The vertebrate eye

• Where are photoreceptor cells found?

  the retina of the eye
• What are the two types of photoreceptor cell?

  Rods and Cones
• Rods


  function in dim light but don't allow for colour perception
• Cones


  function only in bright light but are responsible for colour vision
• Retinal


  light sensitive molecule that captures light energy. Comes from vitamin A e.g carrots
• Opsin


  the polypeptide retinal is a prosthetic group to.
• Rhodopsin complex


  Retinal + opsin = rhodopsin

  Embedded in the cell membrane inside photoreceptor cells
• Photopsin


  Cones contain photopsin to detect colours
• Rhodopsin


  Rods contain rhodopsin to detect low light levels
• how does the eye work when In darkness?


  Rhodopsin is inactive so rod cells produce cyclic GMP which binds to ligand-gated sodium channels keeping channels open so that sodium ions flow across the membrane. This depolarises the membrane and no nerve impulse is generated
• How is a nerve impulse generated in the eye?

  Retinal absorbs light causing a conformational change and making the rhodopsin active (photoexcited rhodopsin) This change activates hundreds of molecules of G-protein called TRANSDUCIN. This activates hundreds of molecules of the enzyme PHOSPHODIESTERASE which causes the breakdown of cGMP causing sodium channels to close and prevents sodium ions from moving in. Sufficient build up of sodium ions causes hyperpolarisation and generates a nerve impulse
• Protein cascade in rod cells when activated


  Rhodopsin- transducin- phosphodiesterase- channels
• why are rod cells extremely sensitive to light?

  This is a method that has a high degree of amplification, meaning a single photon results in a large effect. This means rod cells are extremely sensitive to light even in low light
• Four types of cone cell

  Green, blue and red and UV
• Photopsin and wavelength


  All photopsin have different sensitivities to different wavelengths of light
• describe the role of photoreceptors in triggering a nerve impulse

  Rhodopsin is the light sensitive molecule in rod cells

  Rhodopsin is ade through the combination of retinal and opsin

  Cone cells are sensitive to colour

  In cone cells different forms of opsin combine with retinal

  One photon can stimulate rhodopsin

  Rod cells have a very high degree of amplification and are extremely sensitive to light even in low light

  A cascade of proteins amplifies the signal

  Hundreds of G-protein molecules called transducin are activated