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