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Lecture 14

BIOL120 Lecture Notes - Lecture 14: Thylakoid, Football Association Of Indonesia, P680

Course Code
Simon Chuong

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Photosynthesis – allows plants to convert carbon dioxide and water into sugars that form
the basis of other organic molecules
Photosynthetic organisms underlie almost every food chain
Chemoautotrophs use carbon dioxide as a carbon source and inorganic chemical
for an energy source
Every carbon atom in your body has existed at some point in a photosynthetic
94% of the dry weight of living organisms is made up of carbon-containing
molecules produced by photosynthesis
Oxygen is a by-product of photosynthesis (ozone, O3, block UV)
Two series of reactions – the light reactions and the Calvin Cycle (the Calvin cycle is also
called the dark reactions but it light dependent therefore the name “dark reactions”
doesn’t work because it gives the idea that it could sustain itself in the dark which is not
Light Reactions – occur within the thylakoid membranes and are the “photo” part of
photosynthesis because they capture light energy
The Calvin cycle – occur within the stroma and are the “synthesis” part of photosynthesis
because they assemble simple 3-carbon sugars
To produce one glucose molecule (C6H12O6)
6CO2 + 12H2O + light energy  C 6H12O6 + 6O2 + 6H2O

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Photosynthesis and Respiration
Are interdependent processes
All organisms, including plants, must extract energy from food through
Respiration takes places in mitochondria where the breakdown of organic
molecules in the presence of oxygen converts stored energy into ATP
ATP is the energy currency of the cell thus, all organism are solar-powered
The Light Reactions – in the light reactions, light energy is captured by light absorbing
molecules, the chlorophyll pigments (chlorophylls a and b)
Each type of pigment absorbs light energy at certain wavelengths
Chlorophyll absorbs photons in the blue (430nm) and red (680 nm) portions of the
visible spectrum (400-700 nm)
Green light is reflected
Chlorophyll a – is the only pigment directly involved in the light reactions
Chlorophyll b tranfers energy to chlorophyll a molecules but does not directly
take part in the light reactions
Chlorophyll b is an accessory pigment along with carotenoids
**Blue light is used indirectly
Energy absorbed from the blue portion of the spectrum must partially dissipate
until the electron ends up at the same energy as an electron energized by a photon
from the red portion of the spectrum
Chlorophyll a primarily absorbs blue-violet light and red
NOTE: the lower the wavelength the higher the energy where excess energy is going to
be lost as heat
Accessory Pigments
Carotenoids which are also accessory pigments, primarily absorb blue-green light
and reflect yellow or yellow-orange light
They are not visible until chlorophyll a breaks down (usually in the fall due to
colder temperatures)
Lower plants and higher plants absorb light in the blue-green region and reflect red
and yellow-orange colour through the accessory pigments. It exists early on but in the
fall you have degradation of chlorophyll a so the accessory pigments then can show
The Action Spectrum – a profile of how effectively different wavelengths of light
promote photosynthesis by measuring oxygen release.
**Thomas Englemann, 1883, used bacteria to monitor oxygen production. In the
solution he had oxygen loving bacteria and he looked for where they would
accumulate and found that they gathered around certain areas along the spectrum that
had been broken up – more readily around the blue and red wavelengths and barely
any in the green area
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