Transport Across Biomembranes Lecture Notes PDF

3 Transport across the cell membrane ILOs By the end of this lecture, students will be able to 1. Demonstrate the different transport mechanisms across the cell membrane 2. Compare movement through channels to movement by simple or facilitated diffusion 3. Differentiate between...

3 Transport across the cell membrane ILOs By the end of this lecture, students will be able to 1. Demonstrate the different transport mechanisms across the cell membrane 2. Compare movement through channels to movement by simple or facilitated diffusion 3. Differentiate between primary and secondary active transport Substances generally move across cellular membranes via transport processes that can be classified as passive or active, depending on whether they require cellular energy. In passive process, a substance moves across the cell membrane along any type of gradient without expenditure of energy. On the other hand, in active transport, the transport of substances takes place against any type of gradient by the utilization of energy. Passive transport The transport of the substances along the concentration or electrochemical gradient or both is called passive transport or diffusion. The substances are transported from region of higher concentration to a lower concentration. This type of transport doesn’t require energy but it may needs the help of carrier proteins of the cell membrane. 1)-Simple Diffusion Simple diffusion is a passive process in which substances move freely through the lipid bilayer of the plasma membranes of cells without the help of membrane transport proteins. The lipid layer is only permeable to lipid- soluble substances such as oxygen, carbon dioxide, and nitrogen gases; fatty acids; steroids; and fat-soluble vitamins (A, D, E, and K). Small, uncharged polar molecules such as water, urea, and small alcohols also pass through the lipid bilayer by simple diffusion. 2)-Facilitated Diffusion a)-Channel-mediated facilitated diffusion: A solute moves down its concentration gradient across the lipid bilayer through a membrane channel. Most membrane channels are ion channels, integral transmembrane proteins that allow passage of small, inorganic ions. Water soluble substances can diffuse through protein channels. The characteristic feature of these protein channels is the selective permeability. That is, each channel can permit only one type of ion to pass through it such as sodium channels, potassium channels, etc… Diffusion of ions through channels is generally slower than free diffusion through the lipid bilayer because channels occupy a smaller fraction of the membrane’s total surface area than lipids. Some of protein channels are continuously opened (leak channels) and most of channels are always closed (gated channels). The gated channels are opened only when required and it is of 2 types: Page 1 of 4  Voltage gated channels: these channels open when there is a change in the electrical potential. Ligand gated channels: are opened by binding to a ligand as some hormonal substances. b)-Carrier-mediated facilitated diffusion: The water soluble substances having larger molecules cannot diffuse through the protein channels. They diffuse through cell membrane with the help of carrier proteins (transporters). These transporters are proteins that are part of the cell membrane. In carrier-mediated facilitated diffusion, a carrier moves a solute down its concentration gradient across the plasma membrane. The solute binds to a specific carrier on one side of the membrane and is released on the other side. Binding of the solute with the carrier leads to changes in the shape of the carrier. This physical change propels the solute into the interior of the cell. Substances that move across the plasma membrane by carrier mediated facilitated diffusion include glucose, fructose, galactose, amino acids and some vitamins. Figure 1: The difference between simple and facilitated diffusion. 3)-Osmosis Osmosis is a type of diffusion in which there is net movement of water or any other solvent through a semipermeable membrane. Osmosis occurs only when a membrane is permeable to water but is not permeable to certain solutes. Like the other types of diffusion, osmosis is a passive process. In living systems, the solvent is water, which moves by osmosis across plasma membranes from an area of higher water concentration to an area of lower water concentration. Another way to understand this idea is to consider the solute concentration: In osmosis, water moves through a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration. During osmosis, water molecules pass through a plasma membrane in two ways: (1) by moving between neighboring phospholipid molecules in the lipid bilayer via simple Page 2 of 4 diffusion, and (2) by moving through aquaporins, integral membrane proteins that function as water channels. Active transport It is the movement of substances against the chemical or electrical or electrochemical gradient. Active transport requires energy and a carrier protein. Active transport is considered an active process because energy is required for carrier proteins to move solutes across the membrane against a concentration gradient. Two sources of cellular energy can be used to drive active transport: (1) Energy obtained from hydrolysis of adenosine triphosphate (ATP) is the source in primary active transport; (2) energy stored in an ionic concentration gradient is the source in secondary active transport. Figure 2: The difference between passive and active transport 1)-Primary Active Transport This is the type of active transport in which the energy is liberated directly from hydrolysis of ATP. The carrier protein has ATPase activity which causes breakdown of ATP. The energy derived from hydrolysis of ATP changes the shape of a carrier protein, which “pumps” a substance across a plasma membrane against its concentration gradient. Indeed, carrier proteins that mediate primary active transport are often called pumps. The most prevalent primary active transport mechanism expels sodium ions (Na") from cells and brings potassium ions (K") in. Because of the specific ions it moves, Page 3 of 4 this carrier is called the sodium–potassium pump. Because a part of the sodium-potassium pump acts as an ATPase, an enzyme that hydrolyzes ATP, another name for this pump is Na!–K! ATPase. All cells have thousands of sodium–potassium pumps in their plasma membranes. 2)-Secondary Active Transport This is the type of active transport in which the substance moves against its electrochemical gradient. However the energy supplied for this transport doesn’t come directly from ATP, but it comes from the movement of another substance along its electrochemical gradient. When Na+ is transported by a carrier protein passively, another substance is also transported by the same protein simultaneously, either in the same direction of Na+ movement (co-transport) or in the opposite direction (counter- transport). The energy stored in a Na+ concentration gradient is used to drive other substances across the membrane against their own concentration gradients such as glucose and amino acids. A carrier protein (transporter) simultaneously binds to Na" and another substance and then changes its shape so that both substances cross the membrane at the same time. Types of carrier: Symporters: the carriers move two substances in the same direction at the same time as Na+ and glucose. Antiporters: the carriers move two substances in opposite directions across the membrane at the same time as Na+ and H+ in renal epithelial cells. Figure 3: Types of active transport Page 4 of 4