Membranes 9/15/2016. Phospholipids. Phospholipid bilayer

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1 Membranes Phospholipids Type of complex lipid that forms biological membranes. Have a polar hydrophilic head and two nonpolar hydrophobic tails. Amphipathic. This causes the tails to cluster together in water and the head to point outward. Structure: Glycerol head Two fatty acid tails Micelles lipid molecules orient with polar (hydrophilic) head toward water and nonpolar (hydrophobic) tails away from water. 3 4 Phospholipid bilayer Phospholipid bilayer more complicated structure where 2 layers form Hydrophilic heads point outward Hydrophobic tails point inward toward each other 5 1

2 Phospholipid bilayer Bilayers are fluid. Lipids and proteins can move through it. Hydrogen bondingof water holds the 2 layers together as well as amphipathic nature of the molecule. Semipermeable: Not everything can cross. Large water-soluble molecules (ex. Glucose) can t pass through on their own. Phospholipid bilayer = cell membrane. Basic Membrane Structure Phospholipids arranged in a bilayer that is fluid. Globular proteinsare imbeddedin and attachedto the lipid bilayer. Fluid Mosaic Model Fluid mosaic model Mosaic of different proteins float in or on the fluid lipid bilayer. Nonpolar segments of the proteins are in contact with the nonpolar interior of the bilayer (tails). Integral membrane proteins are embedded in the membrane. Peripheral proteins are associated with the surface of the membrane. Singer and Nicolson Model: A representation of reality. 9 Singer and Nicolson

3 9/15/2016 Nicolson 1976 Movement and anchoring Jacobson et al Movement Escriba et al Domains Nicolson 2014 Crowded Membrane Transmembrane Proteins Membrane protein function types Embedded proteins that span the membrane. Can float freely or be anchored. 3

4 Lipid Rafts Plasma membrane is not homogenous. Contains domains with distinct lipid and protein composition. Lipid Rafts are lipid micro-domains heavily enriched in cholesterol and sphingolipids. Interact with proteins forming an organized structure. Thicker and less fluidthan neighboring domains. Mainly for signaling and communication. Transport across membranes Membranes are selectively permeable. Molecules can cross the membrane in three basic ways: 1. Passing through the phospholipids. 2. Through channels in the membrane. 3. By carriers. Passive or active. Passive = no energy needed. Diffusion Osmosis Active = energy required. Passive Transport Movement of molecules through the membrane in which: No energy is required. Molecules move in response to a concentration gradient. Diffusion Diffusionis movement of molecules from high concentration to low concentration. Will continue until the concentration is the same in all regions. 21 4

5 Major barrier to crossing a biological membrane is the hydrophobic interior that repels polar molecules but not nonpolar molecules. Nonpolar molecules will move until the concentration is equal on both sides. Limited permeability to polar molecules. Facilitated diffusion Molecules that cannot cross membrane easily may move through proteins (polar or too large). Channel proteins - Hydrophilic channel when open Carrier proteins -Bind specifically to molecules they assist Osmosis Cytoplasm of the cell is an aqueous solution. Water is solvent. Dissolved substances are solutes. Osmosis net diffusion of a solvent (usually water) across a membrane toward a higher solute concentration. 27 Compare Diffusion Movement of solute molecules from high concentration to low concentration. No membrane required. Osmosis Net diffusion of solvent across a membrane toward a higher solute concentration. Membrane required. 29 5

6 Osmotic concentration Solutions can have different osmotic concentrations. Hypertonic solution has a higher solute concentration. Hypotonic solution has a lower solute concentration. Isotonic solution has an equal solute concentration. Water flow in cells is facilitated by channels that are specialized for water: Aquaporins. Animal cells need an isotonic solution (osmotic balance). Cells of plants (and a few others) have cell walls which are stronger and can handle different osmotic concentrations. 31 Animal cells need an isotonic solution (osmotic balance). Cells of plants (and a few others) have cell walls which are stronger and can handle different osmotic concentrations. 33 Osmosis in plant cells Isotonic conditions (normal): 6

7 Osmosis in plant cells Osmosis in plant cells Active Transport Sometimes a cell needs molecules moved against the concentration gradient rather than along with it (as in diffusion). Stockpiling certain molecules for use, thus creating a greater concentration inside the cell. To work against the concentration gradient takes energy often in the form of ATP (energy currency of the cell). Passive versus Active Transport Passive Transport Active Transport 7

8 Types of Active Transport Pumps Coupled transport Uses ATP indirectly. Uses the energy released when a molecule moves by diffusion to supply energy to active transport of a different molecule. Glucose Na + symporter captures the energy from Na + diffusion to move glucose against a concentration gradient. Bulk Transport Two types: Endocytosis: Movement of substances into the cell. Exocytosis: Movement of substances out of the cell. 8

9 Bulk Transport - Endocytosis Movement of substances into the cell. Phagosytosis cell takes in particulate matter. Pinocytosis cell takes in only fluid (solute). Receptor-mediated endocytosis specific molecules are taken in after they bind to a receptor. Bulk Transport - Exocytosis Movement of materials out of the cell. Used in plants to export cell wall material. Used in animals to secrete hormones, neurotransmitters, digestive enzymes, and waste. Scientists watch as bacteria evolve antibiotic resistance Growth patterns reveal E. coli spath to becoming superbugs By Laurel Hamers 2:00pm, September 8, 2016 PETRI PLATTER A petri dish more than a meter long helped scientists visualize the evolution of antibiotic resistance in E. coli bacteria.bacteria placed on the outer edges had to adapt to higher and higher levels of antibiotics as they moved toward the center of the plate. Harvard Medical School 9