Photosynthesis Literally photosynthesis means ‘synthesis using light’. Photosynthetic organisms use solar energy to synthesize carbon compound.
• Photosynthesis (Photon = Light, Synthesis = Putting together) is an anabolic, endergonic process by which green plant synthesize carbohydrate from carbon dioxide and water in presence of pigments and sunlight. In other words, we can say that photosynthesis is transformation of solar energy/radiant energy/light energy (ultimate source of energy for all living organisms) into chemical energy. Photosynthesis Radhika Sapkota Photosynthesis Literally photosynthesis means ‘synthesis using light’. Photosynthetic organisms use solar energy to synthesize carbon compound.
• Photosynthesis (Photon = Light, Synthesis = Putting together) is an anabolic, endergonic process by which green plant synthesize carbohydrate from carbon dioxide and water in presence of pigments and sunlight. In other words, we can say that photosynthesis is transformation of solar energy/radiant energy/light energy (ultimate source of energy for all living organisms) into chemical energy. • • Simple general equation of photo synthesis is as follows
• 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O
• According to Van Neil and Robert Hill, oxygen liberated during photosynthesis comes from water and not from carbon dioxide. chlorophyll sunlight Site of Photosynthesis
• Chloroplast in green plants constitute the photosynthetic apparatus and act as site of photosynthesis. Chloroplasts of higher plants are discoid or ellipsoidal in shape measuring 4 —6 μ in length and 1—2 μ in thickness. It is a double membranous cytoplasmic organelle of eukaryotic green plant cells. The thickness of the two membranes including periplastidial space is approximately 300Ã….
• Ground substance of chloroplast is filled with a hydrophilic matrix known as stroma. It contains cp-DNA (0.5%), RNA (2—3%), Plastoribosome (70S), enzymes for carbon dioxide assimilation, proteins (50—60%), starch grains and osmophilic droplets, vitamin E and K, Mg, Fe, Mn, P, etc. in traces. In stroma are embedded a number of flattened membranous sacs known as thylakoids. Photosynthetic pigments occur in thylakoid membranes.
• Aggregation of thylakoids to form stacks of coin like structures known as granna. A grannum consists near about 20 — 30 thylakoids. Each thylakoid encloses a space known as loculus. The end of disc shape thylakoid is called as margin and the area where the thylakoids membranes are appressed together is called partition.
• Some of the granna lamella are connected with thylakoids of other granna by stroma lamella . In photosynthetic prokaryotes (bluegreen algae and Bacteria) chloroplast is absent. Chromatophore is present in photosynthetic bacteria and photosynthetic lamellae in blue-green algae. Photosynthetic pigments
• Photosynthetic Pigments: • Photosynthetic pigments are substances that absorb sunlight and initiate the process of photosynthesis.
• Photosynthetic pigments are grouped into 3 categories •
(i) Chlorophyll: • These are green coloured most abundant photosynthetic pigments that play a major role during photosynthesis. Major types of chlorophylls are known to exist in plants are Chlorophyll a, b, c, d and e, .
• Chemically a chlorophyll molecule consists of a porphyrin head (15 x 15Ã…) and phytol tail (20Ã…). Porphyrin consists of tetrapyrrole rings and central core of Mg.
• Out of various types of chlorophyll, chlorophyll a and chlorophyll b are the most important for photosynthetic process. Chlorophyll a is found in all photosynthetic plants . For this reason it is designated as Universal Photosynthetic Pigment or Primary Photosynthetic Pigment. • •
ii) Carotenoids: • These are yellow, red or orange colour pigments embedded in thylakoid membrane in association with chlorophylls but their amount is less. These are insoluble in water and precursor of Vitamin A. These are of two of types viz., Carotene and Xanthophyll (Carotenol/Xanthol).
• Carotenes are pure hydrocarbons, red or orange in colour and their chemical formula is – C40H56 . • Xanthophylls are yellow coloured oxygen containing carotenoids and are most abundant in nature. The ratio of xanthophyll to carotene in nature is 2:1 in young leaves.
• iii) Phycobilins (Biliproteins): • These are water soluble pigments and are abundantly present in algae. There are two important types of phycobilins-Phycoerythrin (Red) and Phycocyanin (Blue). Mechanism of photosynthesis
• The process of photosynthesis can be divided into two main phases: • Light reaction (or light dependent reaction) • Dark reaction (or light independent reaction Light reaction
• Light reaction is the first stage of photosynthesis process in which solar energy is converted into chemical energy in the form of ATP and NADPH. The protein complexes and the pigment molecules help in the production of NADPH and ATP. • In light reaction, the addition of phosphate in the presence of light or the synthesizing of ATP by cells is known as photophosphorylation.
