Here is a compilation of essays on ‘Photosynthesis’ for class 6, 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Photosynthesis’ especially written for school and college students.
Essay on Photosynthesis
Essay # 1. Meaning of Photosynthesis:
Life on earth ultimately depends on energy derived from the Sun. Photosynthesis is the only process of biological importance that can harvest this energy. The term “Photosynthesis” literally means “synthesis using light”. It is the ability of the green plants to utilise the energy of light to produce carbon containing organic material from inorganic matter.
It is from the carbohydrate produced by photosynthesis that directly or indirectly all the countless number of organic compounds which compose the living world is derived. The oxidation of organic compounds release stored energy to be utilized by organisms to drive essential metabolic processes. Any energy released during the oxidation of organic compounds is ultimately derived from light energy intercepted by green plants during photosynthesis.
Photosynthesis may be defined as the process by which the carbohydrates are synthesized from carbon dioxide and water by green plants (chlorophyllous cells) in the presence of sunlight, oxygen being a byproduct OR it can also be defined as the physiological phenomenon where the green plants have the capacity to synthesize their own food in the presence of sunlight using CO2 and H2O as the raw materials with the help of chlorophyll pigments.
Earlier it was thought that oxygen released during the process came from carbon dioxide. Van Niel (1930) proposed that oxygen liberated in photosynthesis comes from water and not from carbon dioxide. Robert Hill (1939) discovered that the isolated chloroplasts when illuminated were capable of liberating oxygen in the presence of an electron acceptor. Ruben and Kamen (1941) using isotopes of oxygen (O18) in water (H2O18) confirmed that oxygen liberated is from water.
Hence, the equation of photosynthesis is corrected as follows:
Because 6 molecules of water are insufficient for the release of 6 molecules of oxygen, more water molecules (at least 12, per molecule of hexose formed) have to be incorporated in the equation.
Essay # 2. Photosynthetic Pigments:
Photosynthetic pigments are of three types namely:
1. Chlorophylls.
2. Carotenoids.
3. Phycobilins.
These pigments are present on the membranes of thylakoids. They are concerned with the absorption of light in the visible spectrum.
1. Chlorophylls:
The chlorophyll contains porphyrin structure in which four pyrrole rings are united by their nitrogen atoms to magnesium. Each chlorophyll has a fifth ring containing only carbon atom and the long chain phytol alcohol.
There are different types of chlorophylls. They vary in the structure of the side chains attached to the pyrrole rings. Chlorophylls a and b differ in that chlorophyll a has a methyl group on ring 3, while chlorophyll b has an aldehyde in the same place. Cholorophyll a is the chief pigment associated with photosynthesis.
Most plants contain two or three times more chlorophyll a than chlorophyll b. The molecular structures were largely solved by the Germans R. Wilstatter, A. Stoll and by the famous organic chemist Hans Fisher. In 1960, Robert Woodward synthesized chlorophyll a for which he was awarded Nobel Prize in Chemistry in 1965.
2. Carotenoids:
These pigments are a group of pigments which are usually red, orange, yellow or brown and are associated with chlorophyll in the chloroplast.
The carotenoids are divided into two chemical groups, the carotenes and the xanthophylls. These are insoluble in water but soluble in organic solvents such as ether or acetone.
The carotenes are characterised chemically by the presence of a short chain of unsaturated hydrocarbon which makes them completely hydrophobic. Xanthophylls have several hydroxyl groups.
The carotenoids are important to photosynthesis for two reasons:
a. The carotenes appear to prevent a destruction of chlorophyll in the presence of light and oxygen. Such a destructive phenomenon is called photooxidation.
b. The second function of carotenoids particularly the xanthophylls has a role in absorption of light and transfer the absorbed energy to chlorophyll molecules.
3. Phycobilins:
The phycobilins are the third group of photosynthetic pigments occurring in blue- green and red algae. These are classified into two types- the red phycoerythrin and the blue phycocyanin.
These phycobilins are tetrapyrroles. These are open pyrroles attached to proteins in the living cell and they are water soluble.
Phycobilins absorb light and help in photosynthesis as accessory pigments.
The phycobilins are like carotenoids and transfer the excitation energy to chlorophyll a, which is utilised by chlorophyll a to drive photosynthesis.
Such pigments as carotenoids and phycobilins, which absorb and transfer energy to an absorption sink like chlorophyll a, are called accessory pigments.
