The “Examples from the Internet” do, in fact, come from the Internet. We are able to identify trustworthy translations with the aid of automated processes. The main sources we used are professionally translated company, and academic, websites. In addition, we have included websites of international organizations such as the European Union. Because of the overwhelming data volume, it has not been possible to carry out a manual editorial check on all of these documents. So, we logically cannot guarantee the quality of each and every translation. This is why they are marked “not verified by PONS editors”.
There are two main classes of transport proteins that are found within the plasma membrane. Channel proteins aid in passive transport, a process called facilitated diffusion. During this process, they serve as a tunnel for certain ions and small molecules. Examples of channel proteins include chloride, sodium, calcium, and potassium ion channels. Carrier proteins are used in both passive and active transport and change shape as they move their particular molecule across the membrane. Examples of carrier proteins within our cells include the sodium potassium pump and glucose transporters.
Cell membranes are impermeable to charged and polar molecules, meaning that these molecules cannot cross them spontaneously. Some transport proteins are intrinsic to cell membranes and facilitate the transport of polar molecules across the membranes. Each cell of the human body needs glucose , a very polar molecule, and human beings have five different glucose transport proteins (known as GLUT1 through GLUT5) that all serve a similar function: They carry glucose molecules across membranes and into cells. Without these transport proteins, the rate of glucose entry into cells would be very low indeed. Other membrane-linked transport proteins carry other molecules across membranes, including amino acids, ions, and vitamins .