Samacheer Class 11 Bio Botany - Photosynthesis

Group A, exposed to light of wavelength 400–450 nm, will show a significantly higher photosynthetic rate compared to Group B. The wavelength range of 400–450 nm corresponds to the blue region of the electromagnetic spectrum, where chlorophyll a exhibits its maximum absorption peak at approximately 450 nm. Since chlorophyll a is the primary photosynthetic pigment in the reaction center, maximum light absorption in this region leads to efficient excitation of electrons and optimal photosynthetic activity. In contrast, Group B, exposed to light of wavelength 500–550 nm, falls within the green region of the spectrum. Although chlorophyll and other photosynthetic pigments are present in plant leaves, they do not absorb light effectively in the green region; instead, they reflect green light, which is why plants appear green to our eyes. Consequently, the photosynthetic rate in Group B will be negligible or very low because the incident light is not being absorbed by the photosynthetic machinery. Therefore, Group A will demonstrate a substantially higher rate of photosynthesis due to optimal light absorption by chlorophyll a in the blue region of the spectrum.

This statement is partially true, but the explanation requires clarification. A tree can release oxygen during nighttime, but this occurs only in certain types of plants, specifically CAM (Crassulacean Acid Metabolism) plants, not in typical C3 plants. During the day, CAM plants keep their stomata closed to minimize water loss in arid environments. At night, when temperatures are cooler and water loss is reduced, CAM plants open their stomata and fix carbon dioxide using the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase). This CO2 fixation produces oxaloacetic acid, which is subsequently converted into malic acid through a series of reactions similar to the C4 pathway. During the day, the stored malic acid is decarboxylated, releasing CO2 that enters the Calvin cycle for sugar synthesis. The oxygen released during this process at night is a byproduct of the metabolic breakdown of malic acid and other organic compounds. However, in most C3 plants (typical trees), photosynthesis occurs only during the day when light is available, so oxygen release is primarily a daytime phenomenon. Therefore, the statement is true specifically for CAM plants but not for conventional C3 trees.

The photorespiratory losses are checked by certain grasses by having physiological adaptation. The process of photosynthesis occurs in mesophyll cells and bundle sheath cells.
Mesophyll cells:
* Initially, CO 2 is taken up by Phosphoenolpyruvate (PEPA) (3C) and changed to oxaloacetate (4C) in the presence of PEP carboxylase.
* Oxaloacetate is reduced to Malate/Aspartate. The product formed reaches the bundle sheath.
Bundle Sheath:
* The oxidation of Malate and Aspartate occurs with the release of carbon dioxide and the formation of Pyruvate (3C)
* Due to increased CO 2 concentration RUBISCO functions as a carboxylase and not as Oxygenase.
* The photosynthetic losses are prevented.
* RUBP operates now under the Calvin cycle and pyruvate transported back to Mesophyll cells is changed into Phosphoenolpyruvate to keep the cycle going.

C 4 Plants are more advantageous than C 3 plants because of the following reasons:
C 4 Plants
C 3 Plants
CO 2 fixation occurs in mesophyll cells only
CO 2 fixation occurs in mesophyll and bundle sheath cells
RUBP is the only CO 2 acceptor
PEPA Phosphoenol pyruvate in mesophyll is the acceptor in the first phase
Fixation of CO 2 occurs if the atmospheric concentration of C0 2 is 50 ppm only
It can fix carbon dioxide even if the atmospheric concentration of CO 2 is below 10 ppm
Optimum temperature is 20° to 25°C
Optimum temperature is 30° to 45°C and is thus effective in tropical regions.
RUBP carboxylase enzyme also functions as oxygenase if the 0, concentration is higher than carbon dioxide
PEP carboxylase enzyme functions even at low carbon – dioxide concentrations.
Higher rate of photorespiration and hence rate of photosynthesis is reduced.
Minimal rate of photorespiration is seen is C 4 plants.

When there is plenty of light and a higher concentration of oxygen, the plant undergoes the photorespiratory pathway, also known as the Warburg effect. Under these conditions, the rate of photosynthesis actually decreases despite the abundance of light. This inhibitory effect of oxygen on photosynthesis was first discovered by Otto Warburg in 1920 using the green alga Chlorella. The mechanism behind this phenomenon involves the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase), which catalyzes the first step of the Calvin cycle. When oxygen concentration is high relative to carbon dioxide concentration, RuBisCO catalyzes the oxygenation of ribulose-1,5-bisphosphate (RuBP) instead of its carboxylation. This oxygenation reaction initiates photorespiration, a metabolic pathway that consumes ATP and NADPH without producing any net gain of carbohydrates. Photorespiration involves the oxidation of organic compounds and the release of CO2, which represents a loss of carbon that could otherwise be fixed into sugars. Therefore, under high oxygen and high light conditions, the plant's net photosynthetic rate decreases because the energy and reducing power generated by the light reactions are consumed in photorespiration rather than being used productively in the Calvin cycle for carbohydrate synthesis.

