In this study, a new concept is introduced - gravitational cells. The body of a black hole consists of a huge number of such cells. This hypothesis from particle physics has been organically built into string theory. As a result of this addition, using the Schwarzschild radius formula and the Coulomb formula, a formula for the gravitational constant in the region of black holes was obtained on the basis of two fundamental constants: the speed of light and elementary charge. Its value has been determined, and the value of the usual gravitational constant has been confirmed. In this study, the mass of the gravitational cell of a black hole was obtained. The introduction of the hypothesis of gravitational cells into string theory made it possible to apply Planck's formula to gravitational interaction. As a result, a formula for the energy of a gravitational quantum and a formula for the vibrational velocity of a gravitational string were obtained. On this basis, the formula for the mass of the electron was obtained and its value was calculated, which coincided with the experimental mass of the electron. The exact minimum distance of the gravitational interaction was determined by the formula for the vibrational velocity of the gravitational string. This calculated minimum distance completely coincided with the known experimental data obtained when determining the Casimir effect (force).