Contrary to popular belief, not all cells require insulin in order to transport glucose
inside of a cell. Some organs and tissues are insulin-independent, meaning insulin is not required, whereas others are insulin-dependent, meaning they require insulin.  GLUT 4 transporters are located on skeletal muscle, adipose tissue, and the heart and do require insulin. In order for glucose to be transported into these cells, insulin must
bind to insulin receptors and signal for this process to occur. Insulin is produced by beta cells in the pancreas in response to high blood glucose levels. Once it is released, it binds to insulin receptors and triggers a response to allow
glucose to be transported into the cell. GLUT 1 Insulin independentBloodBlood-brain-barrierHeart (partially) GLUT 2Insulin-independentLiverPancreasSmall IntestineGLUT 3Insulin-independentBrain NeuronsSpermGLUT 4Insulin-dependentSkeletal MuscleAdipose tissue (fat)HeartDysfunctions in Glucose RegulationGlucose regulation can become dysfunctional at many different steps. If insulin is not produced sufficiently, glucose is unable to enter insulin-dependent cells and these cells can starve. This is known as type 1 diabetes mellitus. If insulin is produced sufficiently, but receptors are damaged or are insulin resistant, they can not signal to allow glucose transport, and again cells become starved
for energy. This is known as type 2 diabetes.