The correct answer is (b) II, III & IV. Let us examine each matched pair: I) Green non-sulphur bacteria – Clostridium & Lynbya is incorrect because Clostridium is not a photosynthetic bacterium; it is an anaerobic heterotroph. Lyngbya is a cyanobacterium, not a green non-sulphur bacterium. II) Green sulphur bacteria – Chlorobacterium & Chlorobium is correct. Both Chlorobacterium and Chlorobium are authentic green sulphur bacteria that perform anoxygenic photosynthesis using hydrogen sulphide as an electron donor. III) Purple sulphur bacteria – Thiospirillum & Chromatium is correct. Both Thiospirillum and Chromatium are genuine purple sulphur bacteria capable of photosynthesis in anaerobic conditions. IV) Purple non-sulphur bacteria – Rhodopseudomonas & Rhodospirillum is correct. Both Rhodopseudomonas and Rhodospirillum are authentic purple non-sulphur bacteria that can perform photosynthesis heterotrophically. Therefore, options II, III, and IV contain correctly matched pairs of photosynthetic bacteria.

PAR refers to Photosynthetically Active Radiation, which is the portion of the electromagnetic spectrum that is utilized by plants for photosynthesis. PAR encompasses wavelengths between 400 and 700 nanometers, corresponding to the visible light region of the spectrum. Within this range, different wavelengths are absorbed with varying efficiencies by photosynthetic pigments. The photosynthetic rate is maximum in blue light (around 400–450 nm) and red light (around 650–700 nm), where chlorophyll a shows strong absorption peaks. These two regions of the spectrum are most effective at driving the light-dependent reactions of photosynthesis. In contrast, green light (around 500–550 nm) induces the lowest rate of photosynthesis because chlorophyll and other photosynthetic pigments absorb green light poorly; instead, they reflect it, which is why plants appear green to our eyes. However, green light is not entirely useless for photosynthesis, as it can penetrate deeper into the leaf tissue and be utilized by lower leaf layers. The concept of PAR is important in agriculture and horticulture for optimizing light conditions for plant growth and productivity.

• Glycine and Serine synthesized during this process are precursors of many biomolecules like Chlorophyll, Proteins, Nucleotides.
• It consumes excess NADH + H + generated.
• Glycolate protects cells from Photooxidation.

• During Photosynthesis oxygen is evolved from water.
• Electrons for the reduction of CO 2 are obtained from water.
• A reduced substance produced, later helps to reduce CO 2
• 2H 2 O + 2A→ 2 AH 2 + O 2

Xanthophylls are yellow pigments with the molecular formula C40H56O2. They are chemically similar to carotenes but differ in that they contain oxygen atoms in their structure. Xanthophylls are accessory pigments in photosynthesis that absorb light energy and transfer it to chlorophyll. Common examples of xanthophylls include lutein, violaxanthin, and fucoxanthin. Lutein is particularly notable as it is responsible for the yellow color changes observed in leaves during the autumn season when chlorophyll breaks down and the yellow xanthophyll pigments become visible. These pigments play important roles not only in light harvesting but also in protecting the photosynthetic apparatus from excess light damage through non-photochemical quenching mechanisms.

• They are proteinaceous pigments.
• They are soluble in water.
• Lack ‘Mg’ and phytol tail.
• There are 2 forms 1. Phycocyanin 2. Phycoerythrin.
• Phycocyanin occur in Cyanobacteria.
• Phyco erythrin occur in Rhodophycean Algae.

The absorption spectrum is a graphical representation that shows how much light of different wavelengths is absorbed by a particular pigment. It is obtained by plotting the amount or percentage of light absorption on the y-axis against the wavelengths of light on the x-axis. Different pigments absorb different wavelengths of light depending on their chemical structure and the energy levels of their electrons. For example, chlorophyll absorbs strongly in the blue and red regions of the electromagnetic spectrum but reflects green light, which is why plants appear green. The absorption spectrum is specific to each pigment and provides information about which wavelengths of light are most effectively captured by that pigment for use in photosynthesis and other light-dependent processes.

• Carotenoids are yellow to orange pigments mostly tetraterpens and absorb light strongly in the blue to violet region of the visible spectrum.
• These pigments protect chlorophyll from photosynthetic oxidative damage.

• They are physiological photosynthetic units, located on the inner membrane of thylakoid lamellae of size 180A X 160 A length & breadth.
• It was named by Park &Pickins( 1964).
• One quantosome contains about 230 chlorophyll molecules.
• It constitutes a photosynthetic unit responsible for the production of one O 2 molecule or reduction of one CO 2 molecule.

