Photosynthesis in Higher Plants PDF

# Chapter 8: Photosynthesis in Higher Plants ## Level-l (For Board & Competitive Exams) ### Chapter Contents - Introduction - Importance of Photosynthesis - What do we know? - Historical account - Early experiments - Where does Photosynthesis take place? - Photosynthetic Pigments - What is the...

# Chapter 8: Photosynthesis in Higher Plants ## Level-l (For Board & Competitive Exams) ### Chapter Contents - Introduction - Importance of Photosynthesis - What do we know? - Historical account - Early experiments - Where does Photosynthesis take place? - Photosynthetic Pigments - What is the Light Reaction? - Photosynthetic Units/Pigment Systems - The Electron Transport - Non-Cyclic Photophosphorylation - Splitting of water - Cyclic Photophosphorylation - Chemiosmotic Hypothesis - Where are ATP and NADPH used? - The C₄ Pathway (Hatch and Slack Pathway) - Photorespiration - Factors affecting Photosynthesis ## Introduction Photosynthesis is a process in which green parts of plants synthesize complex organic food substances (carbohydrates) using carbon dioxide and water in the presence of sunlight and release oxygen as a by-product. In this process, energy from the sun is converted into chemical energy. It is an anabolic, endergonic (requiring energy) and oxido-reduction process. Sunlight plays a much larger role in our sustenance than we may expect, as all the food we eat and all the fossil fuels we use, the air we breathe, they are all products or byproducts of photosynthesis. Photosynthesis converts the radiant energy to forms of energy that can be used by the biological systems. A simple equation representing the process is: $6CO_{2} +12H_{2}O \xrightarrow{Light} C_{6}H_{12}O_{6} + 6H_{2}O +6O_{2}$ ## Importance of Photosynthesis The use of energy from sunlight by plants for photosynthesis is the basis of life on earth. Food represents the stored energy of sun rays and is manufactured by green plants with the aid of sunlight during photosynthesis. Photosynthesis is important due to two reasons: 1. It is the primary source of food on earth. 2. It's responsible for the release of oxygen into the atmosphere by the green plants which is needed by mostly all life forms. **Note:** According to former estimates, only 10% of dry matter is produced by land plants while 90% of it is formed in oceans. However, the present estimates put the productivity of land plants to be 68% of the total. ## What do we know? Study on photosynthesis started around 300 years ago. On the basis of what we have studied in our earlier classes, simple experiments have shown that chlorophyll (green pigment of the leaf), light and $CO_{2}$ are required for photosynthesis to occur. **Experiment to demonstrate light & chlorophyll is necessary for photosynthesis:** Take a destarched potted plant having variegated leaves and cover 2-3 leaves with the black paper. Expose the potted plant to sunlight for 1-2 hours. Pluck one covered leaf and one exposed leaf and test them for starch. The covered leaf does not show positive starch test showing that photosynthesis cannot occur in the absence of light. The exposed leaf shows blue and yellow parts where the blue colour or positive starch test occurs in the chlorophyll-containing parts. **Experiment to demonstrate CO2 is necessary for photosynthesis (Moll's Half leaf experiment):** A part of leaf was enclosed in a test tube containing some KOH soaked cotton (which absorbs CO₂), while the other half of leaf was exposed to air. When the two halves of leaf were tested for starch, it was found that only the exposed part of leaf tested positive for starch. This showed us that CO₂ is required for photosynthesis. ## Historical Account - Early Experiments There have been several simple experiments done which led to a gradual development in our understanding of photosynthesis. 1. Joseph Priestley (1733-1804) in 1770 revealed the essential role of air in the growth of green plants through several experiments. He discovered oxygen in 1774. In an experiment done, Priestley observed that a candle burning in a closed space i.e., a bell jar, soon gets extinguished. Similarly, a mouse would die of suffocation in a closed space. But, when a mint plant was placed in the same bell jar, the mouse stayed alive and the candle continued to burn. Thus, Priestley concluded that the plants restore to the air whatever the breathing mouse and the burning candle remove. 