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Photosynthesis (Photo = light; synthesis = to join) is the single most important process on earth on which depends the existence of human beings and almost all other living organisms. It is a process by which green plants, algae and chlorophyll containing bacteria utilize the energy of sunlight to synthesize their own food (organic matter) from simple inorganic molecules. An innumerable number of organic molecules which compose the living world are derived directly or indirectly from the photosynthetic organic matter. The oxidation of organic compounds releases stored energy to be utilized by the living organisms to carry out essential metabolic processes. It is important to note that photosynthesis is the only natural process which liberates oxygen to be used by all living forms for the process of aerobic respiration.
- Green plants possess the green pigment, chlorophyll which can capture, transform, translocate and store energy which is readily available for all forms of life on this planet.
- Photosynthesis is a process in which light energy is converted into chemical energy.
- Except green plants, no other organism can directly utilize solar energy to synthesize food, hence they are dependent on green plants for their survival.
- Green plants which can prepare organic food from simple inorganic elements are called autotrophic while all other organisms which cannot prepare their own food are called heterotrophic.
- During photosynthesis, oxygen liberated into the atmosphere makes the environment livable for all aerobic organisms.
- Simple carbohydrates produced in photosynthesis are transformed into lipids, proteins, nucleic acids and other organic molecules.
- Plants and plant products are the major food sources of almost all organisms on the earth.
- Fossil fuels like coal, gas, and oil represent the photosynthetic products of the plants belonging to early geological periods.
What is photosynthesis?
Photosynthesis is the process by which green plants, in the presence of light combine water and carbon dioxide to form carbohydrates. Oxygen is released as a by product of photosynthesis. Current knowledge of photosynthesis has resulted from discoveries made over 300 years of work. Some landmark experiments are given below
- Joseph Priestley (1772) and later Jan Ingenhousz (1779) showed that plants have the ability to take up CO2 from the atmosphere and release O2.
- Ingenhousz also discovered that release of O2 by plants was possible only in presence of sunlight and by the green parts of the plant.
- Robert Hill (1939) demonstrated that isolated chloroplasts evolve O2 when they are illuminated in the presence of electron acceptor which gets reduced. This reaction called Hill reaction accounts for the use of water as a source of electrons and protons for CO2 fixation and release of O2 as the by-product.
Photosynthesis is represented by the following overall chemical equation:
6CO2 + 12H2O —-→ C 6H 12O6 +6H2O +6O 2
In photosynthesis, CO2 is fixed (or reduced) to carbohydrates (glucose C6H12O6). Water is split in the presence of light (called photolysis of water) to release O2. Note that O2 released comes from the water molecule and not from CO2
Where does photosynthesis occur?
Photosynthesis occurs in green parts of the plant, mostly the leaves, sometimes the green stems and floral buds. The leaves contain specialized cells called mesophyll cells which contain the chloroplast– the pigment-containing organelle. These are the actual sites for photosynthesis.
The thylakoids of the chloroplast contain the pigments which absorb light of different wavelengths and carry out the photochemical reaction of photosynthesis.The role of the pigments is to absorb light energy, thereby converting it to chemical.
The role of the pigments is to absorb light energy, thereby converting it to chemical energy. These pigments are located on the thylakoid membranes and the chloroplasts are usually so arranged within the cells that the membranes are at right angles to the light source for maximum absorption. The photosynthetic pigments of higher plants fall into two classes the chlorophyll and carotenoids.The photosynthetic pigment chlorophyll is the principle pigment involved in
The photosynthetic pigment chlorophyll is the principle pigment involved in photosynthesis. It is a large molecule and absorbs light maximally in the violet blue and in the red region of the visible spectrum and reflects green light and thus leaves appear green in colour. Carotenoids (carotene and xanthophyll) absorb light in the regions of the spectrum not absorbed by the chlorophylls and transfer that energy to chlorophyll to be used in photosynthesis.
Chlorophyll-a (a special type of chlorophyll) is the main pigment that traps solar Plants and animals energy and converts it into chemical energy. Chlorophyll-a is present in all autotrophic plants except photosynthetic bacteria. Thus Chl-a is called the essential photosynthetic pigment responsible for representing the reaction centre.
All other pigments such as chlorophyll b and carotenoids are collectively called accessory pigments since they pass on the absorbed light energy to chlorophyll a (Chl-a) molecule to be utilized for photosynthesis. These pigments, that is the reaction centres (Chl-a) and the accessory pigments (harvesting centre) are packed into functional clusters called photosystems. Photosystems are of two types PSI and PSII.
