Can life exist with rigid cell membranes?

Question

Acrylonitrile has been suggested as an alternative membrane-forming material for conditions like those on the surface of Titan, which might allow living cells to develop in liquid methane or ethane solvents. As a fairly small partially-polar molecule, it has a distinct advantage over the long-tailed phospholipids that our biology uses in that it remains liquid at liquid methane temperatures! And supposedly, the liquid nature of cellular membranes is very important for allowing cells to grow and divide, by simply manufacturing more membrane molecules which can slip into the membrane and self-organize.

Unfortunately, acrylonitrile in particular seems (according to this article) to not actually self-assemble into micelles or azotosomes (vesicles formed from nitrogen-containing components) in Titan-like conditions! Based on this earlier article, it seems that acrylonitrile, while getting the most popular press, is not the only potential option for forming low-temperature azotosomes, so all is not necessarily lost... but I have also been thinking that, for example, plants have rigid cell walls in addition to their fluid membranes--and somehow their cells still manage to grow and divide!

So, are fluid membranes really necessary at all? Could very low-temperature life perhaps rely on compartmentalization based on rigid, solid structures formed from larger molecules?

EDIT: Note that, while plant cell walls were an inspiration for this question, they are not actually an answer--plants, fungi, bacteria, and archaea all have cell walls, so clearly it's a useful structure, but they all have different, convergently-evolved, cell walls, and they all have very complex cell walls with a lot of cellular machinery to manage their maintenance and growth. A good answer to this question would provide a plausible explanation for how a simple rigid compartment, analogous to a cell membrane, could arise during abiogenesis--or explain why such a thing is not plausible after all.

Answer 1

the liquid nature of cellular membranes is very important for allowing cells to grow and divide, by simply manufacturing more membrane molecules which can slip into the membrane and self-organize.

First, a minor clarification of definitions: cell membranes are not liquid. In a liquid, molecules freely move around each other, but in the case of cell membranes, molecules are ionically bound together in a fixed, but very flexible structure that allows it to flow sort of like a liquid at macroscopic scales. But at a molecular level, they very much behave like a solid. The term you are probably looking for is fluid.

Semantics aside, the actual detail of this question you need to focus on is if it allows more molecules to "slip into the membrane and self-organize". A functional membrane, regardless of how ridged it is, must allow new molecules to slip in and become more of the membrane allowing an organism to grow. Also, in order for life to evolve, the first version of a cellular membrane must be able to "self assemble" into some sort of sheet. So, your articles are not criticizing how fluid Acrylonitrile is, but how well it can self form into the repeating patterns necessary to make a contiguous membrane at all under the conditions seen on Titan. We have examples of life on earth that have very ridged cellular walls such as plant cells or gram positive bacteria, but these structures do not naturally form into thin sheets. They are created by life, not something life can be created from.

The thing is: it is very hard to create a cellular membrane without a flexible structure to build on. The limiting factor is not that it must be flexible, but that it must self organizes into sheets. However, compounds that can self-form into sheets more than 2 molecules thick can generally form into infinitely thick crystalline structures. It is weird to think about like this, but the lipid based cell membranes we use on Earth are crystals. They are very ridged for how thick they are, but at only 2 molecules thick, they are thin enough to be flexible. Even diamond becomes flexible when organized into thin enough of sheets. For a crystal to be ridged, it needs to naturally form in 3 dimensions instead of 2.

Since self organizing into 2-d vs 3-d lattices seems to be mutually exclusive, it may be impossible for any single compound to be able to form a ridged membrane. So in order for life to exist, you need the availability of at least one compound that self organizes into 2-d sheets, then on top of that, 3-d reinforcement becomes optional using other materials. However, you can probably not risk using a self organizing molecule for your wall or else it would simply grow in all directions. If crystals were to self grow on the outside of a cell, they would likely expand to block off your proteins and kill the cell. Instead, life that uses ridged cell walls build thier walls in a controlled, non-automatic way making sure that the ridged part is the exact shape needed to not block off anything important.