2. Jan Ingenhousz (1730-1799) through his experiments showed that sunlight is essential for the plant process that helps to somehow purify the air fouled by the breathing mouse and the burning candle. In another experiment, with an aquatic plant (Hydrilla) he showed that in bright sunlight, small bubbles were formed around the green parts of plant while in the dark, no bubbles were formed. He identified those bubbles to be of oxygen. Therefore, he showed that in the presence of sunlight it is only the green parts of the plants that could release oxygen. 3. Julius von Sachs (1854) found that the green parts in plants is where glucose is made and glucose is usually stored as starch. Later, he showed that the green substance in plants (now called chlorophyll) is located in special bodies (now called chloroplasts) within the plant cells. 4. T.W. Engelmann (1843-1909) experimented on Cladophora. Using a prism he split light into its spectral components and then he illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria. The bacteria were used to detect the sites of oxygen evolution. He found that the bacteria accumulated mainly in the region of blue and red light of the split spectrum. And thus, the first action spectrum of photosynthesis was described. The empirical equation representing the total process of photosynthesis for organisms evolving oxygen was understood as: $CO_{2} + H_{2}O \xrightarrow{Light} [CH_{2}O]+O_{2}$ where [CH₂O] represented a carbohydrate. 5. Cornelius van Niel (1897–1985) a microbiologist, based on his studies of purple and green sulphur bacteria demonstrated that during photosynthesis, hydrogen released from a suitable oxidisable compound reduces carbon dioxide to carbohydrates and he inferred that oxygen evolved by the green plants comes from H₂O (water) and not from carbon dioxide. This hypothesis was later proved by using radioisotopic techniques. $2H_{2}A + CO_{2} \xrightarrow{Light} 2A + CH_{2}O + H_{2}O$ where H₂A is the oxidisable compound (H₂O or H2S). The correct equation to represent the overall process of photosynthesis could thus be summed as: $6CO_{2}+12H_{2}O \xrightarrow{Chlorophyll} C_{6}H_{12}O_{6} + 6H_{2}O + 6O_{2} \uparrow$ where C6H12O6 is glucose and O₂ is released from water. Ruben, Kamen et.al. used heavy but non-radioactive, stable isotope of oxygen 180 to prove that O₂ evolve during light reaction comes from H₂O and not from CO₂. **Light:** Sunlight is like a rain of photons of different frequencies. Visible light consists of radiations having a wavelength between 390–760 nm. Part of spectrum used in photosynthesis has a wavelength between 400-700 nm. It is called photosynthetically active radiation (PAR). ## Where does Photosynthesis take place? Photosynthesis takes place in the green leaves of plants and other green parts of plants like stem etc. The most active photosynthetic tissue in higher plants is the mesophyll of leaves. Mesophyll cells have many chloroplasts, which contain the specialised light-absorbing green pigments, the chlorophylls. ## Chloroplasts In photosynthetic eukaryotes, photosynthesis occurs in the subcellular organelle known as the chloroplast. This double membrane-enclosed organelle possesses a third system of membranes called thylakoids. A stack of thylakoids forms a granum. Adjacent grana are connected by unstacked membranes called stroma lamellae. The fluid compartment surrounding the thylakoids, called the stroma. There is a clear division of labour within the chloroplast. 1. Proteins and pigments (chlorophylls and carotenoids) that function in the photochemical events of photosynthesis, i.e., trapping the light energy and synthesis of ATP and NADPH, are embedded in the thylakoid membrane. 2. In stroma, enzymatic reactions synthesise sugar, which in turn forms starch. The former set of reactions, since they are directly light-driven are called light reactions. The latter are not directly light-driven but are dependent on the products of light reactions (ATP and NADPH). Hence, to distinguish the latter they are called by convention, as dark reactions (carbon reactions). However, this should not be construed to mean that they occur in darkness or that they are not light-driven. ## Photosynthetic Pigments Pigments are substances that have an ability to absorb light, at specific wavelengths. A chromatographic separation of the leaf pigments shows that the colour of leaves is due to four pigments: 1. **Chlorophyll a:** Bright or blue green in the chromatogram. 