• Photosystems, large complexes of proteins and pigments (light-absorbing molecules) that are optimized to harvest light, play a key role in the light reactions. There are two types of photosystems: photosystem I (PSI) and photosystem II (PSII).
• Both photosystems contain 200-300 light harvesting molecules that help collect light energy, as well as a special pair of chlorophyll molecules found at the core (reaction center) of the photosystem. The special pair of photosystem I is called P700, while the special pair of photosystem II is called P680. • Light reaction occurs in following steps
1. Photoexcitation of chlorophyll- a • Light photon energy is absorbed by antenna molecules transfer it to the core molecules then to reaction centers. Due to the gaining of light energy, the electron present in reaction centers are excited and move to outer orbitals, this electron enters into the electron transport chain. 2. Photolysis of water ( Photo- light , lysis – break or split) where light energy and chlorophyll help in the splitting of water molecules into H and OH ions which give rise to protons (H+), electrons, and oxygen gas. Electrons are supplied to chlorophyll molecule in PS II to replenish its lost electrons, H+ help to reduce NADP. Oxygen is byproduct. + - ( • 3. photophosphorylation • Photophosphorylation may be defined as the production of Adenosine triphosphate (ATP) by the reaction of Adenosine diphosphate (ADP) and inorganic phosphate by the utilization of light energy.
• Light energy + ADP + inorganic phosphate (ip) → ATP • Photophosphorylation is of two types; - • 1.Cyclic Photophosphorylation and • 2.Non-cyclic Photophosphorylation Cyclic Photophosphorylation
• It is a process of photophosphorylation in which an electron expelled by the excited photo-centre is returned to it after passing through a series of electron carriers.
• The electron is expelled from P700 passes through a series of carriers including primary electron acceptor , ferredoxin (Fd), plastoquinone (PQ), cytochrome b – f complex and plastocyanin before returning to photo Centre. While over the cytochrome complex, the electron energises for synthesis of ATP from ADP and inorganic phosphate. ADP ATP Non –cyclic photophosphorylation
• It is the normal process of photophosphorylation in which the electron expelled by the excited photo-centre does not return to it. Non-cyclic photophosphorylation is carried out in collaboration of both photosystems I and II. Electron released during photolysis of water is picked up by photo-centre of PS II called P680 .
• It passes through a series of electron carriers— phaeophytin, PQ, cytochrome b – f complex and plastocyanin. While passing over cytochrome complex, the electron loses sufficient energy for the synthesis of ATP. The electron is handed over to photo Centre P700 of PS I by plastocyanin. P700 extrudes the electron after absorbing light energy. The extruded electron passes through FeS, ferredoxin, to finally reach NADP+ . The latter then combines with H+ (released during photolysis) with the help of NADPreductase to form NADPH. This is called Z scheme due to its characteristic zig-zag shape . Dark reaction or Blackman reaction
• The pathway by which all photosynthetic eukaryotic organisms reduce CO2 into carbohydrate is known as carbon fixation or photosynthetic carbon reduction (PCR.) cycle or dark reactions. The dark reactions are independent of light hence it is called dark reaction, however it depends upon the products of light reaction of photosynthesis, i.e., NADPH2 and ATP.
• Because of the need for NADPH2 as a reductant and ATP as energy equivalent, CO2 fixation is closely linked to the light reactions. During evolution three different ecological variants have evolved with different CO2 incorporation mechanism: C3, C4 and CAM plants. Calvin or C3 Cycle or PCR (Photosynthetic Carbon Reduction Cycle):
• It is the basic mechanism by which CO2 is fixed (reduced) to form carbohydrates. It was proposed by Melvin Calvin.
• For this work Calvin was awarded Nobel prize in 1961. To synthesize one glucose molecule Calvin cycle requires 6CO2, 18 ATP and 12 NADPH2.
• Calvin cycle completes in 4 major phases: •
1. Carboxylation phase • 2. Reductive phase •
3. Glycolytic reversal phase (sugar formation phase) •
4. Regeneration phase •
1. Carboxylation phase: • CO2 enters the leaf through stomata, combines with Ribulose bisphosphate (or RuBP). This reaction is catalyzed by an enzyme, called RUBISCO. The reaction results in the formation of a temporary 6 carbon compound (2-carboxy 3-keto 1,5- biphosphorbitol) Which breaks down into two molecules of 3- phosphoglyceric acid (PGA) and it is the first stable product of dark reaction (C3 Cycle).
