Here is a compilation of term papers on ‘Photosynthesis’. Find paragraphs, long and short term papers on ‘Photosynthesis’ especially written for school and college students.
Term Paper # 1. Meaning of Photosynthesis:
The combination of CO2 and H2O by the plant to form carbohydrates is termed carbon- assimilation and, since the chemical process involved requires light as a means of supplying energy, this process is usually known as photosynthesis (photo = light; synthesis = combination). This process is almost always accompanied by the evolution of oxygen and takes place in the presence of chlorophyll.
The entire process can be represented by the following equation:
Studies by Ruben and Kamen (1941) indicated that the oxygen molecules released during this process are contributed by water which means that 6 molecules of water are insufficient for releasing 6 molecules of oxygen.
Therefore the above equation can be modified as follows:
Term Paper # 2. Mechanism of Photosynthesis:
The overall process of photosynthesis can be divided into two phases:
A. Light reaction
B. Dark reaction.
A. Light Reaction:
The light reaction includes a series of reactions which takes place in the presence of light only. These reactions take place in the grana of the chloroplasts. In this phase, light energy is utilized to produce an energy rich compound, and a hydrogen carrier. The chlorophyll molecules are excited by the light energy and release energy rich electrons.
A smaller part of the energy of the electrons is used in converting ADP into ATP (photophosphorylation) which is stored for immediate supply, but the greater part is used in breaking the water molecules (photolysis) into oxygen and hydrogen. Oxygen is released as a bye-product while hydrogen is picked up by hydrogen acceptor NADP (Nicotinamide adenine dinucleotide phosphate) which becomes reduced to NADPH2.
The reaction of photolysis (Hill’s reaction) can be represented in the equation form as follows:
The protons (4H+) are used to reduce NADP to NADPH2 after they are joined by the electrons (4e–) which travel through a system of different electron acceptors.
During this passage of electrons ADP and inorganic phosphate are combined to form ATP. This process has been called as photophosphorylation by Arnon. The O2 released during this process is a bye-product.
Emerson reported the existence of two distinct light driven processes one of these was driven by wavelengths exceeding 680 nm and the other by shorter wavelengths. These two light-driven reactions were named as Photosystem-I (PS-I) and Photosystem-II (PS-II) respectively by Duysens et al. (1961).
The photosystems (or, pigment systems) are concerned with harvesting the energy of sun light. Each photosystem has reaction centre (core complex or trap) represented by a molecule of chlorophyll ‘a’. It is intimately associated with several other molecules of chlorophyll and accessory pigments. These collectively form the light harvesting complex or antenna complex. The energy gathered by antenna complex is ultimately transferred to the reaction centre.
Photosystem I:
Its reaction centre consists of a single molecule of chlorophyll ‘a’ which absorbs light of 700 nm wavelength. It is designated as P700. The antenna complex is made up of several molecules of chlorophyll, phycobilins, xanthophylls plastocyanin, etc. It is rich in chlorophyll ‘a’, iron and copper. It is active both in red as well as far-red lights. It takes part both in cyclic and non-cyclic photophosphorylation.
Photosystem II:
Its reaction centre is a single molecule of chlorophyll ‘a’ 680 designated as P680. Its antenna complex contains a much higher number of chlorophyll ‘b’ molecules than chlorophyll ‘a’, carotenoids and plastoquinone. It absorbs lights of shorter wavelengths only. It participates in photolysis and non- cyclic photophosphorylation.
Production of Assimilatory Power:
NADPH2 is called as the reducing power of cell because it can donate hydrogen. NADPH2 and ATP are together called as assimilatory power by Arnon (1956). NADPH2 is produced by reducing NADP with the help of electron transport system present in chloroplast membranes while ATP is produced with the help of ADP and Pi using light energy, hence called photophosphorylation by Arnon. The entire process can be explained with the help of Z-scheme, proposed by Hill and Bendall (1960).
1. Z-Scheme (Non-Cyclic Photophosphorylation):
When a quantum of red light of wavelength longer than 680 nm is absorbed by PS-I, the energy of photon is transferred to the reaction centre P700 through various components of the antenna complex. After absorbing the energy of photons, P700 gets excited and loses an electron. This energy-rich electron is accepted by Fe-S centre, rich in iron-sulphur proteins. The electrons then pass via ferredoxin- reducing-substance to ferredoxin (Fd). The reduced ferredoxin then reduces NADP to NADPH2 in presence of enzyme ferredoxin-NADP reductase.
