Cell Membrane Transport PDF

Cell membrane transport Cystic fibrosis disease  It is a genetic disorder.  Caused by mutation in a gene encoding a chloride channel (Cl- channel).  The malfunctioning chloride channels disrupt the transport of chloride ions across cell membranes.  This leads to...

Cell membrane transport Cystic fibrosis disease  It is a genetic disorder.  Caused by mutation in a gene encoding a chloride channel (Cl- channel).  The malfunctioning chloride channels disrupt the transport of chloride ions across cell membranes.  This leads to thick, sticky mucus buildup in various organs such as lungs and the digestive system). The transport of inorganic ions and small organic molecules across the plasma membrane  Two main types of proteins are involved in the transport of inorganic ions and small organic molecules across the plasma membrane: channels and transporters.  Channels (water and ion channels)  Most channels are ion channels.  Channels form tiny hydrophilic pores and allow substances to pass by diffusion.  Transporters (active and passive)  Transporters transfer inorganic ions or small organic molecules.  The conditions under which channels and transporters perform their functions  Na+, K+, Ca2+, Cl-, and H+ (protons) are among the most important inorganic ions for cells.  The ion concentrations inside a cell are very different from those outside. Na+, Cl- and Ca2+ are most abundant outside of the cell (10 times higher for Na+, 14 times higher for Cl- and 12 times higher for Ca2+). K+ is most abundant inside the cell (25 times higher inside). The conditions under which channels and transporters perform their functions Electrical charges inside and outside the cell are generally balanced (Na+ by Cl- and K+ by negatively charged organic molecules). However, small charge differences exist on both sides of the plasma membrane (more negative charges on the inner side of the membrane). This difference in charge is known as the membrane potential (measured in mV).  The movement of inorganic ions is determined by their electrochemical gradient Molecules spread from areas of high concentration to areas of low concentration. This type of movement is called diffusion. Diffusion of inorganic ions or charged molecules is determined not only by their concentration but also by charge differences between the two area (here, outside/inside of the cell). Therefore, the diffusion of an inorganic ion or charged molecule depends on both the concentration gradient and the electrical gradient (the electrochemical gradient).  Ion channels allow the diffusion of inorganic ions along their electrochemical gradient The movement of inorganic ions along their electrochemical gradient does not require energy. This is called passive transport. Ion channels perform a passive transport and facilitate diffusion.  Ion channels are highly selective Most channels in the cell are ion channels. Most channels are narrow and highly selective. Ion channels facilitate the passage of select inorganic ions. Two characteristics distinguish Ion channels from pores :  Ion selectivity  Open/closure state Ion channels are not continuously open  A typical ion channel fluctuates between closed and open conformations  Most ion channels are gated  A specific stimulus triggers gated-ion channels to switch between a closed and an open state by a change in their conformation.  Voltage-gated ion channels are controlled by changes in the voltage across the membrane.  Mechanically-gated ion channels are controlled by a physical stimuli (light, sound waves, pressure, stretch, touch and vibration).  Ligand-gated ion channels are controlled by the binding of a molecule.  Water channels facilitate osmosis Osmosis is the diffusion of water across a membrane from an area of lower solute concentration to an area of higher solute concentration. Water molecules diffuse rapidly through aquaporin channels in the plasma membrane of cells in the nephron tissue.  Transporters are also involved in the transport of inorganic ions and small molecules Transporters are different from ion channels in several ways:  Transporters change conformation to mediate transport across the membrane.  Transporters can move one substance (uniporters) or two substances (symporters and antiporters) at a time.  Some transporters can use energy to move inorganic ions and small molecules against their electrochemical gradient.  Some transporters move inorganic ions and small molecules against their electrochemical gradient There are two types of transporters: passive and active. Transporters that transport molecules along their electrochemical gradient with no energy requirement are called passive transporters. Transporters that transport molecules against their electrochemical gradient and require energy are called active transporters or pumps. Three different sources of energy are used in active transport Three types of energy sources are used in active transport:  Light  ATP  Electrochemical Example of light- driven transporter: bacteriorhodopsi n Bacteriorhodopsin is found in certain bacteria. It uses light to pump protons from the inside of the cell to the outside. A molecule called retinal absorbs light and triggers a conformational change in the transporter.  Example of ATP-driven transporter: Ca2+ transporters The activity of Ca2+ transporter in the endoplasmic reticulum membrane keeps cytosolic Ca2+ concentration low.  An influx of Ca2+ into the cell serves as an intracellular signal. Ca2+ triggers cell processes, such as muscle contraction, fertilization and nerve cell communication. Example of ATP-driven transporter: Na+-K+ pump ATP-driven Na+ transporters use energy supplied by ATP to expel Na+ and bring in K+.  Example of gradient-driven transporter: glucose-Na+ transporters Glucose–Na+ transporters use the electrochemical Na+ gradient to drive the active import of glucose.  Active transporters cooperate to move inorganic ions and molecules across the membrane The transporter that uses ATP and creates the electrochemical gradient which is used by another transporter is called the primary active transporter (ex: Na+-K+ pump). The transporter that uses the electrochemical gradient created by another transporter is called the secondary active transporter (ex: Na+-glucose  Active and passive transporters are involved in glucose transfer across the epithelial lining of the gut  Membranes without channels and transporters are impermeable to most molecules Lipid bilayers are impermeable to most water-soluble molecules. The molecule’s size and solubility determine the diffusion rate.  Summary of how substances cross the plasma membrane There are four different ways that a substance can move across the plasma membrane:  through simple diffusion  a channel  a passive transporter  an active transporter Each cell membrane has its own characteristic set of transporters  Large molecules enter and leave the cell by endocytosis and exocytosis Proteins, polysaccharides, and nucleic acids are too large and too charged or polar to pass through biological membranes. They take the pathways of endocytosis and exocytosis.  Insulin is secreted form pancreatic β cells by exocytosis. There are three types of endocytosis: receptor-mediated endocytosis, pinocytosis and phagocytosis.  Receptor-mediated endocytosis is responsible for LDL uptake LDL LDL (low density lipoprotein) transports cholesterol in the  LDL receptors on the cell blood. surface bind LDL particles in the blood. Cholesterol is extracted from the LDL in the lysosome after Smaller cells and large particles are taken up by phagocytosis Red blood cells macrophage Learning outcomes  Recall how the concentration of ions in the cell differs from that outside the cell and define membrane potential.  Predict rates of diffusion of substances under different conditions.  Describe the ways in which water can move across the cell membrane.  Compare and contrast osmosis, simple diffusion, facilitated diffusion, active and passive transport.  Compare and contrast pores, channels, active and passive transporters in terms of structure, function and energy requirements. Provide examples of each category.  Differentiate between primary and secondary active transport. Differentiate between a symport, an antiport, and a uniport.  Compare the uniport glucose transporter and the glucose-sodium symport in terms of their activities and roles in glucose transport in intestinal epithelial cells.  Describe the properties that govern the rate at which a given solute can cross a protein-free lipid bilayer.  Explain how large particles are transported into and out of the cell. Contrast different types of endocytosis. Provide examples of particles taking these pathways.   Keywords you need to know before studying this lecture  Anion/Cation  Hydrophilic  Ion  Inorganic ion  Organic molecule  Polar/non polar  Protein conformational change  Solute/solven/solution/Concentration  Water -soluble molecules  Macromolecules