Bioluminescence is the production and emission of light by living organisms through biochemical reactions. It is a phenomenon where certain organisms generate light through the oxidation of specific biochemical substances, typically involving the enzyme luciferase and a substrate called luciferin. This light emission occurs without the generation of significant heat, making it a cold light process. Bioluminescence is found in various organisms including fireflies, certain deep-sea fish, dinoflagellates, bacteria, and some fungi. The light produced serves various biological functions such as attracting mates, luring prey, communication, and defense mechanisms. The biochemical pathway involves the oxidation of luciferin in the presence of luciferase enzyme and ATP, which results in the emission of photons of light in the visible spectrum.

Photorespiration, although often considered a wasteful process due to its consumption of oxygen and release of carbon dioxide, possesses several significant roles within plant metabolism. Firstly, the amino acids glycine and serine, which are synthesized during photorespiration, serve as crucial precursors for various essential biomolecules. These include chlorophyll, which is vital for photosynthesis, as well as proteins and nucleotides, which are fundamental components of cellular structure and genetic material. Secondly, photorespiration plays a role in consuming excess NADH + H+ generated during the light reactions of photosynthesis, particularly under conditions of high light intensity and low CO2 availability. This consumption helps to prevent photoinhibition and oxidative damage to the photosynthetic apparatus. Lastly, glycolate, an intermediate product of photorespiration, is believed to protect cells from photooxidation. By metabolizing glycolate, plants can mitigate the harmful effects of reactive oxygen species that might otherwise accumulate under stressful conditions, thus safeguarding cellular integrity and function.

The splitting of water molecule, mechanism was studied by KoK et, al (1970).
It consists of a series of 5 states so, s 1, s 2, s 3, s 4.
Each sate acquires positive charge by a photon (hv) and after the state s 4 – if acquires 4 positive charges 4 electron and evolution of oxygen.
Two molecules of water go back to the so.
At the end of photolysis 4H +, 4e – and O 2 are evolved from water.
4H 2 O → 4H + + + 40H –
40H – → 2H 2 O+O 2 +4e –
2H 2 O → 4H + + O 2 + 4e –.

Photophosphorylation and oxidative phosphorylation are two distinct processes of ATP synthesis that occur in different metabolic pathways. Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate that occurs during photosynthesis using light energy. In this process, light-excited electrons from photosystem II are used to generate a proton gradient across the thylakoid membrane, which drives ATP synthesis through chemiosmosis. Photophosphorylation can be cyclic, where electrons cycle back to photosystem I, or non-cyclic, where electrons flow from water through both photosystems to reduce NADP+. In contrast, oxidative phosphorylation is the synthesis of ATP that occurs during cellular respiration through the oxidation of reduced coenzymes such as NADH and FADH2 in the electron transport chain. In this process, electrons from these reduced coenzymes are transferred through a series of protein complexes, releasing energy that pumps protons across the inner mitochondrial membrane, creating a gradient that drives ATP synthesis. While both processes use chemiosmotic mechanisms to generate ATP, photophosphorylation is light-dependent and occurs in chloroplasts, whereas oxidative phosphorylation is dependent on the oxidation of organic molecules and occurs in mitochondria.

• Non-Cyclic Photo Phosphorylation occurs, when.
• There is the availability of NADP + for reduction.
• Two molecules of water go back to the so.
• When there is the splitting of water molecules.
• When both PSI and PS II are activated.

The light reaction of photosynthesis is a complex process that can be broadly divided into two main phases: the Photooxidation phase and the Photochemical phase. The Photooxidation phase, often referred to as POP, involves the initial capture and transfer of light energy. This phase begins with the absorption of light energy by various photosynthetic pigments, including accessory pigments like carotenoids and phycobilins. This absorbed energy is then efficiently transferred from these accessory pigments to the reaction center chlorophyll 'a' molecule. Upon receiving this energy, the chlorophyll 'a' molecule becomes activated, meaning an electron within it is excited to a higher energy level. The Photochemical phase, or PCO, follows the photooxidation phase and involves the conversion of light energy into chemical energy. A key event in this phase is the photolysis of water, where water molecules are split using light energy, releasing electrons, protons (H+), and molecular oxygen. These electrons are then transported through an electron transport chain, driving the synthesis of assimilatory power in the form of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). These energy-rich molecules are then utilized in the subsequent dark reactions to fix carbon dioxide into sugars.

• Yes green light induces the lowest rate of photosynthesis because it is not coming under photosynthetically
• Active reduction – (400 – 700 nm) known as PAR.
• PAR – (Photosynthetic rate is maximum in blue and red light not in green Light.

