Monday, September 21, 2009

Not the ghost in the machine, but the machine in the membrane

Here are a few highlights from Chapter 11 of Nelson's Biological Physics. It is entitled, "Machines in Membranes."

Indirect physical arguments led to the hypothesis of the existence of the active ion pumps, long before the biochemical identity of these amazing molecular machines was known.
This is analogous to how the existence of a molecular carrier of genetic information (DNA) was proposed to exist long before its chemical or physical structure was known.

Biological question:
The internal and external chemical composition of cells is very different. Why doesn't osmotic flow burst (or shrink) the cell?

Physical idea:
This nonequilibrium, osmotically regulated state can be maintained by active ion pumps located in the cell walls.

The invention of batteries (i.e., voltaic cells) was stimulated by Volta's skepticism towards Galvani's claim that muscles could be a source of electricity.

The Nernst potential is the voltage that must be applied across a membrane to maintain a concentration gradient of a particular ion species across the membrane.
All animal cells have a sodium anomaly, i.e., the Nernst potential for sodium is much more positive than the actual membrane potential, i.e., the ion concentrations deviate significantly from equilibrium values.

This non-equilibrium is maintained by an ion current across the membrane which is proportional to the conductance per unit area of the membrane.

Ion pumps are embedded in cell membranes and hydrolyze ATP to obtain the free energy they use to pump sodium ions out of the cell.

Consider the industrial factory below. It generates, distributes, and utilizes energy.

Mitochondrion are like factories, systems of coupled machines. See the figure below.
They act as bus bars to generate, distribute, and utilize energy in a cell. The chemiosmotic mechanism proposes that ATP synthesis is indirectly coupled to respiration:

NADH + H+ 1/2 O2 -> NAD+ + H2O

Note that proton transfer plays a key role here.

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