About 250-400 Chl-a molecules constitute a single photosystem. Two different photosystems contain different forms of chlorophyll a in their reaction centres. In photosystem I (PSI), chlorophyll– a with maximum absorption at 700 nm (P700) and in photosystem II (PSII), chlorophyll– a with peak absorption at 680 nm (P680), act as reaction centres. (P stands for pigment). The primary function of the two photosystems, which interact with each other is to trap the solar energy and convert it into the chemical energy also called assimilatory power (ATP and NADPH2).The differences between them are given in the following Table
ROLE OF SUNLIGHT IN PHOTOSYNTHESIS
Light consists of small particles or packages of energy called “photons”. A single photon is also called quantum. What does the chlorophyll do? It absorbs light energy. Chlorophyll molecules absorb light energy and get into an excited state and lose an electron to the outer orbit. No substance can remain in an excited state for long, so the energised and excited chlorophyll molecule comes down to a low energy state known as ground state and releases the extra amount of energy. This energy can be lost as heat, or as light (fluorescence) or can do some work. In photosynthesis, it works by splitting water moelcule to produce H+ and OH– ions.
Carotene is orange-yellow pigment present along with chlorophylls in the thylakoid membrane. A carotene molecule breaks down into the vitamin A molecules. It is this pigment which gives carrot its colour.
Absorption and Action Spectra
For investigating a process such as photosynthesis that is activated by light, it is important to establish the action spectrum for the process and to use this to identify the pigments involved. An action spectrum is a graph showing the effectiveness of different wavelengths (VIBGYOR) of light in stimulating the process of photosynthesis, where the response could be measured in terms of oxygen produced at different wavelengths of light. An absorption spectrum is a graph representing the relative absorbance of different wavelengths of light by a pigment. An action spectrum for photosynthesis is shown in Fig. together with an absorption spectrum for the combined photosynthetic pigments. Note the close similarity, which indicates that the pigments, chlorophyll-a in particular, are responsible for absorption of light used in photosynthesis.
All wavelengths of light are not equally effective in photosynthesis i.e. the rate of photosynthesis is more in some and less in others.
Photosynthesis occurs maximum in blue and red region of spectra.Photosynthesis is very little in green and yellow light because these rays are reflected back from the leaf.
PHOTOCHEMICAL AND BIOSYNTHETIC PHASE
The entire process of photosynthesis takes place inside the chloroplast. The structure of chloroplast is such that the light dependent (light reaction) and light independent (Dark reaction) reactions take place at different sites in the same organelle.
The thylakoids have the pigments and other necessary components to absorb light and transfer electrons to carry out the light reaction or Electron Transport Chain (ETC). In ETC upon absorption of light, the electrons from PSII and PSI are excited to a higher energy level i.e. the electrons acquire excitation energy.
As the electrons gain this energy, they are accepted by the electron acceptor which in turn is reduced, leaving the reaction centers of PSII and PSI i.e. P680 and P700 molecules in an oxidized state. This represents the conversion of light energy into chemical energy. The electrons then travel downhill in energy terms, from one electron acceptor to another in a series of oxidation-reduction reaction.
This electron flow is ‘coupled’ to the formation of ATP. In addition, NADP is reduced to NADPH2. The product of light reaction is called the reducing power or assimilatory power (ATP and NADPH2) which move out of the thylakoid into the stroma of the chloroplast.
In the stroma, the second step called as dark reaction or biosynthetic pathway occurs, where CO2 is reduced by the reducing power generated in the first step and carbohydrates are produced.
Electron transport chain in photosynthesis
After receiving light PSII absorbs light energy and passes it on to its reaction center, P680. When P680 absorbs light, it is excited and its electrons are transferred to an electron acceptor molecule (Primary electron acceptor i.e. pheophytin) and it itself comes to the ground state. However by losing an electron P680 is oxidized and in turn, it splits water molecule to release O2. This light-dependent spliting of water is called photolysis. With the breakdown of water electrons are generated, which are then passed on to the electron deficient P680 (which had transferred its electrons earlier). Thus the oxidised P680 regains its lost electrons from water molecules.
