Question

plants are mainly affected by the biochemical cycle, carbon cycle and phosphorus cycle. describe them and how plants are affected by them.

(subject: Plant Ecology)

Answer 1

Carbon reactions of the photosynthesis

Solar radiant energy (ca. 3 x 1021 Joules/year) is converted via endergonic reactions in plants into carbohydrates (ca. 2 x 1011 tonnes of carbon/year). The capture of sunlight energy for transformation into various forms of chemical energy is one of the oldest biochemical reactions on Earth. One billion years ago, heterotrophic cells acquired the ability to convert sunlight into chemical energy through primary endosymbiosis with a cyanobacterium. The original endosymbiosis has given rise to an enormous variety of organelles. In general, the transition from endosymbiont to organelle involved both the loss of functions unnecessary in the protected milieu of the host cell and the gain of other metabolic pathways. The chloroplast is the place of both the light and carbon reactions of photosynthesis.

The products of the light reactions, ATP and NADPH, flow from thylakoid membranes to the surrounding fluid phase (stroma) and drive the enzyme-catalyzed reduction of atmospheric CO2 to carbohydrates and other cell components. Because the stroma-localized reactions depend on products of the photochemical processes and are also known to be regulated directly by light, they are more properly referred to as carbon reactions of photosynthesis. The incorporation of atmospheric CO2 into organic compounds appropriate for life is accomplished by the Calvin-Benson cycle. There are two major products of the photosynthetic fixation of CO2: starch, the reserve polysaccharide that accumulates transiently in chloroplasts; and sucrose, the disaccharide that is exported from leaves to developing and storage organs of the plant.

The Calvin-Benson cycle

The Calvin-Benson cycle is found in many prokaryotes and in all photosynthetic eukaryotes, from the most primitive algae to the most advanced angiosperms. It is also aptly named the reductive pentose phosphate cycle.

The Calvin-Benson cycle has three stages

The Calvin-Benson cycle was elucidated by M. Calvin, A. Benson and their colleagues in the 1950s. It proceeds in three stages that are highly coordinated in the chloroplast (Figure 2.11):

1. Carboxylation of the CO2 acceptor molecule. The first committed enzymatic step to generate two molecules of a 3-carbon intermediate (3-phosphoglycerate).

2. Reduction of 3-phosphoglycerate.

3. Regeneration of the CO2 acceptor ribulose 1,5-bisphosphate.

In the first step three molecules of CO2 and three molecules of H2O react with three molecules of ribulose 1,5-bisphosphate to yield six molecules of 3-phosphoglycerate. This reaction is catalyzed by the chloroplast enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, referred to as rubisco. The reduction stage of the Calvin-Benson cycle reduces the carbon of the 3-phosphoglycerate coming from the carboxylation stage. To prevent depletion of Calvin-Benson cycle intermediates, the continuous uptake of atmospheric CO2 requires constant regeneration of the CO2 acceptor ribulose 1,5-bisphosphate.

Triose phosphates are formed in the carboxylation and reduction phases of the Calvin-Benson cycle at the expense of energy (ATP) and reducing equivalents (NADPH) generated in the thylakoid membranes of chloroplasts:

3CO2 + 3 ribulose 1,5-bisphosphate + 3H2O + 6NADPH + 6H+ +6ATP ---> 6 triose phosphates + 6NADP+ + 6ADP +6Pi

From these six triose phosphates, five are used in the regeneration phase that restores ribulose 1,5-bisphosphate, the CO2 acceptor, while the sixth triose phosphate represents net synthesis from CO2 and is used as a building block for other metabolic processes.

5 triose phosphates + 3ATP ---> 3 ribulose 1,5-bisphosphate + 3ADP

In summary, the fixation of three CO2 into one triose phosphate utilizes 9ATP and 6NADPH; that is, the ratio of ATP:NADPH required for the fixation of one CO2 in the Calvin-Benson cycle is 3:2.

When leaves are kept in darkness for long periods (e.g., at night), the stromal concentration of most biochemical intermediates of the Calvin-Benson cycle is low. Therefore, when leaves are transferred to the light, almost all stromal triose phosphates are committed to the production of the intermediates necessary to regenerate ribulose 1,5-bisphosphate. The fixation of CO2 starts after a lag, called the induction period, and the rate of photosynthesis increases with time in the first few minutes after the onset of illumination.

Carbon cycle plays a very important role in the lives of every living organism:-

Explanation:

Carbon Cycle is the movement of Carbon molecules from one phase to the another in the atmosphere as shown in the below figure :-
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Co2 from atmosphere is taken up by the organic compounds or plants which use it to make their food. From here we can see that

If Co2 would not be there in the atmosphere then plants would not be able to make their food and the greenery would die from the face of earth.

