1. What is photosynthesis? Explain the role of light in photosynthesis.
Answer: Photosynthesis is the biological process by which green plants, algae, and some bacteria convert light energy into chemical energy, synthesizing organic compounds (glucose) from carbon dioxide and water, and releasing oxygen as a byproduct. The overall equation is: 6CO₂ + 12H₂O + light energy [source symbol unavailable] C₆H₁₂O₆ + 6O₂ + 6H₂O. It is the primary source of all organic matter and oxygen on Earth.
The Role of Light in Photosynthesis: Light plays a critical role in the light-dependent reactions of photosynthesis. It is absorbed by pigment molecules (chlorophyll a, chlorophyll b, carotenoids) in the antenna complex of photosystems I and II.
Excitation of Electrons: When a pigment molecule absorbs a photon of light, its electrons become excited and jump to a higher energy level. This energy is transferred to the reaction center chlorophyll a molecule. Charge Separation: In the reaction center, the excited electron is transferred to a primary electron acceptor, creating a charge separation (an electron hole in the chlorophyll and a reduced acceptor). Water Splitting (Photolysis): In Photosystem II, the electron hole is filled by an electron from water. This splitting of water (photolysis) releases oxygen, protons (H⁺), and electrons. Electron Transport Chain: The excited electrons are passed through a series of electron carriers (cytochromes, plastoquinone, etc.), which generate a proton gradient across the thylakoid membrane. ATP Synthesis: The proton gradient drives ATP synthase to produce ATP (photophosphorylation). NADPH Production: The electrons ultimately reduce NADP⁺ to NADPH. Thus, light energy is converted into the chemical energy of ATP and NADPH, which are used in the Calvin cycle to fix CO₂ and synthesize sugars.
2. Describe the structure of chlorophyll.
Answer: Chlorophyll is a green pigment found in the chloroplasts of plants, algae, and cyanobacteria. It is a complex organic molecule composed of a porphyrin ring, a central magnesium ion, and a hydrocarbon tail.
Porphyrin Ring: The core of chlorophyll is a porphyrin ring, a large, flat, cyclic structure made of four pyrrole rings linked together. This ring system is responsible for absorbing light energy. Central Metal Ion: In the center of the porphyrin ring is a magnesium ion (Mg²⁺). This ion is essential for the function of chlorophyll and is coordinated to the nitrogen atoms of the pyrrole rings. Hydrocarbon Tail: Attached to the porphyrin ring is a long hydrocarbon tail (a phytol chain). This tail is hydrophobic and anchors the chlorophyll molecule to the hydrophobic regions of the thylakoid membrane. Variations: Different types of chlorophyll (a, b, c, d) have slight variations in the substituents on the porphyrin ring. For example, chlorophyll a has a methyl group (-CH₃) at a specific position, while chlorophyll b has an aldehyde group (-CHO) at that position. This structure allows chlorophyll to efficiently absorb light, particularly in the blue and red regions of the spectrum, and transfer the energy to the reaction center.
3. Write a note on the photosynthetic pigment carotene.
Answer: Carotene is a type of carotenoid, a class of accessory pigments found in the chloroplasts of plants, algae, and some bacteria. It is an orange-yellow pigment that absorbs light in the blue-green region of the spectrum (around 400-500 nm).
Chemical Structure: Carotenes are composed of isoprene units, forming a long, conjugated hydrocarbon chain. The most common carotene is beta-carotene, which consists of two retinyl groups. It is highly hydrophobic and is embedded in the thylakoid membrane. Role in Photosynthesis: Carotene acts as an accessory pigment. It absorbs light energy and transfers it to chlorophyll a in the reaction center, thereby broadening the range of light that can be used for photosynthesis. It also protects chlorophyll from photo-oxidation damage by quenching reactive oxygen species (ROS) and dissipating excess energy as
heat. Other Functions: Beta-carotene is a precursor for vitamin A in animals. It is also an antioxidant, protecting cells from oxidative stress. Absorption Spectrum: Carotene absorbs blue and green light but transmits yellow, orange, and red light, which is why it appears orange. In summary, carotene is an essential accessory pigment that enhances light absorption and protects the photosynthetic apparatus from damage.
