Membrane Structure and Function Chapter 5 Membrane Structure and Function

CO 5 organelles nuclear envelope plasma membrane 2.7 mm nucleolus Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. organelles nuclear envelope plasma membrane 2.7 mm nucleolus © Professors P. Motta & T. Naguro/Science Photo Library/Photo Researchers, Inc.

Cell or plasma membrane Phospholipid bilayer with embedded proteins Amphipathic molecule- having both hydrophilic (water-loving) region and hydrophobic (water-fearing) region Proteins are scattered and can vary from cell to cell Integral proteins which usually span the membrane Some protrude from one surface, others from both surfaces Hydrophobic region within phospholipid membrane Hydrophilic region protruding beyond membrane

Peripheral proteins occur only on cytoplasmic side Only animal cells have extracellular matrix Proteins can be held in place by attachments of the cytoskeleton (inside) and fibers of the extracellular matrix (outside)

Fig. 5.1 plasma membrane Carbohydrate Chain extracellular matrix (ECM) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. plasma membrane Carbohydrate Chain extracellular matrix (ECM) Outside hydrophobic tails hydrophilic heads glycoprotein phospholipid bilayer glycolipid filaments of cytoskeleton Inside peripheral protein integral protein cholesterol

integral protein peripheral proteins Page 87 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. integral protein hydrophobic region cholesterol hydrophilic regions peripheral proteins

Fluid-Mosaic Model Membrane is fluid because of lipid with the consistency of olive oil at body temperature The greater the concentration of unsaturated fatty acid residues the more fluid is the bilayer This fluidity means cells are pliable Also prevents membranes from solidifying when temperatures drop Cholesterol molecules in membrane affects fluidity At low temperature prevents freezing by keeping phospholipid tails apart Proteins are bound to ECM and move little

Phospholipids and proteins can have attached carbohydrate chains on outer surface making the membrane asymmetrical Called glycolipids and glycoproteins Glycocalyx or “sugar coat” of animal cells protects, helps adhesion between cells, receives signal molecules, and allows cell-to-cell recognition

Channel Protein: Allows a particular molecule or ion to Fig. 5.3a Copyright © The McGraw-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.

Selectively interacts with a specific molecule or ion so Fig. 5.3b Copyright © The McGraw-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 sodium- potassium (Na+–K+) transport has been suggested as the cause of their obesity. b.

complex) glycoproteins are different for each person, so organ Fig. 5.3c Copyright © The McGraw-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.

Receptor Protein: Is shaped in such a way that a specific Fig. 5.3d Copyright © The McGraw-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.

ATP metabolism. Cholera bacteria release a toxin Fig. 5.3e Copyright © The McGraw-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.

can fulfill a function, as when a tissue pinches off the neural tube Fig. 5.3f Copyright © The McGraw-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.

Fig. 5A Cellular response: Altered shape or movement of cell receptor Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. egg embryo newborn 1. Receptor: Binds to a signaling molecule, becomes activated and initiates a transduction pathway. 3. Response: Targeted protein(s) bring about the response(s) noted. Targeted protein: Cellular response: signaling molecule plasma membrane Altered shape or movement of cell structural protein receptor activation Altered metabolism or a function of cell enzyme 2. Transduction pathway: Series of relay proteins that ends when a protein is activated. unactivated receptor protein nuclear envelope Altered gene expression and the amount of a cell protein gene regulatory protein Cytoplasm Nucleus b. (Human egg): © Anatomical Travelogue/Photo Researchers, Inc.; (Embryo): © Neil Harding/Stone/Getty Images; (Baby): © Photodisc Collection/Getty Images

Cell membranes are differentially or selectively permeable Passage of molecules into and out of cells: Diffusion from high concentration to low concentration without the use of energy Facilitated diffusion from high concentration to low concentration without the use of energy, but requires the help of a carrier molecule Active transport from low concentration to high concentration with the use of energy Bulk transport toward inside or outside of cell using vesicles

– + + – Fig. 5.4 charged molecules and ions H2O noncharged molecules Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. charged molecules and ions – + H2O noncharged molecules macromolecule + – phospholipid molecule protein

Aquaporins are channel proteins that allow water to cross a membrane very quickly A solution contains a solute, usually a solid, and a solvent, usually a liquid Diffusion is movement of molecules and ions through a solution from high to low concentration to come to an equalibrium Diffusion rates are affected by temperature, pressure, electrical currents, and molecular size

Diffusion Fig. 5.5 time time crystal dye Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. time time crystal dye a. Crystal of dye is placed in water b. Diffusion of water and dye molecules c. Equal distribution of molecules results

Gas exchange in lungs oxygen Fig. 5.6 bronchiole alveolus capillary O2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. O2 O2 O2 O2 O2 O2 O2 oxygen O2 O2 O2 O2 O2 bronchiole alveolus capillary

Osmosis is diffusion of water across a selectively permeable membrane Fig. 5.7 Osmosis is diffusion of water across a selectively permeable membrane Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. less water (higher percentage of solute) more water (lower percentage of solute) <10% 10% water solute more water (lower percentage of solute) 5% thistle tube >5% less water (higher percentage of solute) a. c. differentially permeable membrane beaker b.

