Organismal Physiology Lecture No. 6: Photosynthesis
Thursday September 27 , 2012
Introduction:
-Autotrophs make food from sunlight (and other energy sources), while heterotrophs have to use the
food to do work. Photosynthesis is by far the most important biological process on Earth due to carbon
fixation happening because of algae hotspots in the oceans and Canada’s boreal forest. There is an
annual cycle of CO concentration in the atmosphere that constantly increases, and then decreases. This
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is referred to as “planetary breathing” and it is the respiring of CO 2n the atmosphere.
Efficiency Of Photosynthesis:
-Plants are far more efficient than solar panels and other commercial photovoltaic cells.
Light Is Energy:
-The range of wavelengths and frequencies of the visible spectrum of light is the only kind used in
photosynthesis.
Pigments:
- Molecules that absorb photons are generally observed as being the same colour as the wavelengths
they are reflecting. In the case of green chlorophyll, it reflects green light. The absorption of the photon
increases the energy level of the molecule. As chlorophyll absorbs a photon of blue light, it gets to a
higher excited state, which will be lost due to heat loss. As chlorophyll absorbs a photon of red light, it
reaches its lowest excited state, which returns to ground state by fluorescence (the loss of energy by the
emission of light of longer wavelength).
Absorption Spectrum Of A Leaf:
-The absorption spectrum of a plant (or leaf pigment extract) describes how much a pigment is
absorbing a particular wavelength of light. The action spectrum describes the O evolution rate (relative
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rate of photosynthesis) at each wavelength of light.
Photosynthetic Structures Of The Chloroplast:
-The chloroplast contains a double membrane (an outer envelope and an inner envelope) with an
intermembrane space in between. Inside the chloroplast, within the space of the stroma, the thylakoids
are stacked as grana lamellae (the site of Photosystem II) and connected by stroma lamellae (the site of
Photosystem I). Under weak blue light, chloroplasts will try to maximize their exposure to photons, while
under strong blue light, chloroplasts will try to minimize their exposure to photons (hiding behind cell
walls). Energy Harvesting & Reaction Centres:
-A group of pigment molecules will form an antenna complex with the reaction centre denoting a special
pair of chlorophyll molecules. This reaction centre (comprised of either P680 or P700) will receive the
transfer of energy (light) by other chlorophylls in the antenna complex in order to accept and then
donate electrons (electron transfer). The combination of a reaction centre and its antenna complex is
known as a photosystem. The chlorophyll-a molecules (comprising the antenna complex) each have an
Mg-containing porphyrin group (related to the Heme group in haemoglobin).
Two Reactions Of Photosynthesis:
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-Light-Dependent Reactions: Uses water and produces oxygen, uses ADP and NADP and produces
ATP and NADPH, energy input from the transfer of photons. Can use the rate of O production to
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measure the photosynthetic rate.
-Light-Independent Reactions: Uses CO and p2oduces sugars, uses ATP and NADPH and produces ADP
and NADP , energy input from light-dependent reactions. Can use the rate of net CO consump2ion to
measure the photosynthetic rate.
-A coupled system of reactions.
Photosynthetic Electron Transport Chain:
-As red light hits PSII and is transferred to the P680 (strong oxidant) reaction ce
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