In this article we will discuss about:- 1. Introduction to Photosynthesis 2. Discovery of Photosynthesis 3. Site 4. Regulation 5. Process 6. Pigments 7. Phases 8. Conditions 9. Factors.
Contents:
- Introduction to Photosynthesis
- Discovery of Photosynthesis
- Site of Photosynthesis
- Regulation of Photosynthesis
- Process of Photosynthesis
- Photosynthetic Pigments
- Phases of Photosynthesis
- Conditions Influencing Photosynthesis
- Factors Affecting Photosynthesis
1. Introduction to Photosynthesis:
Every animal and most microorganisms on Earth, rely continuously on the uptake of large amounts of organic compounds from their environment. These compounds are used to build biomolecules of various structures and to obtain the metabolic energy that drives a cellular process.
The earliest life forms obtained their raw materials and energy from simple organic molecules dissolved in their aqueous environment. These organic molecules formed abiotically, that is, as a result of non-biological chemical reactions. Organisms that depend on an external source of organic compounds are called heterotrophs.
The early heterotrophs were severely restricted, because spontaneous production of organic molecules occurs very slowly. Only those life forms that could employ a new metabolic strategy had better survival fitness. Unlike their predecessors, these organisms could manufacture their own organic nutrients from the simplest types of inorganic molecules, such as carbon dioxide (CO2) and hydrogen sulphide (H2S). Organisms capable of surviving on CO2 as their principal carbon source are called autotrophs.
The manufacture of complex organic molecules from CO2 requires the input of large amounts of energy. Over the course of evolution, two main types of autotrophs have evolved and can be distinguished by their source of energy. Chemo-autotrophs utilize the energy stored in inorganic molecules (such as ammonia, hydrogen sulphide, or nitrites) to convert CO2 into organic compounds, while photo-autotrophs utilize the radiant energy of the sun to accomplish this task.
Chemo-autotrophs are prokaryotes and their relative contribution to the formation of biomass on earth is very small. Photo-autotrophs (plants, eukaryotic algae, various flagellated protists and members of several groups of photosynthetic bacteria) on the other hand, are responsible for capturing the energy that fuels the activities of most organisms on earth. All these organisms carry out photosynthesis, a process by which energy from sunlight is transformed into chemical energy and stored as carbohydrates and other organic molecules.
i) About 90% of the world’s photosynthesis is carried out by marine and fresh water algae.
ii) From Aristotles’s time to 17th century it was generally believed that plant and animal debris of the soil was the source of plant nutrition.
iii) According to Van Helmont in early 17th century, it was water and soil which contributed to the plant growth.
iv) Stephan Hales (1727) – Green plants may get part of their nourishment through their leaves and sunlight may have to do something with it.
v) Priestley (1772) – Idea of gas exchange taking place in photosynthesis.
vi) Ingenhouz (Austria, 1779) – Plants purify the air only in the presence of light. Only the green parts of the plant produce the purifying agent (O2) while non-green tissue contaminates the air. Thus Ingenhouz recognised the participation of chlorophyll and light in the photosynthetic process.
vii) Jean Senebier (1800) – Oxygen, liberated from the plants in this process, comes directly from CO2 which was absorbed by plants.
Red wavelengths of light is the most effective in this process.
viii) de Saussure (1840): Confirmed the finding of Ingenhouz regarding the gas exchange one in light and other in darkness (respiration). He also discovered that water was also utilised in the process.
ix) Dutrochet (1837) – Green part of the plant is essential for photosynthesis.
x) Liebig (1840) – The sole source of ‘C’ in plants was CO2 of the air.
xi) Robert Mayer – Law of conservation of Energy (1845) and idea of organic synthesis and energy transformation (1848).
xii) Sachs (1887) – Green Chloroplast, were the organs where CO2 was used up and O2 was liberated. And starch was the first visible product of photosynthesis.
xiii) Moll’s half leaf experiment showed that CO2 was necessary for photosynthesis.
2. Discovery of Photosynthesis:
Chemically, photosynthesis is the process of conversion of CO2 and water in to carbohydrates in presence of light energy.
It can be represented as:
CO2 + 2H2O + Light Energy —> [CH2O] + O2 + H2O …………….. (1)
Where [CH2O] represents a carbohydrate molecule. The standard free energy needed for the reduction of one mole of CO2 to the level of glucose is + 478 kJ/mol. Since glucose, a six carbon sugar, is often an intermediate product of photosynthesis.
The net equation can be written as:
6CO2 + 12H2O + Light Energy — C6H12O6 + 6O2 + 6H2O…………. (2)
The standard free energy for the synthesis of glucose is + 2,870 kJ/mol. In 1930s Van Niel, proposed that photosynthesis depends on electron donation and acceptor reactions and that the O2 released during photosynthesis comes from the oxidation of water and in some photosynthetic bacteria they could use hydrogen sulfide (H2S) instead of water for photosynthesis and that they release sulfur instead of oxygen.
