Why are cells small?
burrito.bio105 points by mailyk 7 hours ago
105 points by mailyk 7 hours ago
> Cell sizes are not fixed, however, even within a single species. Cells often swell as they increase their production of proteins and metabolites in preparation for division. This is in line with biology’s only rule: namely, there are exceptions to every rule!
> Case in point: a giant bacterium called Thiomargarita magnifica can extend about one centimeter in length, so large that it can be seen by the naked eye. It does so by breaking the surface area-to-volume rule, filling between 65–80 percent of its internal volume with an empty vacuole. In other words, it pushes most of its molecules to the cell periphery, thus shortening diffusion distances.
There is also a captioned image of bubble algae in the post.
> This is in line with biology’s only rule: namely, there are exceptions to every rule!
Nice paradox
> The entire cell contains several cytoplasmic domains, with each domain having a nucleus and a few chloroplasts.
It uses container based virtualization under a single host kernel instead of VM based virtualization.
relatedly, foraminifera are single cellular organisms that can grow up to 20 cm! https://en.wikipedia.org/wiki/Xenophyophorea
Isn't the ovum supposed to be a single cell? Eggs of various species can be substantially larger than this.
Yes. I remember reading that Ostrich eggs are the largest single cells (in terms of mass/volume; Blue Whale nerve cells are longer).
Those still seem kind of small? Why not the size of an mature olive tree for example? I'm guessing the article may answer this, haven't gotten that far yet.
When they invade your saltwater aquarium, you won't think they're small. They can get up just slightly larger than a marble
There’s also the one that almost ate the Enterprise. https://en.wikipedia.org/wiki/The_Immunity_Syndrome_(Star_Tr...
I feel like keeping the amount of molecules the same within the simulation needs to be justified. How would it look like if the average amount of molecule was the same across a um?
Nitpick maybe, but I don't think oocytes are the largest cells, it pretty much has to be some sort of neuron. A sensory neuron for eg. someplace in the foot will be almost as long as the person is tall, and even if the neuron is extremely thin, it's gotta beat the oocyte for volume.
Some back of the envelope math says this is true. A conservative estimate for the size of an alpha motor neuron axon is 10μm diameter and 1m long, which already puts it over an order of magnitude larger than the 4,000,000µm³ oocyte quoted in the article.
This almost feels like cheating. Why not count hair follicles with hair attached then?
That's very different; hair doesn't perform membrane transport along its length. The surface of an axon is critical to the cell's functioning.
In addition to what mbauman said, hair follicles and the hair itself are not single-cell. I can't immediately find the composition and average cell size, but even a long and thick strand of hair is less than 2 orders of magnitude larger than the largest neurons. I doubt any individual hair cell is very large.
I agree, except the Squid Gian Axon https://en.wikipedia.org/wiki/Squid_giant_axon that can "1mm diameter and almost 1m long" https://www.understandinganimalresearch.org.uk/using-animals...
But neurons are electrical no? I suppose maybe that's why they're not in the comparison.
Or does that work with diffusion too?
Giraffes neurons can be up to 15 feet long. Blue whales are speculated to have neurons up to 100 feet long, though they've never been directly observed (dissected).
> A simplistic answer is that evolution has made each cell the size best suited to its function.
Yeah. That's probably it. Really, it probably is the right answer.
That just kicks the can forward one step. What parameters control the optimal size of a given cell?
there is likely evolutionary pressure against large cell size (selfish genes; larger cell takes energy away from replication, provides more opportunity for infiltration by other genes, fewer gene backups in other cells, etc) while occupying a niche puts pressure to be a certain size. it lands somewhere in the middle.
I've recently gotten into microscopy as a hobby and comparing the relative size of microbes is really interesting. There are entire animals (tardigrades for one) which can be smaller than some single celled organisms.
There are even single celled organisms which will prey upon and eat multicellular animals.
Reminds me of: "Gravity plays a role in keeping cells small" [0]
[0] https://www.princeton.edu/news/2013/10/24/gravity-plays-role...
This reminds me also of this paper: https://www.pnas.org/doi/pdf/10.1073/pnas.1115585109
"The allocation of all metabolic resources to maintenance purposes limits the size of the smallest prokaryotes and largest unicellular eukaryotes, whereas an inability to meet the ever-increasing biosynthesis rates limits the largest prokaryotes and smallest unicellular eukaryotes. Metabolic constraints for larger eukaryotes are relieved by alternative reproductive strategies and multicellularity."
Cells are small? compared to what? An ostrich egg is a single cell
I never bought into the egg thing. There’s clearly a distinct cell in the center that’s going to divide and grow inside the egg. The egg itself isn’t undergoing mitosis.
I had to go look this up, as I had heard the egg thing my whole life and just accepted it.
It turns out the oocyte is the single cell inside the egg, which for birds is significantly larger than a typical cell. So in that respect, the cell in a bird egg is very large. However, compared to the egg itself, it's tiny. The yolk and whites in the egg are all to provide nutrients as it grows, if fertilized.
The yolk is an energy/vitamin source, not a 'cell'. The division happens outside the yolk.
From Wikipedia:
> The yolk is not living cell material like protoplasm, but largely passive material
One of the fascinating things about biology I think is this - that if the cells of your body were the size of an egg, they'd be way, way too big and you'd probably die.
I don't know for sure here, but isn't the ostrich IN the egg a multicellular animal? I would assume the first point where the egg contains anything that will become the ostrich, mitosis is happening to make more ostrich cells. I'm assuming there's always cell walls and nucleuses every step of the way here, and the egg and ostrich are never just one big cell.
I could be off base here though, I'm really channeling grade 9 bio class from decades ago!
Unfertilized bird eggs are single cells, fertilized eggs should be multicellular by the time they are laid.
The trick is that the egg is a ball with one small cell (the ovum) that happens to have also a huge reservoir of food for the future ostrich. There is a moment when there is only once cell in the egg, just after the fussion of the ovum and the sperm cell.
You're correct, but only for fertilized eggs. Unfertilized eggs are single cells.
Surrounded by a bunch of stuff that isn’t the ovum. “There is at most one cell in an unfertilized bird egg” is not the same as “an unfertilized bird egg is one cell and nothing more”.
skeletal muscle cells can be many cm in length
A neuron can be more than 1 meter long in humans, more than 20 meter in a whale.
maybe god is small too?
Surface area to volume ratio?
That's literally the first thing in the article.
You got me. Usually I read them.
edit: Huh. Actually not a bad read. It even mentions ' On Growth and Form' which is interesting, if outdated. There are more modern texts like 'Shapes', 'Flow', and 'Branches' by Philip J Ball.
Another answer is: They're not - at least in some plants:
Cells are small compared to humans because we're made up by around 3×10¹³ cells.