Crossing-over pathways

The interactions between the endocrine and neural systems of the GIT are complex, include shared signaling molecules, and can be hard to separate. Therefore, it is likely that the influence(s) of some phytochemicals will cross over between neural and non-neural pathways. Corresponding with this, lumenal administration of capsaicin interacts with visceral neurons to increase GIT motility (Zittel et al., 2001 Topcu et al, 2002) and abrogates the decreased electrolyte and fluid secretion caused by piperine (Capasso et al., 2002). 

The intrinsic and extrinsic pathways cross over at several points. Hageman factor, activated by the intrinsic pathway, can switch on proconvertin of the extrinsic pathway. Convertin can then feed back into the intrinsic pathway to help activated PTA activate Christmas factor. Thrombin itself can trigger both branches of the clotting cascade by activating antihemophilic factor, which is required to help activated Christmas factor in the conversion of Stuart factor to its active form, and also by activating proconvertin. ... 

The areas in the electron transport pathway where energy conservation is observed are termed coupling sites. One site, between plastoquinone and Cyt/, was originally identified in thylakoid preparations by the cross-over phenomenon [14] when ADP, for example, is added to illuminated chloroplasts when electron flow is severely limited by the phosphorylation reaction, all electron carriers which precede the coupling site will be oxidized, while all carriers which follow the coupling site will be reduced. 

Fignre 20.16 schematically illustrates these two pathways for loss of energy by radiation from an electronically excited molecnle, along with one nonradiative pathway. In the nonradiative pathway, the molecnle may cross over to the gronnd electronic state and gradually lose its energy as heat to the snrronndings as it cascades down to lower vibrational and rotational levels. 

Deactivation of the State in Solids. In frozen solutions (say, at 77 K) or in a dry solid at room temperature, Ti molecules are not quenched instead, they return to the So state by two slower pathways (1) phosphorescence emission, and (2) non-radiative intersystem crossing. In the latter pathway, excess electronic energy is converted to vibrational energy as the Tj molecule crosses over to some higher vibrational level (GV ) of the ground state. This is faster than the internal conversion pathway for Si molecules because much less electronic energy is involved. 

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