Team:Paris Bettencourt/Modeling/Assisted diffusion

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Team IGEM Paris 2011

Introduction to the model

The diffusion through the nanotubes is a fast process. This speed can be partially explained by the passive diffusion through the tubes. But what if it is faster? The article from Dubey and Ben-Yehuda suggests that the diffusion is an active process. Several points can be opposed to this statement:

First, the diffusion is happening with molecule of different natures, that have nothing to do with the natural compoments of a cell
Unlike the mamalian cells, the tube seems to have no "railroad" design for such a transport

The question is: can we immagine a process that is faster than passive diffusion but does not rely on specific interractions?

The answer is probably yes, and in this page we propose a new model, really challenging for the mind, but that can play a role in the diffussion process through the nanotubes.

General physical concepts and Hypothesis

Starting from an analogy

Immagine two bottles of gaz connected by a tube. The fist one have an higher pressure than the second one. In the first one, diluted in the gaz, there are a few molecules of another nature. We follow the destiny of these molecules.

When you open the tape, the bottle with a higher pressure will equillibrate with the other one my moving a certain quantity of it's particles through the tube in the direction of the second bottle. These moving molecules will drag with them the copoment diluted in the gaz and a few of these molecules will be transported to the other bottle.

From the analogy to the biology

Of cource, the cell is not a bag of liquid under pressure. The water is equillibrated at both sides of the exterior membrane. The pressure we are dealing with is not related to water or osmotic pressure (that is a "passive diffusion thermodynamical pressure"). There is a part of the cell we are not used to think about that undergo a huge variablilty of pressure: the phospholipid membrane!

Let's evaluate the constraints that the membrane underdo. First, as it is a Gramm positive bacteria, the external sugar envelopp impose the shape of the bacteria. On the other hand, the osmotic pressure is pushing the membrane against the sugar wall. Inside the membrane, the number of phospho-lipids is fixed by the state of division on the cell.

Though, a phospholipid is somehow behaving as a gaz trapped in a bottle. The ospotic pressure and the sugar layer are the bottle, and the number of particle trapped is giving the pressure. This variation of pression can be important if the cell has just devide or if the cell is about to devide.

When the cell enter of communication, a flow of molecule can pass from the cell that have the highest membrane pressure to the other one. To pass from one cell to another, phospholipids runs around the tube.

The tube is small and water has its intrinsic viscosity. If the pipe is moving, the water inside will follow, pumping the water from one cell to another. The sliding will look like a Poiseuil flow. Constituents diluted in water will move from one cell to another unidirectionally and faster than simply diffusing. This is a fast process that we have named after the "active" and "passive" diffusion, the "assisted diffusion"

When membranes behave like a 2D Van der Waals fluid

Trapped between the osmotic pressure and the cell wall, the phospholipids particle are moving in a double layered environement. Their motion is contricted into a two dimentional motion, following the shape of the membrane. At the scale of the phospholipid, the motion can be approximated to a motion in 2D. Using the formalism of the Statistical Physics, we can say that the phase space has 4 dimentions: two dimentions of impulsion and two dimentions of coordinate.

Can we descripe the interraction between two phospholipids in the membrane? Well, the question is complicated, but we will demonstrate that describing properly the pholpholipid, it is reasonable to fit a Lennard-Jones potential energy to this interraction. The enery of binding is smaller and the speed of sliding of one phospholipid against another is slower than the internal vibration of the chemical bounds and the internal conformers rotation of the CH2 tails. Though, in Intermolecular Forces by Israelacviili, the author shows that a phospholipid trapped in a bilayer can be aproximated by a section of cone. The section of cones is an individual, and the others are section of cone joining the previous one. The shape of the sections are giving the shape of the global structure.

Speaking in term of energy, once we have these cones, it is reasonable to fit a Lennard-Jones potential to this interraction. If the cones are interpenetrating, a sterical repulsion keeps the molecules appart, and the hydropatic inteccation that joins one lipid with the other act as an attractive force keeping the coherence of the mombrane. We will discuss later about the value of the two coefficients.

In the liquid physics, we use to call any system behaving with a Morse-like potential a Wan der Waals fluid, because once the statistical aspect is solved, we come back to the famous thermodynamical equation of Van der Waals. The terms in the developpement of the viriel is then often discussed.

Let's start the Maths

In stastical physics, in the canonic ensemble, the partition function for a classical particule is given by:

Where H(p,r) is the Hamiltonian of the system, given by:

where LJ(r1,r2) is the potential energy given by the Lennard Jones formulea. As usual in statiscical physics, we devide the total potential energy of the system into the pair-to-pair interraction between particles.

Once calculated, the partition function has the form of:

The next step is to calculate the free-energy associated with the partition function. Free energy is the value that links the statiscical physics to classical thermodynamics. The classical formulea is:

Which, once applicated to our system gives:

The pressure of the membrane, that correspond to the tension, is given by the formulea

... ...

And so on Oleg !!!