4. Explain the arrangement of photosystems.
Answer: Photosystems are large, multi-protein complexes embedded in the thylakoid membrane of chloroplasts. They are the functional units of the light-dependent reactions of photosynthesis. There are two photosystems: Photosystem I (PSI) and Photosystem II (PSII). They are arranged in a specific orientation to facilitate the flow of electrons and protons.
Photosystem II (PSII): PSII is located primarily in the grana thylakoids. It consists of a core complex, an antenna complex, and an oxygen-evolving complex (OEC). The OEC splits water into oxygen, protons, and electrons. The reaction center of PSII is P680 (chlorophyll a that absorbs light at 680 nm). Photosystem I (PSI): PSI is located in the stroma thylakoids and the edges of grana. It consists of a core complex and an antenna complex. The reaction center is P700. PSI accepts electrons from the electron transport chain and uses light energy to re-excite them, ultimately reducing NADP⁺ to NADPH. Arrangement: The two photosystems are connected by an electron transport chain (cytochrome b6f complex, plastocyanin). PSII is located closer to the inner side of the thylakoid membrane, while PSI is closer to the outer side. This arrangement allows for the vectorial flow of electrons and the pumping of protons into the thylakoid lumen, creating a proton gradient used for ATP synthesis. The spatial arrangement ensures efficient energy transfer and electron flow.
5. Describe the role of water in photosynthesis.
Answer: Water plays a crucial role in photosynthesis, serving as both a reactant and a source of electrons and protons.
Electron Donor: Water is the primary electron donor in the light-dependent reactions. In Photosystem II, water is split (photolysis) by the oxygen-evolving complex (OEC). This splitting provides electrons to fill the electron hole in the reaction center chlorophyll (P680⁺). Source of Protons: The splitting of water releases protons (H⁺) into the thylakoid lumen. This contributes to the proton gradient that drives ATP synthesis via ATP synthase. Source of Oxygen: The oxygen released as a byproduct of photosynthesis comes entirely from water. This is essential for aerobic respiration in most organisms. Reactant in the Calvin Cycle: Although water is not a direct reactant in the Calvin cycle, it is involved in the hydrolysis of ATP and in the overall reaction. Medium for Reactions: Water acts as the solvent for the biochemical reactions of photosynthesis, providing the medium for enzymes and substrates to interact. Turgor Pressure: Water is essential for maintaining turgor pressure in plant cells, which keeps leaves erect and facilitates gas exchange. In summary, water is indispensable for photosynthesis, providing electrons, protons, and oxygen, and serving as the medium for the process.
6. Describe the mechanism of photosynthesis.
Answer: Photosynthesis is a complex process that occurs in two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle).
1. Light-Dependent Reactions (in the thylakoid membrane):
Light Absorption: Pigments in the antenna complex absorb light energy and transfer it to the reaction center.
Water Splitting: In PSII, water is split into oxygen, protons, and electrons. The electrons replace those lost by P680. Electron Transport: Excited electrons from P680 are passed through an electron transport chain (plastoquinone, cytochrome b6f, plastocyanin), generating a proton gradient. ATP Synthesis: Protons flow through ATP synthase, driving the synthesis of ATP (photophosphorylation). NADPH Production: Electrons from PSI (P700) reduce NADP⁺ to NADPH.
2. Light-Independent Reactions (Calvin Cycle, in the stroma):
Carbon Fixation: CO₂ is fixed by RuBisCO onto RuBP to form 3-PGA. Reduction: 3-PGA is reduced to G3P using ATP and NADPH from the light reactions. Regeneration: Most of the G3P is used to regenerate RuBP, while some is used to synthesize glucose.