Essay # 3. Mechanism of Photosynthesis:
Photosynthesis involves two stages:
1. Light reactions.
1. Light Reactions:
Light reaction is also known as path of electrons in photosynthesis. Light reaction is the first phase of photosynthesis which is a light dependent reaction. It takes place in the thylakoid membranes of grana and the stroma thylakoids or stroma lamellae of the chloroplast. It is a photochemical reaction during which the solar energy is converted into chemical energy which is made available in the form of ATP and NADPH molecules.
They constitute the assimilatory power of photosynthesis. The various events that take place during light reaction have been initially studied in detail by a team of scientists led by Robert Hill (1937). Hence, light reaction is also known as Hill reaction. Subsequent investigations suggested clearly the details of light reactions.
Light Reaction involves:
i. Photolysis of water which is a light mediated oxidation process associated with PS-II characterised by splitting of water molecules into electrons (e–) ions, H+ ions and oxygen.
ii. Photophosphorylation is the conversion of ADP into ATP by the addition of an inorganic phosphate using the light energy. It is of two types, namely- non-cyclic photophosphorylation and cyclic photophosphorylation.
In most of the green plants, the light reaction is found to occur in the form of an elaborate process called non cyclic photophosphorylation under normal conditions. The cyclic photophosphorylation which is a simple process occurs under specific conditions.
Electron Transport/Non-Cyclic Photophosphorylation:
It involves photosystem I as well as photosystem II. In this process, electrons from water pass through a series of intermediate electron carriers before reducing NADP to NADPH. Thus, electrons are transferred to NADP and they are not cycled. (This is coupled with ATP synthesis by chemiosmosis). Hence, the name noncyclic photophosphorylation.
The pathway of electron transport from water to NADPH involving two photosystems is often called “Z-scheme”. This is named so, as graphic representation of the transport pathway according to the redox potential of various molecules involved in the process appears like the alphabet ‘Z’.
The steps involved during non-cyclic photophosphorylation are:
i. Photoexcitation of chlorophyll.
ii. Photolysis of water and evolution of molecular oxygen.
iii. Formation of ATP.
iv. Formation of the NADPH molecules (reducing power).
When solar electromagnetic radiation (SEMR) strikes the photosystem II (PS II), the antenna molecules (LHCs) receive the radiation first and then pass to other pigment molecules, finally to the reaction centre of PS II (P680) by inductive resonance or resonance transfer. An electron in P680 molecule gets excited after absorbing the radiation energy of 680 nm (two P680 molecules release two electrons).
The excited electrons or the energy rich electrons are transferred to pheophytin (Ph). They are then passed through a series of electron acceptors which are at lower energy levels namely plastoquinone (PQ), cytochrome b6/f complex, plastocyanin (PC) (a mobile electron carrier) and then to P700 of PS I which would have excited and would have lost an electron by now.
The excited electron or the energy rich electron from PSI is then transferred to a ferredoxin reducing substance (FRS) which is at a high energy level and then to ferredoxin (Fd). Ferredoxin which is thus reduced, transfers the electrons (2 electrons) to the oxidised NADP+ which gets reduced to NADPH+ H+ (are called NADPH molecules) by picking up protons from the surrounding medium.
At cytochrome b6/f complex, the energy supplied by its electron is used to transport protons across the membrane into the thylakoid space. The concentration of H+ ions thus increases within the thylakoid space. This proton gradient helps in the synthesis of ATP by chemiosmosis.
As P680 of PS II has become oxidised by the loss of electrons, accepts the electrons which comes from photolysis of water.
A continuous flow of electrons from photosystem I to ferredoxin can take place only when its electron deficiency is filled up. This is done by pigment system II (P680).
The flow of electrons from photosystem II to photosystem I and finally to NADP leaves an electron ‘hole’ in photosystem II. This hole is filled up by electrons from water.
Photoloysis/Splitting of Water:
PS II has a component called water splitting complex (oxygen evolving complex). This complex splits water molecules in the presence of solar energy. Hence, it is called photolysis of water or photoionisation of water.
The water molecules are split into protons and electrons with the release of molecular oxygen. These protons and electrons are used by NADP to form NADPH molecules.
Photolysis of water plays an important role in replenishing the electrons lost by P680 during non-cyclic photophosphorylation.
Cyclic Photophosphorylation:
This is an alternative pathway of electron flow in which PSI alone is involved. It is believed to occur when there is enough NADPH but is in need of more ATP molecules.
