Membrane transport

• Passive transport


  Type of transport that does not require energy to occur
• Concentration gradient


  A region of space over which the concentration of a substance changes
• Permeability


  The quality of a membrane that allows substances to pass through it
• Equilibrium


  The state at which a substance is equally distributed throughout a space
• Diffusion


  When substances move from an area of high concentration to an area of low concentration until the concentration gradient has reached equilibrium
• Because the cell membrane is only semi-permeable, only small, nonpolar molecules like O2 and CO2 can easily diffuse across without help
• Carrier protein

  
• Channel protein

  
• Many polar substances need the help of membrane proteins to diffuse across the membrane
• Even though equilibrium has been reached, substances still move across the membrane, it's just roughly the same going in as going out, so it stays in equilibrium
• Active transport

  When substances move from an area of low concentration to an area of high concentration, against the concentration gradient. This is a process that requires energy to carry out
• ATP

  Adenosine triphosphate, the primary energy carrier in living things
• Energy from ATP hydrolysis causes a transport protein to change shape, binding substances on one side of the membrane and releasing them on the other
• The carrier proteins used in active transport are different than the ones used in passive transport, because they need ATP to activate
• Active transport does not use channel proteins, because molecules being carried through them only go along the gradient
• Macrophages devouring undesirable particles whole has 2 functions:

  • Lets it recycle useful macromolecules
  • If it's a pathogen, fragments of it are put on its surface, alerting immune cells to an intruder and triggering a response
• Bulk transport

  When large particles or large quantities of smaller particles are moved across a cell membrane
• Endocytosis


  Meaning internal transport mechanism
• Endocytosis


  General term for the type of bulk transport where the plasma membrane invaginates itself around the desired particles, creating a pocket, which pinches itself off with specialized proteins to create a new vesicle or vacuole inside the cell
• Phagocytosis


  Literally meaning cell eating
• Phagocytosis


  Type of endocytosis where once the target particles are engulfed, their pocket pinches off to form a food vacuole, which gets attached to a lysosome and digested by the cell so that it's components can be used
• Pinocytosis

  Literally meaning "cell drinking"
• Pinocytosis


  Form of endocytosis where cells take in small amounts of extracellular fluid. It happens continuously, with cells sampling and re-sampling the surrounding fluid to get the nutrients they need and whatever other molecules happen to be present
• Pinocytosed material is held in small vesicles, much smaller than the large food vacuoles produced by phacocytosis
• Coat proteins


  Layer of proteins on the cytoplasmic side of the pit in receptor-mediated endocytosis
• Clathrin


  The best studied coat protein used in receptor-mediated endocytosis
• Receptor-mediated endocytosis


  Form of endocytosis where receptor proteins (which are transmembrane proteins) cluster in coated pits and are used to grab a specific type of target molecule, and then the receptors and their attached molecules are taken into a vesicle while the coat proteins help pinch it off from the membrane and give it its rounded shape
• Receptor-mediated endocytosis


  Intended and useful for taking up large amounts of molecules that are rarer in the ECM, some viruses also use this method to gain entry into the cell
• Exocytosis


  Literally meaning "external transport"
• Exocytosis


  When vesicles of particles that a cell doesn't want inside of it anymore fuse with the plasma membrane and release them into the extracellular space
• Some of the particles released from exocytosis are proteins directly from the Golgi apparatus made specifically for ECM release, like signalling molecules, while other vesicles contain waste that the cell needs to dispose of, like the remains of digested phagocytosis leftovers
• Exocytosis has to release models:

  • Full fusion : vesicles fuse completely and are incorporated into the membrane
  • kiss-and-run model: vesicles fuse just enough to release their contents, then pinch off again and return to the cell interior