Transport in Cells

• Diffusion

  The movement of particles in a gas or a liquid from an area of high concentration to an area of low concentration down a concentration gradient
• Partially permeable membrane

  Lets small soluble substances through but not others
• Factors affecting rate of diffusion

  • distance
  • surface area
  • concentration gradient / difference
  • temperature
• What increases the rate of diffusion

  • decreased distance
  • increased surface area
  • increased concentration gradient
  • increased temperature
• Osmosis

  Diffusion of water molecules from a dilute to a more concentrated solution through a partially permeable membrane down a concentration gradient
• Red blood cell placed in pure water

  Water moves into cell by osmosis and bursts
• Cell placed in a liquid the same concentration

  No net movement of water by osmosis
• Red blood cell placed in a concentrated solution

  Water moves out of the cell and shrivels
• Turgid

  When water has moved into the cell by osmosis and cell contents is pushed outwards against the wall
• Flaccid and plasmolysed

  When water has moved out of a cell and becomes shrivelled ONLY IN PLANTS CELLS WHEN CONTENTS OF CELL COMES AWAY FROM CELL WALL
• Active transport

  The movement of molecules and ions across a cell membrane against the concentration gradient - to a region of higher concentration
• Why plants rely on active transport

  To obtain mineral ions through root hair cells
• Cells involved in active transport

  Lots of mitochondria because aerobic respiration takes place here and releases energy needed for active transport
• SA to volume ratio when size of object increases

  Decreases
• SA to volume ratio when size of object decreases

  Increased
• How are root hair cells (in plants) specialised to carry out their function?

  1. Large surface area - To speed up osmosis
  2. Thin wall - To absorb water more quickly
• How is the alveoli specialised to maximise diffusion?

  • Large surface area
  • Thin walls
  • Good blood supply
• How is villi specialised to maximise absorption of nutrients?

  1. Increases the surface area of the small intestines
  2. Short diffusion distance
  3. Network of capillaries