It takes place in the stroma thylakoids or stroma lamellae or fret membranes of chloroplasts. During this process, the electrons expelled by the reaction centre of PS I returns to the reaction centre after passing through different electron carriers. So, electron transfer is cyclic. This is coupled with H+ transport to the thylakoid space building up a proton gradient, which later forms ATP by chemiosmosis. Hence the process is called cyclic photophosphorylation.
The light energy is absorbed by the chlorophylls and other pigments of PS I. The majority of the pigments serve as antenna molecules which absorb the radiant energy and transfer to the reaction centre (P700) by inductive resonance or resonance transfer.
So the reaction centre of PS I get excited, expelling an electron from its molecule. This electron is at a high energy level which is accepted by ferredoxin reducing substance (FRS) and then by ferredoxin (Fd). From ferredoxin, electrons are transferred to cytochrome b6/f complex, plastocyanin (PC) and then back to PS I. During this process, H+ is dragged into thylakoid which is the reason for ATP synthesis by chemiosmosis.
Significance of Cyclic Photophosphorylation:
i. Cyclic photophosphorylation generates ATP molecules and no NADPH molecules.
ii. The deficiency of ATP molecules in non-cyclic photophosphorylation is made up by the operation of cyclic photophosphorylation.
iii. It is an important process in providing ATPs for the synthesis of starch, proteins, lipids, nucleic acids, and pigments within the chloroplasts.
Chemiosmotic Hypothesis:
Peter Mitchell proposed chemiosmotic hypothesis in 1961 to explain the ATP synthesis in photosynthesis (and also in respiration). This process is observed in chloroplasts during photosynthesis and in mitochondria during respiration. In honour of his pioneering work, Mitchell was awarded the Nobel Prize for chemistry in 1978.
Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient. More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane due to the development of a proton gradient across a membrane.
In chloroplasts, proton gradient develops across the thylakoid membranes of grana of chloroplast. Organization of the protein complexes of the thylakoid membrane shows that photosystem II is located predominantly in the stacked regions of thylakoid membrane whereas photosystem I and ATP synthase are found in the stroma lamellae and in the unstacked regions of grana.
The steps that cause the development of proton gradient which favours the synthesis of ATP are:
a. During the electron transport through the photosystems, protons or hydrogen ions are pumped across the membrane by cytochrome b6/f complex (proton pump) from stroma into the thylakoid lumen.
b. Splitting of water molecules takes place on the inner-side of the membrane which results in the accumulation of protons or hydrogen ions in the lumen of the thylakoids.
c. The NADP reductase enzyme is located on the stroma side of the thylakoid membrane. This enzyme reduces NADP+ to NADPH by utilizing the electrons that come from the acceptor of electrons of PSI and protons from the stroma.
As a result, there will be decrease in the number of protons in the stroma and increase in the number of protons in the thylakoid lumen. This creates a proton gradient across the thylakoid membrane. The stroma becomes more alkaline (fewer H+ ions) and the lumen becomes more acidic (more H+ ions), as a result of electron transport. This pH gradient across the membrane (proton motive force- from thylakoid lumen to stroma) provides chemical potential energy for the synthesis of ATP.
The ATP is synthesized by a large (400 kDa) enzyme complex called by several names namely ATP synthase, ATP ase (after the reverse reaction of ATP hydrolysis) and CF0 – CF1 or coupling factor. This enzyme consists of two multipeptide complexes.
A hydrophobic complex called CF0 is largely embedded in the membrane. Attached to CF0 on the stroma side is a hydrophilic complex called CF1. CF0 (proton translocator) forms a channel across the membrane through which protons flow under the pressure of proton motive force (pmf- from thylakoid lumen to stroma) by facilitate diffusion. This movement is coupled to ATP synthesis by the CF1 component of the complex that synthesize ATP.
The reaction catalysed by the ATP synthase complex can therefore be summarised as:
2H+Lumen + ADP + Pi → 2H+Stroma + ATP + H2O
Significance of Light Reaction:
The ATPs and NADPH molecules (assimilatory power) formed during light reaction drive the dark reaction of photosynthesis.
2. Dark Reaction:
The dark reaction is the second phase of photosynthesis. It is purely enzymatic and slower than the photochemical reaction (light reaction). It takes place in the stroma region of the chloroplast and is independent of light i.e., it can occur either in the presence or in the absence of light provided that the assimilatory power is available.