Van Niel’s generalized equation is:
CO2 + 2H2A + Light energy [CH2O] + 2A + H2O………………… (3)
In oxygenic photosynthesis, 2A is O2, whereas in anoxygenic photosynthesis, which occurs in some photosynthetic bacteria, the electron donor can be an inorganic hydrogen donor, such as H2S (in which case A is elemental sulfur) or an organic hydrogen donor such as succinate (in which case, A is fumarate).
The biochemical conversion of CO2 to carbohydrate is a reduction reaction that involves the rearrangement of covalent bonds between carbon, hydrogen and oxygen. The energy for the reduction of carbon is provided by energy rich molecules that are produced by the light driven electron transfer reactions. Carbon reduction can occur in the dark and involves a series of biochemical reactions that were elucidated by Melvin Calvin, Andrew Benson and James Bassham in the late 1940s and 1950s. The intermediate steps were traced by using the radioisotope 14C. Calvin was awarded the Nobel Prize for Chemistry in 1961 for this work.
In 1954 Daniel Arnon and coworkers discovered that plants and photosynthetic bacteria use light energy to produce ATP, an organic molecule that serves as an energy source for many biochemical reactions. Robert Emerson, Bessel Kok, L.N.M. Duysens, Robert Hill and Horst Witt, proved that plants, algae and cyanobacteria require two reaction centers, photo system II and photo system I, operating in series.
In 1961 Mitchell’s proposal that energy is stored as an electrochemical gradient across a vesicular membrane, opened the door for understanding energy transformation by membrane systems. Most of the proteins required for the conversion of light energy and electron transfer reactions of photosynthesis are located in membranes. A key element in photosynthetic energy conversion is electron transfer within and between protein complexes and simple organic molecules.
The electron transfer reactions are rapid (as fast as a few picoseconds) and highly specific. Much of our current understanding of the physical principles that guide electron transfer is based on the pioneering work of Rudolph A Marcus, who received the Nobel Prize in Chemistry in 1992 for his contributions to the theory of electron transfer reaction in chemical systems.
3. Site of Photosynthesis:
At some point of time, one of these ancient oxygen- producing cyanobacteria took up residence inside a mitochondria-containing, non-photosynthetic pro-eukaryotic cell. Over time, the symbiotic cyanobacterium was transformed from a separate organism living within a host cell into a cytoplasmic organelle, the chloroplast.
Since the early days of primitive earth, most of the organic compounds required by the living cells have been provided by the photosynthetic organisms, including photosynthetic bacteria. The most advanced photosynthetic bacteria (existing since early days) are the cyanobacteria, which have minimal nutrient requirements. They derive electrons from water and energy from sunlight, when they convert atmospheric CO2 into organic compounds by a process called carbon fixation.
At present, plants and algae are the major source of photosynthetic activities. The chloroplast—in which photosynthesis occurs in a unique and highly specialized intracellular organelle—developed much later. Chloroplasts perform photosynthesis in the presence of sunlight (daylight hours).
The primary products of photosynthesis are NADPH and ATP, which are used by the photosynthetic cells to produce many organic compounds. In plant cells, the next immediate products include a low-molecular weight sugar (usually sucrose) that is exported to meet the metabolic needs of the many non-photosynthetic cells of the organism.
4. Regulation of Photosynthesis:
Plants adapt to a wide variety of environmental conditions and therefore, the mechanism of regulation of photosynthesis is complex. Although all the regulatory mechanisms of the photosynthetic process are not known, some of them have been well-characterized.
Light Control of Photosynthesis:
Since, the photosynthetic rate depends on temperature, cellular CO2 concentration and light, investigation becomes complicated. Light is the most important factor that regulates photosynthesis. Most of the enzymes involved in the Calvin cycle for catalyzing CO2 fixation are rapidly inactivated in the dark, thereby conserving ATP that is synthesized in the dark for other reactions, such as lipid and amino acid biosynthesis.
Light assists in activation of certain photosynthetic enzymes and deactivation of several enzymes in degradative pathways. Among the light-activated enzymes are ribulose-1,5-bisphosphate carboxylase, NADP+- glyceraldehyde-3-phosphate dehydrogenase, fructose- 1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase and phosphoribulokinase. Examples of light- inactivated enzymes include phosphofructokinase and glucose-6-phosphate dehydrogenase. Light affects enzymes by indirect mechanisms. Among the best- studied are pH, Mg2+, the ferredoxin-thioredoxin system and phytochrome.
