Final answer:
Carrier proteins in cell membranes facilitate both active and passive transport of ions and molecules. In active transport, they use energy to move substances against their concentration gradient and include uniporters, symporters, and antiporters. They also enable facilitated diffusion, which does not require energy, by changing shape to transport molecules down their concentration gradient.
Step-by-step explanation:
Carrier proteins, which include pumps and transporters, are integral membrane proteins capable of facilitating the movement of ions, molecules, or groups of substances across the cell membrane. These proteins operate in two main types of transport: active transport and passive transport. Active transport requires energy, often in the form of ATP, to move substances against their concentration gradient.
There are three main types of carrier proteins involved in active transport:
- Uniporters: Transport a single ion or molecule across the membrane.
- Symporters: Carry two different ions or molecules in the same direction.
- Antiporters: Transport two different ions or molecules but in opposite directions.
Examples of carrier proteins functioning as pumps for active transport include Na+-K+ ATPase and H+-K+ ATPase, which are antiporters; as well as Ca2+ ATPase and H+ ATPase, which transport only calcium and only hydrogen ions, respectively. These transport systems are essential for the regulation of cellular processes, such as nerve impulse transmission and muscle contraction.
Carrier proteins are not limited to active transport; they are also involved in facilitated diffusion, which is a form of passive transport and does not require energy. Instead, molecules move down their concentration gradient with the help of carrier proteins that undergo conformational changes during the transport process.