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Reaction Kinetics in Food Systems
Published in Dennis R. Heldman, Daryl B. Lund, Cristina M. Sabliov, Handbook of Food Engineering, 2018
Ricardo Villota, James G. Hawkes
Other factors influencing the stability of chlorophylls include light, oxygen, water activity, irradiation, pH, presence of metal traces, and enzymatic activity. Lajolo and Lanfer Marquez (1982) indicated higher rates of degradation with water activity in a spinach model system at 38.6°C. Similarly, the authors observed an increase in the rates of chlorophyll degradation upon a decrease in pH for the range 5.9–6.8. These results confirm the well-known and most common mechanism for chlorophyll degradation through its acid-catalyzed transformation into pheophytin (Figure 3.26). This reaction has been reported by several authors to follow first-order kinetics. The mechanisms by which chlorophylls degrade, of course, depend upon the process under consideration. For instance, Minguez-Mosquera et al. (1989) found that chlorophyllides were intermediary products in the fermentation of olives, and that the ratio of the various degradation products, including chlorophyllides a, b; pheophytins a, b; and pheophorbides a, b, were very dependent upon pH of the system (Figure 3.26).
Methods for Biological Determination
Published in V. Dean Adams, Water and Wastewater Examination Manual, 2017
Three chlorophyll pigments are commonly found in algae: chlorophylls a, b, and c. In this procedure, the chlorophyll is extracted in acetone. The optical density (absorbance) of the extract is determined spectro-photometrically at four different wavelengths. The optical density reading at 750 nm is used as a turbidity correction. Chlorophylls a, b, and c are calculated from the optical density measurements at the other wavelengths. Chlorophyll a may be overestimated if pheophytin a, which fluoresces near the same wavelength, is present. Pheophytin a is a common degradation product of chlorophyll a; measurements for both pigments must be made in order to correct for the pheophytin concentration. The ratio of chlorophyll a to pheophytin a is a good indicator of the physiological condition of the phytoplankton. In the sample, acidification with dilute acid releases the magnesium atom from the chlorophyll a molecule, converting it to pheophytin a. The fluorescence is read before and after acidification and concentrations for both chlorophyll a and pheophytin a are calculated.
Photosynthesis
Published in Thomas M. Nordlund, Peter M. Hoffmann, Quantitative Understanding of Biosystems, 2019
Thomas M. Nordlund, Peter M. Hoffmann
In plants and green algae, the light-dependent photosynthetic reactions occur in the thylakoid membranes of the chloroplasts. The primary products of these reactions are ATP and NADPH. ATP, of course, is the common energy currency of life: its stored chemical free energy can be used for almost any energy-requiring reaction in the cell. NADPH also represents stored energy but in a form more specific to electrical processes—the addition of an electron (usually with an accompanying proton to ensure charge neutrality) to another molecule or the replenishment of charge that may have been pumped across a membrane. The photons are absorbed by light-harvesting antenna protein complexes of photosystem II by chlorophyll and accessory pigments. When the reaction center chlorophyll (actually, a chlorophyll dimer) at the core of the photosystem II unit accepts this excitation energy, the center transfers an electron to an electron-accepting molecule, pheophytin. The electron is then shuttled through the Z scheme’s first electron transport chain, which can either generate a chemiosmotic potential across the thylakoid membrane or transfer to the PS-I photosystem. Note that in this electron transport chain are no less than six electron acceptors, one of which is actually a pool of reduced molecules that can be used as needed. Besides shuttling the electrons to the correct places, these multiple acceptors act as a buffered pool for stored energy and reducing power. An ATP synthase enzyme uses the membrane potential energy obtained from this pool of stored energy to make ATP. Some electrons, however, enter the PS-I system. The electron gains energy from light absorbed by the P700 chlorophyll protein of PS-I. A second electron transport chain accepts the electron. The electron-donating energy carried by the electron acceptors is used to move H+ ions across the thylakoid membrane into the lumen, the aqueous phase inside the thylakoid (see Figures 7.5, 7.7, and 7.12), as well as to reduce NADP.
Responses of Eucalyptus globulus and Ficus nitida to different potential of heavy metal air pollution
Published in International Journal of Phytoremediation, 2020
A. A. El-Khatib, N. A. Youssef, N. A. Barakat, N. A. Samir
Chlorophyll pigments are organized in a specific state. When they are exposed to stress, they exhibit many photochemical reactions (Puckett et al.1973). Then, any change in chlorophyll content leads to change in the morphology, physiology and biochemistry of the plant. Many workers observed that the degradation of photosynthetic pigments is enhanced by air pollution (Bansal 1988; Sandelius et al.1995). Both chlorophyll (a) and (b) were decreased at polluted area. The decline in chlorophyll content has been reported as a result of foliar damage caused by heavy metal pollution (Ali 1993; Cañas et al.1997). Rabe and Kareeb (1979) also reported that air pollutants had high impact on the concentration of chlorophyll contents. De Filippis and Pallaghy (1994) cited that heavy metals have an inhibitory effect on chlorophyll pigments biosynthesis and enzymes responsible for its biosynthesis. This agrees totally with the present results. There was a clear reduction in chlorophyll content in trees growing in both traffic and industrial area. According to Stobart et al. (1985), chlorophyll is degraded into phaeophytin by losing magnesium ions leading to reduction in chlorophyll content. Concentration of chlorophyll changes with different time periods under various conditions of heavy metal stress and various meteorological conditions. The most important heavy metal that affects biosynthesis of chlorophyll is Cd. It also causes inhibition of protochlorophyll reductase and synthesis of aminolevulinic acid (ALA).