What Are Carotenoids | Carotenoid | Photosynthesis

It has been estimated that perhaps as many as half of available herbicides have a mode of action that involves interaction with a few components in the chain of II. Remember that transfer of electrons from Photosystem II to Photosystem I is essential for the production of photosynthetic energy. ( for animation review--Link currently broken, please come back after 12-31-12). A key step in this electron transfer chain is the of plastoquinone by the D1 protein in the thylakoid membrane. Herbicides with a mode of action involving this site act as inhibitors of plastoquinone binding. These herbicides bind to the D1 protein and block the binding of PQ. By inhibiting the binding of PQ, the process of photosynthetic electron transfer is interrupted, and the synthesis of ATP and NADPH in the is compromised. This results in an inability to fix CO2 and produce the nutrients needed for the plant to survive. The block in electron transfer also causes an oxidative stress and the generation of radicals which cause rapid cellular damage.

Much effort has gone into the design of this class of inhibitors. Since the mode of action involves competition for a binding site within a membrane environment, the effectiveness of the herbicide will be greatly affected by small changes in its structure. Thus, small modifications in the molecular structure of a herbicide may cause differential sensitivity in different species of plants. Also, because the D1 protein in different plant species will have slightly different sequences of amino acid residues, differential effectiveness is possible with the same herbicide molecule. Plants also have detoxification systems that may greatly affect the response of different crops to herbicides.

Redox functions of carotenoids in photosynthesis.

oung, A. and G. Britton (eds.). 1993. Carotenoids inphotosynthesis. Chapman & Hall, London.

Importance of Pigments in Photosynthesis | Sciencing

Plant pigments are important cues to humans and other herbivorous animals in helpingidentify plants, find plant parts such as fruit, leaves, stems, roots, or tubers, anddetermine stages of plant development such as fruit ripeness or overall senescence. It wasrealized early in this century that many of these pigments play a positive role in humanhealth. In 1919, Steenbock noted that yellow corn ( L.) and"yellow" vegetables (carrots ( L.) and sweetpotato ( L.)) eliminated the symptoms of vitamin A deficiency in rats while white cornand "white" vegetables (parsnip ( L.), potato ( L.), and beets ( L.) did not (; ). Since then, approximately 40 carotenoids have been found to bevitamin A precursors (). When provitamin Acarotenoids are consumed, they are enzymatically broken down to retinol (vitamin A). Inthis way, consumption of horticultural crops provides over 80% of the vitamin A for theworld (). Vitamin A deficiency worldwide isthe most common specific dietary deficiency as it afflicts millions of children each yearwith xerophthalmia, blindness, or death. Subclinical deficiency also reduces immunefunction to increase the risk of severe and fatal infections (). Therefore, the development of new andmore potent sources of provitamin A carotenoids in horticultural crops, and improvement ofproduction, shelf life and consumer acceptance of these crops can make an importantcontribution to improved human health.

Carotenoid pigments are found in many photosynthetic organisms, ..

Very soon after, more pieces of the puzzle were found by two chemists working in Geneva. Jean Senebier, a swiss pastor, found that "fixed air" (CO2) was taken up during photosynthesis, and Theodore de Saussure discovered that the other reactant necessary was water. The final contribution to the story came from a German surgeon, Julius Robert Mayer (right), who recognised that plants convert solar energy into chemical energy. He said:

569-579Carotenoids are usually considered to perform two major functions inphotosynthesis.

Role of plant pigments in photosynthesis ..

Protoporphyrinogen oxidase (PPO) is an enzyme in the of the plant cell that oxidizes protoporphyrinogen to produce IX. This product is important because it is a precursor molecule for both chlorophyll (needed for photosynthesis) and heme (needed for chains) (Figure: Protoporphyrin lX). However, inhibitors of the oxidase enzyme do more than merely block the production of chlorophyll and heme. When the enzyme is inhibited, the protoporphyrinogen substrate accumulates and is slowly oxidized by the high concentrations of O2 being produced in the chloroplast, producing protoporphyrin IX. This spontaneous production of the product may seem to bypass the inhibitor and let the cell function normally, but it has dire consequences.

Protoporphyrin is a very effective photosensitizing molecule. Normally, the concentrations of protoporphyrin in the cell are kept very low and it is channeled from its production site in the chloroplast to other locations in the cell where it is needed for heme biosynthesis. In the presence of the herbicide inhibitors, the concentrations of protoporphyrin increase and begin to accumulate throughout the cell. As with most of the herbicides discussed in this course, if the plant is maintained in the dark or in dim light, the effects of the herbicide are not observed. However, when exposed to light the protoporphyrin released in the cell is excited to the with a high efficiency and interacts with molecular O2 to produce singlet oxygen. Singlet oxygen is toxic to cells because it is much more destructive than molecular oxygen in the normal triplet state. Favorite targets of singlet oxygen include the double bonds of fatty acids and amino acids. Membranes, sites with high concentrations of unsaturated fatty acids, are particularly vulnerable to peroxidation (molecular damage from free radicals). The plasma membrane of the plant cell is considered to be the vulnerable component most impacted by the photodynamic damage from herbicides that inhibit protoporphyrinogen oxidase. Here is a velvetleaf plant that has been sprayed with Blazer, a PPO inhibitor (Figure: Blazer Damage). Notice the necrotic (localized dead tissue) lesions that have formed in response to the herbicide. Inhibition of PPO has caused release of protoporphyrin throughout the plant cell. In the presence of light singlet oxygen is produced, causing rapid cell death.

Plant Pigments and Photosynthesis - Biology Junction

Protoporphyrinogen oxidase (PPO) is an enzyme in the of the plant cell that oxidizes protoporphyrinogen to produce IX. This product is important because it is a precursor molecule for both chlorophyll (needed for photosynthesis) and heme (needed for chains) (Figure: Protoporphyrin lX).

carotenoids serve as accessory pigments in photosynthesis, ..

Carotenoid triplets play a photoprotective role in natural photosynthesis. The main process of carotenoid triplet formation is known to be triplet-triplet energy transfer from chlorophyll triplets. The structural requirements for high transfer yields are still a matter of discussion and the presence of competitive triplet formation pathways has not been excluded. Transient EPR measurements of triplet states formed by photoexcitation allow detection of the initial spin polarization. This pattern derives from the mechanism of triplet formation. In the case of triplet-triplet energy transfer, if the condition of spin angular momentum conservation is fulfilled, simulation of the EPR spectra gives information about the donor-acceptor mutual orientation. We describe transient EPR experiments on two artificial photosynthetic dyads, consisting of a carotenoid covalently-linked to a free-base or zinc substituted pyropheophorbide moiety and we discuss the results in terms of possible dyad conformations.