• 2. Reductive Phase: • The PGA molecules are now phosphorylated by ATP molecule and reduced by NADPH2 to form 3-phospho-glyceraldehyde (PGAL). •
3. Glycolytic Reversal (Formation of sugar) Phase: • Out of two mols of 3-phosphoglyceraldehyde one mol is converted to its isomer 3-dihydroxyacetone phosphate.
• 4. Regeneration Phase: • Regeneration of Ribulose-5-phosphate takes place through number of biochemical steps. •
• 2. Reductive Phase: • The PGA molecules are now phosphorylated by ATP molecule and reduced by NADPH2 (product of light reaction known as assimilatory power) to form 3-phospho-glyceraldehyde (PGAL) Factors Affecting Photosynthesis • Photosynthesis is affected by both environmental and internal factors. The environmental factors are light, CO2, temperature, oxygen, water, nutrients etc. Internal factors are all related with leaf and include protoplasmic factors, chlorophyll contents, structure of leaf, accumulation of end product etc.
• Light: • The ultimate source of light for photosynthesis in green plants is solar radiation, which moves in the form of electromagnetic waves. Out of the total solar energy reaching to the earth, about 2% is used in photosynthesis and about 10% is used in other metabolic activities. Light varies in intensity, quality (wavelength) and duration.
• The effect of light on photosynthesis can be studied under following three headings: •
(i) Intensity of Light: • The total light perceived by a plant depends on its general form (viz., height of plant and size of leaves, etc.) and arrangement of leaves. Of the total light falling on a leaf, about 80% is absorbed, 10% is reflected and 10% is transmitted. • Effect of light intensity varies from plant to plant, e.g., more in heliophytes (sun loving plants) and less in sciophytes (shade loving plants). For a complete plant, rate of photosynthesis increases with increase in light intensity, except under very high light intensity where phenomenon of Solarization’ occurs, (i.e., photo-oxidation of different cellular components including chlorophyll). • It also affects the opening and closing of stomata thereby affecting the gaseous exchange. The value of light saturation at which further increase is not accompanied by an increase in CO2 uptake is called light saturation point. • iii) Duration of Light: • Longer duration of light period favours photosynthesis. Generally, if the plants get 10 to 12 hrs. of light per day it favours good photosynthesis. Plants can actively exhibit photosynthesis under continuous light without being damaged. Rate of photosynthesis is independent of duration of light.
• 2 . Temperature: • The rate of photosynthesis markedly increases with an increase in temperature provided other factors such as CO2 and light are not limiting. The temperature affects the velocity of enzyme controlled reactions in the dark stage. When the intensity of light is low, the reaction is limited by the small quantities of reduced coenzymes available so that any increase in temperature has little effect on the overall rate of photosynthesis. • At high light intensities, it is the enzyme-controlled dark stage which controls the rate of photosynthesis and there the Q10 = 2. If the temperature is greater than about 30°C, the rate of photosynthesis abruptly falls due to thermal inactivation of enzymes. • Concentration of Carbon dioxide: • Being one of the raw materials, carbon dioxide concentration has great effect on the rate of photosynthesis. The atmosphere normally contains 0.03 to 0.04 per cent by volume of carbon dioxide. It has been experimentally proved that an increase in carbon dioxide content of the air up to about one per cent will produce a corresponding increase in photosynthesis provided the intensity of light is also increased. • Minerals: • Presence of Mn++ and CI– is essential for smooth operation of light reactions (Photolysis of water/evolution of oxygen) Mg++, Cu++ and Fe++ ions are important for synthesis of chlorophyll. •
Water: • Although the amount of water required during photosynthesis is hardly one percent of the total amount of water absorbed by the plant, yet any change in the amount of water absorbed by a plant has significant effect on its rate of photosynthesis. Under normal conditions water rarely seems to be a controlling factor as the chloroplasts normally contain plenty of water. • Many experimental observations indicate that in the field the plant is able to withstand a wide range of soil moisture without any significant effect on photosynthesis and it is only when wilting sets in that the photosynthesis is retarded. Some of the effect of drought may be secondary since stomata tend to close when the plant is deprived of water. A more specific effect of drought on photosynthesis results from dehydration of protoplasm. • Oxygen: • Excess of O2 may become inhibitory for the process. Enhanced supply of O2 increases the rate of respiration simultaneously decreasing the rate of photosynthesis by the