The Calvin cycle, also known as the C3 cycle, requires multiple turns to synthesize one glucose molecule. One molecule of CO2 is fixed in one turn of the Calvin cycle, so six turns of the cycle are required to fix six molecules of CO2 and produce one molecule of glucose (C6H12O6). The balance sheet for six turns of the Calvin cycle is as follows: Inputs are six molecules of CO2, eighteen molecules of ATP, and twelve molecules of NADPH. Outputs are one molecule of glucose (C6H12O6), eighteen molecules of ADP, and twelve molecules of NADP+. The ATP and NADPH required for the Calvin cycle are produced during the light reactions of photosynthesis. The energy from eighteen ATP molecules and the reducing power from twelve NADPH molecules are used to reduce and assimilate the six CO2 molecules into one glucose molecule. This demonstrates the energy cost of carbon fixation and the dependence of the dark reactions on the products of the light reactions.

Photosynthesis is an indispensable biological process with profound significance for life on Earth. Firstly, photosynthetic organisms, primarily plants, algae, and some bacteria, form the foundation of nearly all food webs. They produce organic matter from inorganic substances, thereby providing food and energy for all other living organisms, either directly as primary consumers or indirectly through complex trophic levels. Without photosynthesis, the vast majority of life forms, including humans, would not have a sustained food supply. Secondly, photosynthesis is the only natural process that liberates oxygen into the atmosphere. This continuous release of oxygen is crucial for aerobic respiration, the metabolic process used by most organisms to extract energy from food. Photosynthesis thus maintains the atmospheric oxygen level, which is vital for the survival of oxygen-breathing life. Lastly, photosynthesis plays a critical role in balancing the oxygen and carbon cycles in nature. It removes carbon dioxide from the atmosphere, incorporating it into organic compounds, and simultaneously releases oxygen. This exchange helps to regulate the concentrations of these two vital gases, preventing excessive accumulation of carbon dioxide, which is a greenhouse gas, and ensuring a stable oxygen supply, thereby contributing to the overall stability of Earth's climate and ecosystems.

• Light is a transverse electromagnetic wave.
• It consists of ocillating electric and magnetic fields that are perpendicular to each other and perpenticular to the direction of propagation of the light.
• Light moves at a speed of 3 x 108 ms -1
• Wave length is the distance between successive crests of the wave.
• Light as a particle is called photon. Each photon contains an amount of energy known as quantum.
• The energy of a photon depends on the frequency of the light.

Absorption spectrum and action spectrum are two related but distinct concepts in photosynthesis. The absorption spectrum is a curve obtained by plotting the amount or percentage of light absorption on the y-axis against different wavelengths of light on the x-axis for a particular pigment. It shows which wavelengths of light are most effectively absorbed by a pigment molecule. The absorption spectrum is specific to individual pigments and is determined by their chemical structure and electron energy levels. In contrast, the action spectrum is a curve that shows the rate of photosynthesis at different wavelengths of light. It is obtained by measuring the photosynthetic rate (such as oxygen evolution or CO2 uptake) at various wavelengths and plotting these rates against the wavelengths. The action spectrum reflects the combined absorption of all photosynthetic pigments present in the plant and shows which wavelengths are most effective in driving photosynthesis. While the absorption spectrum of individual pigments may show peaks at specific wavelengths, the action spectrum typically shows a broader response because multiple pigments work together in photosynthesis, and energy absorbed by one pigment can be transferred to another through resonance energy transfer.

Fluorescence and phosphorescence are two distinct processes by which excited molecules release absorbed light energy. Fluorescence is the immediate emission of absorbed radiation in the form of light, typically in the red region of the spectrum. When a pigment molecule absorbs a photon and becomes excited to a higher energy state (S1 or higher singlet state), it rapidly returns to the ground state (S0) by emitting a photon, releasing the absorbed energy as light almost instantaneously. This process occurs within nanoseconds. In contrast, phosphorescence is the delayed emission of absorbed radiation in the form of light, also typically in the red region of the spectrum. In phosphorescence, the excited electron undergoes a transition to a triplet state (T1) through intersystem crossing from the singlet state (S2 to S1 to T1). The return from the triplet state to the ground state (T1 to S0) is a forbidden transition and occurs much more slowly, resulting in delayed light emission that can persist for seconds or longer. While fluorescence involves direct transitions between singlet states, phosphorescence involves the longer-lived triplet state, making it a much slower process. Both phenomena are important in photosynthesis, where fluorescence from chlorophyll is used to assess photosynthetic efficiency, and phosphorescence can dissipate excess energy to protect the photosynthetic apparatus.