The reduced primary acceptor now donates electrons to the down stream components of the electron transport chain. The electrons are finally passed onto the reaction centre P700 or PSI. During this process, energy is released and stored in the form of ATP. Similarly, PSI also gets excited when it absorbs light and P700 (Reaction Similarly, PSI also gets excited when it absorbs light and P700 (Reaction centre of PSI) gets oxidised as it transfers its electrons to another primary acceptor molecule. While the oxidised P700 draws its electrons from PSII, the reduced primary acceptors molecule of PSI transfers its electrons via other electron carriers to NADP (Nicotinamide Adenine Dinucleotide Phosphate) to produce NADPH2 a strong reducing agent. Thus we see that there is a continuous flow of electrons from the H2O molecules to PSII to PSI, and finally to the NADP molecule which is reduced to NADPH2. NADPH2 is then utilized in reduction of CO2 to carbohydrates in the biosynthetic pathway.
Reduction of CO2 to carbohydrate also requires ATP, which too are generated via electron transport chain. As the energy-rich electrons pass down the electron transport system, it releases energy which is sufficient to bind inorganic phosphate (Pi) with ADP to form ATP. This process is called photophosphorylation. Since this takes place in presence of light it is called Photo- Plants and animals phosphorylation. It occurs in chloroplast in two ways:
- Non-cyclic photophosphorylation where electrons flow from water molecule to PSII and then to PSI and ultimately reduce NADP to NADPH2. Since the electron flow is unidirectional and the electrons released from one molecule do not return to the same molecule, it is called non-cyclic photosphorylation
- Cyclic photophosphorylation occurs in photosynthetic bacteria which lack PS-II, and it involves PSI only. During this process electrons from PSI are not passed on to NADP. Instead the same electrons are returned to the oxidised P700 molecule. During this downhill movement of electrons ATP formation takes place. Thus this is termed as cyclic photophosphorylation
Differences between cyclic and non-cyclic photophosphorylation
In higher photosynthetic plants, extra ATP can be made via cyclic photophosphorylation if cyclic and non-cyclic photophosphorylation occur side by side. The efficiency of energy conversion in the light reactions of photosynthesis is high and estimated at about 39%.
BIOSYNTHETIC PATHWAY (DARK REACTION)
- Both NADPH2 and ATP produced during light reaction are essential requirements for synthesis of carbohydrates.
- These series of reactions which catalyse the reduction of CO2 to carbohydrates (also called fixation of CO2) take place in the stroma of the chloroplast.
- These reactions are independent of light i.e. light is not necessary but can continue in light as well if products of the light reaction are available. Thus it is also called dark reaction.
- The carbon fixation reactions produce sugar in the leaves of the plant from where it is exported to other tissues of the plant as a source of both organic molecule and energy for growth and metabolism.
- There are two major pathways by which CO2 fixation (Dark reaction) takes place.
C3 cycle (also called Calvin cycle after the name of its discoverer, Melvin Calvin)
In this cycle, initially, the atmospheric CO2 is accepted by a 5-carbon sugar ribulose bisphosphate (RuBP) resulting in the generation of two molecules of 3-carbon compound, 3-phosphoglyceric acid (PGA). This 3-carbon molecule is the first stable product of this pathway and hence the name C3 cycle is given. Formation of PGA is called carboxylation. This reaction is catalyzed by an enzyme called ribulose bisphosphate carboxylase/oxygenase or Rubisco. This enzyme is probably the most abundant protein on earth.
- In the next step, PGA is reduced to 3-carbon carbohydrate called triose Plants and animals phosphate using NADPH2 and ATP (from light reaction). Much of these molecules are then diverted from the C3 cycle and used for synthesis of other carbohydrates such as glucose and sucrose.
- To complete the cycle, the initial 5-carbon acceptor molecule, RuBP is regenerated from the triose phosphates using ATP molecule thus the C3 cycle continues to regenerate the CO2-acceptor (RuBP).
C4 Cycle (or Hatch Slack Cycle)
- The C4 cycle seems to be an adaptation for plants growing under dry hot environment. Such plants can photosynthesise even in the conditions of very low CO2 concentration and under partial closure of stomata.
- Such plants can thus grow at low water content, high temperature and high light intensity. Sugarcane, and maize are some examples.
- Photorespiration (oxidation of RuBP in presence of O2) is absent in these plants. So the photosynthetic rate is high.
- The leaves of C4 plants show presence of dimorphic chloroplasts, called Kranz anatomy.
