Naked Science Forum
Life Sciences => Cells, Microbes & Viruses => Topic started by: Tomassci on 28/05/2017 06:18:53
-
I just asked for reversion of cellular membrane. Is that possible? How and why?
-
In endocytosis, a cell engulfs something in the external environment.
The cell membrane forms an indentation which then closes off, forming a "vesicle" on the inside of the cell, surrounded by part of the cell membrane.
In this vesicle, the "outside" of the cell membrane is "inside" the vesicle, which is in turn inside the cell.
This is one case where a cell membrane can turn inside-out.
See: https://en.wikipedia.org/wiki/Endocytosis
-
I asked for whole-membrane reversion. But thanks.
-
I must admit that I don't actually understand the question. What do you mean by "reverse the cellular membrane?"
-
The cell membrane is an equilibrium configuration, albeit, in dynamic equilibrium. There are potentials at work that maintain the configurations and orientations of things in the membrane.
If you were to reverse the membrane, you would create a state of non-equilibrium. The cell will not do this in entity, since it would take a lot of energy to go against itself. When there is small reversal, to form vesicles, this non equilibrium state is expressed by breaking off. It is easier to get rid of it.
If we could reverse the membrane, in the lab, there would be an internal rearrangement of orientations of proteins and other materials, in attempt to reform the old configuration, so it can right itself to the cellular equilibrium.
-
Experiments have been done where the cell membrane were removed from cells to form naked cells. It turns out that naked cells, without their membranes, will still concentrate potassium ions, even without ion pumps. This has to do with an equilibrium induction between protein, water and potassium ions that minimizes the water potential.
Pure water has a certain degree of order and disorder at equilibrium. The impact of the proteins inside the cell is to add too much order to the water, due to the hydrophilic surfaces. This is too much of a good thing in terms of the energy needs of water, with water the dominant phase of the cell.
Potassium ions in water are chaotropic or they will add chaos to the water. This chaos compensates for the extra order induced by the proteins, placing the water back into its sweet spot. This is why potassium will accumulate inside the naked cell without any apparent energy expenditure. It helps to restore the water back to balance.
Say we took our naked cell that is accumulating potassium ions, for balance, and add a randomly orientated membrane. We also have sodium ions on the outside. Sodium ions are kosmotropic in water or they will create order in water, like the proteins. Sodium ions will not spontaneously accumulate in naked cells, because they make it worse for the water. They will stay outside.
The membrane is fluid so its contents will start to rotate and align with the gradient potential; kosmotropic ends will try to point outside and chaotropic ends will try to point inside, so the potential of the membrane water is as low as possible at all points. Before long we have a working membrane.
Water is the shaping of the cell.
Conceptually, the evolution of ion pumps would have driven the rapid development of the interior of the cell. If we started with a membrane with ion pumps and potassium ions accumulated inside, protein would be selectively retained, extracted and concentrated to balance out the chaotropic impact of the potassium ions, so the water can reach its sweet spot. There are many places to start but they all try to minimize the water potential.
-
whole-membrane reversion
The general structure of a cell membrane is a lipid bilayer. This simple underlying structure is symmetrical, so would work just as well if it were turned inside-out.
See: https://en.wikipedia.org/wiki/Cell_membrane#Lipid_bilayer
However, some cells have an additional outer layer which is not symmetric.
All cells have an internal skeleton, which anchors various proteins that penetrate this cell membrane to various degrees, to transport ions or other nutrients, signal other cells, detect food, etc. These proteins can make up 50% of the volume of the cell membrane, and they are quite asymmetric.
Some cells have a difference in electrical potential between the inside and the outside, and this won't work if the cell wall is reversed.
Since the cell wall as a whole is asymmetric, the cell won't survive if you turned the cell wall inside-out.
-
whole-membrane reversion
The general structure of a cell membrane is a lipid bilayer. This simple underlying structure is symmetrical, so would work just as well if it were turned inside-out.
See: https://en.wikipedia.org/wiki/Cell_membrane#Lipid_bilayer
However, some cells have an additional outer layer which is not symmetric.
All cells have an internal skeleton, which anchors various proteins that penetrate this cell membrane to various degrees, to transport ions or other nutrients, signal other cells, detect food, etc. These proteins can make up 50% of the volume of the cell membrane, and they are quite asymmetric.
Some cells have a difference in electrical potential between the inside and the outside, and this won't work if the cell wall is reversed.
Since the cell wall as a whole is asymmetric, the cell won't survive if you turned the cell wall inside-out.
The asymmetry of the membrane is a result of a hydrogen bonding gradient within the membrane water, with water able to freely diffuse through the cell membrane. This hydrogen bonding gradient, is induced by the different impacts of sodium and potassium ions on water.
