Mie Theory Describes the Association of Light Through the Absorption (Cabs), Scattering (Csca), and Extinction (Cext) Cross-Sections With a Gold Nanoparticle (Aunp) Such Parameters Were Determined For Aunps Distributed In Homogeneous Media, But Not For Particular Tissues. the Goal of This Research Was to Obtain Theoretically, Through Mie Theory, the Optical Cross Sections (Cabs, Csca, and Cext) of Functionalized Aunps In Liver and Colon Tissues and Compare Them With the Temperature Increase Observed Experimentally In Tissues Containing Aunps Under Plasmonic Photothermal Irradiation Using a Laser Nd-Yag (Α = 532 Nm). Calculations Have Shown That Cabs Account For 98.96 ± 0.03 Percent of Cext at 532 Nm. the Cext Value For a Practical Aunp In Water Was 365,66 Nm2 (94 Percent of the Maximum Theoretical Value at 522,5 Nm), 404,24 Nm2 In Colon (98 Percent of the Maximum Theoretical Value at 525 Nm), and 442,39 Nm2 In Liver (96 Percent of the Maximum Theoretical Value at 525 Nm). Nanoparticles Irradiated at 532 Nm Therefore Come Very Close to Their Resonance Value. Such Findings Were Consistent With the Experimental Irradiation of Functionalized Aunps In Different Tissues, Where the Average Rise In Temperature Showed Liver Pattern > Colon > Water. the Observed Increase In Temperature (Available Up to 13∘C) Is Necessary to Produce Cell Demise.