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Phospholipid Bilayer

plasma membrane - skin of lipids w/ embedded proteins covering cells 

  • protein determines what substances can pass through
  • only 2 phospholipids thick

phospholipids - glycerol + 2 fatty acids + phosphorylated alcohol 

  • normal fatty acids aren't soluble, nonpolar all over
  • phosopholipids have polar, organic heads
  • forms bilayer sheets so that nonpolar fatty acid tails never touch the water
  • phospholipid bilayer - forms spontaneously due to water's tendency to form the max number of hydrogen bonds
  • stops any water-soluble substances from passing through
  • certain proteins act as passageways through the membrane

fluidity of bilayer - phospholipids have weak interactions w/ each other  

  • parts of membrane can freely move
  • less fluidity where phospholipid tails align close together
  • some phospholipids don't align well due to double carbon bonds
  • membranes w/ steroid lipids (ex. cholestrol) increase/decrease in fluidity depending on temperature

fluid mosaic model - embedded proteins also have nonpolar parts 

  • nonpolar parts of phospholipids/proteins come in contact w/ each other; polar parts on the surface
  • developed by Singer/Nicolson, disproved the Davson-Danielli model
  • phospholipid bilayer - impermeable, flexible matrix
    • other parts of the membrane are embedded in it
    • nonpolar interior stops polar substances from getting through
  • transmembrane proteins - float on/in the membrane
    • can move around in the membrane freely
  • interior protein network - reinforces the membrane shape
    • spectrin links - proteins that give red blood cells their biconcave shape
    • anchors some important membrane proteins
  • cell surface markers - sugar coating aka glycocalyx
    • used as identity markers
    • microdomain - distinct areas of the membrane
    • plasma membrane not homogeneous
    • lipid raft - heavily enriched w/ cholestrol, saturated fats; more tightly packed than surrounding area

examining cell membranes - must prepare specimens before viewing w/ electron microscopy 

  • epoxy shavings - transparent peelings from a block of tissue embedded in hard matrix
  • microtome - machine w/ very sharp blade
  • freeze-fracturing - tissue is quick frozen w/ liquid nitrogen
    • crack between phospholipid layers form when cracked
    • thin coating of platinum used to creat a cast of the surface

membrane proteins - 6 main groups of proteins let cell interact w/ environment 

  • transporters - allow only certain substances to enter, usually through a channel or on a carrier
  • enzymes - certain reactions use proteins in the membrane
  • cell surface receptors - detects chemical messages
  • cell surface identity markers - ID tag for each cell
  • cell adhesion proteins - glue cells to each other (temporary/permanent bonds)
  • attachments to cytoskeleton - surface proteins linked to cytoskeleton

membrane protein structure - some proteins anchored in the membrane, others move freely 

  • anchored proteins connected to phospholipids by molecules w/ nonpolar region and chemical bonding domains that link to the protein
  • nonpolar helices/beta-pleated sheets of amino acids keep proteins within the membrane, though polar ends stick out
  • single-pass anchors - receptor proteins w/ single-pass anchors
    • binds to specific hormones outside the cell
    • sends messages into the cell, causing changes inside
  • multiple-pass channels/carriers - uses several helices to form a channel
    • only way that water-soluble substances can pass into the cell
  • pores - beta-pleated sheets forming a barrel to allow water and other substances through
Subject X2: 

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