Carrier proteins are used to transport larger molecules, such as glucose, across the membrane. Due to the larger size of these molecules, it is not possible to use simple pore or channel proteins, so carrier proteins must be used. These transport proteins can be active or passive, allowing transport with or against concentration gradients. In the case of glucose, the GLUT1 transporter is a carrier protein used to passively absorb glucose down a concentration gradient. This occurs through the binding of a proton, increasing its affinity for glucose, before a glucose then binds to the high affinity binding site. A conformational change then transfers, or carries, the glucose across the membrane and the glucose is released. This release is due to the binding site changing affinity, allowing the glucose to ‘fall’ out to the cytoplasm. The proton is then released and the original conformation is adopted, allowing the transport of another glucose molecule. This allows for a much increased rate of transport than would be naturally possible over the plasma membrane, transporting some $$50;000$$ glucose molecules per second. Glucose is too large to effectively transport across the membrane, and so for a cell to uptake adequate levels of glucose as required for glycolysis and the Krebs cycle, it is imperative a transport protein is used.