The two stages are interconnected: the light reactions provide ATP and NADPH for the Calvin cycle, and the Calvin cycle provides ADP and NADP⁺ for the light reactions. This mechanism allows plants to convert light energy into chemical energy efficiently.
7. Explain in detail the light dependent phase of photosynthesis?
Answer: The light-dependent phase of photosynthesis occurs in the thylakoid membranes of the chloroplasts and requires light energy to produce ATP and NADPH. It involves the following steps:
1. Excitation of Photosystem II (PSII):
Light is absorbed by pigment molecules in the antenna complex of PSII. The energy is transferred to the reaction center chlorophyll P680. P680 absorbs a photon and an electron is excited to a higher energy level. This electron is transferred to the primary electron acceptor (pheophytin). P680 is now oxidized (P680⁺) and needs an electron to return to its ground state.
2. Photolysis of Water:
The oxygen-evolving complex (OEC) associated with PSII splits water into oxygen, protons (H⁺), and electrons. The electrons are transferred to P680⁺, reducing it back to P680. The protons are released into the thylakoid lumen. Oxygen is released as a byproduct.
3. Electron Transport Chain:
The excited electron from PSII is passed through a series of electron carriers, including plastoquinone (PQ), cytochrome b6f complex, and plastocyanin (PC). As electrons pass through the cytochrome b6f complex, protons are pumped from the stroma into the thylakoid lumen, creating a proton gradient.
4. Excitation of Photosystem I (PSI):
Light is absorbed by PSI, exciting P700. An electron is transferred to the primary electron acceptor (ferredoxin). The electron hole in P700⁺ is filled by the electron from plastocyanin.
5. Reduction of NADP⁺:
The electrons from ferredoxin are transferred to NADP⁺ reductase, which reduces NADP⁺ to NADPH.
6. ATP Synthesis (Photophosphorylation):
The proton gradient generated by the electron transport chain drives ATP synthase to produce ATP from ADP and Pi. This is called photophosphorylation.
The products of the light-dependent phase are ATP, NADPH, and oxygen. These products are used in the Calvin cycle.
8. Explain in detail the light independent phase of photosynthesis?
Answer: The light-independent phase of photosynthesis, also known as the Calvin cycle or the dark reactions, occurs in the stroma of the chloroplasts. It does not require light directly but uses the ATP and NADPH produced in the light-dependent phase to fix carbon dioxide into carbohydrates.
1. Carbon Fixation:
CO₂ from the atmosphere diffuses into the stroma. The enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the attachment of CO₂ to a 5-carbon sugar, ribulose-1,5-bisphosphate (RuBP). This forms an unstable 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA), a 3-carbon compound.
2. Reduction:
Each molecule of 3-PGA is phosphorylated by ATP to form 1,3-bisphosphoglycerate. NADPH then reduces 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar. For every three CO₂ molecules fixed, six molecules of G3P are produced. One of these G3P molecules is used to synthesize glucose and other carbohydrates.
3. Regeneration of RuBP:
The remaining five molecules of G3P are used to regenerate RuBP through a series of reactions. This regeneration requires ATP. The cycle continues, allowing for the continuous fixation of CO₂.
The Calvin cycle is the primary pathway for carbon fixation in C₃ plants. It is regulated by light through the activation of enzymes by the changes in pH and ion concentrations in the stroma. The overall product is glucose, which can be used for energy or converted into other organic molecules.
9. Describe cyclic photophosphorylation.
Answer: Cyclic photophosphorylation is a type of photophosphorylation that occurs in the thylakoid membrane of chloroplasts, involving only Photosystem I (PSI). It is called "cyclic" because the electron that is excited by light is eventually returned to the reaction center from which it originated.
Mechanism:
Light Absorption: Light is absorbed by PSI (P700). An electron is excited and transferred to the primary electron acceptor (ferredoxin). Electron Transport: The electron is not passed to NADP⁺ but is instead transferred to the cytochrome b6f complex via plastoquinone (PQ). Proton Pumping: As electrons pass through the cytochrome b6f complex, protons are pumped from the stroma into the thylakoid lumen, creating a proton gradient. Return to PSI: The electron is then transferred back to P700 via plastocyanin (PC), completing the cycle. ATP Synthesis: The proton gradient drives ATP synthase to produce ATP. No NADPH is produced, and no oxygen is released because water is not split.
