What is the main reason the interior of the cell is electronegative at rest?

The reason why the interior of the cell is electronegative at rest is that the membrane is more permeable to K+ than to other ions. The difference in the concentration of charges in the inner and outside of the cells is what brings about the term membrane potential.

At rest, cells are more electronegative because of there more negative ions in the inner side of the cells than the outside. This causes an increase in the concentration of potassium ions in the inner side of the cell. As a result, the cell strives to maintain the balance in the concentration of the ions by ensuring that both the inner and outer sides of the cell have an equal number of ions.

This process is known as depolarization. The process of depolarization causes the membrane to be permeable to potassium ions so that they can diffuse out of the cell. The above is just an explanation of why the cell is electronegative at rest.

The membrane is going to be more permeable to K+ as compared to other ions. This means that the inside cell will become more electronegative so that the electrical gradient will also increase little by little. When you say that a cell is electronegative, this means that the atoms will be able to attract other electrons so that they can form chemical bonds.

The cell membrane on the other hand is something that cells have. It will contain different layers that are all meant to help the cell function normally. The lipid bilayer of the cell membrane will be in charge of making the inside of the cell become more electronegative.