Osmotic pressure is pressure that develops in a system due to osmosis Isotonic means water concentration is equal on both sides of the membrane 0.9% solution of NaCl is isotonic to red blood cells Hypotonic means solution has lower concentration of solutes in fluid outside the cell Causes turgor pressure in plant cells Hypertonic means solution has higher concentration of solutes in fluid outside the cell Causes red blood cells to shrink or crenate Causes plasmolysis (wilting) in plant cells

In an isotonic solution, there is no net movement of water. Fig. 5.8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. plasma membrane Animal cells nucleus In an isotonic solution, there is no net movement of water. In a hypotonic solution, water mainly enters the cell, which may burst (lysis). In a hypertonic solution, water mainly leaves the cell, which shrivels (crenation). Plant cells cell wall nucleus central vacuole 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.

Faciliated transport Fig. 5.9 Inside plasma membrane carrier protein Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Inside plasma membrane carrier protein solute Outside

Proteins involved in active transport are often called pumps They use energy to move molecules against the concentration gradient Sodium-potassium pump is associated with nerve and muscle cells Moves sodium ions to outside of cell Moves potassium ions to inside of cell

Fig. 5.10 carrier protein Outside Inside Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. carrier protein Outside K+ K+ K+ K+ K+ Na+ Na+ K+ K+ K+ Na+ K+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Inside 1. Carrier has a shape that allows it to take up 3 Na+. Na+ Na+ P Na+ Na+ Na+ K+ K+ ATP ADP Na+ 6. Change in shape results and causes carrier to release 2 K+ inside the cell. 2. ATP is split, and phosphate group attaches to carrier. Na+ K+ Na+ K+ Na+ Na+ Na+ K+ Na+ K+ K+ K+ K+ K+ Na+ K+ Na+ Na+ K+ P Na+ K+ Na+ P K+ Na+ Na+ 5. Phosphate group is released from carrier. 3. Change in shape results and causes carrier to release 3 Na+ outside the cell. P Na+ Na+ 4. Carrier has a shape that allows it to take up 2 K+.

During exocytosis, a vesicle fuses with the plasma membrane as secretion occurs Golgi apparatus (body) produces the vesicles that contain materials produced in the cell for export to other parts of the body

plasma membrane Outside Inside secretory vesicle Fig. 5.11 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. plasma membrane Outside Inside secretory vesicle

During endocytosis, cells take in substances by vesicle formation as part of the plasma membrane invaginates Phagocytosis is taking in large things like a food particle or another cell White blood cells in the immune system move like amoeba and take in invading forms and debris by phagocytosis Pinocytosis is taking in a liquid or small particle Receptor-mediated endocytosis is pinocytosis for a specific molecule like a vitamin, peptide hormone, or lipoprotein

Fig. 5.12 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. plasma membrane paramecium pseudopod vacuole forming vacuole a. Phagocytosis 399.9 mm vesicles forming solute vesicle b. Pinocytosis 0.5 mm receptor protein coated pit coated vesicle solute coated vesicle coated pit c. Receptor-mediated endocytosis (Top): © Eric Grave/Phototake; (Center): © Don W. Fawcett/Photo Researchers, Inc.; (Bottom, both): Courtesy Mark Bretscher

Extracellular structures are built from materials produced and secreted by the cell Extracellular matrix is proteins and polysaccharides around the cell Collagen (resists stretching), elastin (give resilience) Fibronectin is an adhesive protein that binds to integrin that hooks to the cytoskeleton

Outside (extracellular matrix) Fig. 5.13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Inside (cytoplasm) actin filament integrin elastin fibronectin proteoglycan collagen Outside (extracellular matrix)

Junctions between cells: There are three types: Adhesion junctions mechanically attach adjacent cells Come in two types: In desmosomes, internal cytoplasmic plaques attach to cytoskeleton in each cell which are joined by intercellular filaments Sturdy but flexible sheet of cells in stretchable organs like heart, stomach, and bladder. In hemidesmosomes, a single point of attachment connects the cytoskeletons of adjacent cells

Tight junctions even more closely join adjacent cells when plasma membranes attach to each other in a zipperlike fastening Found in organs like intestine and kidneys where leakage between cells is not welcome Gap junctions allow cells to communicate Two identical membrane channels join Allow small molecules and ions to pass between them Important in heart (cardiac) and smooth muscles where the flow of ions is necessary for cells to contract as a unit

Adhesion junctions, Tight junctions, and Gap junctions Fig. 5.14 Adhesion junctions, Tight junctions, and Gap junctions Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. plasma membranes cytoplasmic plaque plasma membranes plasma membranes tight junction proteins membrane channels filaments of cytoskeleton intercellular filaments intercellular space 100 nm intercellular space 50 nm intercellular space 20 nm a. Adhesion junction b. Tight junction c. Gap junction a: From Douglas E. Kelly, Journal of Cell Biology 28 (1966): 51. Reproduced by copyright permission of The Rockefeller University Press; b: © David M. Phillips/Visuals Unlimited; c: Courtesy Camillo Peracchia, M.D.

Plant cell membranes are surrounded by cell walls The living cells are connected by plasmodesmata They contain numerous narrow, membrane-lined channels passing through cell walls Allows water and small solutes to pass freely from cell to cell

© E.H. Newcomb/Biological Photo Service Fig. 5.15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. plasmodesmata cell wall cell wall middle lamella plasma membrane plasma membrane cell wall cell wall cytoplasm cytoplasm plasmodesmata Cell 1 Cell 2 0.3 mm © E.H. Newcomb/Biological Photo Service