Membrane Structure and Function

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as "self" versus foreign Most are glycoproteins Carbohydrate chains vary between species, individuals, and even between cell types in a given individual. Glycolipids also play a role in cell recognition ● Integrins are a type of integral protein The cytoskeleton attaches to integrins on the cytoplasmic side of the membrane Integrins strengthen the membrane Intercellular junction proteins - help like cells stick together to form tissues ● Many membrane proteins are enzymes This is especially important on the membranes of organelles. Copyright © 2009 Pearson Education, Inc. What's an enzyme? Signal transduction (receptor) proteins bind hormones and other substances on the outside of the cell. ● Binding triggers a change inside the cell. Called signal transduction Example: The binding of insulin to insulin receptors causes the cell to put glucose transport proteins into the membrane. Fig. 5-1c Messenger molecule Activated molecule Copyright © 2009 Pearson Education, Inc. Receptor Transport Proteins Passive Transport Proteins allow water soluble substances (small polar molecules and ions) to pass through the membrane without any energy cost Active Transport Proteins The cell expends energy to transport water soluble substances against their concentration gradient Fig. 5-1d Copyright © 2009 Pearson Education, Inc. ATP 1. 2. 3. Transport of Substances Plasma Membrane (PM) Passive Transport Across the (Simple) Diffusion (5.3) Facilitated diffusion (5.6) Osmosis (5.4, 5.5) Active Transport (5.8) Bulk Flow (5.9) Endocytosis Exocytosis Passive Transport In passive transport substances cross the membrane by diffusion • Diffusion - net movement of substances from an area of high concentration to low concentration • no energy required Factors Affecting Diffusion Rate Steepness of concentration gradient Steeper gradient, faster diffusion Molecular size Smaller molecules, faster diffusion Temperature Higher temperature, faster diffusion ● ● ● 9 Simple Diffusion Nonpolar, hydrophobic molecules diffuse directly through the lipid bilayer • Simple diffusion does not require the use of transport proteins. • Examples: 0₂, CO₂, steroids ● Polar, hydrophilic substances cannot pass directly through the lipid bilayer Examples: water, ions, carbohydrates Simple Diffusion small, nonpolar molecules (ex. O₂, CO₂) LIPID-SOLUBLE LIPID-SOLUBLE ©2006 Brooks/Cole - Thomson Polar molecules (ex. Glucose, water) ions (ex. H+, Na+, K+) WATER-SOLUBLE X Facilitated Diffusion In facilitated diffusion small polar molecules and ions diffuse through passive transport proteins. No energy needed ● Most passive transport proteins are solute specific Example: glucose enter/leaves cells through facilitated diffusion Facilitated Diffusion ooooo Higher concentration of Passive transport protein Copyright © 2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved. Lower concentration Osmosis Osmosis - diffusion of water across a selectively permeable membrane Water moves from an area of high water concentration to an area of low water concentration Water travels in/out of the cell through aquaporins ● Osmosis Terms Consider two solutions separated by a plasma membrane. Hypertonic solution with a relatively high concentration of solute Hypotonic solution with a relatively low concentration of solute Isotonic • solutions with the same solute concentration ● Lower concentration of solute Solute molecule Selectively permeable membrane Water molecule H₂O Higher concentration of solute 21 Equal concentration of solute Solute molecule with cluster of water molecules Net flow of water Osmosis When a Cell is Placed in a Hypotonic Solution Water will enter the cell. The cell will swell/burst (for animal cells.) Osmosis When a Cell is Placed in a Hypertonic Solution • Water will leave the cell. The cell will shrivel. Osmosis and Animal Cells Cells placed in distilled water H₂O Cells swell and burst H₂O Cells placed in concentrated salt solution Cells shrink and shrivel Osmosis and Plant Cells Cell wall Nucleus Plant cell placed in distilled water Cell stiffens but generally retains shape Plasma membrane Cytoplasm Vacuole Plant cell placed in concentrated salt solution Cell body shrinks and pulls away from cell wall Animal cell Plant cell Isotonic solution H₂O H₂O (1) Normal 000 (4) Flaccid H,O H₂O H₂O Hypotonic solution (2) Lysed (5) Turgid Copyright © 2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved. H₂O traging Hypertonic solution (3) Shriveled Plasma membrane H₂O See page 83 H₂O (6) Shriveled (plasmolyzed) Osmosis Summary When a cell is placed in a Hypotonic solution: Cell gains water through osmosis Animal cell lyses; plant cell becomes turgid (firm) ➤When a cell is placed a Hypertonic solution: Cell loses water through osmosis Animal cell shrivels; plant cell plasmolyzes Active Transport Active transport proteins move substances across the PM against their concentration gradient. ● ● Requires energy (ATP) Active transport proteins are highly selective Active transport is needed for proper functioning of nerves and muscles Active Transport of "X" Active transport proteins span the plasma membrane They have openings for "X" on only one side of the membrane "X" enters the channel and binds to functional groups inside the transport protein. Cytoplasmic ATP binds to the transport protein O Active Transport of "X" A phosphate group is transferred from ATP to the transport protein protein is energized by the added -P. The energized transport protein changes shape and releases "X" on the other side of the cell. The phosphate group is released from the transport protein and it resumes its original shape. Process repeats. Fig. 5-8-1 Transport protein (8 Solute Copyright © 2009 Pearson Education, Inc. Fig. 5-8-2 Transport protein G Solute Copyright © 2009 Pearson Education, Inc. ATP P ADP Fig. 5-8-3 Transport protein G Solute Copyright © 2009 Pearson Education, Inc. ATP P ADP P Protein changes shape Fig. 5-8-4 Transport protein Solute Copyright © 2009 Pearson Education, Inc. ATP P ADP CP P Protein changes shape H Phosphate P detaches Active Transport tell the story... ATP P ADP C5 2 3 Copyright © 2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved. 4 Bulk Flow Vesicles are used to transport large particles across the PM. Requires energy Types: Exocytosis Endocytosis Phagocytosis, pinocytosis, receptor-mediated Fluid outside cell Vesicle Exocytosis Copyright © 2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved. Protein Cytoplasm O Bulk Flow Exocytosis • Cytoplasmic vesicle merges with the PM and releases its contents . Example: Golgi body vesicles merge with the PM an release their contents How nerve cells release neurotransmittors Endocytosis Copyright © 2005 Pearson Education, Inc. Publishing s Pearson Benjamin Cummings. All rights reserved. Vesicle forming Endocytosis can occur in three ways Phagocytosis ("cell eating") Pinocytosis ("cell drinking") Receptor-mediated endocytosis Endocytosis Endocytosis PM sinks inward, pinches off and forms a vesicle ● ● . Vesicle often merges with Golgi for processing and sorting of its contents Phagocytosis - cell eating Membrane sinks in and captures solid particles for transport into the cell ● Endocytosis - terms ● Examples: Solid particles often include: bacteria, cell debris, or food Pinocytosis - cell drinking . Cell brings in a liquid Endocytosis - comments Phagocytosis and pinocytosis are not selective • Membrane sinks inward and captures whatever particles/fluid present. Vesicle forms and merges with the Golgi body... Receptor Mediated Endocytosis Receptor Mediated Endocytosis is a highly specific form of endocytosis. Receptor proteins on the outside of the cell bind specific substances and bring them into the cell by endocytosis Receptor Mediated Endocytosis Receptor proteins on PM bind specific substances (vitamins, hormones..) 2. 3. Membrane sinks in and forms a pit Called a coated pit Pit pinches closed to form a vesicle around bound substances Cytoskeleton aids in pulling in the membrane and vesicle formation Fig. 5-9c Receptor Specifico molecule Copyright © 2009 Pearson Education, Inc. Coat protein Coated pit Coated vesicle Coat Ligand binds to receptor Clathrin & adaptor proteins bind to receptor Early coated pit Receptor Mediated Endocytosis Components continue to accumulate FOX Coated pit Transport vesicle CON K Coated vesicle Uncoating O'Day Fig. 5-9 EXTRACELLULAR FLUID Receptor Specific molecule Pseudopodium CYTOPLASM "Food" or other particle Plasma membrane Copyright © 2009 Pearson Education, Inc. Coat protein Coated pit Food vacuole Vesicle Coated vesicle Food bing ingested