Significance:
Cyclic photophosphorylation provides additional ATP for the Calvin cycle, which requires more ATP than NADPH. It occurs when the cell needs more ATP than NADPH, or when NADP⁺ is unavailable. It is also important in protecting the photosynthetic apparatus from photo-damage by dissipating excess energy.
10. Describe Calvin cycle.
Answer:
The Calvin cycle, also known as the light-independent reactions or the dark reactions, is a series of biochemical reactions that occur in the stroma of chloroplasts. It uses the ATP and NADPH produced by the light-dependent reactions to convert carbon dioxide into glucose.
Stages of the Calvin Cycle:
1. Carbon Fixation:
The cycle begins with the fixation of CO₂. The enzyme RuBisCO catalyzes the reaction between CO₂ and ribulose-1,5- bisphosphate (RuBP), a 5-carbon sugar. This forms an unstable 6-carbon intermediate that quickly splits into two molecules of 3-phosphoglycerate (3-PGA), a 3- carbon compound. This is the first stable product of the Calvin cycle.
2. Reduction:
Each molecule of 3-PGA is phosphorylated by ATP to form 1,3-bisphosphoglycerate. NADPH then reduces 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate (G3P). For every three CO₂ molecules fixed, six molecules of G3P are produced. One G3P molecule is used to synthesize glucose and other carbohydrates.
3. Regeneration of RuBP:
The remaining five molecules of G3P are used to regenerate RuBP. This process requires ATP. The regeneration of RuBP ensures that the cycle can continue to fix more CO₂.
Overall Equation: 3CO₂ + 9ATP + 6NADPH + 6H⁺ [source symbol unavailable] G3P + 9ADP + 8Pi + 6NADP⁺ + 3H₂O
The Calvin cycle is essential for the production of organic molecules in plants and is the primary pathway for carbon fixation in C₃ plants.
11. Describe the kinds of cellular respiration.
Answer: Cellular respiration is the process by which cells break down glucose to produce ATP. There are two main types: aerobic respiration and anaerobic respiration.
1. Aerobic Respiration:
Definition: Aerobic respiration requires oxygen to completely oxidize glucose into carbon dioxide and water. Location: It occurs in the cytoplasm (glycolysis) and mitochondria (Krebs cycle and electron transport chain). Stages: Glycolysis, Pyruvate Oxidation, Krebs Cycle, and Electron Transport Chain/Oxidative Phosphorylation. ATP Yield: It produces a large amount of ATP (approximately 36-38 ATP per glucose molecule). Equation: C₆H₁₂O₆ + 6O₂ [source symbol unavailable] 6CO₂ + 6H₂O + ATP
2. Anaerobic Respiration (Fermentation):
Definition: Anaerobic respiration occurs in the absence of oxygen. It partially breaks down glucose to produce ATP. Location: It occurs entirely in the cytoplasm. Stages: Glycolysis followed by fermentation (lactic acid or alcoholic). ATP Yield: It produces a small amount of ATP (2 ATP per glucose molecule). Types:
Lactic Acid Fermentation: Pyruvate is converted to lactic acid (e.g., in muscle cells during intense exercise). Alcoholic Fermentation: Pyruvate is converted to ethanol and CO₂ (e.g., in yeast).
In summary, aerobic respiration is highly efficient and requires oxygen, while anaerobic respiration is less efficient and occurs without oxygen. Both processes are essential for energy production in different organisms and conditions.
12. Give an account of 'Glycolysis'.
Answer: Glycolysis is the first stage of cellular respiration and occurs in the cytoplasm of the cell. It is an anaerobic process (does not require oxygen) that breaks down one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons).
