Plasma membranes

• Fluid mosaic model


  Idea that the plasma membrane is a mosaic of components, mainly phospholipids, cholesterol, and proteins which flow freely in the membrane plane
• Because the plasma membrane is fluid, if you stick a fine enough needle through it, the membrane will simply flow around it
• Phospholipid


  A lipid made of glycerol, two fatty acid tails, and a phosphate-linked head group
• Phospholipid bilayer


  Two layers of phospholipids with their tails pointing inward to form a semi-permeable membrane
• Cholesterol

  A lipid made up of four fused carbon rings, is found along phospholipids at the core of the membrane
• Membrane proteins may extend partway through the membrane, cross it entirely, or be loosely attached to its inside or outside face
• Carbohydrates


  Only attached to the outside layer of the plasma membrane, hanging on to lipids or proteins
• 50 / 40 / 10 ratio of proteins to lipids to carbohydrates in the human cell's plasma membrane
• Amphipathic


  Structures that have both hydrophobic and hydrophilic regions
• The head of the phospholipid (phosphate and R) is hydrophilic, due to a partial negative charge, and so readily interacts with water
• The tail of the phospholipid (long, nonpolar fatty acid strings) is hydrophobic, because of its nonpolarity, and interacts poorly with water but well with each other, so the tails stick together on the inside of the membrane
• A phospholipid bilayer is good at protecting the cell because the hydrophilic ends interact well with their surroundings, but it's hard for things to penetrate it because of the inner layer of hydrophobic tails
• Phospholipid molecule

  
• Liposome


  A hollow droplet of bilayer membrane made naturally in an aqueous solution when phospholipids have bulkier tails
• Micelle


  A small, single-layered sphere of phospholipids with small tails formed naturally in an aqueous solution
• Integral membrane proteins


  Proteins that have at least one hydrophobic part that anchors it to the hydrophobic core of the phospholipid bilayer
• Transmembrane proteins


  Proteins that pass through the phospholipid bilayer completely and stick out on both ends
• The parts of integral membrane proteins that are inside of the membrane are generally hydrophobic, and the parts inside of the cell or touching the cytoplasm are generally hydrophyllic
• Transmembrane proteins can cross the membrane just once, or up to 12 times, like when hydrophobic amino acids are arranged in an alpha helix
• Peripheral membrane proteins


  Found on the inside or outside of the phospholipid membrane, attached either to integral membrane proteins or phospholipids. They tend to be more loosely attached
• Glycoprotein


  Carbohydrates found on the outside surface of cells bound to proteins in the membrane
• Glycolipid


  Carbohydrates bound to the outside surface of cells bound to lipids
• Carbohydrates and membrane proteins form distinctive cellular markers that immune cells use to know which cells to attack
• At cooler temperatures, the straight tails of saturated fatty acids pack closely together to form a dense and fairly rigid membrane
• At cooler temperatures, the kinky tails of unsaturated fatty acids cannot pack together as tightly, so membranes made up of unsaturated fatty acids stay liquid for longer
• Most cell membranes have a mix of saturated and unsaturated fatty acid tails; some have two saturated tails, others have one saturated and one unsaturated
• Cholestorol


  Lipid embedded within the phospholipids of a membrane to help extend its range of fluidity (reduces fluidity at high temperatures, and keeps the phospholipids from packing too close together at lower temps)