In diabetic patients, what molecule is missing in the transport system

The answer to this question would be: insulin

Insulin is a hormone that produced by the pancreas. Insulin makes sugar go inside cells. Sugar is the food for cells. Without insulin, the sugar can't enter the cell and the cell food is depleted. This will cause the body feel that the sugar is not enough and trying to increase the sugar concentration. This will result in the increases of blood sugar level.