chapter 6.docx

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Biological Science
BISC 160
Dr.Beckie Symula

Chapter 6 Alzheimer’s occurs when abnormal deposits called plaques are present in brain cells. Plaques are clumps of the protein amyloid beta. Amyloid beta is produced by Beta-secretase and Lambda-secretae cutting amylouid precursor proteins. 6.1 What is the structure of a biological membrane?  Lipids, proteins, carbohydrates  Fluid Mosaic Model o Proteins are noncovalently embedded in the phospholipid bilayer by their hydrophobic regions, but their hydrophilic domains are exposed to the water conditions on either side of the bilayer o Protein functions:  Moving materials through the membrane  Receiving chemical signals from the cell’s external environment o Carbohydrates are located on the outside of the cell where they remain intact with the cell’s external environment o Carbohydrate functions:  Recognizing specific molecules  Lipids form the hydrophobic core of the membrane o Phospholipids  Hydrophilic regions  Phosphorus-contianing head is electrically charged and therefore polar  Hydrophobic regions:  Nonpolar fatty acid tail do not dissolve in water or associate with hydrophilic substances  8nm thick biloayer, which is twice the length of a typical phospholipid o All biological membranes have a similar structure, but differ in the kinds of proteins and lipids they contain  Fatty acid chain length(# of carbon atoms)  Degree of unsaturation(double bonds) in fatty acids  Polar(phosphate-containing) groups present o Most common fatty acids and their chain length with degree of unsaturation  Palmitic: C , no double bonds, saturated 14  Palmitoleic: C 16one double bond  Stearic: C 18no double bonds, saturated  Oleic: C 18one double bone 2 [Type text]  Linoleic: C18 two double bonds  Linolenic: C18 three double bonds o Fatty acids with kinks make a less dense membrane allowing for more fluid packing and accommodation of cholesterol  Important for membrane integrity o The fluiditiy of a membrane is affected by its lipid composition and by its temperature  Cholesterol interacts hydrophobically with the fatty acid chains  A membrane with long-chain saturated fatty acids and cholesterol is less fluid than one with shorter- chain fatty acids, unsaturated fatty acids, or less cholesterol  Membrane proteins are asymmetrically distributed o Plasma membranes have about one protein molecules for every 25 phospholipids  Mitochondria have one protein for every 15 lipids  Myelin have one protein for every 70 lipids o Peripheral membrane proteins: lack exposed hydrophobic groups and are not embedded in membrane  Have polar regions that interact with exposed parts of membrane proteins or with polar heads of phospholipid molecules o Integral membrane proteins: at least partly embedded in the membrane, have both hydrophilic and hydrophobic regions  Hydrophilic domains  Interact with water and stick out into the aqueous environment inside or outside the cell  Hydrophobic domains  Interact with the fatty acids in the interior of the phospholipid bilayer o According to the fluid-mosaic model, the proteins and lipids in a membrane are somewhat independent of each other and interact only noncovalently o Some proteins have fatty acids covalently attached to them—anchored membrane proteins o Transmembrane protein  An integral protein that extends all the way through the phospholipid bilayer Chapter 6  Localized on one side of the membrane; gives the membrane great functional significance  Membranes are constantly changing o Fragments of membrane move, in the form of vesicles, from the ER to the Golgi to the plasma membrane o Secondary lysosomes form when primary lysosomes from the Golgi fuse with phagosomes from the plasma membrane  Plasma membrane carbohydrates are recognition sites o Located on outer surface o Membrane-associated carbs may be covalently bonded to lipids or proteins  Glycolipid: carbohydrate covalently bonded to a lipid  May serve as a recognition signal for interactions between cells  Glycoprotein: carbohydrate covalently bonded to a protein  Oligosaccharides; function as signaling sites 6.1 Summary: The fluid mosaic model applies to both the plasma membrane and the membranes of organelles. An integral membrane protein has both hydrophilic and hydrophobic domains, which affect its position and function in the membrane. Carbohydrates that attach to lipids and protein on the outside of the membrane serve as recognition sites 6.2 How is the plasma membrane involved in cell adhesion and recognition?  Cell recognition: one cell specifically binds to another cell of a certain type  Cell adhesion: the connection between the two cells is strengthened  Sponge cells can be separated and then placed in a container and the cells will reorganize into the original sponge  Cell recognition and cell adhesion involve proteins at the cell surface o A protein has a specific shape and chemical groups that allow it to interact with other substances and allow binding to other specific molecules o Homotypic: the same molecules stick out of both cells, and the exposed surfaces bind to each other o Heterotypic: binding of cells with different proteins  Three types of cell junctions connect adjacent cells 4 [Type text] o Cell junctions seal intercellular spaces, reinforce attachments to one another, and communicate with each other o Tight junctions seal tissues  link adjacent epithelial cells  result from mutual binding of specific proteins in the plasma membranes of the cells  arrayed in bands so they form a series of joints encircling each cell  found in lining of lumens in organs  Functions:  Prevent substances from moving from the lumen through the spaces between cells  Define specific functional regions of membranes by restricting the migration of membrane proteins and phospholipids from one region of the cell to another  Help ensure the directional movement of materials into the body o Desmosomes hold cells together  Connect adjacent plasma membranes  Has a dense structure called a plaque on the cytoplasmic side of the plasma membrane  Plaque—membrane—intercellular space— membrane—other plaque  Attached to intermediate filaments in the cytoplasm  Made up of keratin o Gap junctions are a means of communication  Made of specialized channel proteins, called connexins, which interact to form a connexon that spans the plasma membrane and the intercellular space between them  Water, small molecules, and ions can pass through  Cell membranes adhere to the extracellular matrix o Integrin: a transmembrane protein that often mediates the attachment of epithelial cells to the extracellular matrix o 24 integrins have been described in human cells; all bind to a protein in the EM on the outside of the cell, and to actin filaments, on the inside of the cell o Binding of integrin to the extracellular matrix is noncovalent and reversible Chapter 6 6.2 Summary: In multicellular organisms, cells arrange themselves in groups by two processes: cell recognition and cell adhesion. Both proesses are mediated by integral proteins in the plasma membrane. Cell membrane proteins also interact with the extracellular matrix. 6.3 What are the passive processes of membrane transport?  Selective permeability: o allows the membrane to determine what substances enter or leave a cell or organelle o Passive transport: does not require input of outside energy to drive  Difference between its concentrations on either side of membrane o Active transport: requires outside chemical energy source  Diffusion and facilitated diffusion  Diffusion is the process of random movement toward a state of equilibrium o A solution in which the solute particles are uniformly distributed is said to be at equilibrium o Diffusion: the process of random movement toward a state of equilibrium o Speed of diffusion depends on three factors:  The diameter of the molecules or ions (small=fast)  Temperature of solution: higher is faster  Concentration gradient: greater concentration = faster diffusion o Diffusion within cells and tissues  Where distances are very short, solutes distribute themselves rapidly by diffusion  One organelle to another could be a millisecond  One centimeter maybe an hour  A meter could be years o Diffusion across membranes  In a solution without barriers, all the solutes diffuse at rates determined by temperature, their physical properties, and their concentration gradients  Molecules continue to move back and
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