Biology/ Photosynthesis term paper 41768

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Photosynthesis occurs in two stages. The first stage is known as Light Dependent Process or Light Reactions, and it occurs in the �grana� region of the chloroplasts. In this stage, when light strikes the leaf, it excites chlorophyll electrons to a higher energy state. The energy is stored by forming a chemical called ATP (adenosine triphosphate). It is a chemical compound that is made up of the nucleotide adenine - ribose sugar bond, and that is bonded to three phosphate radicals. (ATP molecule is very similar to the building blocks of human DNA). The stored energy is used in the second process. Water is chemically disintegrated in the process, releasing oxygen as a by product. The second stage is known as Light Independent Process or Dark Reactions) and takes place in the stroma region of the chloroplasts.. In this stage, the products of LDP are used to form carbohydrates with C-C covalent bonds (carbon dioxide to sugar). Carbon dioxide from the atmosphere is �trapped� and �modified� by hydrogen molecules to form carbohydrates. Such incorporation of carbon dioxide into carbohydrates, by plants, is known as carbon fixation. The chain of reactions in LIP is known as Calvin cycle. The process is schematically illustrated in fig.(i).

The chemical reaction involved in photosynthesis is as follows:

6CO2 + 6H2O (+ light energy) C6H12O6 + 6O2.

6 molecules of carbon dioxide reacts with 6 molecules of water, in the presence of light to produce 1 molecule of glucose and 6 molecules of oxygen.

In simple terms, photosynthetic efficiency of a plant is a measure of how efficiently solar energy is converted by its leaves into chemical energy. It can be defined as the �ratio of the energy stored to the energy of light absorbed� by the leaf¹. Another way of looking at it is to use a graphical method. In this method, the rate of photosynthesis and the irradiance level of light are monitored and plotted with the rate of photosynthesis on the y axis as shown in fig. (i). (In graphs, dependent variable is considered on the y axis, and the independent variable, on the x-axis). Irradiance level is the amount of light that falls on the leaf�s surface. Strictly speaking, irradiance level is different from the amount of light. The slope of the straight line part of the photosynthesis light response curve is considered as a measure of photosynthetic efficiency. It is the gradient of the curve and not the maximal rate of photosynthesis that determines the photosynthetic efficiency.

Factors that influence photosynthetic efficiency are: (i) maximum potential speed of physical processes (such as electron transport), (ii) concentration level of the reactants (including water, O2, CO2, ATP, etc) and (iii) temperature. Under normal soil and atmospheric conditions, any shortage in the reactants slows down the process. This applies to light at a very significant level: As the irradiance level of light is increased, more ATP and other chemicals are produced, and the rate of photosynthesis increases. This continues to happen until some reactant runs short of supply. From here, the corresponding light response curve begins to reach the "light saturation point" and the curve begins to be linearly parallel to the x axis, showing that the rate of photosynthesis is zero.

Photosynthetic efficiency can be improved by any of the following approaches:

(i). By changing the way the products of photosynthesis are used and distributed around the plant. E.g., seeds and tubers normally contain a mixture of starches, proteins and oils, in varying proportions from one plant species to another. An understanding of how these proportions are retained and controlled within the plants can help to increase the quantity and quality of the produces as well as to reduce the waste products.

(ii). By growing shorter (dwarf or semi-dwarf) plants. Shorter plants use more energy for storing food than larger ones, as they need less energy to produce stems and leaves.

(iii).By understanding how some plants (such as corn), use carbon dioxide and water more efficiently than other plants.

As more and more research works are taken up, on a global scale, we may have better methods of making use of natural resources in an eco-friendly manner, by using a wider understanding of photosynthesis, the marvelous process taking place in incredibly wonderful factories called �chloroplasts�.



� ¹ "photosynthesis." Encyclopædia Britannica. 2006. Encyclopædia Britannica Online. 5 Nov. 2006 .


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