Series Resistance Equation

$$\text{Total Resistance} = R_T = \sum^{n}_{i=1}\left({R_i}\right)$$

Resistors in Series = Cytoplasm / Cytosol

Info

Total Resistance = Sum of All Resistors
Voltage = Sum of all the individual voltage drops
$Current\ (I) = \frac{Voltage_{total}}{Resistance_{total}}$
https://www.allaboutcircuits.com/textbook/direct-current/chpt-5/simple-series-circuits/

Why does voltage change with resistors in series but not in parallel?

Due to Kirchhoff's Law , the total voltage in a series circuit is the sum of each voltage drop
When in parallel , there are multiple branches the current can take , and electric fields are conservative

Total Resistors