2. **Chlorophyll b:** Yellow-green 3. **Xanthophylls:** Yellow 4. **Carotene:** Yellow to yellow-orange Of these, **chlorophyll-a is the primary photosynthetic pigment.** ### Chlorophyll Pigments Chlorophyll has a tadpole like structure. It consists of a porphyrin head and a phytol tail. #### Porphyrin head: 1. All chlorophylls have a complex ring structure chemically related to the porphyrin-like groups found in haemoglobin and cytochromes. 2. Site of the electrons rearrangements when the chlorophyll is excited. 3. A cyclic tetrapyrrolic structure with non-ionic magnesium atom. #### Phytol tail: 1. A long hydrocarbon tail is almost always attached to the ring structure. 2. Anchors the chlorophyll to the hydrophobic portion of the thylakoids. Major types of chlorophylls are chlorophyll a, b, c, d, e; bacteriochlorophyll a and b etc. ## Accessory Pigments All pigments other than chlorophyll a are called accessory pigments. These have two major roles in photosynthesis: 1. They absorb light of different wavelengths and transfer the energy to chlorophyll molecules, thus they are also called antenna molecules. This enables a wider range of wavelength of incoming light to be utilised for photosynthesis. Chlorophyll b accounts for about one-fourth of total chlorophyll content. 2. Carotenoids protect plant from excessive heat and prevent photo-oxidation (oxidative destruction by light) of chlorophyll pigments. Thus, they are also called "Shield Pigments". Let us study the graph showing ability of pigments to absorb lights of different wavelengths. ### Absorption spectrum: The graphic curve showing the amount of energy of different wavelengths of light absorbed by a substance/pigments. ### Action spectrum: The graphic curve showing the relative rates of photosynthesis at different wavelengths of light. These graphs, together, show that most of the photosynthesis takes place in the blue and red regions of the spectrum, some photosynthesis does take place at the other wavelengths of the visible spectrum. These graphs depict that maximum photosynthesis occurs at the wavelength at which there is maximum absorption by chlorophyll a i.e., in the blue and red regions. ## Photosynthetic Units/Pigment Systems These are group of pigments molecules which take part in the conversion of light energy into the chemical energy. The photosynthetic units are called Photosystem I (PS-I) and Photosystem II (PS-II). Each unit has a reaction centre of a specific chlorophyll a molecule which absorbs light energy of long wavelength. These center can release electron upon absorption of energy. In PS-I, the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P₇₀₀, while in PS-II, reaction centre has an absorption maxima at 680 nm and is called P₆₈₀. ## The Electron Transport Electron transport chain is a series of electron carriers over which electrons pass in a downhill journey releasing energy at every step that is used in generating an electrochemical proton gradient which helps in synthesising ATP. **Note:** Redox Potential: It is the measure of the tendency of a chemical species to acquire electrons and thereby get reduced. Also, called oxidation-reduction potential, it is measured in volts (V) or milli volts (mV). Based on path of electron, associated photophosphorylation can be identified as non-cyclic and cyclic photophosphorylation. ### Non-cyclic Photophosphorylation It involves both photosystem I and photosystem II. These two photosystems work in series, first PS II and then PS 1. The two photosystems are connected through an electron transport chain. Both ATP and NADPH + H are synthesised by this kind of electron flow. First in PS II, the P₆₈₀ molecule absorbs 680 nm wavelength of red light causing electrons to become excited and jump into an orbit which is farther from the atomic nucleus. These electrons are picked up by an electron acceptor which passes them to an electron transport system of cytochromes. This movement of electrons is downhill on redox potential scale. The electrons are then passed onto the pigments of PS I, without being used as they pass through the electron transport chain. Simultaneously, electrons in the reaction center of PSI (P₇₀₀) are excited when they receive light wavelength 700 nm and these electrons are transferred to another acceptor molecule that has a greater redox potential. These electrons are then moved downhill again to a molecule of NADP*. The addition of these electrons reduces the NADP* to NADPH + H². The whole scheme of transfer of electrons, starting from the PS II, uphill to the acceptor, down the electron