The ground state refers to the lowest energy state of a molecule, in which all electrons occupy their most stable orbitals. When a pigment molecule absorbs a photon of light, the energy of the photon excites electrons to higher energy orbitals, moving the molecule from the ground state to an excited state. In the excited state, the molecule possesses higher energy and is in an unstable condition. When the light source is turned off or when the excited molecule releases its excess energy through various mechanisms such as fluorescence, phosphorescence, or heat dissipation, the high-energy electrons return to their original low-energy orbitals. This return of the excited molecule to its original stable, low-energy condition is called the ground state. The ground state is the most thermodynamically stable state for the molecule under normal conditions. In photosynthesis, the excitation of chlorophyll molecules from the ground state to excited states by absorbed photons is the crucial first step that initiates the light reactions and allows the capture of light energy for conversion into chemical energy.

• Photosynthetic organisms provide food for all living organisms on earth either directly or indirectly All other organism depend on them for energy.
• It liberates oxygen in the atmosphere and balances.
• Fuels such as coal, petroleum, and other fossil fuels are preserved forms got only from photosynthetic plants.
• It also provides fodder, fibre, firewood, timber useful medicinal products and these sources come by the act of photosynthesis.

• It is a chemical herbicide having an inhibiting effect on photosynthesis.
• It is Dichloro phenyl D 1 Methyl Urea.
• It can inhibit electron flow during light reactions of photosynthesis.
• It is a herbicide that blocks the plastoquinone binding site of P.S II and inhibits electron flow from plastoquinone to cytochrome.

Blackman's law of limiting factor is a modification of Liebig's law of minimum that applies specifically to photosynthesis. According to this law, at any given point in time, the rate of photosynthesis is determined by the factor that is in the shortest supply among all the essential factors required for the process. In other words, whichever essential factor is present in the minimum amount will limit the overall rate of photosynthesis, regardless of how abundant the other factors may be. For example, if both light intensity and carbon dioxide concentration are low, the factor that is lowest among these two will act as the limiting factor and restrict the photosynthetic rate. If carbon dioxide concentration is lower than light intensity in such a scenario, then CO2 becomes the limiting factor. This principle helps explain why increasing one factor beyond a certain point does not increase photosynthesis if another factor remains deficient. The limiting factor can change depending on environmental conditions, and identifying it is crucial for optimizing photosynthetic efficiency in agricultural and horticultural practices.

The dicarboxylic acid pathway, commonly known as the C4 pathway, is an alternative photosynthetic route found in certain plants. This pathway is completed in two distinct phases. In the first phase, which occurs in the stroma of mesophyll cells, carbon dioxide is fixed by combining with a three-carbon compound called phosphoenolpyruvate (PEP) to form a four-carbon compound known as oxaloacetic acid (OAA). Because the first stable product of CO2 fixation is a four-carbon compound, this cycle is named the C4 cycle. Oxaloacetic acid is a dicarboxylic acid, meaning it contains two carboxyl groups, and therefore this pathway is also referred to as the dicarboxylic acid pathway. The C4 pathway is particularly advantageous in plants adapted to hot, dry environments because it concentrates CO2 around the enzyme RuBisCO, reducing photorespiration and improving photosynthetic efficiency compared to the C3 pathway found in most plants.

• C 4 cycle is an alternative path way for CO 2 fixation.
• It occur in nearly 1000 plant species 300 dicots but mostly 700 monocots (tropical and sub tropical grasses)
• It represent about 5% earths biomass and 1% of its known plants
• 30% terrestrial carbon fixation on earth is due to C 4 So, if C 4 plants on earth is increased, then by carbon sequestration by thus strategy severe climate change would be avoided in the near future.

Kranz anatomy, also known as dimorphism of chloroplasts, is a distinctive structural feature observed in C4 plants that is absent in C3 plants. In C4 plants, the leaves show a characteristic two-celled photosynthetic system with different types of chloroplasts in different cell types. The bundle sheath cells, which surround the vascular bundles, contain larger chloroplasts with thylakoids that are free and not arranged in grana, appearing more like loose stacks. In contrast, the mesophyll cells contain smaller chloroplasts with thylakoids arranged in grana, similar to those found in C3 plants. This anatomical differentiation is functionally significant because the two cell types perform different roles in the C4 photosynthetic pathway: mesophyll cells perform the initial CO2 fixation, while bundle sheath cells carry out the subsequent reactions. C3 plants lack this specialized anatomy and possess only one type of chloroplast distributed uniformly in both mesophyll and bundle sheath cells. The Kranz anatomy in C4 plants is an adaptation that enhances photosynthetic efficiency and reduces photorespiration in hot and dry environments.

