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Basic process of photosynthesis

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Basic process of photosynthesis

Algae (phytoplankton) are the primary source of photosynthetically generated DO in the study area (). As algal biomass increases, the amount of oxygen produced by algae through photosynthesis also increases. Studies conducted in the DWSC have shown that algal photosynthesis can add considerable amounts of oxygen to the near-surface portion of the water column. As a result, DO concentrations can be high during the day (when photosynthesis is occurring) in the near surface waters of the DWSC but low near the channel bottom where light cannot penetrate sufficiently to support it (; ; ).

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Practically speaking, there is no direct way to distinguish between DO declines caused by BOD (i.e., algal respiration, bacterial decay, or nitrification) or SOD. The conceptual model includes both sources of oxygen demands, although the relative magnitude of the sediment and suspended sources is uncertain. In addition, the bottom materials that produce the measured SOD are organic particulates that have settled from the water column.

photosynthesis the basic process of food making in …

Photosynthesis usually takes place in infoldings or extensions derived from the cytoplasmic membrane., The cell is the basic unit of life.

The current body of information indicates that algae and algal detritus are the primary sources of BOD in the DWSC (). Each 1 mg/L of algae will yield a theoretical total oxygen demand of 1.2 mg/L, which represents the sum of CBOD and NBOD. The CBOD from the decomposition of 1 mg/L of algal biomass is 0.9 mg/L, or 75% of the total oxygen demand, while the NBOD is 0.3 mg/L or 25% of the total oxygen demand (). The algae pigments chlorophyll a and pheophytin are used to represent the concentrations of living and dead algae, respectively, in a water column. Therefore, the pheophytin concentrations indicate the quantity of dead algae that might be a source of CBOD. However, the chlorophyll a and pheophytin content of algal biomass is somewhat variable.

Enzyme activity is very responsive to temperature, as is enzyme synthesis, activation and stability. However the response of plant growth to temperature is the result of a number of complex processes involving many enzyme systems, and most likely governed by the response of the enzymes involved in CO2 fixation. The different temperature responses of C3 versus C4 photosynthesis are described next, along with temperature effects on assimilate transport and on the basic concepts of enzyme activity including the Q10: the increase in rate of respiration for a 10°C rise in temperature.

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Concept 1: An Overview of Photosynthesis - …

One of the most characteristic changes associated with cold acclimation are increases in the degree of saturation of membrane fatty acids, leading to greater membrane fluidity and an increase the functionality of membrane-bound substrate transporters and membrane-bound proteins at low temperatures. Moreover, cold-induced modifications in gene expression, subsequent changes in the abundance and activity of proteins, and alterations in other processes such as source-sink relationships and accumulation of metabolic intermediates, collectively lead to increases in the concentration of many metabolites. Thus, sustained exposure to cold leads to major modifications to the metabolome of plants. Enhanced concentrations of soluble sugars are foremost amongst these, the most studied being the monosaccharides glucose and fructose, the disaccharide sucrose, and the trisaccharide raffinose; levels of each rise within a few hours of the onset of a cold treatment. Starch also accumulates in cold-exposed leaves. The increase in sucrose is particularly rapid, despite the fact that cold often has strong inhibitory effect on one of the enzymes (sucrose phosphate synthase, SPS) responsible for its synthesis. Levels of sucrose continue to rise during the first few days of cold acclimation, aided by a long-term increase in the abundance and activity of SPS. Moreover, overall sugar levels generally remain elevated in plants exposed to sustained cold; this accumulation reflects shifts in source:sink relationships, underpinned by changes in the balance between carbon uptake by photosynthesis, carbon use by catabolic processes (e.g. respiration) and carbon export to other parts of the plant. In some plants, increased concentrations of sugars may convey cryoprotective properties, reducing the incidence of membrane lesions and increasing survival during freezing (see Section 14.6).

Cold is known to markedly inhibit rates of photosynthesis, via both its kinetic effects on protein activity and membrane fluidity. Rapid increases in soluble sugars also contribute to an inhibition of photosynthesis through feedback inhibition and down-regulation of nuclear-encoded photosynthetic gene expression. Extended cold-treatment of pre-existing leaves can also result in photodamage; low temperature slows the consumption of ATP and NADPH by the Calvin cycle and the resulting over-reduction of the photosynthetic electron transport chain may cause oxidative damage to the light-harvesting machinery. A further (and important) limitation is the exhaustion of chloroplastic orthophosphate (Pi) required for ATP regeneration and maintenance of the thylakoid membrane proton gradient, which in turn drives the chloroplastic electron transport chain. Calvin cycle turnover also decreases (explaining the observed decline in carbon assimilation) as this relies on ATP to drive the regeneration of ribulose-1,5-bisphosphate (RuBP) reduction.

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Cold developed leaves can accumulate soluble sugars without the suppression of carbon assimilation typically associated with an abundance of photosynthates, and the transcript and protein abundance of most of the Calvin cycle enzymes have been found to be greatly increased in these leaves. The abundance of ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) increases, whereas the other Calvin cycle enzymes tend to decline with cold acclimation relative to Rubisco. Further recovery of sucrose synthesis in cold developed leaves assists in the recovery from cold stress and photoinhibition; SPS transcript abundance, protein and the activity of the enzyme increases relative to the Calvin cycle and starch synthesis during acclimation. Associated with the increase in photosynthetic capacity in cold developed leaves is a change in electron transport capacity relative to carboxylation capacity, and an alleviation of triose phosphate utilization (TPU) limitation. Alleviation of TPU limitation during acclimation to low temperatures has been ascribed to increasing activity and transcription of SPS and cFBPase, and the redistribution of inorganic phosphate between cellular compartments. The net result of these changes is an increase in photosynthetic capacity in newly developed, relative to pre-existing, leaves. This phenomenon may therefore optimise the function of these new leaves to their environment.

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The photosynthetic phenotype of cold-developed leaves has been suggested to be distinct from that of pre-existing leaves. Cold-developed leaves exhibit marked physical changes compared to warm-grown leaves (Figure 14.23). Leaves developed in the cold tend to be thicker, with a higher leaf mass per area and higher nitrogen and protein concentrations than leaves developed at warmer temperatures.


During a frost episode in nature, plants experience a sequence of events similar to that summarised in Figure 14.25. Tissue water supercools, and cell sap freezing point is depressed by osmotically active material. Moreover, plants are also equipped with ice nucleating agents that can be of either plant or bacterial origin (e.g. Pseudomonas syringae). As with solutions, formation of ice in plants is accompanied by a release of heat. This exothermic response can be detected by sensitive infrared thermography, and has been used to trace ice propagation during a freezing event in leaves and shoots (Wisniewski et al. 1997).

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