Transport Proteins (Pumps, Carriers, and Channels) Lecture Notes PDF

TRANSPORT PROTEINS (PUMPS, CARRIERS, AND CHANNELS) Transport proteins play a crucial role in regulating the movement of molecules across the cell membrane, maintaining homeostasis. These proteins can be classified into three main categories: pumps, carriers, and channels. Each type of protein has s...

TRANSPORT PROTEINS (PUMPS, CARRIERS, AND CHANNELS) Transport proteins play a crucial role in regulating the movement of molecules across the cell membrane, maintaining homeostasis. These proteins can be classified into three main categories: pumps, carriers, and channels. Each type of protein has specific mechanisms for facilitating the transport of ions, nutrients, and other molecules. 1. Pumps (Active Transport) Pumps are membrane proteins that use energy to move molecules or ions against their concentration gradient (from low to high concentration). This process requires ATP. a) Primary Active Transport Directly uses ATP hydrolysis to drive the transport. Na+/K+ ATPase is the most well-known example. It pumps 3 Na+ out of the cell and 2 K+ into the cell for every ATP hydrolyzed. b) Secondary Active Transport Does not directly use ATP. Instead, it relies on an electrochemical gradient created by primary active transport. Symporters and antiporters fall under this category: 2. Carriers (Facilitated Diffusion and Active Transport) Carrier proteins facilitate the movement of molecules across the membrane by undergoing conformational changes. They may mediate passive transport or secondary active transport. a) Facilitated Diffusion Carriers assist in moving molecules down their concentration gradient, which does not require energy. Example: GLUT transporters move glucose into cells depending on the concentration of glucose inside and outside the cell. b) Mechanism of Action: A specific molecule binds to the carrier protein on one side of the membrane. The carrier undergoes a conformational change that allows the molecule to pass to the other side. The transporter then returns to its original shape to bind more molecules. Types: Uniporters: Transport a single type of molecule (e.g., GLUT-1 for glucose). Symporters: Transport two molecules in the same direction. Antiporters: Transport two molecules in opposite directions. 3. Channels (Passive Transport) Channel proteins create hydrophilic pores in the membrane that allow specific ions or water molecules to move across the membrane by passive transport, following their electrochemical gradient. a) Ion Channels Highly specific for particular ions, such as Na+, K+, Ca2+, Cl-. Ion movement through channels is much faster than via carriers. Some ion channels are always open and are referred to as leak channels or non-gated channels. These channels allow ions to flow passively across the cell membrane at all times, driven by their concentration or electrochemical gradients. Channels can be gated, opening or closing in response to stimuli: o Voltage-gated channels: Respond to changes in membrane potential. o Ligand-gated channels: Open when a specific molecule (ligand) bind o Mechanically gated channels: Respond to mechanical forces (stretch or pressure) on the membrane. b) Aquaporins These are channels specific for water transport, allowing rapid movement of water across cell membranes. Aquaporins play a key role in regulating osmotic balance in cells.