vertebrae eye

• define the term retina


  the retina is the area within the idea that detects light
• make the two types of photoreceptor cells on the retina


  rods and cones
• explain the difference between rods and cones


  rods function in dim light but do not allow colour perception

  
  

  cones are responsible for colour vision and only function in bright light
• describe the 2 components of the photoreceptors on the membrane of rods and cones


  retinal: a light sensitive molecule is combined with

  opsin: a membrane protein
• name the retinal-opsin complex in rod cells


  rhodopsin
• describe the initial event in the detection of light by rods

  retinal absorbs a photon of light and rhodopsin changes conformation to a photoexcited rhodopsin
• explain why a protein cascade must amplify the signal from rhodopsin


  a very high degree of amplification results in rod cells being able to respond to low intensities of light
• describe (in detail) how the signal from photoexcited rhodopsin is amplified


  1. a single photoexcited rhodopsin activated hundreds kf molecules of G-protein (transducin)

  2. transducin activates ONE MOLECULE of the enzyme phosphodiesterase (PDE)

  3. PDE catalyses the hydrolysis of a molecule called cyclic GMP (cGMP)

  4. each active PDE molecule causes hydrolysis and breakdown of 1000s of cGMP per second
• state the number of PDE activated by each G-protein


  ONE
• state the effect of reduction in cGMP concentration as a result of its hydrolysis


  affects the function of ion channels in the membrane of rod cells by closing them
• state what triggers nerve impulses in neurons in the retina


  closure of ion channels in the membrane of the rod cells
• name the retinal-opsin complex in cone cells


  photoreceptor proteins
• explain why different forms of opsin combine with retinal


  to give different receptor proteins, each with a maximal sensitivity to specific wavelengths: red, green, blue or UV
• explain why come cells have different receptor proteins


  cone cells have specific receptor proteins sensitive to certain wavelengths of light but when all cones are present an animal will be sensitive to the full visual light spectrum