Transport Across Cell Membranes PDF

Transport across cell membrane Dr. Menna ELmenyawi  Cell membranes are made of PHOSPHOLIPIDs Cell membranes are composed primarily of lipids and proteins. The lipid component consists of phospholipids, cholesterol, and glycolipids Protein Function...

Transport across cell membrane Dr. Menna ELmenyawi  Cell membranes are made of PHOSPHOLIPIDs Cell membranes are composed primarily of lipids and proteins. The lipid component consists of phospholipids, cholesterol, and glycolipids Protein Functions  Movement of Substances across Cell Membranes passive transport (Diffusion ) : Simple diffusion Facilitated diffusion Osmosis Active transport Primary active Secondary active Exocytosis and Endocytosis  Passive vs. Active Diffusion (passive transport) It is the movement of substance molecules across membrane without expending energy. Active transport It is the movement of substances across cell membrane by expending energy.  Simple diffusion Kinetic movement of molecules or ions through lipid bilayer membrane spaces( intermolecular spaces) or through openings in membrane. Rate of diffusion depends on:  Simple diffusion through protein channels Factors affecting the rate of diffusion: 1.Concentration gradient Electrochemical (from high to low concentration) gradient Electric gradient 2.velocity of kinetic motion of molecules 3. Number and sizes of openings in cell membrane. Simple diffusion through lipid bi-layer  Diffusion through protein pores and channels The pores are integral cell membrane protein, they are always open pathways Their sizes and electrical charges inside provide selective permeability. The channels are highly selective to ions. They are open or closed by gates which are regulated by electrical signals (voltage gated channels) or chemicals binding (ligand gated channels) Simple diffusion through protein channels  Facilitated diffusion Diffusion of a substance with the help of a specific carrier protein. A mechanism of transport of relatively large organic molecules such as glucose and amino acids  Osmosis Osmosis is the net diffusion of water across membrane caused by a concentration difference of water (from a region of high water concentration to a region of low water concentration) Water concentration is inversely related to the number of solute particles (not related to the size of particles)  Osmosis Terms related to osmosis Osmotic Pressure It is the amount of pressure required to stop osmosis.  Osmosis Terms related to osmosis Osmotic Pressure The force exerted by the particles in solution. This pressure is related to number of particles rather than the mass. The greater the solute concentration of a solution, the greater its osmotic pressure. The greater the size of particles, the slower the motion they move So, pure water has an osmotic pressure of zero. 360-g/L glucose solution has twice the osmotic pressure of a 180-g/L glucose solution.  Osmosis Terms related to osmosis Osmolarity is the number of osmoles / liter of solution Osmolality is the number of osmoles / Kg of solution Normal osmolarity of body fluids: 290 mosm/L  Osmosis Terms related to osmosis  Osmosis Terms related to osmosis Isotonic solution: is having the same concentration of solutes as the extracellular fluid. Hypertonic: …higher concentration than… Hypotonic: … lower concentration than…  Movement of Substances across Cell Membranes Passive transport (Diffusion): Simple diffusion through lipid bi-layer Simple diffusion through protein channels Facilitated diffusion Osmosis Active transport Primary active Secondary active Exocytosis and Endocytosis  Active transport Active transport is the transport of a substance against its concentration gradient with the help of a specific carrier protein that utilizes energy Types of active transport: 1. Primary active transport. 2. Secondary active transport a. Co-transport (symport). b. Counter-transport (antiport). Primary active transport Sodium-Potassium Pump  Primary active transport The ion or molecule is transported against concentration gradient. The concentration of Na outside the cell membrane is higher than inside. The concentration of K outside the cell membrane is lower than inside This concentration gradient is the basis of nerve cell functions and other excitable cells like muscles.  To maintain this concentration gradient of both Na and K It needs active transport mechanism called Na K ATPase pump. This pump is formed of protein molecule has 2 binding sites, one for binding to 3 Na and other for binding 2 K. The binding of ions to their sites activates ATPase activity breaking phosphate bond in ATP into ADP and Phosphate.  The released energy causes conformational change in protein molecule to release Na outside and K inside. thus, a great concentration gradients of both ions develops across the membrane which is a storehouse of energy.  Secondary active transport This stored energy of concentration gradients created by primary active transport used to transport other molecules or solutes. 2 solutes bind to the same transporter protein, one solute (NA) is transported along its gradient and the other is transported against its gradient.  Secondary Active transport Co-transport (symport): transport of 2 solutes in the same direction Examples: Na+/glucose co- transport (luminal border of intestinal epithelium). Na+/amino acids Counter-transport (antiport): transport of 2 solutes in opposite directions. Examples: Na+/Ca+ counter- transport. Na+/H+ counter- transport. Types of Active Transport  Endocytosis and Exocytosis Is the process by which large molecules, particles, or fluid droplets can be transported across the cell membrane.  Endocytosis Types of endocytosis: 1. Phagocytosis (cell eating). example: phagocytosis of bacteria, damaged cells and tissue debris by phagocytes. 2. Pinocytosis (cell drinking). example: transport of proteins or fats as well as part of extracellular fluid).  Exocytosis Exocytosis is the process by which membrane- bound vesicles, in the cytoplasm, fuse with the plasma membrane and release their contents to the extracellular fluid. `