The conversion of CO2 to carbohydrate with the help of assimilatory power (NADPH and ATP) in dark reaction of photosynthesis is most thoroughly analysed part of photosynthesis. Main credit for investigating the sequences of dark reaction in photosynthesis goes to Melvin Calvin who was awarded Nobel Prize in 1961.
So, the dark reaction cycle is called Calvin cycle. It is also known as C3 pathway because the first stable intermediate compound formed during this cycle is a 3-carbon organic compound called phosphoglycerate (PGA).
The five major steps involved in Calvin cycle are:
a. Carboxylation or Carbon Dioxide Fixation
b. Reduction
c. Hexose formation
d. Regeneration
e. Phosphorylation
a. Carboxylation or Carbon Dioxide Fixation:
The first step in the Calvin cycle is carboxylation of ribulose 1, 5-bisphosphate (RuBP, earlier known as ribulose diphosphate i.e., RuDP) by atmospheric CO2 in the presence of an enzyme RuBP carboxylase/ oxygenase (RuBisCO). This RuBP after combining with CO2 results in the formation of an unstable-6-carbon compound. It splits into 2 molecules of a 3-carbon compound called 3-phosphoglycerate (3-PGA).
b. Reduction:
During this process, the 3-PGA is first phosphorylated to 1-3 bisphosphoglycerate (1, 3-bisPGA) in the presence of an enzyme phosphoglycerokinase. The ATP produced during light reaction provides the phosphate group for the phosphorylation. The 1 -3 bisphosphoglycerate is reduced to glyceraldehyde-3-phosphate (G-3-P) in the presence of an enzyme glyceraldehyde-3-phosphate dehydrogenase. Reduction requires hydrogen atoms and the same are provided by NADPH molecules (produced during light reaction).
Some of the glyceraldehyde-3-phosphate (G-3-P) molecules are converted to their isomer dihydroxyacetone phosphate (DHAP) molecules in the presence of an enzyme triosephosphate isomerase.
c. Hexose Formation (Sugar Formation):
Some of the G-3-P formed during the reduction process are used for (sugar) hexose formation and others are used for regeneration of phosphorylated pentose sugar i.e., RuMP. One molecule of G-3-P and one molecule of DHAP combine to form one molecule of fructose 1, 6-bisphosphate in the presence of an enzyme aldolase.
Through number of enzymatic reactions, fructose 1, 6-bisphosphate is finally converted into glucose with the release of two molecules of phosphates. The glucose molecules are then polymerised into starch, a storable carbohydrate.
[For every six molecules of CO2 fixed during the dark reaction, 12 molecules of triose phosphate are formed (6 G-3-P + 6 DHAP), one molecule of G-3-P and one molecule of DHAP combine to form one molecule of fructose 1,6-bisphosphate which later on get converted into glucose].
d. Regeneration:
The remaining molecules of G-3-P undergo many enzymatic reactions resulting in the formation of several intermediates which are different kinds of phosphorylated sugars. The final product RuMP is formed (regeneration of RuMP, Ribulose monophosphate) in the presence of an enzyme ribulose-5-phosphate kinase which is required for the production of glucose molecules continuously through the Calvin cycle.
e. Phosphorylation:
Each RuMP molecule gets phosphorylated to form RuBP in the presence of an enzyme phosphopentose kinase. Phosphorylation takes place with the help of ATP molecules (produced during light reaction).
All the carbon atoms of glucose should come from the CO2 molecules. The glucose is a 6-C sugar and to produce a molecule of it, 6 molecules of carbon dioxide are required. To accept 6 molecules of carbon dioxide, 6 molecules of RuBP are required. After carboxylation of RuBP, 6 molecules of 6-carbon unstable compound are formed.
From this, 12 molecules of glycerate 3-phosphate are produced. The PGA is reduced to G-3-P. Out of 12 G-3-P molecules, 2 are utilised in the formation of fructose 1, 6-bisphosphate that later becomes glucose 1-phosphate, which then becomes glucose. The remaining 10 G-3-P molecules are utilized for regeneration of RuMP.
During dark reaction (Calvin cycle), for one molecule of hexose (glucose) formation, 6 molecules of CO2 are absorbed. During this process, there is consumption of 18 molecules of ATP and 12 molecules of NADPH.
The entire process of dark reaction can be represented by the following equation:
6RuBP + 6CO2 + 18ATP + 12 NADPH ⇌ 6RuBP + C6H12O6 +18 (ADP + Pi) + 12NADP+ + 6H2O
Essay # 4. Conditions Influencing Photosynthesis:
Law of Limiting Factors:
According to Blackman “When a process is conditional as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor.” This law is called as Blackman’s law of limiting factors.
