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Lecture 8

Lecture 8: "Photosynthesis I"

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Western University
Biology 1002B
Tom Haffie

Biology Lecture No.8: Photosynthesis I st Wednesday February 1 , 2012 Photosynthesis I: -Photosynthetic organisms are capable of such processes due to the high amounts of chlorophyll in their physiology. -Photosynthesis, where light energy is converted to chemical energy, powers the biosphere of Earth. Carbon Dioxide & Sugar: -Photosynthesis can be classified as an endergonic process as energy is needed to power this light-dependent reaction in order to convert energy into a chemical form. -Redox (Reduction/Oxidation) reactions are the transfer of electrons which are either oxidized (lost) or reduced (gained). Oxidation reactions are coupled with reduction reactions and vice versa. -CO 2oes not have a lot of energy associated with it as the oxygen atom is highly electronegative. Glucose on the other hand, is associated with lots of energy as molecules with carbon-hydrogen bonds carry much energy. -This is due to the fact that neither carbon nor hydrogen are very electronegative and therefore are great for the production of ATP. -Cellular respiration is the reverse reaction of photosynthesis and is endergonic as well. Two Stages Of Photosynthesis: -Photosynthesis can be separated into two processes: the light -dependent reaction and the Calvin cycle. -The light-dependent reaction is responsible for synthesizing NADPH and indirectly producing ATP as well. NADPH provides reducing power along with more electrons, while ATP provides the free energy used to drive the endergonic processes. -The reduction of CO i2to glucose and sugars is an example of one such endergonic process. Light As A Means Of Energy & Information: -Light is converted by organisms for photosynthesis or absorbed by organisms for information on the surrounding environment. Chlamydomonas is an example of an organism that can perform both. -This photosynthetic phytoplankton uses light to acquire information using its primitive eyespot and possesses what is known as a phototactic response; it can move toward and away from the light. -On the electromagnetic spectrum, the frequency (amount of energy) of a photon (a discrete particle of light) is inversely proportional to its wavelength. Absorption Of Light: -Organisms often absorb light through the use of pigments which are exceptional when it comes to trapping photons. -All functioning pigments display conjugated double bonds, whether they are in a ring system or in a linear arrangement. Using this configuration, electrons do not participate in bonding and are therefore able to be easily excited. This makes photon interaction more likely. -Pigments are bound very specifically to proteins and thus being another example of post-translational modulation. Once pigments are attached to a particular protein the portion responsible for congregating light is known as a chromophore. Light Absorption & Emission: -In photosynthesis two things should be quite clear: a photon may excite only one other photon and that chlorophyll has the capability to absorb both blue and red light (both of which have distinct excited states). -The higher energy state (for blue photons) eventually decays to a lower energy state (for red photons) in the form of heat. This excites the red photon at this energy level. -There are four possibilities that may arise from this excited electron: 1. It could lose energy as heat and therefore photosynthesis would not occur. 2. It could lose small amounts of heat and then re-emit light as fluorescence (a lower frequency, but greater wavelength than the initial photon; it emits a deep-red colour of light). 3. It could engage photosynthesis where the energy of the excited state leads to a biochemical reaction which eventually yields the oxidation of a molecule. This is known as photochemistry. 4. It could pass that energy to a neighbouring chlorophyll. This is known as resonance energy transfer. Chlorophyll & Colour: -The energy necessary to produce the excited state correlates perfectly with the energy state of the photons of red and blue light. -As there is no possible excited state that matches so perfectly with the energy state of photons of green light, they are instead reflected and form the familiar green colour of chlorophyll. -Fluorescence within these biochemical pathways occurs frequently, but it cannot be seen clearly as all the processes involved are competing reactions. It is possible to detect the fluorescence of such photosynthetic organisms, but most of the light goes into photochemistry and energy transfer. Absorption Spectra: -There is strong (but not equivalent) absorption of wavelengths of blue and red light by photosynthetic organisms. Chlorophyll b (a strong absorber of blue light) has a different excited state than Chlorophyll a (a stronger absorber of red light) due to variations in molecular structure. Importance Of Visible Light: -Visible light is the frequency/wavelengths used for photosynthesis and vision. Everything known in existence uses this very narrow spectrum of light for energy and information. -Photons that fall under the spectrum of visible light are frequently seen as they are part of the most dominant form of electromagnetic radiation that hits Earth. This is because much of the UV light that is derived from the sun is depleted by the ozone layer. -The energy of visible light is perfect in that it is not too strong or too weak for the Earth. Excitation of e
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