Final answer:
Glucose requires transport proteins to cross cell membranes, with the GLUT protein enabling this process through facilitated diffusion. Once inside, hexokinase phosphorylates glucose into glucose-6-phosphate, which is trapped inside the cell for metabolic processes.
Step-by-step explanation:
Glucose cannot cross a cell membrane into a cell in the absence of transport proteins. Specifically, glucose transport proteins like the GLUT protein (glucose transporter) facilitate the transport of glucose into cells via a process known as facilitated diffusion. These transporters allow glucose to move down its concentration gradient and into the cytoplasm. For instance, in kidney cells and small intestine cells, specific glucose transporters known as SGLT (Sodium-Glucose Transporter) work by symporting glucose alongside sodium ions. Once inside the cell, glucose is often immediately phosphorylated by the enzyme hexokinase, which adds a phosphate group to the glucose molecule using ATP, producing glucose-6-phosphate. This phosphorylated glucose is then unable to leave the cell because it cannot cross the lipid bilayer of the cell membrane through any transport proteins. Hexokinase is crucial in this process as the phosphorylation traps the glucose within the cell, making it available for cellular processes such as glycolysis.