In diabetes mellitus, your body has trouble moving glucose, which is a type of sugar, from your blood into your cells.
This leads to high levels of glucose in your blood and not enough of it in your cells, and remember that your cells need glucose as a source of energy, so not letting the glucose enter means that the cells starve for energy despite having glucose right on their doorstep.
In general, the body controls how much glucose is in the blood relative to how much gets into the cells with two hormones: insulin and glucagon.
Insulin is used to reduce blood glucose levels, and glucagon is used to increase blood glucose levels.
Both of these hormones are produced by clusters of cells in the pancreas called islets of Langerhans.
Insulin is secreted by beta cells in the center of the islets, and glucagon is secreted by alpha cells in the periphery of the islets.
Insulin reduces the amount of glucose in the blood by binding to insulin receptors embedded in the cell membrane of various insulin-responsive tissues like muscle cells and adipose tissue.
When activated, the insulin receptors cause vesicles containing glucose transporters that are inside the cell to fuse with the cell membrane, allowing glucose to be transported into the cell.
Glucagon does exactly the opposite, it raises the blood glucose levels by getting the liver to generate new molecules of glucose from other molecules and also break down glycogen into glucose so that it can all get dumped into the blood.
Diabetes mellitus is diagnosed when the blood glucose levels get too high, and this is seen among 10% of the world population.