exam questions

• Describe the role of sodium ions in the functioning of a mammalian rod cell (3)

  -        sodium ions are pumped out of the rod cell 

  -       In the light/when rod cells are stimulated Sodium ions do not move back into rod cell in the light

  -       In the dark/when rod cells are not stimulated sodium ions move back into rod cell

  -       so, in the light the rod cell is hyperpolarised but in the dark the rod cell is depolarised
• explain the effect of increasing light intensity on the current production by a rod cell (5)

  -       increasing light intensity decreases the current

  -       because rhodopsin is broken down into opsin and retinal, opsin is released 

  -       thus, more opsin binds to the channel proteins

  -       sodium gated voltage channels close 

  -       stopping the influx of sodium ions, thus no depolarisation of rod cells.
• Describe how absorption of light by rhodopsin results in an action potential in the optic nerve (3)

  -       opsin released via bleaching of rhodopsin causing sodium ion channels to be blocked 

  -       which causes hyperpolarisation in the rod cell 

  -       causing ap in bipolar cell- resulting in action potential in the optic nerve 
• describe how light causes a change in the voltage across the cell surface membrane of a rod cell (4)

  -       when light intensity increases, light is absorbed by rhodopsin 

  -       rhodopsin changes shape 

  -       rhodopsin is converted to retinal and opsin 

  -       opsin binds with cell surface membrane so fewer sodium ions enter rod cell 

  -       whilst also being pumped out of rod cell, leading to hyperpolarisation
• suggest why a change in voltage across the cell surface membrane of a rod cell may not lead to formation of AP in an optic nerve (2)

  -       not enough rhodopsin is bleached, not enough opsin binds to membrane 

  -       so, change in voltage is insufficient 

  -       so inhibitory NT is still released from rod cell

  -       so, depolarisation in bipolar neurone is insufficient 
• explain the sequence of events that occur in rod cells in both light and dark conditions (6)

  in the light:

  • Light bleaches rhodopsin (into retinal and opsin)
  • This causes Na+ channels in the rod cell membrane to close
  • Therefore, Na+ can no longer diffuse back into the rod cell
  • Active transport of Na+ out of the cell is occurring
  • so inside of the rod cell becomes more negative/is hyperpolarised
  • Therefore, the inhibitory neurotransmitter is no longer released from rod cell 
  • This generates an action potential in the next cell (bipolar cell)
  • An impulse is sent along the optic nerve
• explain the sequence of events that occur in rod cells in both light and dark conditions (6)

  In the dark:

  • Sodium ions/Na+ is actively transported out of the cell 
  • A concentration gradient is established
  • Sodium ions/Na+ diffuses back into the cell down a concentration gradient
  • The cell is depolarised (more positive) as the potential difference between the inside and outside the cell is minimal
  • The rod cell releases an inhibitory neurotransmitter
  • Action potentials are prevented in the bipolar cells
  • No impulse was sent to the optic nerve
• describe how movement of sodium ions in a rod cell affects depolarisation in a bipolar neurone(4)

  • sodium ions actively transported out of rod cells
  • in bright light, rhodopsin is broken down and opsin prevents NA+ entering, causing hyperpolarisation in the rod cell
  • hyperpolarisation in the rod cell prevents inhibitory NT being released
  • thus depolarisation can occur in the bipolar neurone