Chapter 5 Membrane Structure and Function 1 5

Chapter 5 Membrane Structure and Function 1

5. 1 Plasma Membrane Structure and Function • The plasma membrane is common to all cells • Separates: § Internal cytoplasm from the external environment of the cell • Phospholipid bilayer: § External surface lined with hydrophilic polar heads § Cytoplasmic surface lined with hydrophilic polar heads § Nonpolar, hydrophobic, fatty-acid tails sandwiched in between 2

Plasma Membrane Structure and Function • Components of the Plasma Membrane § Three components: • Lipid component referred to as phospholipid bilayer • Protein molecules – Float around like icebergs on a sea – Membrane proteins may be peripheral or integral » Peripheral proteins are found on the inner membrane surface » Integral proteins are partially or wholly embedded (transmembrane) in the membrane • Cholesterol affects the fluidity of the membrane 3

Plasma Membrane Structure and Function • Carbohydrate Chains § Glycoproteins • Proteins with attached carbohydrate chains § Glycolipids • Lipids with attached carbohydrate chains § These carbohydrate chains exist only on the outside of the membrane • Makes the membrane asymmetrical 4

Plasma Membrane of an Animal Cell Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Plasma membrane carbohydrate chain extracellular Matrix (ECM) glycoprotein phospholipid glycolipid hydrophobic hydrophilic tails heads phospholipid bilayer filaments of cytoskeleton peripheral protein Outside cell Inside cell integral protein cholesterol 5

Plasma Membrane Structure and Function • Functions of Membrane Proteins § Channel Proteins: • Allow passage of molecules through membrane via a channel in the protein § Carrier Proteins: • Combine with the substance to be transported • Assist passage of molecules through membrane § Cell Recognition Proteins: • Glycoproteins • Help the body recognize foreign substances (Immune System) 6

Plasma Membrane Structure and Function • Functions of Membrane Proteins (continued) § Receptor Proteins: • Bind with specific molecules • Allow a cell to respond to signals from other cells § Enzymatic Proteins: • Carry out metabolic reactions directly § Junction Proteins: • Attach adjacent cells 7

Membrane Protein Diversity Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Channel Protein: Allows a particular molecule or ion to cross the plasma membrane freely. Cystic fibrosis, an inherited disorder, is caused by a faulty chloride (Cl–) channel; a thick mucus collects in airways and in pancreatic and liver ducts. a.

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Carrier Protein: Selectively interacts with a specific molecule or ion so that it can cross the plasma membrane. The inability of some persons to use energy for sodiumpotassium (Na+–K+) transport has been suggested as the cause of their obesity. b.

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Cell Recognition Protein: The MHC (major histocompatibility complex) glycoproteins are different for each person, so organ transplants are difficult to achieve. Cells with foreign MHC glycoproteins are attacked by white blood cells responsible for immunity. c.

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Receptor Protein: Is shaped in such a way that a specific molecule can bind to it. Pygmies are short, not because they do not produce enough growth hormone, but because their plasma membrane growth hormone receptors are faulty and cannot interact with growth hormone. d.

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Enzymatic Protein: Catalyzes a specific reaction. The membrane protein, adenylate cyclase, is involved in ATP metabolism. Cholera bacteria release a toxin that interferes with the proper functioning of adenylate cyclase; sodium (Na+) and water leave intestinal cells, and the individual may die from severe diarrhea. e.

Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Junction Proteins: Tight junctions join cells so that a tissue can fulfill a function, as when a tissue pinches off the neural tube during development. Without this cooperation between cells, an animal embryo would have no nervous system. f.