transport chain to PS I, excitation of electrons, transfer to another acceptor and finally downhill to NADP* reducing it to NADPH + H* is called *Z-scheme*. This shape is formed when all the carriers are placed in sequence on a redox potential scale. ### Splitting of Water The electrons that were removed from PS II must be replaced. This is achieved by electrons available due to splitting of water. The water splitting complex or oxygen evolving complex (OEC) is associated with the PS II, which itself is physically located on the inner side of the membrane of the thylakoid. Water is split into H', [O] and electrons. The protons and oxygen formed by splitting of water is released within the lumen of the thylakoids. The oxygen produced is released as one of the net products of photosynthesis. $2H_{2}O→ 4H* + O_{2} + 4e*$ ### Cyclic Photophosphorylation The process of cyclic photophosphorylation involves only PS I and this process takes place in the stroma lamellae membrane. When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs, due to cyclic flow of electrons. **Note:** Hill and Bendall proposed Z-scheme. Reaction centre is involved in "quantum conversion" where energy of light is converted to chemical energy possessed by excited electron. Some important differences between Cyclic and Non-cyclic photophosphorylation are as follows: | Cyclic Photophosphorylation | Non-cyclic Photophosphorylation | |---|--- | It is performed by photosystem I independently. | It is performed by collaboration of both photosystems II and I. | | An external source of electrons is not required. | The process requires an external electron donor. | | It is not connected with photolysis of water. Therefore, no oxygen is evolved. | It is connected with photolysis of water and liberation of oxygen occurs. | | It synthesises ATP only. | It is not only connected with ATP synthesis, but also with production of NADPH. | | It operates under low light intensity, anaerobic conditions or when CO₂ availability is poor. | Non-cyclic photophosphorylation takes place under optimum light, aerobic conditions and in the presence of carbon dioxide. | | The system does not take part in photosynthesis except in certain bacteria. | The system is connected with CO₂ fixation in green plants. | | It occurs mostly in stroma lamellae membrane. | It occurs in the granal thylakoids. | ## Chemiosmotic Hypothesis Chemiosmotic hypothesis was explained by P. Mitchell. This mechanism explains how ATP is synthesised in the chloroplast. ATP synthesis is linked to the development of a proton gradient across the membrane of the thylakoid and the proton accumulation is towards the inside of the membrane i.e., in the lumen. There are several processes that take place during activation of electrons and their transport within chloroplasts: 1. **Photolysis of water towards thylakoid lumen:** The splitting of the water molecule takes place on the inner side of the membrane and so the hydrogen ions (protons) that are produced accumulate within the lumen of the thylakoids. 2. **Transfer of H+ from stroma to lumen as electrons move through photosystems:** The electron acceptor of electron located towards the outer side of the membrane transfers its electron to another carrier, and this molecule then removes a proton from the stroma while transporting an electron. When this molecule H+ is released into the lumen of the membrane. 3. **NADPH reductase reaction occur towards stroma:** The NADPH reductase enzyme is embedded in the thylakoid membrane. Protons are necessary for the reduction of NADP+ to NADPH and these protons are removed from the stroma of the membrane. So, within the chloroplast, protons in the stroma decrease in number, while in the lumen accumulation of protons. This causes a decrease in pH in the lumen and creates a proton gradient across the thylakoid membrane. This gradient is important because the breakdown of this gradient leads to release of energy. This gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel of the CF₀ of the ATP synthase. The ATP synthase consists of two parts: one called the CF₀ is embedded in the membrane and forms a transmembrane channel that carries out facilitated diffusion of protons across the membrane. The other portion called CF₁ and protrudes on the outer surface of the thylakoid membrane on the side that faces the stroma. The breakdown of this gradient provides enough energy to cause a conformational change in the CF₁ part of the ATP synthase, which makes the enzyme