CH 7 Membrane Structure, Function, and Use

CH 7 Membrane - Protobionts as Cell Precursors • Protobionts – Are aggregates of abiotically produced molecules surrounded by a membrane or

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Unformatted text preview: Protobionts as Cell Precursors? • Protobionts – Are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure • In earliest cell, the first genetic material – Was probably RNA, not DNA (?) – RNA can function as genetic code carrying material and sometimes as an enzyme (ribozymes) – But we’ll save this topic for a different time • Laboratory experiments demonstrate that protobionts – Could have formed spontaneously from abiotically produced organic compounds • For example, small membrane-bounded droplets called liposomes – Can form when lipids or other organic molecules are added to water Liposomes have the same basic lipid bilayer composition as membranes Glucose-phosphate 20 µ m Glucose-phosphate Phosphorylase Starch Phosphate Amylase Maltose Figure 26.4a, b (a) Simple reproduction. This liposome is “giving birth” to smaller liposomes (LM). Maltose (b) Simple metabolism. If enzymes—in this case, phosphorylase and amylase—are included in the solution from which the droplets self-assemble, some liposomes can carry out simple metabolic reactions and export the products. Membrane Structure & Function Membrane Structure & Function • Fluid mosaics of lipids and proteins • Selectively permeable • Transport mechanism – Passive transport (uses Diffusion) – Active transport (uses Energy) – Bulk transport (endo- and exocytosis) Membrane is composed largely of a phospholipid bilayer • Hydrophobic core of fatty acid tails (hydrocarbon) • Hydrophilic “headgroups” containing phosphate facing aqueous solution (extracellular fluid or cytoplasm) • Membrane defines cell, forms boundary between – Interior (cytoplasm) – Exterior (extracellular space) Membrane Structure & Function • Fluid mosaic of lipids and proteins • Selectively permeable • Transport mechanism – Passive transport (uses Diffusion) – Active transport (uses Energy) – Bulk transport (endo- and exocytosis) Fluidity Factors • What makes lipids liquid at room temperature? – double bonds (unsaturated HC tail) – Causes kink in chain – Increases spacing – More movement – More fluidity • Add cholesterol? – When warm, as in membranes– less movement; less fluidity; “lipid rafts” form – (Cold – increases spacing, more fluid ) Membrane Model Lipid Rafts: recent addition to membrane model • Membrane has regions of low fluidity (raft) and high fluidity (“the sea”?) • Lipid raft regions are relatively less fluid than other membrane regions • Rafts contain relatively high concentrations of cholesterol and sphingolipids (e.g. glycolipids) • Role in bringing membrane receptor components together, role in signal transduction (kinases, etc.) Lipid Rafts Lipid raft research is bridging a gap between lipid biochemistry and signal transduction. Figure 6-3 Janeway’s Immunobiology, 2005 Integral Membrane Proteins • Hydrophilic portions of proteins extend into aqueous areas • Hydrophobic portions are in the interior of the bilayer away from water Making more membrane • Membrane or secreted proteins (made in ER) • Leave ER as vesicles • In the Golgi , proteins and lipids are glycosylated (sugars added) • Vesicles fuse with plasma membrane releasing internal contents to extracellular fluid • Inside-Out: Portion of membrane proteins facing into the vesicle face externally when they become part of the plasma membrane Membrane Structure & Function • Fluid mosaic of lipids and proteins • Selectively permeable • Transport mechanism – Passive transport (uses Diffusion) – Active transport (uses Energy) – Bulk transport (endo- and exocytosis) Remember: • Molecules will only move by diffusion if they are going down their concentration gradients (net movement) • -That is, from an area of high concentration to an area of low concentration • Energy is required to move against concentration gradient (active transport) What is diffusion? What makes it happen? Does it require external energy? What is the goal of this process? Osmosis is covered in Bio 115. It involves membranes and is a very important concept, but we will not focus on it here. This membrane is selectively permeable. Solute particles cannot permeate the membrane. Water can. What moves? What drives the movement? What is the goal? Do we need energy? Membrane Structure & Function • Fluid mosaics of lipids and proteins • Selectively permeable • Transport mechanism – Passive transport (uses Diffusion) – Active transport (uses Energy) – Bulk transport (endo- and exocytosis) Net Movement of Molecules Across Membrane Unassisted Passive Transport – Only molecules that are relatively hydrophobic (or partially hydrophobic) can diffuse across membrane without assistance). – One example is a steroid hormone (remember steroids are lipids) – Of course, there will only be net movement into the cell if the cytoplasm has a lower concentration than the extracellular fluid (movement is down their concentration gradient ) Net Movement of solute Facilitated passive transport: Sometimes channels through membranes facilitate movement of hydrophilic molecules across the hydrophobic region of the bilayer. Why is this example not active transport? • Transport mechanism – Passive transport (uses Diffusion) – Active transport (uses Energy) – Bulk transport (endo- and exocytosis) Sodium-Potassium Pump (Na+-K+ Pump) • Goal: Move Na+ and K+ against their conc. gradient to maintain cytosol composition • Energy in the form of ATP required • Notice the Na+ binding site: – Wide & Round (binds Na+) – Narrow & Round with phosphate group + - Cotransport (Passive coupled to Active Transport) • H+ ion is actively pumped (ATP requ.) • H+ diffuses back through facilitated passive diffusion • H+ diffusion provides the force for sucrose to enter the membrane • Transport mechanism – Passive transport (uses Diffusion) – Active transport (uses Energy) – Bulk transport (endo- and exocytosis) Extracellular fluid enters the cell as a vesicle defined by formerly plasma membrane components invaginates and is internalized. ...
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This note was uploaded on 05/01/2011 for the course BIO 120 taught by Professor Annapowolny during the Fall '10 term at Spelman.

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CH 7 Membrane - Protobionts as Cell Precursors • Protobionts – Are aggregates of abiotically produced molecules surrounded by a membrane or

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