2.6 Exchanging substances

• How do substances cross a membrane?

  • diffusion
  • facilitated diffusion
  • osmosis
  • active transport
• Diffusion


  A passive mode of transport where molecules move down a concentration gradient from an area of high concentration to one of low concentration
• How do you calculate the rate of diffusion?

  Fick's law

  
• What is the rate of diffusion determined by?

  • size of concentration gradient
  • thickness of membrane
  • distance between 2 areas
  • size of molecule
• facilitated diffusion


  • same as diffusion but with carrier/channel proteins
  • aids larger molecules
• carrier proteins


  1. molecule attaches
  2. carrier changes shape
  3. moves across membrane
• channel proteins


  • forms pores in membrane
  • for charged molecules
  • only open if molecule is present
• factors affecting facilitated diffusion

  • conc gradient
  • number of carrier or channel proteins
• osmosis


  net movement of water through a partially permeable membrane from less negative water potential to more negative water potential
• water potential


  tendency of water to move from one place to another
• hypotonic solution


  less negative water potential outside the cell

  
• isotonic solution


  equal water potential in and out of cell

  
• hypertonic solution


  more negative water potential outside cell

  
• What does it mean when a plant cell is turgid?


  It's in a hypotonic solution:

  • water is moving into the cell
• What happens when a cell is plasmolysed?

  In a hypertonic solution:

  • water moves out of cell
  • protoplast pulls away from cell wall
• Animal cell in hypotonic solution

  cell burst

  
• animal cell in hypertonic solution


  shrivelled

  
• Active transport


  • uses energy
  • moves against conc grad
  • uses carrier proteins (use ATP)
• steps of active transport

  1. Molecule attaches to carrier protein
  2. ATP molecule provides energy to go against conc grad
  3. carrier changes shape and moves molecule to other side
• is energy required for simple diffusion?

  no
• is energy required for facilitated diffusion?

  no
• Is energy required for active transport?

  yes
• is energy required for osmosis?

  no
• What substances does diffusion move?

  oxygen, carbon dioxide, lipid soluble molecules
• What substances does facilitated diffusion move?

  glucose, polar molecules
• What substances does active transport move?

  many molecules
• What substances does osmosis move?

  water
• Where is simple diffusion found in the cell membrane?

  phospholipid bilayer
• Where is facilitated diffusion found in the cell membrane?

  protein carriers/ channels
• where is active transport found in the cell membrane?

  protein carriers/ channels
• Where is osmosis found in the cell membrane?

  phospholipid bilayer
• Factors affecting speed of simple diffusion

  conc grad, surface area, temp, mol size
• Factors affecting facilitated diffusion

  • conc gradient
  • number of carrier or channel proteins
• factors affecting speed of active transport

  ATP, carriers, speed binding, temp
• factors affecting speed of osmosis

  temp, surface area, water potential, conc grad