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, using the formula for the Schwarzschild radius and the Coulomb formula, a formula was obtained to determine the gravitational constant in the region of black holes and its value was determined. The value of the usual gravitational constant has been confirmed. Also, a new physical constant was obtained - the mass of the gravitational cell of a black hole. The introduction of the hypothesis of gravitational cells into string theory allowed us to apply Planck's formula to gravitational interaction. As a result, the formula for the quantum of the gravitational field was obtained and the frequency of vibrations of gravitational strings was calculated. Based on this, a formula was obtained to determine the mass of an electron. The electron mass calculated by the new formula coincided with the known experimental value. In this work, it was also proved that the vibration frequency of gravitational strings is directly proportional to the ratio of the mass of an electron and a proton inside the gravitational cell (and inside the atom). The formula for the dependence of the gravitational constant on the magnitude of the electron mass was obtained and confirming calculations were made.