synthesise several molecules of ATP ## Where are ATP and NADPH Used? The products of light reaction i.e., ATP and NADPH are essential for assimilation of $CO_{2}$ to carbohydrates. This is the biosynthetic phase of photosynthesis. These reactions take place in the stroma of chloroplast where all the enzymes required are present. This process does not depend directly on the presence of light but is dependent on the products of light reaction i.e., ATP and NADPH. This could also be verified as immediately after light becomes unavailable, this biosynthetic process continues for some time and then stops. But, if then, light is made available again, the synthesis starts again. Hence, calling the biosynthetic phase as the dark reaction is a misnomer. The dark reaction occurs through Calvin cycle. Calvin cycle may be supported by C₁ cycle or Crassulacean Acid Metabolism (CAM) in certain plants. ## Calvin Cycle or C₃ Cycle Melvin Calvin used radioactive "C in algal photosynthesis studies. This led to the discovery that the first CO₂ fixation product was a three-carbon organic acid. He also helped to mark out the complete biosynthetic pathway, hence it is called Calvin Cycle. The first stable product identified was 3-phosphoglyceric acid (PGA), hence it is named C3 pathway. Calvin cycle occurs in all photosynthetic plants whether they have C3 or C₄ pathway. ### Primary Acceptor of CO₂ The primary acceptor molecule during the C₃ cycle is a five-carbon ketose sugar-Ribulose bisphosphate (RuBP). The enzyme for CO₂ fixation is RuBisCO (Ribulose Bisphosphate Caboxylase Oxygenase). It is the most abundant enzyme on earth. It is characterised by the fact that its active site can bind to both CO₂ and $O_{2}$, hence the name. RuBisCO has a much greater affinity for CO₂ than for $O_{2}$ and the binding is competitive. It is the relative concentration of $O_{2}$ and CO₂ that determines which of the two will bind to the enzyme. Before the scientists discovered the 5-carbon ketose sugar as primary acceptor it was believed that since the first product was a C3 acid, the primary acceptor would be a 2-carbon compound. ### Stages of Calvin Cycle Calvin cycle can be described under three stages: 1. **Carboxylation:** It is the fixation of CO₂ into a stable organic intermediate. In this, CO₂ is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBisCO and it results in the formation of two molecules of 3-PGA (3-Phosphoglyceric acid). 2. **Reduction:** These reactions lead to the formation of glucose. The steps involve utilisation of two molecules of ATP for phosphorylation and two of NADPH for reduction, per molecule of CO₂ fixed. The fixation of six molecules of CO₂ and six turns of the cycle are required for the removal of one molecule of glucose from the pathway. 3. **Regeneration:** For the cycle to continue uninterrupted, regeneration of the CO₂ acceptor molecule is crucial. This step requires one ATP for phosphorylation to form RuBP. To make one molecule of glucose six turns of the cycle are required. 18 ATP and 12 NADPH molecules are used to make a molecule of glucose. Hence, for every CO₂ molecule entering the Calvin cycle, three molecules of ATP and two molecules of NADPH are required. For every CO₂ molecule entering Calvin cycle, three molecules of ATP and two molecules of NADPH are required. It is to meet this difference in number of ATP and NADPH that the cyclic phosphorylation takes place. **Note:** RuBisCO and many other enzymes of Calvin cycle are regulated by light. ## The C₄ Pathway (Hatch and Slack Pathway) Most of the plants that are adapted to dry tropical regions have the C₄ pathway. e.g., Sugarcane, Maize, Sorghum, Amaranthus etc. In these plants, double fixation of carbon dioxide occurs. The initial fixation product of this pathway is a four carbon compound-Oxaloacetic acid (OAA) and hence the name. Two Australian botanists Hatch and Slack discovered that tropical plants are much more efficient in CO₂ utilization. Hence, Hatch-Slack cycle was named. C₄ plants are special as they have a special type of leaf anatomy, they can tolerate higher temperatures, they show a response to high intensities of light, they lack a wasteful process called photorespiration, they show greater productivity and higher yield as compared to the C₃ plants. The C₄ pathway requires the presence of two types of cells i.e., mesophyll cells and bundle sheath cells. The particularly large cells around the vascular bundles of C₄ plants are called bundle sheath cells. The bundle sheath cells