Control of Ribulose-1,5-Bisphosphate Carboxylase:
Rubisco is found within the chloroplast (the L subunit) and the nucleus (the S subunit). The activation of these genes is mediated by an increase in light intensity (illumination). Once the S subunit is transported from the cytoplasm to the chloroplast, both subunits assemble to form the L8S8 holoenzyme. The activity of rubisco is modified by a number of metabolic signals. Rubisco is spontaneously activated in the presence of high CO2 and Mg2+ concentrations.
In order to activate rubisco, the active site of the L subunit must be carbamoylated at a specific lysine residue-191, forming a carbamate group that then binds a Mg2+ ion. The rate of carboxylation is dependent on the CO2 concentration and an alkaline pH.
Under normal conditions, however, with ambient levels of CO2, the reaction requires catalysis by rubisco activase, an enzyme that simultaneously hydrolyzes ATP and uses the energy to attach a CO2 to the lysine. The level of activation is cooperative and increases as more of the eight subunits are modified. This regulatory mechanism ensures that CO2 fixation only occurs at an appreciable rate when the concentration of CO2 and available energy are high.
5. Process of Photosynthesis:
The preparation of food by the leaves of green plant and micro-organism in presence of sunlight, chlorophyll, water and “CO2“ is called photosynthesis. In this process, the CO2 from the atmosphere combines with water and light energy to produce carbohydrates (i.e., sugars, starches etc.) and oxygen.
The photosynthesis process can be represented by the following reaction:
6CO2 + 6H2O + light energy C6H12O6 + 6CO2
Biomass does not add CO2 to the atmosphere as it absorbs the same amount of carbon in growing the plants as it is released when consumed as fuel. It is a superior fuel as the energy produced by biomass is ‘carbon cycle neutral’.
The conditions necessary for photosynthesis are:
(i) Light:
The intensity of solar radiation of 400-700°A wavelength is one of the important inputs for biomass production; this range of light is called ‘Photosynthetically active radiation (PAR)’. The upper limit of photosynthesis efficiency is about 5 per cent.
(ii) CO2 Concentration:
CO2 is the primary raw material for photosynthesis.
The main sources of CO2 are:
a. Animal respiration;
b. Combustion of fuel;
c. Decay of organic matter by bacteria;
d. Ocean (respiration of marine plants and animals releases CO2 into the water).
(iii) Temperature:
The process of photosynthesis is restricted to temperature range of 0°C to 60°C which can by tolerated by proteins.
The energy stored in the plants by way of carbon fixation in the form of chemical bond energy when expressed as a fraction of total insolation falling on the plant, is called as photosynthetic efficiency.
6. Photosynthetic Pigments
:
i. Chlorophylls occur mostly in the grana and are associated with the thylacoid membrane.
ii. At least 7 types of chlorophylls are known viz., chl a, b, c, d, e, bacteriochlorophyll and bacterioviridin.
iii. All chlorophyll (Chl a & Chl. b) molecules contain a tetrapyrrole skelton formed into ring with ‘Mg’ at the centre. Thus it has five atoms i.e. 4 carbon and one nitrogen. The base unit of the chlorophyll molecule is a porphyrin ring system made up of 4 simple pyrrole nuclei (tetrapyrrole) joined by carbon linkages. The centre of the porphyrine ring is occupied by a single atom (non-ionic) of Magnesium (Mg). Only chl.a & chl.b contain Magnesium.
iv. Chl. a and chl. b are the most abundant ones found in all autotrophic plants except pigmented bacteria. Other chlorophylls (viz., chl. c, chl. d, chl. e) are found only in algae and in combination with chl. a.
v. Precursor of chlorophyll is protochlorophyll but according to recent view the immediate precursor is chlorophyllide. Protochlorophyllide → chlorophyllide → chl. a → chl. b
vi. In fresh green leaves, the proportions of photosynthetic pigments are as follows:
i. Carotene and Xanthophyll are together called carotenoids.
ii. These are fat soluble yellow pigments.
iii. Carotenoids are located in chloroplast and chromoplast.
iv. Yellow colour of etiolated and variegated leaves is due to carotenoids.
v. Such pigments are composed of two 6-membered rings with a hydrocarbon chain stretched between them.
vi. Light energy absorbed by carotenoids is shunted to chl. a and light absorption results in fluorescence of chlorophyll.
vii. Strong absorption takes place in the blue violet and ultraviolet end of spectrum with almost no absorption in the red end.
i. Its colour is Orange-yellow having empirical formula C40H56 (i.e., exclusively of C & H.)
ii. It is abundant in roots of carrot hence the name carotene.
iii. It is insoluble in water.
iv. Its most common form is β-carotene. β-carotene is the precursor of vit. A. Other forms of carotene are α-carotene, γ-carotene.
v. It is quickly oxidised in air and hence the rapid change of colour takes place in the scaped carrot.
i. It is more abundant than carotenes.
ii. It occur in many isomeric forms having colour yellow to brown.
iii. Empirical formula is C40H56O2
iv. It is also called carotenol.
v. The commonent form is Luteol (lutein) followed by violaxanthal (violaxanthin).
vi. The principal yellow pigment of maize is zeaxanthin.
i. Phycobilins are found in blue-green and red algae.
ii. It has tetrapyrrole rings but in straight chain.
iii. Light absorbed by phycobilins is transferred to chl. a where is used in photosynthesis.
iv. It contains no magnesium (Mg.)
v. Red pigment is called phyco-erythrin and blue pigment is phycocyanin found in red algae and blue-green algae respectively.
vi. It is soluble in hot water while chlorophylls and carotenoids are soluble in organic solvent.
vii. It masks the green colour like anthocyanin.