How Cells Talk to One Another • Signaling molecules serve as chemical messengers allowing cells to communicate with one another § Cell receptors bind to specific signaling molecules § Once the signaling molecule and the cell receptor bind a cascade of events occurs that elicits a cellular response § Referred to as Signal Transduction Pathway § Extremely important for cellular communication 14

Cell Signaling Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. a. egg embryo 1. Receptor: Binds to a signaling molecule, becomes activated and initiates a transduction pathway. 3. Response: Targeted protein(s) bring about a cellular response. plasma membrane signaling molecule receptor activation 2. Transduction pathway: Series of relay proteins that ends when a protein is activated. unactivated receptor protein Cytoplasm newborn Nuclear envelope Targeted protein: Cellular response: structural protein Altered shape or movement of cell enzyme gene regulatory protein Nucleus Altered metabolism or cellular function Altered gene expression and the types and amount of proteins produced b. Left: © Anatomical Travelogue/Photo Researchers, Inc. ; Middle: © Neil Harding/Stone/Getty Images; Right: © Photodisc Collection/Getty RF 15

Plasma Membrane Structure and Function • Permeability of the Plasma Membrane § The plasma membrane is selectively permeable • Allows some substances to move across the membrane • Inhibits passage of other molecules § Small, non-charged molecules (CO 2, glycerol, alcohol) freely cross the membrane by passing through the phospholipid bilayer • These molecules follow their concentration gradient – Move from an area of high concentration to an area of low concentration. 16

Plasma Membrane Structure and Function • Permeability of the Plasma Membrane § Water moves across the plasma membrane • Specialized proteins termed aquaporins speed up water transport across the membrane § The movement of ions and polar molecules across the membrane is often assisted by carrier proteins § Some molecules must move against their concentration gradient with the expenditure of energy • Active transport § Large particles enter or exit the cell via bulk transport • Exocytosis • Endocytosis 17

How Molecules Cross the Plasma Membrane Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. – + hyd nonp rop ola hob r, ic c ore – + water inside cell charged molecules and ions water outside cell H 2 O noncharged molecules macromolecule phospholipid molecule protein 18

Passage of Molecules Into and out of the Cell 19

5. 2 Passive Transport Across a Membrane • A solution consists of: § A solvent (liquid), and § A solute (dissolved solid) • Diffusion § Net movement of molecules down a concentration gradient § Molecules move both ways along gradient, but net movement is from high to low concentration § Equilibrium: • When NET movement stops • Solute concentration is uniform – no gradient 20

Process of Diffusion Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. time crystal dye a. Crystal of dye is placed in water b. Diffusion of water and dye molecules c. Equal distribution of molecules results 21

Passive Transport Across a Membrane • Osmosis: § Special case of diffusion § Focuses on solvent (water) movement rather than solute § Diffusion of water across a selectively permeable membrane • Solute concentration on one side is high, but water concentration is low • Solute concentration on other side is low, but water concentration is high § Water can diffuse both ways across membrane but the solute cannot § Net movement of water is toward low water (high solute) concentration • Osmotic pressure is the pressure that develops due to osmosis 22

Osmosis Demonstration Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. more water (lower percentage of solute) less water (higher percentage of solute) <10% water 10% more water (lower percentage of solute) solute thistle tube 5% a. >5% less water (higher percentage of solute) c. differentially permeable membrane beaker b. 23

Passive Transport Across a Membrane • Isotonic Solutions § Solute and water concentrations are equal on both sides of membrane § No net gain or loss of water by the cell • Hypotonic Solutions § Concentration of solute in the solution is lower than inside the cell § Cells placed in a hypotonic solution will swell • Causes turgor pressure in plants • May cause animal cells to lyse (rupture) 24

Passive Transport Across a Membrane • Hypertonic Solutions § Concentration of solute is higher in the solution than inside the cell § Cells placed in a hypertonic solution will shrink • Crenation in animal cells • Plasmolysis in plant cells 25

Osmosis in Animal and Plant Cells Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Animal cells plasma membrane nucleus In an isotonic solution, there is no net movement of water. In a hypotonic solution, water In a hypertonic solution, water mainly enters the cell, which may mainly leaves the cell, which burst (lysis). shrivels (crenation). Plant cells central vacuole nucleus cell wall plasma membrane chloroplast In an isotonic solution, there is no net movement of water. In a hypotonic solution, vacuoles fill with water, turgor pressure develops, and chloroplasts are seen next to the cell wall. In a hypertonic solution, vacuoles lose water, the cytoplasm shrinks (plasmolysis), and chloroplasts are seen in the center of the cell. 26

