Cycle 1: Chlamydomonas and how it uses light

• Facts about chlamy


  Green algae, photosynthetic, unicellular eukaryote
• Chlamy parts


  Eyespot (controls phototaxis), cytoplasm, chloroplast, nucleus, flagellum
• Growing chlamy in the lab


  Growth occurs by binary fission, generation/ doubling time is ~ 10 hours
• Macronutrients


  Required in large quantities
• Micronutrients


  Required in lesser quantities
• Chlamy liquid culture


  A darker green colour means more chlamy cells are present
• Growth curve x-axis


  Independent variable (time)
• Growth curve y-axis


  Dependent variable (number of cells)
• Phase 1 (lag phase)


  Cells do not grow much (culture adapting to new environment)
• Phase 2 (exponential phase)


  Exponential cell growth, very steep (nutrient available for growth, used to calculate growth rate)
• Phase 3 (stationary phase)


  Cells stop dividing (depleted a nutrient called limiting nutrient)
• Microtubule


  Protein polymer (protein chain) of tubulin subunits
• Dynein


  Walks towards the end, causing the microtubules to bend, leading to a whip-like motion
• Motile cilia


  Cilia that move, have dynein (ex: sperm cells)
• Non-motile (sensory) cilia


  Function as sensory receptors and don't move (no dynein), membrane proteins that interact with stimulus
• Ciliopathies


  Genetic mutations that affect cilia structure and/or function, causing disease
• Chlamy is a good model system because...


  It can be used to study a number of human diseases, are collectively called ciliopathies
• Genetically heterogeneous disease


  Same ciliopathy (phenotype) caused by different genetic mutations (genotype)
• Positive phototaxis


  Cells move toward light
• Negative Phototaxis


  Cells move away from light
• Phototaxis


  The ability of the cell to move in relation to light
• Mutation in flagella gene may cause

  Loss of phototaxis in Chlamy
• Chlamy eyespot


  Not involved in photosynthesis, gives chlamy a sense of direction (where the light is coming from)
• Channelrhodopsin


  Allows Chlamy to see
• Light-gated ion channel

  Light causes channel to open, positive ions rush in (depolarization) = action potential. To re-establish -ve charge in the cell, sodium ions pumped out (repolarization)
• Channelrhodopsin = 


  Opsin (protein that has no ability to interact with light) + retinal (pigment)
• Chlorophyll has a conjugated system of double bonds

  Electrons available for light absorption
• Absorption occurs when ...


  Energy from the photo is transferred to an electron within a molecule, moving the electron into the excited state (ex: ChI to ChI*)
• Blue photons of light have


  More energy than red photons (too unstable, decays as heat to red energy level)
• Energy of photon must

  Match energy gap between ground and excited state
• Fluorescence


  The release of energy when the excited electron returns back to ground state
• Why can chlorophyll not absorb green light?

  There is no excited state with that energy level
• Photoisomerization


  Trans-retinal absorbs light and turns into cis-retinal (this drives change in opsin conformation)
• Opsin conformation


  Kink in molecule in cis formation, protein gate opens 
• What cannot absorb light?

  Protein
• Rhodopsin


  Found in human eyes, enables rod cells to be sensitive to light
• Channelrhodopsin and rhodopsin similarities


  Both involve an ion channel, both systems absorb light
• Channelrhodopsin and rhodopsin differences

  Rhodopsin is not the ion channel itself - triggers further downstream reactions. G-protein coupled receptor.
• Action potential in the eye

  Rhodopsin changes shape, activates a G-protein complex, which opens a different channel and allows for an action potential in the eye
• Channelrhodopsin and rhodopsin are not evolutionarily related but are


  Analogous structures, both recruited retinal (the pigment component)