have a number of chloroplasts, grana are absent, thick walls impervious to gases; they are characterised by having a number of chloroplasts, grana are absent, thick walls impervious to gases. This special anatomy of leaves of the C₄plants is called gaseous exchange and means that there is a reflection of the arrangement of cells. ### Process of Hatch-Slack Pathway It is a cyclic process. The primary CO₂ acceptor is a three-carbon molecule phosphoenol pyruvate (PEP) and it is present in mesophyll cells. The enzyme that catalyses this CO₂ fixation is PEP carboxylase (PEPcase). The mesophyll cells of C₁ plants lack the enzyme RuBisCO. The 4-carbon oxaloacetic acid (OAA) is formed in the mesophyll cells. It is then converted to other four-carbon compounds like malic acid or aspartic acid in the mesophyll cells itself, these are then transported to the bundle sheath cells. In the bundle sheath cells, these C₄ acids are broken down to release CO₂ and a three-carbon molecule. The CO₂ released in the bundle sheath cells enters the C₃ or the Calvin pathway. The bundle sheath cells are rich in an enzyme RuBisCO, but lacks PEPcase. The three-carbon molecule is transported back to the mesophyll cells where it is converted to PEP again with the help of a cold sensitive enzyme, called PEP synthetase, thus completing the cycle. Thus, the basic pathway that results in the formation of the sugars, the Calvin pathway is common to the C₃ and C₄ plants. ## Some major differences between C₃ pathway and C₄ pathway are: | C₃ pathway | C₄ pathway | |---|---| | The primary acceptor of CO₂ is RuBP - a five-carbon compound. | The primary acceptor of CO₂ is PEP - a three-carbon compound. | | The first stable product is 3-phosphoglycerate (3C-compound). | The first stable product is oxaloacetic acid (4C-compound). | | It occurs in the mesophyll cells of the leaves. | It occurs in the mesophyll and bundle sheath cells of the leaves. | | It is a slower process of carbon fixation. | It is a faster process of carbon fixation. | | 3 ATP are consumed to fix one CO₂. | 2 ATP are consumed to fix one CO₂ | ## Importance of C₄ Plants 1. They can tolerate saline conditions due to abundant occurrence of organic acids (oxaloacetic acid) in them which lowers their water potential than that of soil. 2. Can perform photosynthesis even when their stomata are closed due to the presence of fixing enzyme i.e. PEPcase. 3. Concentric arrangement of cells in leaf produces smaller area in relation to volume for the utilisation. ## Photorespiration Photorespiration is a process which involves loss of fixed carbon as CO₂ in plants in the presence of light. It is initiated in chloroplasts. This process does not produce ATP or NADPH and is a wasteful process. Photorespiration occurs usually when there is high concentration of oxygen. Under such circumstances, RuBisCO, the enzyme that catalyses the carboxylation of RuBP during the first step of Calvin cycle,, functions as an oxygenase. Some O₂ does bind to RuBisCO and hence CO₂ fixation is decreased. The RuBP binds with $O_{2}$ to form one molecule of PGA (3C compound) and phosphoglycolate (2C compound) in the pathway of photorespiration. There is neither the synthesis of sugar, nor of ATP. Rather, it results in the release of CO₂ with the utilisation of ATP. It leads to a 25 percent loss of the fixed CO₂. $O_{2}$ is first utilized in chloroplast and then in peroxisomes. Photorespiration or C₂ cycle involves three organelles viz., chloroplast, peroxisomes and mitochondria. Loss of CO₂ occurs in mitochondria. In C₄ plants, photorespiration does not occur. This is because these plants have a mechanism that increases the concentration of CO₂ at the enzyme site. During the C₄ pathway, when the C₄ acid from the mesophyll cells is broken down in the bundle sheath cells, it releases CO₂ - this results in increasing the intracellular concentration of CO₂. This in turn, ensures that the RuBisCO functions as a carboxylase minimising the oxygenase activity. Thus, the productivity and yields are better in C₄ plants as compared to C3 plants. In addition, the C₄ plants show tolerance to higher temperature also. ## Factors affecting Photosynthesis The rate of photosynthesis is very important in determining the yield of the plants including crops. An understanding of the factors that affect photosynthesis is very necessary. Photosynthesis is under the influence of both external and internal (plant) factors. ### The external factors include the availability of