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Defending the Christian Worlview, Creationism, and Intelligent Design

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Defending the Christian Worlview, Creationism, and Intelligent Design » Photosynthesis, Protozoans,Plants and Bacterias » How are photosynthetic organisms able to protect themselves from energy-related damage?

How are photosynthetic organisms able to protect themselves from energy-related damage?

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How are photosynthetic organisms able to protect themselves from energy-related damage?

Dissipation of excess excitation energy in the light harvesting antenna of Photosystem II (PSII) gives
photosynthetic organisms an evolutionary advantage by reducing damage in fluctuating light conditions. This dissipation, known as qE, only occurs when excitation energy cannot be used in productive photosynthesis. In order to understand the energetic rearrangement that takes place to switch on this pathway, we have measured the change in fluorescence lifetime of the PSII antenna as qE is turned on in intact cells of Chlamydomonas reinhardtii, a photosynthetic alga.

In order to dissipate excess excitation energy in the PSII antenna while retaining usable excitation energy, photosynthetic organisms have evolved a rapid nonphotochemical quenching mechanism, known as qE, that is rapidly turned on and off through a feedback loop. This feedback loop improves the plantʼs fitness in variable light conditions. Nonphotochemical quenching (NPQ) is the general name for quenching of chlorophyll excitations in the PSII supercomplex by a method other than photochemistry at the reaction center. In healthy plants, the majority of NPQ serves to protect the plant from damage caused by excess excitation energy that the plant cannot use.

Monitoring the Fluorescence Lifetime of chlorophylls in the PSII supercomplex enables us to determine the lifetime of excitations. The lifetime of chlorophyll excitations decreases when PSII is in a quenching configuration. We can measure the fraction of PSIIs in a quenching configuration, as well as the lifetime of an average trajectory to a quenching site. Using our apparatus, we can measure the reduction in fluorescence lifetime as qE is activated in Chlamydomonas reinhardtii. We see that there is a 300 ms offset before qE turns on, followed by a rise of qE with a time constant of 400 ms. This tool provides more information than conventional PAM fluorometers because it enables us to measure the amplitude of qE, rather than the bulk fluorescence yield. We can also simulate the effect of qE on the time-evolution of chlorophyll fluorescence yield. We can do this by modeling the relevant variables by using a series of coupled nonlinear differential equations that can be solved using a stiff differential equations solver.

2How are photosynthetic organisms able to protect themselves from energy-related damage? Empty Non-photochemical quenching Sat Mar 08, 2014 5:59 am


Non-photochemical quenching

Non-photochemical quenching (NPQ) is a mechanism employed by plants and algae to protect themselves from the adverse effects of high light intensity. It involves the quenching of singlet excited state chlorophylls (Chl) via enhanced internal conversion to the ground state (non-radiative decay), thus harmlessly dissipating excess excitation energy as heat through molecular vibrations. NPQ occurs in almost all photosynthetic eukaryotes (algae and plants), and helps to regulate and protect photosynthesis in environments where light energy absorption exceeds the capacity for light utilization in photosynthesis

Last edited by Admin on Sat Mar 08, 2014 10:15 am; edited 1 time in total

3How are photosynthetic organisms able to protect themselves from energy-related damage? Empty Photosynthesis and oxidative stress Sat Mar 08, 2014 6:31 am


Photosynthesis and oxidative stress


Plants and algae have a love/hate relationship with light. As oxygenic photoautotrophic organisms, they require light for life; however, too much light can lead to increased production of damaging reactive oxygen species (ROS) which are by-products of photosynthesis. In extreme cases, photooxidative damage can cause pigment bleaching and plant death. The productivity of plants is therefore influenced to a great extent by environmental conditions: non optimal conditions with respect to abiotic factors such as cold, drought and excessive light are experienced by the plant during the vegetative cycle and they can damage irreversibly the cultivations and adversely affect the production. In the last years many experimental evidences have suggested that the chloroplast is the main site of ROS production and the primary site of damage of abiotic stresses: various abiotic stresses (cold, light, drought) with different mechanisms impair the chloroplast function and lead to the production of reactive oxygen species (due to the overexcitation of the photosystem) that may damage chloroplast proteins, membranes and pigments. The plant antenna system is composed of homologous proteins belonging to the Lhc family (light harvesting complex). These proteins, which bind chlorophyll a, chlorophyll b and xanthophylls and are located in the thylakoid membranes, have multiple functions: besides collecting light energy and transferring it to the photosynthetic reaction centres, they are able to thermally dissipate excitation energy in excess with respect to the capacity of electron transport in order to carry out the essential function of photoprotection.


In order to elucidate the mechanisms of resistance against abiotic/oxidative stress at the chloroplast level, different approaches have been undertaken: 1) Studies of the structure/function of the Lhc proteins: light harvesting and energy dissipation By utilising spectroscopic and biochemical approaches, we investigate in vivo and in vitro the molecular mechanisms mediated by Lhc proteins involved in the photoprotection phenomenon necessary to prevent the oxydative stress.

2) A source of oxidative stress: Photosystem II Particular phenomena of electron transport at the level of the photosystem II are studied in detail in order to understand the complex pattern of energy transfer inside this complex dedicated to the energy transfer and charge separation, the first events of the photosynthetic process that may be strictly regulated to avoid the formation of reactive oxygen species.

3) The function of carotenoids during protection against oxidative stress. These pigments, bound to many proteins of the photosystem apparatus, carry out an indispensable function of photoprotection. The use of spectroscopic techniques on whole organisms and single proteins is utilised to study the molecular mechanisms of action of this class of molecules during the photoprotection process.

4) Electron transfer in photosynthetic bacteria The photosynthetic bacteria are utilised as a model to investigate the Photosystem II turnover rates and the charge separation and recombination.

4How are photosynthetic organisms able to protect themselves from energy-related damage? Empty Mechanism of QE-quenching Sat Mar 08, 2014 10:28 am


Mechanism of QE-quenching

The qE-quenching reflects a mechanism associated with a component of the light-harvesting antenna rather than the reaction center of photosystem (PS) II; we suggest that it occurs through formation of an efficient quencher in one of the minor chlorophyll protein (CP) complexes. b) The minor CPs have glutamate residues instead of glutamines at positions shown in light harvesting complex II(LHCII) to be ligands to chlorophylls near the lumenal interface. We suggest that the quenching reflects a change in ligation of chlorophyll on protonation of these glutamate residues leading to formation of an exciton coupled dimer with a neighboring pigment, in which additional energy levels allow vibrational relaxation of the excited singlet. The model accounts for the dependence on low lumenal pH, the ligand residue changes between LHCII and the minor CPs, the preferential distribution of components of the xanthophyll cycle in the minor CPs, the inhibition of qE-quenching by DCCD, and the specific binding of DCCD to the minor CPs.

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