Stages of Glycolysis:
1. Investment Phase (Energy-Consuming):
Glucose is phosphorylated by ATP to form glucose-6-phosphate. It is isomerized to fructose-6-phosphate. Another ATP is used to phosphorylate fructose-6-phosphate to fructose-1,6-bisphosphate. This 6-carbon molecule is split into two 3-carbon molecules: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).
2. Payoff Phase (Energy-Producing):
G3P is oxidized, and NAD⁺ is reduced to NADH. A phosphate group is added, forming 1,3-bisphosphoglycerate. ATP is produced (substrate-level phosphorylation) to form 3-phosphoglycerate. 3-phosphoglycerate is converted to 2-phosphoglycerate. A water molecule is removed, forming phosphoenolpyruvate (PEP). PEP transfers a phosphate to ADP, forming ATP and pyruvate.
Net Products:
2 Pyruvate 2 ATP (net gain) 2 NADH
Glycolysis is a universal pathway found in almost all organisms. It is the initial step in both aerobic and anaerobic respiration.
13. Explain oxidation of pyruvate.
Answer: The oxidation of pyruvate, also known as the link reaction or the transition reaction, is the second stage of aerobic respiration. It occurs in the mitochondrial matrix and connects glycolysis to the Krebs cycle.
Process:
Pyruvate (3 carbons) produced in glycolysis is transported from the cytoplasm into the mitochondrial matrix. Here, it undergoes oxidative decarboxylation by the enzyme complex pyruvate dehydrogenase. Decarboxylation: A carbon atom is removed from pyruvate as carbon dioxide (CO₂). Oxidation: The remaining 2-carbon fragment (acetyl group) is oxidized. The electrons are transferred to NAD⁺, reducing it to NADH. Coenzyme A Attachment: The acetyl group is attached to coenzyme A (CoA) to form acetyl-CoA.
Equation: Pyruvate + CoA + NAD⁺ [source symbol unavailable] Acetyl-CoA + CO₂ + NADH + H⁺
Significance:
This reaction links glycolysis to the Krebs cycle. It is a crucial step in the aerobic breakdown of glucose. It produces NADH, which carries electrons to the electron transport chain. It is irreversible, committing the carbon atoms to the Krebs cycle.
14. Explain Krebs cycle.
Answer: The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is the third stage of aerobic respiration. It occurs
in the mitochondrial matrix. It is a series of eight enzymatic reactions that oxidize acetyl-CoA to CO₂, producing ATP, NADH, and FADH₂.
Stages of the Krebs Cycle:
1. Formation of Citrate: Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). CoA is released.
2. Isomerization: Citrate is isomerized to isocitrate.
3. First Decarboxylation: Isocitrate is oxidized and decarboxylated to alpha-ketoglutarate (5C). NAD⁺ is reduced to NADH. CO₂ is released.
4. Second Decarboxylation: Alpha-ketoglutarate is oxidized and decarboxylated to succinyl-CoA (4C). NAD⁺ is reduced to NADH. CO₂ is released.
5. Substrate-Level Phosphorylation: Succinyl-CoA is converted to succinate (4C). GDP is phosphorylated to GTP (which can be converted to ATP).
6. Oxidation: Succinate is oxidized to fumarate (4C). FAD is reduced to FADH₂.
7. Hydration: Fumarate is hydrated to malate (4C).
8. Oxidation: Malate is oxidized to oxaloacetate (4C). NAD⁺ is reduced to NADH. Oxaloacetate can then combine with another acetyl-CoA to start the cycle again.
Net Products per Acetyl-CoA:
3 NADH 1 FADH₂ 1 ATP (or GTP) 2 CO₂
The Krebs cycle is a central hub of metabolism, providing precursors for the synthesis of amino acids, fatty acids, and other molecules.
15. Explain electron transport chain.
Answer: The electron transport chain (ETC) is the final stage of aerobic respiration. It is a series of protein complexes and electron carriers embedded in the inner mitochondrial membrane. The ETC transfers electrons from NADH and FADH₂ to oxygen, using the energy released to pump protons and create a proton gradient.