Simultaneously, after getting excited on absorbing a quantum of light of lower wavelength, the reaction centre P680 of PS-II loses an electron which is accepted by phaeophytin (a non-Mg chlorophyll ‘a’ molecule). From phaeophytin the electron travels downhill through a series of carriers to PS-I. The carriers are quinone (Q), substance B, plastoquinone (PQ), cytochrome-f and plastocyanin (PC). During transfer of electrons from PQ to Cyt ‘f’, significantly larger amount of energy is released which is utilised to synthesise ATP from ADP and inorganic phosphate.
The loss of electrons of PS-I is made up by PS-II but this creates a deficit of electrons in PS-II which is overcome by photolysis of water. In this way the electrons released by one photosystem never come back to the same photosystem. Therefore, the synthesis of ATP resulting from this type of electron transport is known as non-cyclic photophosphorylation.
2. Cyclic Photophosphorylation:
During this process, the electrons expelled from P700 of PS-I are accepted by Fe-S Centre, passed on to FRS and then to ferredoxin. From here the electrons cannot be transferred to NADP because PS-II does not operate and therefore photolysis of water does not occur to release H+.
As a result, electrons from ferredoxin are transferred first to PQ, then to cytochrome b6 which ultimately returns these electrons to PS-I via cyt, and plastocyanin. In this process one molecule of ATP is generated between Fd and PQ, and another molecule of ATP is produced between PQ and Cyt ‘f’. However, NADPH2 and oxygen are not produced.
B. Dark Reaction:
The second step of photosynthesis includes the series of reactions for which light is not necessary and so it is called as the dark reaction. Its existence was indicated by F.F. Blackman (1905) hence it is also known as Blackman’s reaction. The reactions are biochemical in nature and are catalyzed by enzymes present in the stroma of chloroplast. During this phase CO2 obtained from the atmosphere is reduced with the help of NADPH2 and ATP which are produced during light reaction. The end product of this reaction is starch or sugar.
This fixation of CO2 can occur by either of the following two pathways:
1. Calvin cycle or C3-cycle
2. Hatch and Slack cycle or C4-cycle
1. Calvin Cycle:
Its main reactions were discovered by Calvin et al on the basis of their experiments with green algae Chlorella pyrenoidosa and Scenedesmus which were provided with 14C-labelled CO2. In recognition of the importance of his work Calvin was awarded Nobel Prize in 1961.
2. Hatch and Slack cycle (C4-Cycle):
This is an alternative pathway of CO2 fixation known to occur in certain tropical grasses (e.g., sugarcane, maize and Sorghum etc.) and some dicots (e.g., Amaranthus edulis, Artiplex rosea etc.). Its existence was first of all detected by Kortschak, Hart and Burr (1965). Later, the entire sequence of reactions was worked out by Hatch and Slack (1967).
They differ from C3-plants in that their first stable product is oxaloacetic acid, a 4-carbon compound, hence this cycle is also called as C4-cycle. The C4-plants are characterised by the presence of Kranz-anatomy in their leaves.
The features of Kranz-anatomy include:
(a) The presence of two concentric layers of bundle sheath ceils around the vascular bundles.
(b) Undifferentiated mesophyll cells.
(c) Presence of Dimorphic Chloroplasts:
The cells of bundle sheath ceils have large chloroplasts which -lack grana. They also contain numerous starch grains. In contrast, the chloroplasts of mesophyll cells are smaller, have normally developed grana, but lack the enzymes of C3-cycle Instead, they contain the enzyme phosphoenol pyruvate carboxylse (PEPC) which catalyzes carboxylation of phosphoenol oxaloacetic acid.
The reactions of C4-cycle can be studied in two phases, the first phase is completed in the chlorloplasts of mesophyll cells and the second phase proceeds in the chloroplasts of bundle sheath cells.
Reactions in Mesophyll Cells:
(1) Phosphoenol pyruvic acid (PEPA-produced in glycolysis) acts as the primary CO2 acceptor. It reacts with CO2 to form oxaloacetic acid in presence of water. This reaction is catalyzed by the enzyme phosphoenol pyruvate carboxylase.
(2) The oxaloacetic acid is reduced to malic acid by the enzyme malic dehydrogenase. NADPH2 acts as the hydrogen donor.
(3) Oxaloacetic acid may sometimes be transaminated into aspartic acid to maintain a physiologically normal level of malic acid.
All the malic acid produced in mesophyll cells is transported into bundle sheath cells through the plasmodesmata.