Anthocyanin:
Anthocyanin: is a purple pigment, soluble in water hence it occurs in solution in the water of the cells means it is actually dissolved in the cell sap and not in cytoplasm; does not take part in photosynthesis; present in sugarbeet.
7. Phases of Photosynthesis
:
It has two phases viz. light phase and a dark phase:
i. Reaction of the light phase is light sensitive hence called photochemical reaction.
ii. The reactions of the dark phase are temperature sensitive and don’t require light. These are purely chemical reaction and called Blackman reactions on the name of FE Blackman who first demonstrated its existence.
Evidences in support of source of O2 is H2O, are given by following scientists:
a) Von Niel:
Experiment on bacteria i.e., purple sulphur bacteria. These bacteria are autotrophic and photosynthetic. (Normally bacteria are hetero-trophs).
b) Ruben:
Experiment by Ruben is more authentic work. His experiment was on Alga (i.e., chlorella) through isotopic studies (O16 and O18).
c) Hill:
Experiment on cell free or isolated chloroplast.
Here CO2 is not supplied to isolated chloroplast although O2 is released. This reaction is called Hill’s reaction.
On the basis of the above evidences, the revised reaction of photosynthesis is-
Empirical Molecular Reaction:
Three inferences are drawn from the empirical molecular formula:
a) Breakdown of water molecule to release of O2.
b) 50% Hydrogen from water combines with C = O of CO2 to produce food.
c) Remaining 50%H of H2O combines with [O] of CO2 to produce water.
Division of Labour:
Light and Dark Reaction:
The conversion of light energy into chemical energy i.e. ATP is called Photophosphorylation.
ATP = Adenosine triphosphate; ADP = Adenosine Diphosphate
The breakdown of water molecule (H2O) into hydrogen and oxygen by light energy is called Photolysis of water. (Photo means light and lysis means to break).
Origin of Food:
Three major reactions are:
8. Conditions Influencing Photosynthesis:
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.
9. Factors Affecting Photosynthesis:
1) Light:
It is the most important factor of photosynthesis. Light affects through its intensity, quality and duration. The amount of light received by the plant depends on its morphology. The actual requirement of the light intensity depends upon the type of plant and its habitat. Generally average sunlight is sufficient except on rainy or cloudy days.
The rate of photosynthesis increases with the increase in light intensity until law of minimum operates. Extremely high light intensity has an inhibitory effect on the photosynthesis and this phenomenon is called solarization. During solarization, photo-oxidation occurs in which certain cell constituents are oxidized by O2 into CO2.
High light intensity also increases the transpiration rate, consequently reducing the water content of mesophyll cells which has also an inhibitory effect on photosynthesis. Low intensity causes stomatal closure which restricts the entry of CO2. The light intensity at which the photosynthetic intake of CO2 is equal to the respiratory output of CO2, is called compensation point. Therefore net photosynthesis is zero at the compensation point.
Visible part of the light spectrum i.e. wavelength between 400 nm to 750 nm is the only need for photosynthesis. Heavy absorption of light takes place in the red and next in the blue & violet. Continuous illumination of light also affects the photosynthesis.
2) CO2:
The percentage of CO2 in the air is 0.03% by volume. At optimum temperature and light intensity, photosynthesis is markedly increased with the increase in CO2 concentration. But relatively high conc. of CO2 reduces the photosynthetic rate.
3) Temperature:
There is a rapid increase in photosynthesis if temp, increases from 10°C-35°C, provided other factors are not limiting. Photosynthetic rate declines with the higher temp, beyond the maximum limit.
4) Water:
It has no direct effect. The decrease in water content of leaves may cause partial or complete closure of stomatal openings which reduces the diffusion of CO2.
5) O2:
Oxygen accumulation may retard the photosynthesis. It is a competitive inhibitor of carboxylase thus inhibits photosynthesis in C3-plants.
6) Mineral Nutrients:
Magnesium is a part of chlorophyll molecule and other nutrients are necessary for enzymic action and plant metabolism.
All the above factors are external but some internal factors like chlorophyll content; protoplasmic factors & hydration, end product of photosynthesis may also affect the photosynthesis.
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