sunlight, temperature, CO₂ concentration and water. Though several factors interact and simultaneously affect photosynthesis rate, at any point the rate is determined by the factors available at sub-optimal levels. ### The plant factors include the number, size, age, and orientation of leaves, mesophyll cells and chloroplasts, internal CO₂ concentration and amount of chlorophyll. The plant factors are dependent on the predisposition and the growth of the plant. In 1905, Blackman gave the *Law of Limiting factors*. When several factors affect any biochemical process, then this law comes into effect. This states that: If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value. It is the factor which directly affects the process if its quantity is changed. **To illustrate the law:** suppose light intensity supplied to a leaf is just sufficient to utilize 5 mg of CO₂ per hour in photosynthesis. As the CO₂ supply is increased, the rate also increases till 5 mg of CO₂ enters the leaf per hour. After that, any further increase in the supply of CO₂ does not have any effect upon the rate of photosynthesis. Light has now become the limiting factor and further increase in rate of photosynthesis will occur only by increasing the intensity of light. ## External factors affecting photosynthesis It is an essential factor for photosynthesis. It affects the rate of photosynthesis as: 1. **Light intensity:** There is a linear relationship between incident light and CO₂ fixation at low light intensities. At higher light intensities, gradually the rate does not show further increase as other factors become limiting. The light saturation occurs at 10 percent of the total sunlight available to plants. Increase in incident light beyond a point causes the breakdown of chlorophyll and thus resulting in decrease in photosynthesis. Hence, except for plants in shade or in dense forests, light rarely becomes a limiting factor. 2. **Light quality:** Light between 400-700 nm wavelength constitute the photosynthetically active radiation (PAR). Maximum photosynthesis takes place in red and blue light of the visible spectrum and minimum photosynthesis takes place in green light. 3. **Duration of light:** Light duration does not affect the rate of photosynthesis, but it affects the overall photosynthesis. ## Carbon Dioxide Concentration It is a major limiting factor influencing the rate of photosynthesis. The concentration of CO₂ is very low in the atmosphere (between 0.03 percent and 0.04 percent). This level of carbon dioxide is far below the requirement for optimum photosynthesis. Increase in concentration up to 0.05 percent can cause an increase in the rate of photosynthesis but beyond this level, it becomes damaging over longer periods. ## Internal factors affecting photosynthesis Photosynthesis is under the influence of several internal (plant) factors. The plant factors include the number, size, age and orientation of leaves, mesophyll cells and chloroplasts, internal CO2 concentration, and the amount of chlorophyll. The plant or internal factors are dependent on the genetic predisposition and the growth of the plant. 1. **Chlorophyll:** Of the internal factors, chlorophyll is the most important because light energy is trapped by only this substance. There is no photosynthesis in the absence of chlorophyll. The non-green parts of variegated leaves (e.g., Croton), therefore, do not have starch. Photosynthetic number or assimilation number shows a relationship between the chlorophyll and photosynthesis. It is the amount of carbon dioxide (in gms) assimilated by one gram of chlorophyll in an hour. Emerson (1923) observed a direct relationship between the chlorophyll content of a leaf and the rate of photosynthesis. If all other factors are favourable, increased chlorophyll leads to an increase in photosynthesis. 2. **Photosynthetic products:** With the accumulation of the end products of photosynthesis, mesophyll cells, there is decrease in their photosynthetic rate because concentration of these products in the cells increases the rate of respiration. ## Try Yourself 1. On which green alga, action spectrum of photosynthetic pigments was studied by Engelmann? - Nostoc - Cladophora - Chlorella - Scenedesmus 2. Dark reactions of photosynthesis occur in - Grana - Thylakoid - Stroma lamellae - Stroma 3. Mark out the incorrect statement. - PS II is found in both grana and stroma lamellae - PS II is involved in photolysis of water - PS I participates in both cyclic as well as non-cyclic flow