- (a) In these plants, the vascular bundles have a sheath of large parenchyma cells around them in the form of a wreath, thus the name Kranz anatomy (Kranz : wreath)
- (b) Leaves possess two types of chloroplasts (dimorphic chloroplasts)
- (c) Chloroplasts in the mesophyll cells are smaller and have well developed grana (granal chloroplasts) but do not accumulate starch.
- (d) Chloroplasts in the bundle sheath cells are larger and lack grana (agranal chloroplasts) but contain numerous starch grains.
- In C4 plants, the initial acceptor of CO2 is phosphoenol pyruvic acid or PEP, a 3 carbon compound. It combines with CO2 in presence of an enzyme Phosphoenol pyruvate carboxylase (PEP carboxylase) and forms a C4 acid, oxaloacetic acid (OAA). This fixation of CO2 occurs in the cytosol of the mesophyll cells of the leaf. OAA is the first stable product of this cycle which is 4 carbon compound and hence the name C4 pathway is given.
- OAA then travels from mesophyll cells to the chloroplasts of bundle sheath cell where it releases the fixed CO2. C3 cycle operates within these cells and this CO2 immediately combines with RuBP in C3 cycle producing sugars.
- Thus in C4 pathway of dark reaction, there are two carboxylase enzymes that take part. PEP carboxylase (PEPCo) in the mesophyll cells and RUBP carboxylase Rubisco) in the bundle sheath cells.
differences between C3 and C4 plants are tabulated below
|C3 Plants||C4 Plants|
|Carbon dioxide fixation||Occurs once||Occurs twice, first in mesophyll cells, then in bundle sheath cells|
|Carbon dioxide acceptor||Only one acceptor, RuBP which occurs in all green cells of the plant||In Mesophyll cells, PEP (Phosphoenol Pyruvic acid), 3-C, compound is CO2 acceptor, but in the bundle sheath cells-RuBP, 5C, compound, is the CO2 acceptor|
|Carbon dioxide fixing enzyme||RuBP carboxylase, which is not efficient when CO2 conc is low||PEP caboxylase which is very efficient, even if CO2 conc. is low RuBP carboxylase, works efficiently because carbon dioxide concentration is high.|
|First product of photosynthesis||The first stable product is 3-C compound phosphoglyceric acid|
The first product is 4-C compound oxaloacetic acid
|Concentration of CO2||Higher CO2 conc. Promotes photosynth]esis||Photosynthetic efficiency is high even if CO2 conc. is low|
|Leaf anatomy||Only one type of chloroplast Kranz’ anatomy is absent||Two types of chloroplasts (dimorphic) or Kranz’ anatomy, i.e., two types of cells. each with its own type of chloroplasts are present|
|Photorespiration||Occurs; excess of oxygen is an inhibitor of photosynthesis||Photorespiration is absent. The photo. synthetic efficiency is further increased|
|Efficiency||Less efficient plotosynthesis than C4 plants. Yields usually much lower.||More efficient photosynthesis as compared to that of the C3 plants. Yields usually much higher.|
- Internal Factors
- Chlorophyll : The amount of chlorophyll present has a direct relationship with the rate of photosynthesis because this pigment is directly involved in trapping light energy responsible for the light reactions.
- Leaf age and anatomy : Newly expanding leaves show gradual increase in rate of photosynthesis and the maximum is reached when the leaves achieve full size. Chloroplast functions decline as the leaves age. Rate of photosynthesis is influenced by variation in (i) number, structure and distribution of stomata, (ii) size and distribution of intercellular spaces (iii) relative proportion of palisade and spongy tissues and (iv) thickness of cuticle.
- Demand for photosynthate : Rapidly growing plants show increased rate of photosynthesis in comparison to mature plants. When demand for photosynthesis is lowered due to poor meristematic activity, the photosynthetic rate declines.
- External Factors The major external factors which affect the rate of photosynthesis are temperature, light, carbondioxide, water, and mineral elements.
- Concept of limiting factors: When a process is affected by various factors, the rate of the process depends upon the pace of the slowest factor. Let us consider three factors like light, carbon dioxide and temperature. It is seen that when all three factors are optimum, the rate of photosynthesis is maximum. However, of the three factors even if one of the factors becomes suboptimal and the other factors remain optimal, the rate of the photosynthetic process declines substantially. This is known as law of limiting factors shown by Blackman in 1905. It is defined as 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 which is known as the limiting factor.