Components of the ETC:
Complex I (NADH dehydrogenase): Accepts electrons from NADH. Complex II (Succinate dehydrogenase): Accepts electrons from FADH₂. Ubiquinone (Coenzyme Q): A mobile electron carrier. Complex III (Cytochrome bc1): Passes electrons to cytochrome c. Cytochrome c: A mobile electron carrier. Complex IV (Cytochrome c oxidase): Transfers electrons to oxygen, forming water.
Process:
Electrons from NADH and FADH₂ are passed through the complexes. As electrons move through Complexes I, III, and IV, protons (H⁺) are pumped from the mitochondrial matrix into the intermembrane space. This creates a proton gradient (high concentration in the intermembrane space, low in the matrix). Oxygen acts as the final electron acceptor, combining with electrons and protons to form water.
Significance:
The ETC is the primary site of ATP production in aerobic respiration. The proton gradient generated by the ETC drives ATP synthesis via ATP synthase (oxidative phosphorylation). It is essential for the efficient extraction of energy from glucose.
16. Explain chemiosmosis and oxidative phosphorylation.
Answer: Chemiosmosis and oxidative phosphorylation are the mechanisms by which ATP is synthesized in the mitochondria during aerobic respiration.
Chemiosmosis:
Chemiosmosis is the movement of ions across a semipermeable membrane, down their electrochemical gradient. In mitochondria, the electron transport chain pumps protons (H⁺) from the mitochondrial matrix into the intermembrane space, creating a high concentration of protons in the intermembrane space. This creates an electrochemical gradient, also called the proton-motive force. The protons then flow back into the matrix through a channel protein called ATP synthase. The flow of protons through ATP synthase provides the energy needed to phosphorylate ADP to ATP.
Oxidative Phosphorylation:
Oxidative phosphorylation is the process of ATP synthesis driven by the transfer of electrons from NADH or FADH₂ to oxygen via the electron transport chain. It is called "oxidative" because it involves the oxidation of NADH and FADH₂. It is called "phosphorylation" because it involves the addition of a phosphate group to ADP to form ATP. The energy for ATP synthesis comes from the proton gradient generated by the electron transport chain. The overall process is: NADH + H⁺ + ½O₂ + ADP + Pi [source symbol unavailable] NAD⁺ + H₂O + ATP.
In summary, chemiosmosis is the mechanism, and oxidative phosphorylation is the process of ATP synthesis in the mitochondria. Together, they are responsible for the majority of ATP produced during aerobic respiration.
17. Describe substrate level phosphorylation.
Answer: Substrate-level phosphorylation is a type of ATP synthesis that occurs when a phosphate group is directly transferred from a high-
energy substrate molecule to ADP, forming ATP. This process does not require the electron transport chain or a proton gradient.
Mechanism:
A substrate molecule with a high-energy phosphate bond (e.g., 1,3-bisphosphoglycerate, phosphoenolpyruvate, succinyl- CoA) donates its phosphate group to ADP. This reaction is catalyzed by a specific enzyme (e.g., phosphoglycerate kinase, pyruvate kinase). The energy for this transfer comes from the high-energy bond of the substrate.
Occurrence:
Glycolysis: Two ATP molecules are produced by substrate-level phosphorylation per glucose molecule (one in the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate, and one in the conversion of PEP to pyruvate). Krebs Cycle: One ATP (or GTP) is produced by substrate-level phosphorylation per acetyl-CoA (in the conversion of succinyl-CoA to succinate).
Significance:
It is a rapid way to produce ATP, especially during anaerobic conditions. It is the only way to produce ATP in the absence of oxygen. It is a simpler process than oxidative phosphorylation but yields much less ATP.
In summary, substrate-level phosphorylation is a direct method of ATP synthesis that occurs during glycolysis and the Krebs cycle, providing a small but essential amount of ATP for the cell.