of electrons - The reaction centre in PS II is P₆₈₀ 4. An external source of electrons is not required in - Cyclic photophosphorylation - Non-cyclic photophosphorylation - Z-scheme of flow of electrons - All of these 5. Regeneration step in C₃ cycle per CO₂ fixation requires - 1 ATP - 6 ATP - 1 NADPH + H* and 1 ATP - 3 ATP and 2 NADPH + H* 6. The primary enzyme necessary for carboxylation in C₄ plants is present in - Chloroplast of mesophyll cells - Cytoplasm of mesophyll cells - Cytoplasm of bundle sheath cells - Chloroplast of bundle sheath cells 7. Fill in the blank columns in the table to bring the differences between C₃ and C₄ plants. | Characteristics | C₃ plants | C₄ plants | Choose from | |---|---|---|---| | Cell type in which the Calvin cycle takes place. | | | Mesophyll / Bundle sheath / both | | Cell type in which the initial carboxylation reaction occurs. | | | Mesophyll / Bundle sheath / both | | How many cell types, do the leaf have, that fix CO₂. | | | Two: Bundle sheath and mesophyll / One: Mesophyll / Three: Bundle sheath, palisade, spongy mesophyll | | Which is the primary CO₂ acceptor? | | | RuBP/PEP/PGA | | Number of carbons in the primary CO₂ acceptor. | | | 5/4/3 | | Which is the primary CO₂ fixation product? | | | PGA/OAA/ RuBP/PEP | | Number of carbons in the primary CO₂ fixation product. | | | 3/4/5 | | Does the plant have RuBisCO? | | | Yes / No / Not always | | Does the plant have PEP Case? | | | Yes / No / Not always | | Which cells in the plant have RuBisCO? | | | Mesophyll / Bundle sheath / none | | CO₂ fixation rate under high light conditions. | | | Low / high / medium | | Whether photorespiration is present at low light intensities. | | | High / neglibible / sometimes | | Whether photorespiration is present at high light intensities. | | | High / neglibible / sometimes | | Whether photorespiration would be present at low CO₂ concentrations. | | | High / neglibible / sometimes | | Whether photorespiration would be present at high CO₂ concentrations. | | | High / neglibible / sometimes | | Temperature optimum | | | 30-40°C / 20-25°C / above 40°C | | Examples | | | Cut vertical sections of leaves of different plants and observe under the microscope for Kranz anatomy and list them in the appropriate columns. | ## Exercise 1. Select the incorrect statement w.r.t. photosynthesis. - Anabolic, endergonic and redox process - Physico-chemical process using light energy to drive the synthesis of organic compounds - Of the total world's photosynthesis, 90% is carried out by fresh water plants - Annually, 4 x 10^13 kg of carbon is fixed through photosynthesis in biosphere 2. Action spectrum of photosynthetic pigments was studied by Engelmann on _ of _ bacteria. - Spirogyra, Anaerobic - Cladophora, Aerobic - Chlorella, Aerobic - Scenedesmus, Anaerobic 3. Anoxygenic and oxygenic photosynthesis are respectively shown by - Green algae & red algae - Red algae & monocots - Pigmented sulfur bacteria & cyanobacteria - BGA & higher plants 4. OEC is located in/on - Outer surface of granal membrane - Lumen of stroma lamellae - Inner surface of thylakoid membrane - Stroma 5. Pigments are organised into two discrete photochemical light harvesting complexes within the PS I and PS II. These are named in - The sequence of discovery - Which they function in light reaction - The sequence of arrangement of chlorophylls - More than one option is correct 6. Select incorrect statement - Each photosystem has all the pigments - Action spectra is greater in blue and red light - Chlorophyll a & b are primary pigments associated with photosynthesis - PS II is involved in evolution of O₂ 7. Primary electron acceptor in cyclic photophosphorylation is - Pheophytin - Fe-S - PC - Cyt-b-f complex 8. Whole scheme of transfer of electrons, starting from the PS II, uphill to the acceptor, down the electron transport chain to PS I, excitation of electrons, transfer to another acceptor, and finally down hill to NADP* causing it to be reduced to NADPH + H+ is called - Oxidative phosphorylation - Cyclic photophosphorylation - PCR cycle - Z-scheme 9. Which of the following is not a requirement of chemiosmosis? - RuBisCO - Membrane - ATP synthase enzyme - Proton pump 10. Stroma lamellae membrane lacks - PS I only - PS II only - PS I and electron carriers - PS II and NADP reductase 11. The most crucial step of the Calvin cycle is - Decarboxylation - Carboxylation - Reduction - Regeneration 12. Which one of the following statement is incorrect for carboxylating enzyme in C