All photosynthetic organisms plants, certain protistans, prochlorobacteria, and cyanobacteria have chlorophyll a. Accessory pigments absorb energy that chlorophyll a does not absorb. Accessory pigments include chlorophyll b also c, d, and e in algae and protistans , xanthophylls, and carotenoids such as beta-carotene.
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Chlorophyll a absorbs its energy from the Violet-Blue and Reddish orange-Red wavelengths, and little from the intermediate Green-Yellow-Orange wavelengths. Molecular model of chlorophyll. Molecular model of carotene. Carotenoids and chlorophyll b absorb some of the energy in the green wavelength. Why not so much in the orange and yellow wavelengths?
Burgess, Lauren / Unit 4. Photosynthesis
Both chlorophylls also absorb in the orange-red end of the spectrum with longer wavelengths and lower energy. The origins of photosynthetic organisms in the sea may account for this. Shorter wavelengths with more energy do not penetrate much below 5 meters deep in sea water.
The ability to absorb some energy from the longer hence more penetrating wavelengths might have been an advantage to early photosynthetic algae that were not able to be in the upper photic zone of the sea all the time. The molecular structure of chlorophylls. The action spectrum of photosynthesis is the relative effectiveness of different wavelengths of light at generating electrons. If a pigment absorbs light energy, one of three things will occur.
Energy is dissipated as heat. The energy may be emitted immediately as a longer wavelength, a phenomenon known as fluorescence.
Energy may trigger a chemical reaction, as in photosynthesis. Chlorophyll only triggers a chemical reaction when it is associated with proteins embedded in a membrane as in a chloroplast or the membrane infoldings found in photosynthetic prokaryotes such as cyanobacteria and prochlorobacteria. Absorption spectrum of several plant pigments left and action spectrum of elodea right , a common aquarium plant used in lab experiments about photosynthesis. Images from Purves et al. The thylakoid is the structural unit of photosynthesis.
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Only eukaryotes have chloroplasts with a surrounding membrane. Thylakoids are stacked like pancakes in stacks known collectively as grana. The areas between grana are referred to as stroma. While the mitochondrion has two membrane systems, the chloroplast has three, forming three compartments. Structure of a chloroplast. Photosynthesis is a two stage process. The first process is the Light Dependent Process Light Reactions , requires the direct energy of light to make energy carrier molecules that are used in the second process. The Dark Reactions can usually occur in the dark, if the energy carriers from the light process are present.
Recent evidence suggests that a major enzyme of the Dark Reaction is indirectly stimulated by light, thus the term Dark Reaction is somewhat of a misnomer. The Light Reactions occur in the grana and the Dark Reactions take place in the stroma of the chloroplasts. Overview of the two steps in the photosynthesis process. In the Light Dependent Processes Light Reactions light strikes chlorophyll a in such a way as to excite electrons to a higher energy state. Water is split in the process, releasing oxygen as a by-product of the reaction.
The incorporation of carbon dioxide into organic compounds is known as carbon fixation. The energy for this comes from the first phase of the photosynthetic process. Living systems cannot directly utilize light energy, but can, through a complicated series of reactions, convert it into C-C bond energy that can be released by glycolysis and other metabolic processes. Photosystems are arrangements of chlorophyll and other pigments packed into thylakoids.
Many Prokaryotes have only one photosystem, Photosystem II so numbered because, while it was most likely the first to evolve, it was the second one discovered. Photosystem I uses chlorophyll a, in the form referred to as P Photosystem II uses a form of chlorophyll a known as P Both "active" forms of chlorophyll a function in photosynthesis due to their association with proteins in the thylakoid membrane.
Action of a photosystem. Photophosphorylation is the process of converting energy from a light-excited electron into the pyrophosphate bond of an ADP molecule. This occurs when the electrons from water are excited by the light in the presence of P The energy transfer is similar to the chemiosmotic electron transport occurring in the mitochondria. Light energy causes the removal of an electron from a molecule of P that is part of Photosystem II.
These O -2 ions combine to form the diatomic O 2 that is released. The electron is "boosted" to a higher energy state and attached to a primary electron acceptor, which begins a series of redox reactions, passing the electron through a series of electron carriers, eventually attaching it to a molecule in Photosystem I. Light acts on a molecule of P in Photosystem I, causing an electron to be "boosted" to a still higher potential. The electron is attached to a different primary electron acceptor that is a different molecule from the one associated with Photosystem II.
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