- Light: The rate of photosynthesis increases with increase of intensity of light within physiological limits or rate of photosynthesis is directly proportional to light intensity. Except on a cloudy day and at nights, light is never a limiting factor in photosynthesis in nature. At a certain light intensity the amount of CO2 used in photosynthesis and the amount of CO2 produced in respiration are the same. This point of light intensity is known as compensation point. Wavelength of light absorbed by photosynthetic pigments affects rate of photosynthesis. Red light and to some extent blue light has an enhancing influence on photosynthesis. The proportion of the total incident sunlight on earth, absorbed by green plants is generally a limiting factor. As per the estimates of the total incident light reaching the green plants, only about 1-2% is actually absorbed, because 70% is transmitted, and 28-29% is reflected back into the atmosphere.
- Temperature : Very high and very low temperature affect the rate of photosynthesis Plants and animals adversely. Rate of photosynthesis will rise with temperature from 5°-37°C beyond which there is a rapid fall, as the enzymes involved in the process of the dark reaction are denatured at high temperature. Between 5°-35°C, with every 10°C rise in temperature rate of photosynthesis doubles or Q10 is 2 (Q = quotient), or slightly less than two.
- Carbon dioxide : Since carbon dioxide being one of the raw materials for photosynthesis, its concentration affects the rate of photosynthesis markedly. Because of its very low concentration (0.03%) in the atmosphere, it acts as limiting factor in natural photosynthesis. At optimum temperature and light intensity, if carbon dioxide supply is increased the rate of photosynthesis increases markedly until CO2 conc. is as high as 3.0%. Thus, CO2 conc. in the atmosphere is always a limiting factor for photosynthesis.
- Water : Water has an indirect effect on the rate of photosynthesis. Loss of water in the soil is immediately felt by the leaves, which get wilted and their stomata close down thus hampering the absorption of CO2 from the atmosphere. This causes decline in photosynthesis.
- Oxygen : Concentration of oxygen as an external factor, is never a limiting factor for photosynthesis because it is a by-product of photosynthesis, and it easily diffuses into the atmosphere from the photosynthesizng organ, the leaf. However, excesss of O2 surrounding a green plant, reduces photosynthetic rate by promoting the rate of aerobic respiraiton.
- Mineral elements: Some mineral elements like magnesium, copper, manganese and chloride ions, which are components of photosynthetic enzymes, and magnesiumas a component of chlorophylls are important, and their deficiency would affect the rate of photosynthesis indirectly by affecting the synthesis of photosynthetic enzymesand chlorophyll, respectively.
When plants utilise light energy to reduce carbon dioxide to carbohydrates, they are called photosynthetic autotrophs. There are some bacteria which can utilise chemical energy released during biological oxidation of certain inorganic substances to reduce carbon dioxide to carbohydrate. These bacteria are called chemosynthetic autotrophs.
This is found in many colourless bacteria and because they use chemical energy to
reduce carbon dioxide, this process of carbohydrate synthesis is known as
Chemosynthesis may be defined as “the method of carbon assimilation when the reduction of CO2 is carried out in darkness, utilising the energy obtained fromoxidation of inorganic substances, such as H2S and NH3.
The common chemosynthetic forms are :
- Nitrifying bacteria. Nitrosomonas and Nitrobactor oxidise NH3 to NO2
- Sulphur bacteria
- Iron bacteria
- Hydrogen and methane producing bacteria
Differences between photosynthesis and chemosynthesis
This is a process in which energy stored as a hydrogen ion gradient across a membrane is used to synthesise ATP from ADP and Pi. The enzyme which uses the energy is ATP synthase and the energy or power source is the difference in the concentration of H+ ions on opposite sides of the membrane. The membrane is the inner membrane of the mitochondrion or the chloroplast. The word ‘osmosis’ in Greek means ‘push’ and here the flow of H+ ions across the membrane provides the energy or push to ATP synthase enzyme which then catalyses the synthesis of ATP.Chloroplasts use chemiosmosis to generate ATP during photosynthesis. The
Chloroplasts use chemiosmosis to generate ATP during photosynthesis. The prokaryotes lack the organelles mitochondria and chloroplast to generate H+ gradients across plasma membranes and cannot use it for ATP synthesis. Peter Mitchell won the Nobel prize in 1978 for proposing the chemiosmotic model for syntheis of ATP.