It can be explained with the help of an example. Let us assume that for maximum rate of photosynthesis 1500 foot candle intensity of light and 15 ml of CO2 are required. If a plant is exposed to appropriate intensity of light but CO2 is not available to it, there will be no photosynthesis. If 5 ml CO2 is provided to plant, photosynthesis starts and acquires a constant rate in a short time.
If we keep on increasing the amount of CO2, there is a corresponding increase in the rate of photosynthesis but beyond the concentration of 15 ml there is no further increase in photosynthesis. Because, even though plant is getting more than the optimum quantity of CO2 the intensity of light now starts behaving as the limiting factor. Therefore, to increase the photosynthetic yield at this stage, now the intensity of light will have to be increased until some other factor becomes limiting.
There are a number of factors which influence the rate of photosynthesis. These factors can be summarized as follows:
(a) External Factors:
(1) Light
(2) Concentration of CO2
(3) Temperature
(4) Water
(5) Oxygen
(6) Pollutants and Inhibitors
(b) Internal Factors:
(1) Chlorophyll contents of the cells
(2) Accumulation of end-products
(3) Hydration of protoplasm
(4) Leaf anatomy
(5) Minerals
We will consider each of these factors separately.
(a) External Factors:
1. Light:
Light influences photosynthesis in following ways:
(i) Intensity of Light:
Very high intensity of light has an inhibitory effect on photosynthesis because it causes photo-oxidation of chlorophyll, which is also called as solarisation. On the average, plant utilizes only 1-2% of the total incident light. The intensity of light at which rates of photosynthesis and respiration become equal, is called light compensation point.
(ii) Wavelength of Light:
Photosynthesis takes place only in visible spectrum (between 380 – 720 nm). Maximum photosynthesis occurs in red light, followed by the blue. In green light minimum photosynthesis occurs.
(iii) Duration of Light:
Photosynthesis is more if light is given in intermittent flashes separated by dark periods of a fraction of second, than if given continuously.
2. Concentration of CO2:
If more of CO2 is artificially added to the atmosphere, the rate of photosynthesis is increased and consequently there is an increase in the dry weight of the plant.
3. Temperature:
The rise in the photosynthetic process takes place between 6°C to 37°C but beyond and below it, there is a fall. In plants like Opuntia, photosynthesis occurs at 55°C. The effects of temperature are seen due to its effects on the activity of enzymes. Temperature coefficient (Q-10) for photosynthesis is equal to or more than 2 if other factors are not limiting.
4. Water:
It has been observed that the photosynthesis decreases as the turgidity of the cells falls.
5. Oxygen:
Under normal circumstances O2 has no direct effect on photosynthesis because it is looked upon only as a bye-product of photosynthesis. However, Warburg (1920) reported that very high concentrations of O2 have an inhibitory effect on the process of photosynthesis. This is called as the Warburg effect.
6. Minerals:
Presence of Mn++ and CI– is essential for smooth operation of light reaction. Mg++, Cu++ and Fe++ -ions are important for synthesis of chlorophyll.
7. Pollutants and Inhibitors:
The oxides of nitrogen and hydrocarbons present in smoke react to form peroxy acetyl nitrate (PAN) and Ozone. PAN is known to inhibit Hill reaction. Diquat and Paraquat (commonly called as Viologens) block the transfer of electrons between Q and PQ in PS-II. Other inhibitors of photosynthesis are monouron or CMU (chlorophenyl dimethyl urea) diuron or DCMU (Dichlorophenyl dimethyl urea), bromacil and atrazine etc., which have the same mechanism of action as that of viologens.
(b) Internal Factors:
1. Chlorophyll Content of the Cells:
The rate of photosynthesis is proportional to the quantity of chlorophyll present.
2. Accumulation of end Products:
In photosynthesis, the final product is starch which is immediately removed from solution. In case this starch is not removed, it collects round the chloroplasts and decreases the rate of photosynthesis.
3. Hydration of Protoplasm:
Loss of water from the protoplasm reduces the rate of photosynthesis.
4. Anatomy of Leaf:
The anatomical features of leaf like the number and distribution of stomata, thickness of cuticle, structure of mesophyll, etc., influence the rate of photosynthesis by regulating the amount of CO2 and light made available to the plant.
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