The Crassulacean Acid Metabolism (CAM) cycle is a specialized form of photosynthesis that offers significant advantages, particularly for succulent plants thriving in arid environments. One of its primary significances is that it allows these plants to obtain carbon dioxide (CO2) from malic acid during the daytime, even when their stomata are closed. This is crucial because CAM plants open their stomata only at night to minimize water loss through transpiration, a process that would be excessive during the hot, dry daytime. By fixing CO2 at night into malic acid and storing it, they can then decarboxylate the malic acid during the day to provide CO2 for the Calvin cycle, thus continuing photosynthesis without opening stomata. Therefore, during the daytime, stomata remain closed, which effectively helps the plants to avoid excessive transpiration and conserve precious water. This adaptation is a key survival mechanism for plants in water-stressed habitats, enabling them to maintain photosynthetic activity while drastically reducing water loss, making the CAM cycle an evolutionary success in such challenging conditions.

Comparing C3 and C4 plants on the basis of ATP production reveals distinct energetic requirements for their respective photosynthetic pathways. In C3 plants, the evolution of one oxygen molecule, which requires the transfer of four electrons, typically necessitates 8 quanta of light energy. For the synthesis of one molecule of glucose, which involves the evolution of 6 molecules of oxygen, C3 plants utilize a total of 18 ATPs and 12 NADPH + H+. This is because for each CO2 fixed, 3 ATPs and 2 NADPH are consumed in the Calvin cycle. Therefore, to fix 6 CO2 molecules for one glucose, 18 ATPs and 12 NADPH are required. In contrast, C4 plants have a more energy-intensive process due to the additional steps involved in concentrating CO2. While the Calvin cycle itself still requires 3 ATPs and 2 NADPH per CO2 fixed, C4 plants incur an extra cost of 2 ATPs per CO2 molecule for the regeneration of PEP carboxylase in the mesophyll cells. This means that for the fixation of one CO2 molecule, C4 plants utilize 5 ATPs and 2 NADPH + H+. Consequently, to evolve 6 molecules of oxygen and synthesize one molecule of glucose, C4 plants require a total of 30 ATPs and 12 NADPH + H+. This higher ATP demand in C4 plants is compensated by their increased photosynthetic efficiency in hot, dry environments where photorespiration is minimized.

The quanta requirement for the complete light reaction is directly related to the number of oxygen molecules released. If the evolution of one molecule of oxygen requires 8 quanta of light, then to release 6 oxygen molecules, a simple multiplication can determine the total quanta needed. Therefore, for 6 oxygen molecules, the calculation would be 6 multiplied by 8 quanta, which equals 48 quanta of light. This total of 48 quanta is required for the complete light reaction to produce 6 molecules of oxygen, which is the amount typically released during the synthesis of one molecule of glucose in photosynthesis.

Photosystem – I:
* The reaction centre is P700.
* PS I is involved in both cyclic and non – cyclic.
* Not involved in the photolysis of water and evolution of oxygen.
* It receives electrons from PS II during non – cyclic photophosphorylation.
* Located in unstacked region granum facing chloroplast stroma.
* Chlorophyll and Carotenoid ratio is 20 to 30: 1.
Photosystem – II:
* Reaction centre is P680.
* PS II participates in Non – cyclic pathway.
* Photolysis of water and evolution of oxygen take place.
* It receives electrons by photolysis of water.
* Located in stacked region of thylakoid membrane facing lumen of thylakoid.
* Chlorophyll and Carotenoid ratio is 3 to 7: 1.

• Thylakoid membrane contains photosystem I (PSI) and photosystem II (PSII)
• PS I is unstacked region of granum tàcing stroma ofchÍoroplast.
• PS II is found in stacked region of thylakoid membrane facing lumen of thylakoid.
• Each photosystem consists of central core complex (CC) and light harvesting complex (LHC) or Antenna molecules.
• The core complex consists of respective reaction centre associated with proteins, electron donors and acceptors.
• PSI – CCI consists of reaction centre P 700 and LHC – I
• PS II- CC II consists of reaction centre P680 and LHC – II
• Light harvesting complex consists of several chiorophylls, carotenoids and xanthophyll molecules.
• The main function of LHC is to harvesting light energy and transfer it to their respective reaction centre.

Photosystem I
Photosystem II
1. The reaction centre is P700
1. Reaction centre is P 680.
2. PSI is involved in Photolysis of water and evolution.
2. PS II participates in Non – Cyclic pathway
3. Not involved in photolysis of water and evolution of oxygen.
3. Photolysis of water and evolution of oxygen take place.
4. It receives electrons from PSII during non – cyclic photophosphorylation.
4. It receives electrons by photolysis of water.
5. Located in unstacked region granum racing chloroplast stroma.
5. Located in stacked region of thylakoid membrane facing lumen of thylakoid.
6. Chlorophyll and carotenoid ratio is 20 to 30:1
6. Chlorophyll and carotenoid ratio is 3 to 7:1