Reactions in Bundle Sheath Cells:
(1) The malic acid, transported from mesophyll cells into the bundle sheath cells, is converted into pyruvic acid, CO2 and NADPH2. It involves simultaneous dehydrogenation and decarboxylation of malic acid. The reaction is catalyzed by an NADP-dependent malic enzyme.
(2) The CO2 and NADPH2 produced in the above reaction are used up for production of starch by C3-cycle.
(3) Pyruvic acid is transported back into mesophyll cells where it is converted into PEPA by enzyme pyruvate phosphate diakinase. One molecule each of ATP and H3PO4 is consumed in this reaction and AMP and inorganic pyrophosphate (PPi) are produced.
The C4-plants are supposed to be more efficient in fixing CO2 than C2-plants due to the following reasons:
(i) PEPC enzyme has a very strong affinity for CO2; therefore, it can trap CO2 even if present in very low concentrations.
(ii) The activity of PEPC is not affected by high concentration of O2, which otherwise has a competitive effect on Rubisco.
(iii) There is no photorespiration in C4-plants.
Term Paper # 3. Factors Affecting Photosynthesis:
The main external factors which influence photosynthesis are:
(i) Light
(ii) Temperature
(iii) Carbon Dioxide Concentration
(iv) Oxygen
(v) Water Content
(vi) Mineral Elements
(i) Light:
Light is the source of energy for photosynthesis. Hence, its intensity, quality and duration control the rate of the process. At low intensity of light, the rate of photosynthesis is low. The rate increases with the increase in intensity of light upto l/6th to l/3rd of the maximum sunlight in the tropical countries (for C3 plants). The increase in the intensity beyond this optimum level is harmful to the plant.
It induces solarization (photo-oxidation). This destroys the chlorophyll pigment and the enzymes. Carotenoids play an important role in protecting the chlorophyll from photooxidation by diverting the excess of light away from chlorophyll. Prolonged exposure to such high intensity is harmful to plants. It can result in death of the plants.
Observe the graph of light intensity on the rate of photosynthesis (Fig. 13.10). In the graph, light is the limiting factor in the region A. Point D represents the intensity of light at which some other factor becomes a limiting factor. Point C represents that the rate of photosynthesis is not increased with increased light intensity.
Fig. 13.10 The graph of light intensity on the rate of photosynthesis
The solar radiation consists of a wide spectrum of individual rays differing in their wavelengths. Longer the wavelength, lesser is the energy and shorter the wavelength, greater is the energy. In the visible spectrum of sunlight, red light has a longer wavelength and the blue light has a shorter wavelength.
Different photosynthetic pigments absorb different wavelengths of white light.
The resulting graph, in which the efficiency of absorption is plotted against wavelength, is known as an absorption spectrum (Fig. 13.14 (a)).
An action spectrum is a graph that shows the effectiveness of light in a particular process plotted as a function of wavelength. The underlying assumption is that light most efficiently absorbed by the responsible pigment will also be most effective in driving the response. In other words, the action spectrum for a light-dependent response should closely resemble the absorption spectrum of the pigment that absorbs the effective light.
A comparison of action spectra with the absorption spectra of suspected pigments can therefore provide useful clues to the identity of the pigment responsible for a photosensitive process. A typical action spectrum for photosynthesis in a given plant is shown in (Fig 13.14 (b)). It is compared with the absorption spectrum for a leaf extract that contains chlorophyll a (Fig.13.14 (c)).
Note that the action spectrum of photosynthesis has pronounced peaks in the red and blue regions of the spectrum showing the wavelengths at which maximum photosynthesis occurs in a plant. The absorption spectrum of chlorophyll an also shows pronounced peaks in the red and blue regions of the spectrum indicating the wavelengths at which there is maximum absorption by chlorophyll a. Hence, it can be concluded that chlorophyll a is the chief pigment associated with photosynthesis.
The full spectrum of visible light (380 nm to 750 nm) is more efficient than any individual colour of the spectrum in inducing the maximum rate of photosynthesis. However among individual colours, the red has the maximum, blue has lesser and green has the least influence on the rate of photosynthesis.
The duration of light also influences photosynthesis. Intermittent light is more efficient than the continuous light in inducing the maximum rate of photosynthesis.
(ii) Temperature:
In non-living systems, the increase in temperature increases the rate of the reaction. But in living systems, at high temperature the denaturation of enzymes occur. The rate of photosynthesis increases with increase in temperature from 0° to 25°C (in mesophytes) of tropical region.