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The plants not only make their food but also give out O2 in the atmosphere which is used by us or the second group of organic compounds which shown in the above figure. And also release some amount of H2O in form of transpiration so from here we can say that

If Co2 would not be there then plants would not be able to make there food and then we human beings would not be able to get the necessary life air or oxygen which we get from plants so
indirectly after plants, animal life would also diminish from the earth.

Also we get from the above figure that
Petroleum, Coal- Which have become necessity nowadays to human life.

Limestones.

Inorganic carbonates or shells- Which we use as decoration material would not be available at all if Co2 would not be present in the atmosphere or on earth.

Phosphorous cycle in plants:

Phosphorus is an essential nutrient for plants and animals in the form of ions PO43- and HPO42-. It is a part of DNA-molecules, of molecules that store energy (ATP and ADP) and of fats of cell membranes. Phosphorus is also a building block of certain parts of the human and animal body, such as the bones and teeth. Phosphorus can be found on earth in water, soil and sediments. Unlike the compounds of other matter cycles phosphorus cannot be found in air in the gaseous state. This is because phosphorus is usually liquid at normal temperatures and pressures. It is mainly cycling through water, soil and sediments. In the atmosphere phosphorus can mainly be found as very small dust particles. Phosphorus moves slowly from deposits on land and in sediments, to living organisms, and than much more slowly back into the soil and water sediment. The phosphorus cycle is the slowest one of the matter cycles that are described here. Phosphorus is most commonly found in rock formations and ocean sediments as phosphate salts. Phosphate salts that are released from rocks through weathering usually dissolve in soil water and will be absorbed by plants. Because the quantities of phosphorus in soil are generally small, it is often the limiting factor for plant growth. That is why humans often apply phosphate fertilizers on farmland. Phosphates are also limiting factors for plant-growth in marine ecosystems, because they are not very water-soluble. Animals absorb phosphates by eating plants or plant-eating animals. Phosphorus cycles through plants and animals much faster than it does through rocks and sediments. When animals and plants die, phosphates will return to the soils or oceans again during decay. After that, phosphorus will end up in sediments or rock formations again, remaining there for millions of years. Eventually, phosphorus is released again through weathering and the cycle starts over. A schematic representation of the phosphorus cycle: For more information on phosphorus, move to the periodic chart or directly to the element phosphorus Phosphorus compounds reside primarily in rocks. Phosphorus does not go through an atmospheric phase, but rather, phosphorus-laden rocks release phosphate (PO4–3) into the ecosystem as the result of weathering and erosion. Phosphorus and plants To plants, phosphorus is a vital nutrient (second only to nitrogen). Plants absorb phosphates through their root hairs. Phosphorus then passes on through the food chain when the plants are consumed by other organisms. Phosphorus is an essential component of many biological molecules, including deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Adenosine triphosphate (ATP), one of the nucleotides that make up DNA and RNA, is also the main energytransfer molecule in the multitude of chemical reactions taking place within organisms. Because phosphorus is a major plant nutrient, massive amounts of phosphate-based fertilizers are either derived from natural sources (in the form of bat or bird guano) or chemically manufactured for use by agriculture. In aquatic systems such as rivers and lakes, where such runoff eventually appears, an infusion of phosphates can cause algal blooms (rapidly forming, dense populations of algae). When the algae die, they are consumed by bacteria. Decomposition by bacteria requires large amounts of oxygen, which soon depletes the available oxygen in the water. If the process is allowed to continue unchecked, fish and other organisms die from lack of oxygen. Both phosphates and nitrates contribute to cultural eutrophication. Phosphates not taken up by plants go into the sedimentary phase, where they are very chemically reactive with other minerals. Some of these reactions produce compounds that effectively remove phosphates from the active nutrient pool. This sedimentary phase is characterized by its long residence time compared to the rapid cycling through the biological phase. Phosphates can remain locked up in rocks for millions of years before being exposed and broken down by weathering, which once again makes them available to plants. Excessive phosphates in a eutrophic lake disrupt the carbon cycle by reacting with bicarbonates, thus increasing the pH. Many freshwater organisms depend on a neutral pH level for their survival. The presence of phosphorus under these oxygen-depleted conditions can also indirectly affect the sulfur cycle, leading to the conversion of sulfate to sulfide. When sulfide combines with hydrogen to form the gas hydrogen sulfide, it takes on the familiar "rotten egg" smell. One of the keys to preventing environmental degradation through the altering of global chemical cycles lies in recognizing the effects of such alterations.With the perception of an environmental crisis in the early 1970’s, more attention was paid to the role of human activity in these cycles. Test lakes were studied to determine why freshwater fisheries were becoming oxygen-depleted at accelerated rates. Dramatic progress has been made in eliminating the problem of algal blooms and oxygen depletion by limiting the phosphorus-laden effluents being discharged into lakes.