movement of molecules

• diffusion -

  • The net movement of particles from a region of high concentration to a region of low concentration.
  • Movement down a concentration gradient.
  • This continues until equilibrium is met
  • Only occurs between different concentrations of the same substance
  • passive
• Factors that increase the rate of diffusion -

  • a Higher Difference in the Concentration Gradient
  • a Higher Temperature
  • a larger surface area
  • a smaller molecule or ion
  • a shorter distance for diffusion
• facilitated diffusion -

  • Diffusion across a membrane, through a protein channel, is facilitated diffusion.
  • These protein channels are only specific to one molecule or Ion so the membrane is selectively permeable.
  • The more proteins to carry out facilitated diffusion, the higher the rate of diffusion
• facilitated diffusion -

  • Allows water-soluble, charged and larger ions and molecules (like glucose and amino acids) across a plasma membrane
  • Protein channels are water filled hydrophilic channels that span the phospholipid bilayer.  The channels are selective and will open only in the presence of a specific ion or molecule.
  • Carrier proteins will change shape when a particular molecule binds to it.  The shape change allows the molecule to be released to the other side of the membrane
• active transport -

  • The movement of molecules or Ions into and/or out of a cell from a region of lower concentration to a region of higher concentration.
  • This process requires energy and uses carrier proteins.
  • Energy is needed as particles are being moved up a concentration gradient (the opposite direction to diffusion)
  • Energy is supplied by ATP
• Both facilitated diffusion and active transport use carrier proteins but facilitated diffusion occurs down a concentration gradient and does not require energy from ATP.
• 1. The molecule or ion to be transported binds to receptors on the carrier protein on the outside of the cell
  2. Inside the cell ATP binds to the carrier protein and is hydrolysed into ADP and phosphate
  3. Binding of this phosphate molecule causes a conformational change in the protein- opening the protein
  4. The molecule or Ion is then released to the inside of the cell
  5. The phosphate molecule is released from the carrier protein and reforms ATP by binding to ADP
  6. The carrier protein returns to its original shape

   
• bulk transport -

  • The carrier protein returns to its original shape
  • Large molecules like hormones are too large to move through channel or carrier proteins so can be moved in and out of the cell by bulk transport.
• endocytosis - bulk transport of material into cells

  exocytosis - bulk transport of material out of cells
• endocytosis -

  Process by which the membrane engulfs material, forming a vesicle around it to allow intake of that material into the cell

  1. cell surface membrane invaginates (bends in) when the material that needs to be transported comes into contact with it.
  2. the membrane enfolds or engulfs this material until the membrane fuses, forming a vesicle.
  3. the vesicle pinches off and moves into the cytoplasm

  
• There are 2 forms of Endocytosis

  • phagocytosis - Bulk intake of solids, By phagocytes, Form phagocytic vesicles
  • pinocytosis - Bulk intake of liquids, Vesicles can be extremely small- this is then micropinocytosis
• exocytosis -

  Process by which materials are transported out of the cell

  1. The material that is to be released is packaged into secretory vesicles. (these are formed from the golgi body)
  2. These travel to the cell membrane and fuse with it.
  3. Contents of the vesicle are released outside of the cell.
• In both endocytosis and exocytosis ATP energy is required for:

  • the movement of vesicles
  • for the changing of the shape of cells to engulf material
  • for fusion to the membrane