common intermediate substances. The concentration for oxygen in the atmosphere is about 21% by volume and it seldom fluctuates. O2 is not a limiting factor of photosynthesis. • An increase in oxygen concentration decreases photosynthesis and the phenomenon is called Warburg effect. [Reported by German scientist Warburg (1920) in Chlorella algae]. This is due to competitive inhibition of RuBP-carboxylase at increased O2 levels, i.e., O2 competes for active sites of RuBP-carboxylase enzyme with CO2. The explanation of this problem lies in the phenomenon of photorespiration. If the amount of oxygen in the atmosphere decreases then photosynthesis will increase in C3 cycle and no change in C4 cycle. • Pollutants and Inhibitors: • The oxides of nitrogen and hydrocarbons present in smoke react to form peroxyacetyl nitrate (PAN) and ozone. PAN is known to inhibit Hill’s 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), bromocil and atrazine etc., which have the same mechanism of action as that of violates. At low light intensities potassium cyanide appears to have no inhibiting effect on photosynthesis . Internal Factors: • The important internal factors that regulate the rate of photosynthesis are: •
1. Protoplasmic factors: • There is some unknown factor in protoplasm which affects the rate of photosynthesis. This factor affect the dark reactions. The decline in the rate of photosynthesis at temperature.above 30°C or at strong light intensities in many plants suggests the enzyme nature of this unknown factor. •
2. Chlorophyll content: • Chlorophyll is an essential internal factor for photosynthesis. The amount of CO2 fixed by a gram of chlorophyll in an hour is called photosynthetic number or assimilation number. It is usually constant for a plant species but rarely it varies. The assimilation number of variegated variety of a species was found to be higher than the green leaves variety. •
3. Accumulation of end products: • Accumulation of food in the chloroplasts reduces the rate of photosynthesis. •
4. Structure of leaves: • The amount of CO2 that reaches the chloroplasts depends on structural features of the leaves like the size, position and behaviour of the stomata and the amount of intercellular spaces. Some other characters like thickness of cuticle, epidermis, presence of epidermal hairs, amount of mesophyll tissue, etc., influence the intensity and quality of light reaching the chloroplast •
5. CO2 Compensation Point: • It is that value or point in light intensity and atmospheric CO2 concentration when the rate of photosynthesis is just equivalent to the rate of respiration in the photosynthetic organs so that there is no net gaseous exchange. The value of light compensation point is 2.5 -100 ft. candles for shade plants and 100-400 ft. candles for sun plants. The value of CO2 compensation point is very low in C4 plants (0-5 ppm), where as in C3 plants it is quite high (25-100 ppm). A plant can not survive for long at compensation point because there is net lose of organic matter due to respiration of nongreen organs and dark respiration. Importance of photosynthesis •
1. Photosynthesis is the most important natural process which sustains life on earth. •
2. The process of photosynthesis is unique to green and other autotrophic plants. It synthesizes organic food from inorganic raw materials. •
3. All animals and heterotrophic plants depend upon the green plants for their organic food, and therefore, the green plants are called producers, while all other organisms are known as consumers. •
4. Photosynthesis converts radiant or solar energy into chemical energy. The same gets stored in the organic food as bonds between different atoms. Photosynthetic products provide energy to all organisms to carry out their life activities (all life is bottled sunshine). •
5. Coal, petroleum and natural gas are fossil fuels which have been produced by the application of heat and compression on the past plant and animal parts (all formed by photosynthesis) in the deeper layers of the earth. These are extremely important source of energy. •
6. All useful plant products are derived from the process of photosynthesis, e.g., timber, rubber, resins, drugs, oils, fibers, etc. •
7. It is the only known method by which oxygen is added to the atmosphere to compensate for oxygen being used in the respiration of organisms and burning of organic fuels. Oxygen is important in (a) efficient utilization and complete breakdown of respiratory substrate and (b) formation of ozone in stratosphere that filters out and stops harmful UV radiations in reaching earth. •
8. Photosynthesis decreases the concentration of carbon dioxide which is being added to the atmosphere by the respiration of organisms and burning of organic fuels. Higher concentration of carbon dioxide is poisonous to living beings. •
9. Productivity of agricultural crops depends upon the rate of photosynthesis. Therefore, scientists are busy in genetically manipulating the crops.