The SA node exhibits automaticity; it can spontaneously generate action potentials without neural input. 

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#Unit 6. System Physiology – Plant

Fo complex, with its proton pore, is composed of three subunits, a, b, and c, in the proportion ab₂ C_n, where n ranges from 8 to 15 in various organisms; in yeast, it is 10.

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#Unit 6. System Physiology – Plant

Subunit c is a small (Mr 8,000), very hydrophobic polypeptide, consisting almost entirely of two transmembrane helices, with a small loop extending from the matrix side of the membrane

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#Unit 6. System Physiology – Plant

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#Unit 6. System Physiology – Plant

Each c subunit has a critical Asp (or Glu) residue about midway across the membrane, which can donate or accept a proton.

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#Unit 6. System Physiology – Plant

The a subunit has a positively charged Arg side chain that forms an electrostatic interaction with the negatively charged carboxylate of Asp on the adjoining c subunit

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#Unit 6. System Physiology – Plant

Arg residue swings aside, forming an interaction with the Asp on the adjacent c subunit in the ring and displacing the proton from that Asp; this adjacent proton exits through the second half- channel to the N side, completing the movement of one proton equivalent from outside to inside the matrix.