Both cations have a single positive charge, but each ion impacts water differently at the atomic level. Sodium ions can bind to water stronger than water can hydrogen bond to other water. While water can hydrogen bond to other water stronger that can potassium can bind to water. The binding forces of water-water are right in the middle between sodium-water and potassium-water.
Hydrogen bonding is mostly polar in nature; electrostatic . Water also shows covalent bonding character; magnetic. Electrostatic and magnetic are two sides of the electromagnetic force; EM force. Hydrogen bonding is a like a binary switch, with two EM force settings, with only a small energy difference between. Although this energy different is small, the properties of water are quite different in terms of entropy, enthalpy and volume.
The impact of the sodium and potassium ions, is each favors one of the two settings, each shifting the distribution of hydrogen bonding switches of water, so the properties of the water, inside and outside the cell become different. Proteins dissolved in the membrane will align themselves with this water gradient, with each end of each protein, better suited to one switch setting or the other.
The experiment that showed that cells will still concentrate potassium, even without a membrane, tells us that if we reversed the membrane, the membrane will not work properly in the short term. However, since the inside of the cell can still accumulate potassium ions, the potassium switch setting will remain, albeit weaker. Therefore, little by little, the protein will start to rotate and flip.
In the short term, the outside of the membrane, by ending on the inside, will cause the water potential to be wrong inside of the cell; relative to normal. The outside membrane protein will flip the switches backwards, relative to where it should be. This will impact the cellular inwards, all the way to the DNA. There will be a disordering impact inside the cell. This should help disrupt the scaffolding protein, helping the membrane protein become more fluid so they shuffle and rotate. There is a fail safe.
Water is a copartner with the organics of life. Water has many jobs, one of which is to help define the shapes and orientations of large organic molecules like protein. Sodium and potassium ions, as well as organics can be used to fine tune the bulk and local water potential, to get intermediate layers of shapes.
The water also supports gradients, such as inside and outside membrane (sodium-water-potassium). It also supports another important gradient, between the membrane to the DNA; potassium to water. The cells inwards know where to go for equilibrium, and also can align with the gradient. The cell can also use active transport of protein and scaffolds to force protein into non equilibrium positions in the water gradient. This gives protein overdrive; turbo boost.
-
The experiment that showed that cells will still concentrate potassium, even without a membrane,
I'm sorry, but I'm not buying this. A mammalian cell has only a cell membrane to separate it from the external environment. If you take the membrane away then there is no cell, because all the bits inside would just float away. The fluid would be one system, so it couldn't concentrate itself.
-
The experiment that showed that cells will still concentrate potassium, even without a membrane,
I'm sorry, but I'm not buying this. A mammalian cell has only a cell membrane to separate it from the external environment. If you take the membrane away then there is no cell, because all the bits inside would just float away. The fluid would be one system, so it couldn't concentrate itself.
Below is where I read this. What keeps the cell together is adhesive ordering within the water. Water has many uses in the cell and is the swiss army knife of life.
Very little of the intercellular water can be considered as solely a medium [2753] but is a metabolic reactant, product, catalyst, chaperone, messenger and controller (see for example, [1194]). Intracellular water is responsible for the conformation and function of all biomolecules through direct interaction with their hydration shells [2845]. However and importantly the water's structuring may also be controlled by some cellular constituents.
The cellular innards; protein surfaces, by being hydrophilic, tend to create low density water, which is more on the covalent side of hydrogen bonding binary switch. This covalent nature of hydrogen bonding becomes the gel/glue that hold things together.
The potassium ions; K+ are chaotropic and create chaos in water. Cells without membrane will still concentration in K+, to shift water back more to the middle switch setting of natural water. Once we add a membrane and ion pumps we can drive the K+ equilibrium even further and induce the inside of the cell to concentrate in hydrophilic protein surfaces; natural selection process.
http://www1.lsbu.ac.uk/water/intracellular_water.html#r1094 (http://www1.lsbu.ac.uk/water/intracellular_water.html#r1094)
Also, in contrast to that written in several undergraduate textbooks, many studies show that cells do not need an intact membrane to function [635]. Instead, the intracellular water tends towards a low density structuring due to the kosmotropic character of the majority of the solutes, the confined space within the cell stretching the hydrogen-bonded water and the extensive surface effects of the membranes [1094]. The ions partition according to their preferred aqueous environment; in particular, the K+ ions partition into the cells. Ion pumps must thus be present for other (perhaps fail-safe) purposes, such as speeding up the partition process after metabolically linked changes in ionic concentration.
635 G. H. Pollack, Cells, gels and the engines of life; a new unifying approach to cell function, (Exner and Sons Publishers, Washington, 2001) .
1094 M. J. Higgins, M. Polcik, T. Fukuma, J. E. Sader, Y. Nakayama and S. P. Jarvis, Structured water layers adjacent to biological membranes, Biophys. J. 91 (2006) 2532-2542.