Transport Across Cell Membrane (BUC Lecture) PDF

Transport across cell membrane Dr. Sahar Greish  Learning Objectives By the end of this lecture, you should be able to: 1. Differentiate between passive and active transport. 2. Mention types of passive transport and give an example for each type. 3. List the mechanisms of wa...

Transport across cell membrane Dr. Sahar Greish  Learning Objectives By the end of this lecture, you should be able to: 1. Differentiate between passive and active transport. 2. Mention types of passive transport and give an example for each type. 3. List the mechanisms of water transport across membranes. 4. Mention types of active transport and give an example for each type. 5. Discuss transport of big molecules across membranes (.i.e. bulk transport)  Transport of molecules Simple diffusion. 1-Passive Simple diffusion of water transport molecules (.i.e. osmosis). Facilitated diffusion. 2-Active Primary transport Secondary 3-Bulk Endocytosis transport Exocytosis  Passive vs. Active Passive transport  is the net movement of molecules and ions across a membrane from higher to lower concentration (down a concentration gradient);  it does not require metabolic energy. Active transport  is net movement across a membrane that occurs against a concentration gradient (to the region of higher concentration).  Active transport requires the expenditure of metabolic energy (ATP) and involves specific carrier proteins.  1-Passive transport (Simple diffusion)  It is the transport of a substance from high concentration to low concentration (i.e. downhill).  It is a passive process (.i.e. no energy cost).  The rate of diffusion of a substance depends on : 1. Concentration gradient of the substance across the membrane. 2. Electrical potential difference: This factor particularly affects the rate of diffusion of ions. 3. Surface area of the membrane. 4. Permeability of the membrane: Increased permeability increase diffusion rate Types of simple diffusion. (a) simple diffusion through the double phospholipid layers of the plasma membrane (Nonpolar molecules) (b) simple diffusion through protein channels (polar molecules & inorganic ions). Simple diffusion through lipid bi-layer Simple diffusion through protein channels  Facilitated diffusion Diffusion of a substance with the help of a carrier protein. A mechanism of transport of relatively large organic molecules such as glucose and amino acids  Osmosis  Osmosisis the net diffusion of water from a region of high water concentration to a region of low water concentration through a membrane which is permeable to water but not to the solute.  Osmosis Terms related to osmosis Osmotic Pressure  Osmosis Terms related to osmosis Osmotic Pressure  The force that would have to be exerted to prevent osmosis.  The greater the solute concentration of a solution, the greater its osmotic pressure.  Pure water thus 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  Normal osmolarity of body fluids: 290 mosm/L  Isotonic solution: is having the same concentration of solutes as the extracellular fluid.  Hypertonic: …higher concentration than…  Hypotonic: … lower concentration than…  2-Active transport it occurs against gradient (from low to high concentration). It needs a carrier. It needs energy (ATP). It has two types; Primary and secondary A-Primary active transport Energy is provided directly by ATP. Phosphorylation of the carrier induce conformational change. The carrier then allows transport of a molecule from low to high concentration. Dephosphorylation of the carrier restore its original conformation. This carrier protein transports Ca2+ from a lower concentration inside the cell to a higher concentration outside of the cell, and is thus known as a Ca2+ pump. (1) Ca2+ within the cell binds to sites in the carrier protein. (2) ATP is hydrolyzed into ADP and phosphate (P i ), and the phosphate is added to the carrier protein; this phosphorylation causes a hinge like motion of the carrier. (3) The hinge like motion of the carrier protein allows Ca2+ to be released into the extracellular fluid. B-Secondary active transport The molecule is transported against concentration gradient but the transporter does not hydrolyze ATP directly. Energy is provided by concentration gradient of Na⁺ produced by Na⁺-K⁺ pump. Secondary active transport a. Co-transport (symport). b. Counter-transport (antiport). Co-transport (symport): The cotransport of Na+ and glucose. This carrier protein transports Na+ and glucose at the same time, moving them from the lumen of the intestine and kidney tubules into the lining epithelial cells. This cotransport requires a lower intracellular concentration of Na+, which is dependent on the action of other carriers, the Na+/K+ (ATPase) pumps. Because ATP is needed to power the Na+/K+ (ATPase) pumps, the cotransport of Na+ and glucose depends indirectly on ATP, and so can be considered secondary active transport. Counter-transport (antiport): transport of 2 solutes in opposite directions. Examples: Na+/Ca+ counter-transport. Na+/H+ counter-transport.  Bulk Transport Polypeptides and proteins are too large to be transported through a membrane by carriers. Endocytosis:It is the process by which molecules are taken into the cell.e.g. removal of lipoprotein molecules from blood stream by liver cells. Exocytosis : It is the process by which the cell SECRETES hormones or neurotransmitters or the process by which the cell EXCRETES waste products. Pinocytosis is cell drinking Phagocytosis is a type of cell endocytosis in which the cells engulf bacteria or large particles such as cell debris from damaged tissues.