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BIO 1002B LECTURE OUTCOMES
Lecture 1: Introduction to Chlamydomonas
Characteristics of Chlamydomonas that make it a useful model system
- Unicellular—single cell organism.
- Sexually active, light-harvesting and carbon-reducing
- Has an eyespot and flagella that work together
o First organism to combine a sense with movement.
o Flagellum makes it a model system for human diseases due to its
similar traits.
- Are both heterotrophs and autotrophs.
- Contains chloroplast and flagella.
Relatedness of Chlamydomonas to plants and animals
- More related to plants than animals.
- Classified as a protist.
Relationship between genome size and protein coding genes
- No relationship between genome size and protein-coding genes.
- Genome size of Chlamy is bigger than E.coli, but much smaller than
humans.
o However, Chlamy has fewer amounts of protein-coding genes than
humans.
- Some genomes simply contain much more junk DNA than others.
Phototransduction from eyespot to flagella
- Phototransduction: the process by which light is converted into electrical
signals.
o The process of absorbing light and creating a response.
- Channel rhodopsin, a protein found in the eyespot, absorbs a photon of light.
o It changes conformation and opens Calcium and Hydrogen pores.
o Ca+2 and H+ leak into the flagellum.
Flagellum moves.
- The plasma membrane around the flagella is depolarized.
, o Results in an action potential that shoots down plasma membrane.
Advantages to Chlamydomonas in being phototactic.
- Phototactic/phototaxis: movement of an organism towards or away from a
source of light.
- Maximized exposure to light for photosynthesis
o By moving towards a source of light, Chlamy is able to maximize the
time spent exposed to sunlight.
Maximized photosynthesis output.
- Decreased excess energy
o By moving away from the light, Chlamy can halt photosynthesis when
it has sufficient amounts of energy.
Distinctions between primitive, complex, simple.
- Primitive: occurred early in evolutionary history and is in the fossil record.
o Chlamy does not appear on the fossil record.
Impossible to determine if the eyespot is a primitive structure or
not.
The eyespot is simple, however.
- Simple & complex are opposites:
o Simple: not complicated.
o Complex: many mechanisms.
- Simple =/= primitive.
Mental Floss:
Reasons why Chlamydomonas might move towards a light source.
- To maximize photosynthesis output.
o More time in light = more photoexcitation = more photosynthesis =
more energy.
Reasons why Chlamydomonas might move away from a light source.
- Minimize excess energy.
o Chlamy has enough food. Prevent macromolecular crowding.
- Minimizing photodamage.
o High light intensities can cause damage to PSII.
, - Prevent heat shock response
o Induces/represses transcript abundance for optimal growth.
Possible mutations that could cause a Chlamydomonas cell not to be
phototactic.
- Mutations to any of the enzymes involved in the phototransduction of the
eyespot.
- Mutations that repress the transcription of proteins involved.
Longer term....think about homology between human eye and
Chlamydomonas eyespot.
- The eyespot uses channelrhodopsin, while the eye uses rhodopsin
Lecture 2: Light - Energy and Information
Relationship between wavelength and energy content of a photon.
- Inversely proportional.
- Higher the wavelength = lower the energy content of a photon.
Molecular characteristic of pigments that make them able to absorb light.
- Light absorption is the excitation of an electron from the ground state to a
higher excited state.
- Conjugated ring structure with alternating double bond/single bond
structure.
o Pi-orbital bonds are non-bonding electrons that are easily excitable.
o These electrons can interact with photons of light.
- Pigments absorb lights at different wavelengths.
o Differ in the energy required to excite its electrons.
Relationship between pigments and associated protein.
- Pigments absorb light, proteins do not.
- Pigments bind to associated proteins.
Four “fates” of the excited state of chlorophyll resulting from absorption of
photons.
, 1. The chlorophyll loses its heat.
o Discards the heat and returns to ground state.
o Fluorescence
Wavelength of fluorescence is always longer (less energy) than
the wavelength of the photon it absorbed.
2. The chlorophyll loses a little heat.
o The electron goes from the higher excited state to the lower excited
state.
3. The chlorophyll uses the energy.
o The energy of the excited state is used for photochemistry.
Photochemistry: the use of the energy from light to change the
structure of the molecule.
4. The chlorophyll transfers the energy.
o Pigment-to-pigment energy transfer.
o Does not occur in eye, occurs in chloroplast.
Relationship between energy of photon and electron excited states to explain
pigment colour and absorption spectrum.
- Pigments can only absorb certain wavelengths of light to excite its electrons
to the excited state.
o Pigments only absorb certain colours.
- The absorption spectrum is the wavelengths of light that a pigment absorbs.
- A pigment’s colour is the reflection of the wavelengths that are not absorbed
by the pigment.
o Chlorophyll cannot absorb the green colour: it reflects it.
Distinctions of photochemistry between phototransduction (vision, eyespot...)
& photosynthesis
- Photochemistry: the use of the energy from light to change the structure of
the molecule.
- Phototransduction: the use of energy from light to change the conformation
of protein.
o Cis-trans conformation changes.
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