**The rate at which molecules diffuse across the cell membrane is directly proportional to the concentration gradient.** This applies to simple diffusion, which is governed by Fick's law.

When the concentration gradient is heavier outside the cell, substances diffuse into the cell where it is lower. Fick's law applies to small nonpolar molecules, which move at a diffusion using the equation (Cout - Cin)/Dx. Cout and Cin are the concentrations inside and outside the cell, and Dx is the thickness of the cell wall. When Cout is higher than Cin, the concentration gradient is positive, so the net volume of the substance moves across the cell wall and into the cell.

To calculate the rate of diffusion, the equation dn/dt = P x A X (dC/dx) is necessary. In this equation, A is the membrane area and P denotes the permeability constant. The constant P depends on the molecule's lipid solubility and size.

In facilitated diffusion, the concentration gradient process is more complex. Increasing the solute's volume once the concentration is high does not alter the rate of diffusion. This is because facilitated diffusion requires carrier proteins, and there is a maximum number (Vmax) present. The variable K influences how fast carrier proteins saturate. Molecule properties influence both K and Vmax.