chapter 5

• Simple diffusion


  The process by which solutes are moved along a concentration gradient in a solution or across a semipermeable membrane
• Simple diffusion

  1. Hydrogen bonds form between water molecules and solutes

  2. Water molecules move to surround individual solute molecules

  3. Hydrogen bonds are extremely temporary

  4. Solution is constantly stirred to distribute solute evenly
• Osmosis


  Water moving along its concentration gradient through a cell membrane, a form of simple diffusion
• Facilitated diffusion


  Cell membranes incorporate specialized membrane proteins to help transport substances across the membrane
• Active transport


  Uses ATP to provide energy to the proteins providing the transport, allowing molecules to be moved against their gradient
• Simple diffusion in biological systems


  • Delivery of oxygen, water, and other small molecules to the cells of the body
• Cells often have protein channels to help speed up the process of simple diffusion
• Simple diffusion of carbon dioxide


  • Carbon dioxide is small enough to move through simple diffusion through tissues and out of the body
  • Accumulation of carbon dioxide causes the "desire to breathe"
  • Many gases can diffuse through the lungs including oxygen, nitrogen, and others in the atmosphere
• Bacteria


  • Rely on simple diffusion to deliver oxygen, water and small nutrients to the cytoplasm
  • Have no specialized organelles to hold or transport substances, so rely on simple diffusion within their cells
• Facilitated transport

  A type of passive transport where materials diffuse across the plasma membrane with the help of membrane proteins
• Concentration gradient

  Allows materials to diffuse into the cell without expending cellular energy
• Facilitated transport proteins


  Shield materials from the repulsive force of the membrane, allowing them to diffuse into the cell
• Facilitated transport


  1. Material is attached to protein or glycoprotein receptors on the exterior surface of the plasma membrane

  2. Substances are passed to specific integral proteins that facilitate their passage
• Facilitated transport proteins

  • Can be channel proteins or carrier proteins
  • Channel proteins have hydrophilic domains and a hydrophilic channel through their core
  • Carrier proteins bind a substance and change shape to move the bound molecule from outside to inside the cell
• Channel proteins in facilitated transport


  • Can be open at all times or "gated" to control opening
  • Found in tissues like the kidney and in nerve and muscle cells to facilitate electrical transmission
• Carrier proteins in facilitated transport


  • Involved in reabsorption of glucose, water, salts, ions, and amino acids in the kidney
  • Glucose transport proteins (GLUTs) transport glucose and other hexose sugars
• Channel proteins vs carrier proteins

  Channel proteins transport much more quickly than carrier proteins
• Active transport


  The process of transferring substances into, out of, and between cells, using energy
• Active transport is required to move substances against their concentration gradient
• Active transport


  Requires energy to move substances from a low concentration to a high concentration
• Channel proteins


  Transport material at a rate of tens of millions of molecules per second
• Carrier proteins


  Transport material at a rate of a thousand to a million molecules per second
• Active transport

  1. Transferring substances into, out of, and between cells, using energy

  2. Passive transport uses no energy

  3. Transporting materials against their concentration gradient
• Active transport is required to transport materials against their concentration gradient
• Active transport process

  1. Substance binds to transport protein

  2. ATP binds to protein

  3. Protein changes shape

  4. Substance is transported across membrane
• Primary active transport


  Transport protein directly uses ATP
• Secondary active transport


  Transport protein does not use ATP directly, but cell must use ATP to keep it functioning
• Endocytosis


  1. Cell folds membrane into pocket around contents

  2. Pocket pinches off, trapping contents inside cell
• Exocytosis


  1. Cell creates vesicle to enclose something inside

  2. Vesicle moves to cell membrane and fuses with it, releasing contents outside cell
• Antiport pumps


  • Pump one substance in one direction, while transporting another substance in the opposite direction
  • Can use one ATP molecule to fuel both tasks
• Symport pumps

  Use diffusion gradients to move substances, with one substance "carrying" another against its concentration gradient
• Sodium-potassium pump


  • Transports potassium into cells and sodium out of cells
  • Vital for nerve and muscle function
  • Consumes 20-25% of body's energy
• Sodium-glucose transport protein


  • Binds to sodium ions and glucose
  • Sodium diffusion facilitates glucose transport into cell
  • Requires sodium gradient maintained by sodium-potassium pump
• White blood cells destroying pathogens


  • Fold membrane around pathogen to engulf it
  • Merge vesicle with lysosome to digest pathogen