Cyclic Photophosphorylation
Non – Cyclic Photophosphorylation
1. PSI only involved
1. PS I and PS II involved.
2. Reaction centre is P 700
2. Reaction centre is P 680.
3. Electrons released are cycled back
3. Electron released are not cycled back.
4. Photolysis of water does not take place
4. Photolysis of water takes place
5. Only ATP Synthesized
5. ATP and NADPH + H + are synthesized.
6. Phosphorylation takes place at two places
6. Phosphorylation takes place at only one place
7. It does not require an external electron donor.
7. Requires external electron donor like H 2 O or H 2 S
8. It is not sensitive to dichloro dimethyl urea (DCMU)
8. It is sensitive to DCMU and inhibits electron flow

Contrast the photosynthetic processes in C 3 and C 4 plants:
C 3 Plants:
* CO 2 fixation takes place in mesophyll cells only.
* CO 2 acceptor is RUBP only.
* First product is 3C – PGA.
* Kranz anatomy is not present.
* Granum is present in mesophyll cells.
* Normal Chloroplast.
* Optimum temperature 20° to 25° C.
* Fixation of CO 2 at 50 ppm.
* Less efficient due to higher photorespiration.
* RUBP carboxylase enzyme used for fixation.
* 18 ATPs used to synthesize one glucose.
* Efficient at low CO 2.
* eg: Paddy, Wheat, Potato and so on.
C 4 Plants:
* CO 2 fixation takes place mesophyll and bundle sheath.
* PEP in mesophyll and RUBP in bundle sheath cells.
* First product is 4C – OAA.
* Kranz anatomy is present.
* Granum present in mesophyll cells and absent in bundle sheath.
* Dimorphic chloroplast.
* Optimum temperature 30° to 45° C.
* Fixation of CO 2 even less than 10 ppm.
* More efficient due to less photorespiration.
* PEP carboxylase and RUBP carboxylase used.
* 30 ATPs to produce one glucose.
* Efficient at higher CO 2.
* eg: Sugar cane, Maize, Sorghum, Amaranthus and so on.

When PS II (P680) gets activated, electrons from a high energy state pass through a series of electron carriers like pheophytin, plastoquinone cytochrome complex, plastocyanin, and finally accepted by PS I (P700).
During this flow ATP is generated:
* PS. I (P 700) is activated by light electrons moved to high energy state and accepted by electron acceptor (FRS) Ferredoxin Reducing Substance, during downhill passes through Ferredoxin. During this process NADPH is reduced by H+ formed during photolysis.
* Electrons released from PS II are not cycled back but used in the reduction of NADPH+ into NADPH+H +.
* During electron transport, it generates ATP and this type of Phophorylation is called.
Non Cyclic Photophosphorylation:
The electron flow looks like the letter ‘Z’ so known as Z scheme. It has 3 stages.
* Electron transport from water to P 680: Electrons lost by the PS II are replaced by electrons from splitting of the water molecule, producing electron, protons, and oxygen.
* Electron transport from P680 to P 700: The flow, through various electron carrier molecules, like pheophytin, plastoquinone (PQ) cytochrome b6 – F complex, plastocyanin (PC) finally reaches P 700 (P.S.I)
* Electron transport from P 700 to NADP +: PSI (P700) is excited now and the electrons pass through ferredoxin, NADP is reduced to NADPH + H +

It follows a light reaction.
* Utilises ATP and NADPH + H + produced during the light reaction, and reduce carbon dioxide carbohydrate.
* These reactions do not require light so named as Dark reactions.
* The first formed product is a 3 carbon compound (Phospho Glyceric Acid) and so-known as C 3 cycle.
* It was found by Melvin, Calvin, and Benson – so known as the Calvin cycle.
* Occur in the stroma of the chloroplast.
* It is temperature-dependent, and so it is also called a thermochemical reaction.
Phase I carboxylation (Carbon fixation):
* The 5 C compound Ribulose 1 – 5 Bisphosphate (RUBP) with the help of (RUBISCO) enzyme accepts one molecule of carbon dioxide → 6 carbon compound (unstable)
* The 6c compound is broken into → 2 molecules of 3 c compound.
Phase II – Glycolytic Reversai/Reduction:
Phase III – Regeneration:
* The regeneration of RUBP involves several intermediate compounds of 6c, 5c, 4c, and 7c compounds.
* Fixation of one CO 2 require 3 ATPs + 2NADPH + + H +
* Fixation of six CO 2 require I8 ATPs+ 12NADPH + + H +
* 6 c compound is the net gain to form hexose sugar.

• It is one of the carbon path ways in succulent plants growing in semi arid or xerophytic condition.
• The stomata are closed during day (scoto active) and open during night.
• This reverse rhythm help to conserve water loss through transpiration and will stop the fixation of CO 2 during day.
• At night time CAM plants fix CO, with help of (PEP) phospho Enol Pyruvic acid and produce (OAA) Oxalo Acetic Acid.
• Subsequently OAA is converted into a Malic acid-like C 4 cycle and get accumulated in the vacuole, increasing the acidity.
• During daytime stomata, are closed and Malic acid is decarboxylated into pyruvic acid resulting in the decrease of acidity.
• CO 2 thus formed enters into the Calvin cycle and produces carbohydrates.

