[Iamyourfather]

This is the last question on my assignment (slit into 4 sub-sections), if you can, can you quickly review it? I'd like to know what you think of it, if you have any suggests please feel free to comment

Context: http://www.flickr.com/photos/8518928...ream/lightbox/

a) Explain how the model violates the second law of thermodynamics [Hint: Consider
what would happen to the entropy of the system if one started with equal concentrations of dye in the astrocyte and neurone connected by the “fish trap” type of gap junctions] (5%)

The neuron and the astrocyte cells are the system. The second law of thermodynamics states that the entropy of closed systems never decreases, because closed systems spontaneously evolve towards thermal equilibrium -- the state of maximum entropy.
The implementation of a gap junction (the ‘fish trap’) between the two cells allow dye molecules to move in a unidirectional manner. This gap junction therefore allows the molecules to increase in concentration. An increased concentration within a closed system will decrease entropy. This is because molecules pack closely together and have less space to move.
This theory can be expressed into an equation:
S = S° - Rln(c)
S is the entropy of the solution, S° is the standard entropy at 1M concentration (or 1 atm partial pressure), and c is the actual concentration (or actual partial pressure, for a gas). If the concentration increases the entropy decreases.


b) Explain why this model cannot work for small molecules, although it may allow one to catch fish. (5%)

Small molecules are more likely to pass through their ‘fish trap’ bi-directionally because they are significantly smaller in size - A ‘fish trap’ model is where a gap junction (i.e. a gap somewhat like a fish trap) separates a system. A ‘fish trap’ model is comprised of a two pores located on opposite regions of the ‘trap’, where one pore is smaller than the other. The larger pore is the opening in which many moving objects can pass. The smaller pore will only allow one or a few objects (depends on the size of the diameter) to pass.

Small molecules can pass through the larger pore very easily; however it may take a longer amount of time for small molecules to pass through the smaller pore (bi-directionally) if they accumulated themselves by it. A/a few fish may by be able to pass through the trap in a bi-directional manner however due to fish being large in size, many fish will struggle to pass through the smaller pore at once. It therefore will take a significantly longer time for all fish in a system to move bi-directionally than all small molecules.


c) Explain why a fish trap does work for fish. (5%)

Kinetic energy from a fish will allow it to move. You could say the fish has high entropy as it moves in arbitrary directions until it comes into contact with a fish trap. The design of a ‘fish-trap’ model is mentioned in b). The larger pore will allow a supple amount of fish to pass through the pore at one time. This is because its diameter will be greater than the size several fish. The size of the larger pore will also increase the chance of fish coming into contact with the trap and moving through it. The smaller pore is built so that one or a small number fish at a time can pass through it. The accumulation of trapped fish on the other side of the system will prevent the fish from escaping because a high concentration of fish cannot pass through the smaller pore at one time. Moreover, as fish move in arbitrary directions the chances they will leave the other side of the system through a small pore are small.

d) Provide a plausible mechanism for the unidirectional transport of dye molecules between the cells that does not violate the second law of thermodynamics. (5%)

Molecules move through cells through protein ion channels between each cell. Rather than a ‘fish trap’ gap junction, the gap junction will have pores that are parallel in size. In order to increase entropy dye molecules will be injected into the astrocyte and will diffuse through the neurone from an area of high free energy (point of injection) to low free energy (end of neurone) unidirectionally. A chemical signal is propagated without decrement. This therefore does not violate the second law of thermodynamics, because the energy for the conduction comes from within (not from the stimulus). As the experiment undergoes exocytosis, the system will eventually reach stability. The equilibrium state of a system (i.e., the most stable accessible state) is the one that maximises the entropy of the system (subject to any constraints placed on the system).