In cardiac muscle, the action potential is caused by opening of three types of channels

Voltage activated fast sodium channels 

L-type calcium channels (slow calcium channels or calcium-sodium channels)

Voltage activated potassium ion channels

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#Unit 6. System Physiology – Plant

PAS-related domain (PRD) repeats mediate phytochrome dimerization. 

The PRD domain targets the Pfr form of phyB to the nucleus, although it lacks a canonical nuclear localization signal (NLS).

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#Unit 6. System Physiology – Plant

In higher plants the phytochrome chromophore is a linear tetrapyrrole called phytochromobilin

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#Unit 6. System Physiology – Plant

Pr chromophore undergoes a cis–trans isomerization between carbons 15 and 16 and rotation of the C14-C15 single bond on red light exposure.

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#Unit 6. System Physiology – Plant

Phytochromobilin is synthesized in plastids from chlorophyll biosynthetic pathway and transported into cytosol by a passive process.

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#Unit 6. System Physiology – Plant

Phytochromobilin is exported from the plastid to the cytosol where it autocatalytically attaches to the PHY apoprotein through a thioether linkage to a cysteine residue.

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#Unit 6. System Physiology – Plant

hy1 and hy2 mutant are chromophore biosynthesis mutant: which fail to supress hypocotyl elongation