The process you are referring to is facilitated diffusion. No additional energy is required to transport glucose across a cell membrane. However, there must be a concentration gradient(difference) outside and inside the cell for transport to occur. If both sides of the membrane are equal, no net movementof glucose molecules will occur. If glucose is outside the membrane in a high concentration relative to inside the membrane, it will bind to a specific carrier molecule in the membrane. A change occurs in the membrane exposing the glucose to the inside of the cell. Once inside, it changes to glucose-6 phosphate. Because of this, the level of glucose inside the cell is close to zero. This maintains the concentration gradient being higher outside the cell compared to inside the cell. This insures that glucose will move into the cell from high concentration to low.