waste product Y, and then a different single-cell organism P eats Y and spits out waste product Z, etc. We can kibbitz from the sideline that Q is being “wasteful”: why spit out Y as waste, when in fact Y was digestible in principle—after all, P just digested it! But from my perspective this is pretty much expected. Maybe digesting Y and digesting X require very different salinity or pH, for example. So in order to digest Y at all, the cell Q would need to be much worse at digesting X, and that tradeoff winds up being not worthwhile.
didn’t immediately see why using potassium ions was the only thing evolution could’ve possibly done to make a signalling thing. Or why using hydrochloric acid would be the only way to do digestion.
It could be chloride ions instead of potassium, as far as I know. But there are only so many elements on the periodic table, and many fewer that are abundant in the evironment, and they all have different properties which may be disadvantageous for a particular function. There are other ways to do digestion, but they don’t all spend identical resources and produce identical results, right? So there are tradeoffs.
There’s another issue that one individual cell, at any one time, can only have one membrane potential, and can only have one concentration of calcium ions in its cytoplasm, and can only have one concentration of any given protein in its cytoplasm, and can only have one areal density of any given integral membrane protein on its outer membrane, and so on. (If I understand correctly.) If the optimal values of any of these parameters (say, potassium ion concentration in the cytoplasm) is different for muscle-type-functionality versus neuron-type-functionality versus immune-system-type-functionality etc. (which seems extremely likely to me), then there would be an obvious benefit to splitting those functions into different cells, each with its own separate cytoplasm and so on.
But why is that so? Why are there no parameter combinations here that let you do well simultaneously on all of these tasks, unless you split your system into parts? That is what we are asking.
Could it be that such optima just do not exist? Maybe. It’s certainly not how it seems to work out in small neural networks, but perhaps for some classes of tasks, they really don’t, or are infrequent enough to not be finable by the optimiser. That’s the direct selection for modularity hypothesis, basically.
I don’t currently favour that one though. The tendency of our optimisers to connect everything to everything else even when it seems to us like it should be actively counterproductive to do so, but still end up with a good loss, suggests to me that our intuition that you can’t do good while trying to do everything at once is mistaken. At least as long as “doing good” is defined as scoring well on the loss function. If you add in things like robustness, you might have a very different story. Thus, MVG.
For example, gastric parietal cells excrete hydrochloric acid into the stomach. The cell is a little chemical factory. For the chemistry steps to work properly, the potassium ion concentration in the cytoplasm needs to be maintained at a certain level.
Meanwhile, neurons communicate information by firing action potentials. These involve the potassium concentration in the cytoplasm dropping rapidly at certain times, before slowly returning to a higher level.
So, now suppose the same cell is tasked with both excreting hydrochloric acid and communicating information by firing action potentials. I claim that it wouldn’t be able to do both well. Either the potassium concentration in the cytoplasm is maintained at the level which best facilitates the acid-producing chemical reactions, or else the potassium level in the cytoplasm is periodically dropping rapidly and climbing back up during action potentials. It can’t be both. Right?
(I’m not an expert on cell biology and could be messing up some details here, but I feel confident that there are lots and lots of true stories in this general category.)
It e.g. wouldn’t use potassium to send signals, I’d imagine. If a design like this exists, I’d expect it to involve totally different parts and steps that do not conflict like this. Something like a novel (to us) kind of ion channel, maybe, or something even stranger.
Does it seem to you that the constraints put on cell design are such that the ways of sending signals and digesting things we currently know of are the only ones that seem physically possible?
This is not a rhetorical question. My knowledge of cell biology is severely lacking, so I don’t have deep intuitions telling me which things seem uniquely nailed down by the laws of physics. I just had a look at the action potential wikipedia page, and didn’t immediately see why using potassium ions was the only thing evolution could’ve possibly done to make a signalling thing. Or why using hydrochloric acid would be the only way to do digestion.
You’re a chemical engineer. Your task is to manufacture chemicals A and B. But you only get one well-mixed tank: all the precursors of chemical A have to mix with each other and with the precursors of chemical B without undesired cross-reactions, and the A and B production process need to take place in the same salinity environment and the same pH environment, etc.
Maybe that’s doable.
Oh hey, now also please manufacture chemicals C,D,E,F,G,H,I,J and K. You still have one well-mixed tank in which to do all those things simultaneously.
Maybe that’s doable too. Or maybe not. And if it is doable at all, presumably the efficiency would be terrible.
At some point, you’re going to throw up your hands and go to your boss and say “This is insane, we need more than one well-mixed tank to get all this stuff done. At least let me have a high-pH tank for manufacturing A,B,E,G,H and a low-pH tank for manufacturing C,D,F,I,J,K. Pretty please, boss?”
Anyway, you can say “Design space is huge, I bet there’s a way to manufacture all these chemicals in the same well-mixed tank.” Maybe that’s true, I don’t know. But my response is: there is a best possible way to manufacture A, and there is a best possible way to manufacture B, etc., and these do not involve the exact same salinity, pH, etc. Therefore, when we put everything into one tank, that’s only possible by making tradeoffs. A chemical plant that has multiple tanks will be way better.
Back to biology. (I’m not an expert on cell biology either, be warned.)
A cell cytoplasm is a well-mixed chemical reactor tank, as far as I understand. It’s not compartmentalized (with certain exceptions).
My strong impression is that there are no one-size-fits-all jack-of-all-trades cells. Not in multicellular life, and not in single-cell life either. Different single-cell species specialize in performing different chemical reactions.
Thus, we can have single-cell organism Q that eats food X and spits out waste product Y, and then a different single-cell organism P eats Y and spits out waste product Z, etc. We can kibbitz from the sideline that Q is being “wasteful”: why spit out Y as waste, when in fact Y was digestible in principle—after all, P just digested it! But from my perspective this is pretty much expected. Maybe digesting Y and digesting X require very different salinity or pH, for example. So in order to digest Y at all, the cell Q would need to be much worse at digesting X, and that tradeoff winds up being not worthwhile.
It could be chloride ions instead of potassium, as far as I know. But there are only so many elements on the periodic table, and many fewer that are abundant in the evironment, and they all have different properties which may be disadvantageous for a particular function. There are other ways to do digestion, but they don’t all spend identical resources and produce identical results, right? So there are tradeoffs.