Photorespiration
Dark respiration
1. It takes place in photosynthetic green cells
It takes place in all living cells
2. It takes place only in the presence of light
It involves only Mitochondria
3. It involves Chloroplast, Peroxisome, and Mitochondria
It involves only Mitochondria
4. It does not involve Glycolysis, Kreb’s cycle, and ETS
It involves Glycolysis, Kreb’s cycle and ETS
5. Substrate is Glycolic acid
Substrate is Carbohydrates protein or fats
6. It is not essential for survival
Essential for survival
7. No phosphorylation and yield of ATP Phosphorylation
produces ATP energy
8. NADH 2 is oxidised to NAD +
NAD + is reduced to NADH 2
9. Hydrogen peroxide is produced
Hydrogen peroxide is not produced
10. End products are CO 2 and PGA
End products are CO 2 and water

1. Carbon dioxide:
330ppm or 0.3% of CO 2 is available in the atomsphere If there is an increase in CO 2 concentration the rate of Photosynthesis increases -If it increases beyond 500 PPm rate of photosynthesis will be inhibited.
2. Oxygen:
When there is increase in oxygen concentration there is unhibition of photosynthesis Warburg – studied this in chlorellain 1920. This effect is known as Warburg effect.
3. Temperature:
* Optimum temperature for photosynthesis vary from plant to plant
* Normally it is 25°C to 3 5°C
* In Opuntia, it is 55°C
* In Lichens it is 20°C
* In Algae growing in hot spring it is 75°C
* At high and low temperature the stomata will close also the enzymes get inactivated.
4. Water:
* Pholysis of water provide electrons and protons for the reduction of NADP – directly.
* Affect stomatal movement and hydration of protoplasm – indirectly.
* During water stress, supply of NADPH + H + affected
5. Minerals:
Deficiency
Effect
Mg, Fe and N
Synthesis of chlorophyll
P
phosphorylation reactions
Mn, Cl-
photolysis of water
CU
Formation of plastocyanin

AIM: To proove that oxygen is evolved during Photosynthesis.
Procedure:
Take some hydrilla plant and place them at the bottom of a beaker containing water – Add, little NaHCO 3 in to the water. Cover plant with an inverted funnel Invert a test tube over the funnel keep this set up in sun light.
Observation: Air bubbles are released from Hydrilla plant and collected in the test tube by downward displace ment of water. Take the test tube carefully by closing with a finger and then introduce a burning match stick, it bum brightly.
Inference: Hydrilla plant perform photosynthesis and oxygen is liberated during photosynthesis.

The Wilmott’s bubbler experiment is a classic method used to determine the rate of photosynthesis by measuring the rate of oxygen evolution from an aquatic plant. The procedure involves setting up the apparatus carefully. Wilmott’s bubbler consists of a wide-mouthed bottle, which is fitted with a single-holed cork. Through this cork, a glass tube is inserted; its lower end has a wider opening designed to accommodate a Hydrilla plant, while the upper end is connected to a narrow bottle filled with water. To begin the experiment, the wide-mouthed bottle is filled with water, and a living Hydrilla plant is carefully inserted into the wider part of the glass tube. It is crucial that the Hydrilla plant is cut inside the water to prevent any air bubbles from entering the plant's vascular system, which could interfere with the experiment. The tube is then securely fixed with the jar, which acts as a water reservoir, ensuring a closed system. Once the apparatus is assembled, it is placed in sunlight. The Hydrilla plant will begin to photosynthesize, releasing oxygen bubbles from its cut end. The rate of photosynthesis is then determined by counting the number of bubbles evolved per unit time, ensuring that the bubbles are of a consistent size for accurate comparison. This simple yet effective experiment provides a quantitative measure of photosynthetic activity under varying conditions.

Photosynthesis
Bacterial photosynthesis
1. Cyclic and Non – Cyclic phosphorylation takes place
Only cyclic phosphorylation takes place
2. Photosystem I and II involved
Photosystem I only involved
3. Electron donor is water
Electron donor is H 9 S
4. Oxygen is evolved
Oxygen is not evolved
5. Reaction centres are P700 and P680
Reaction centre is P890
6. Reducing agent is NADPH + H +
Reducing agent is NADH + H +
7. PAR is 400 to 700 nm
PAR is above 700nm
8. Chlorophyll, Carotenoid and Xanthophyll
Bacterio chlorophyll and Bacterio viridin
9. Photosynthetic apparatus – chloroplast
It is chromosomes and Chromatophores