Beyond 25°C, the rate of photosynthesis decreases. Usually at temperatures beyond 45°C, in most of the plants the photosynthesis fails to occur. This is due to the denaturation of enzymes at high temperature. The photosynthesis occurs in some conifers at -35°C and in the cyanobacteria of hot springs at 70°C.
In conclusion, it can be said that the optimum temperature for photosynthesis of different plants depends on the habitat where they grow. Tropical plants have a higher temperature optimum than the plants which grow in temperate regions. The C4 plants show higher rate of photosynthesis at higher temperatures whereas C3 plants have a much lower temperature.
(iii) Carbon Dioxide Concentration:
This is one of the raw materials of photosynthesis. The atmosphere is the reservoir of carbon dioxide and this is the source of carbon for the photosynthesis in the terrestrial plants. The atmosphere has 0.03% of carbon dioxide. In the tropical and sub-tropical countries, the carbon dioxide is usually a limiting factor controlling the rate of photosynthesis. If the level of carbon dioxide in these regions is raised to 0.1%, then the rate of photosynthesis increases by about 15-30 times. The optimum level of CO2 for long duration is 0.1%. Further increase in CO2 induces stomatal closure.
The response of the C3 and C4 plants are different to CO2 concentration. Both C3 and C4 plants do not respond to high CO2 concentration if there is low light intensity, but both (C3 and C4 plants) show increased rate of photosynthesis with high light intensities. C4 plants show saturation at about 360 µIL-1 whereas C3 plants respond to increased CO2 concentration and saturation at about 450 µIL-1. At present the availability of CO2 level is limiting to the C3 plants.
We have understood that the C3 plants show increased rate of photosynthesis with higher productivity at higher CO2 concentration. Hence, some crops like tomatoes, bell peppers which are grown in greenhouses yield more as greenhouses have high CO2 concentration in their atmosphere.
(iv) Oxygen:
High concentration of oxygen has an inhibitory effect on photosynthesis. This phenomenon, known as Warburg effect, was first observed by O.Warburg in 1920 in Chlorella.
Oxygen may be inhibitory to photosynthesis due to various reasons:
a. Respiration may compete with photosynthesis for some common intermediates.
b. Oxygen may compete with CO2 for hydrogen and becomes reduced in place of CO2.
c. Oxygen may upset the oxidation-reduction balance of the photo-electron transport system.
(v) Water Content:
The entire process of photosynthesis depends on the availability of water. Photolysis of water takes place during light reactions. Water is the source of H+ ions to reduce NADP and release of molecular O2. Water also regulates the opening and closing of stomata and hence the entry of CO2 into the tissues of leaf. Therefore, a decrease in water supply decreases the rate of photosynthesis. However, water rarely becomes a limiting factor because less than 1% of the water absorbed by a plant is used as a raw material for photosynthesis.
If there is water stress, the leaves wilt. As a result, the surface area of the leaves is reduced which results in reduced metabolic activity.
(vi) Mineral Elements:
Mineral elements like Mg, Fe, Cu, CI, Mn, P etc., are closely associated with important reactions of photosynthesis. Therefore, deficiency of these minerals ultimately reduces the rate of photosynthesis.
Blackman’s Law of Limiting Factors:
F.F. Blackman in 1905 formulated the “principle of limiting factors”. This was later known as “Blackman’s law of limiting factors” which states that “When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor”.
OR
When a physiological process is under simultaneous influence of many independent factors, the pace of the slowest factor limits the rate of process.
When the simultaneous influence of many independent factors on photosynthesis is studied, if one factor is present in minimum and others are in the optimum level, the rate of photosynthesis is low. This minimum factor is the limiting factor.
Term Paper # 4. Significance of Photosynthesis:
i. Green plants are autotrophs as they synthesize their own food by photosynthesis. The food synthesized by green plants is not only food for them but also for other organisms (heterotrophs) which depend on plants for their survival.
ii. The oxygen released during photosynthesis is used by all aerobic organisms for respiration including plants, animals and man.
iii. The CO2 released by the organisms during respiration and combustion of carbon containing substances is used by green plants during photosynthesis. Thus, photosynthesis maintains a more or less stable concentration of carbon dioxide in the atmosphere (about 0.03%).
iv. “Plants are lungs of nature” because they take in CO2 and release O2 during photosynthesis, thereby purifying the air. Thus, the green plants by photosynthetic activity maintain the ratio of CO2 and O2 of air.
v. Photosynthesis is the main source of fodder, fire wood, timber, resins, gums and oils that are commercially important.
vi. Fossil fuels like coal, petroleum and diesel are the products of photosynthesis of plants which lived in the past geological era.
Comments are closed.