Passive Transport Across a Membrane § Facilitated Transport • Movement of molecules that cannot pass directly through the membrane lipids • These molecules must combine with carrier proteins to move across the membrane • Follow concentration gradient, moving from high concentration to low concentration 27

Facilitated Transport Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or

5. 3 Active Transport Across a Membrane § Active Transport • The movement of molecules against their concentration gradient – Movement from low to high concentration • Movement is facilitated by carrier proteins • Requires the expenditure of energy in the form of ATP • Ex: sodium-potassium pump – Uses ATP to move sodium ions out of the cells and potassium ions into the cell against their concentration gradients. 29

The Sodium-Potassium Pump Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. carrier protein Outside K+ K+ K+ Na+ + Na Na + K+ K K+ + Na Na + Na + Inside Na + 1. Carrier has a shape that allows it to take up 3 Na + P Na + + Na K+ Na + ATP K+ 2. ATP is split, and phosphate group attaches to carrier. 6. Change in shape results and causes carrier to release 2 K + inside the cell. Na+ + Na K+ Na + K+ Na K+ K+ K+ + Na P Na + K+ + Na P K+ K+ 5. Phosphate group is released from carrier. Na + Na 3. Change in shape results and causes carrier to release 3 Na + outside the cell. P + Na 4. Carrier has a shape that allows it to take up 2 K +. 30

Active Transport Across a Membrane • Macromolecules are transported into or out of the cell inside vesicles via bulk transport § Exocytosis – Vesicles fuse with plasma membrane and secrete contents § Endocytosis – Cells engulf substances into a pouch which becomes a vesicle • Phagocytosis – Large, solid material is taken in by endocytosis • Pinocytosis – Vesicles form around a liquid or very small particles • Receptor-Mediated Endocytosis– Specific form of pinocytosis using receptor proteins and a coated pit 31

Exocytosis Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

Three Methods of Endocytosis Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. plasma membrane paramecium pseudopod vacuole forming vacuole 399. 9 μm a. Phagocytosis vesicles forming solute vesicle 0. 5 μm b. Pinocytosis receptor protein coated pit solute coated pit c. Receptor-mediated endocytosis coated vesicle 33

5. 4 Modifications of Cell Surfaces • Cell Surfaces in Animals § Extracellular Matrix (ECM) • Meshwork of proteins and polysaccharides in close connection with the cell that produced them – Collagen – resists stretching – Elastin – provides resilience to the ECM – Integrin – play role in cell signaling – Proteoglycans – regulate passage of material through the ECM to the plasma membrane 34

Animal Cell Extracellular Matrix Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Inside (cytoplasm) actin filament integrin elastin fibronectin collagen proteoglycan Outside (extracellular matrix) 35

Modifications of Cell Surfaces • Cell Surfaces in Animals § Junctions Between Cells • Adhesion Junctions - Intercellular filaments between cells – Desmosomes – internal cytoplasmic plaques – Tight Junctions – form impermeable barriers • Gap Junctions – Plasma membrane channels are joined (allows communication) 36

Junctions Between Cells of the Intestinal Wall Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. cytoplasmic plaque plasma membranes Filaments of cytoskeleton adhesion proteins 100 nm intercellular space a. Adhesion junction plasma membranes light junction proteins 50 nm intercellular space b. Tight junction plasma membranes membrane channels 20 nm intercellular space c. Gap junction a: From Douglas E. Kelly, J. Cell Biol. 28 (1966): 51. Reproduced by copyright permission of The Rockefeller University Press; b: © David M. Phillips/Visuals Unlimited; c: Courtesy Camillo Peracchia, M. D. 37

Modifications of Cell Surfaces • Plant Cell Walls § Plants have a freely permeable cell wall, with cellulose as the main component • Plasmodesmata penetrate the cell wall • Each contains a strand of cytoplasm • Allow passage of material between cells 38

Plasmodesmata Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. plasmodesmata cell wall middle lamella cell wall plasma membrane cell wall cytoplasmodesmata Cell 1 Cell 2 0. 3 mm 39 © E. H. Newcomb/Biological Photo Service