Photosynthesis 🌱

Light Reactions of Photosynthesis

• Photosystems and their Components:

• Photosystem II (PSII): Located in the thylakoid membranes of chloroplasts, PSII captures light energy and uses it to split water molecules. Reference: NCERT Biology Class 11 Chapter 13 Photosynthesis in Higher Plants

• Photosystem I (PSI): Also present in the thylakoid membranes, PSI uses light energy to reduce NADP+ to NADPH. Reference: NCERT Biology Class 11 Chapter 13 Photosynthesis in Higher Plants.

• Electron Transport Chain and ATP Synthesis:

• The electron transport chain, located in the thylakoid membranes, is a series of electron carriers that transfer electrons from PSII to PSI.

• As electrons move through the chain, energy is released, which is used to pump hydrogen ions (H+) across the membrane, creating a proton gradient.

• The flow of H+ ions back through ATP synthase enzyme drives the synthesis of ATP through a process called photophosphorylation. Reference: NCERT Biology Class 11 Chapter 13 Photosynthesis in Higher Plants.

Calvin Cycle (Dark Reactions)

• Carbon Dioxide Fixation:

  • The Calvin cycle takes place in the stroma of chloroplasts.

• The primary enzyme involved is ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which fixes carbon dioxide to ribulose-1,5-bisphosphate (RuBP) to form two molecules of 3-phosphoglycerate (3-PGA). Reference: NCERT Biology Class 11 Chapter 13 Photosynthesis in Higher Plants.

• Reduction of 3-phosphoglycerate to Glyceraldehyde-3-Phosphate:

• 3-PGA is reduced to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH generated in the light reactions. Reference: NCERT Biology Class 11 Chapter 13 Photosynthesis in Higher Plants.

• Regeneration of RuBP:

  • Some G3P molecules are used to regenerate RuBP, which is necessary to continue the carbon fixation process. Reference: NCERT Biology Class 11 Chapter 13 Photosynthesis in Higher Plants.

Respiration 🌬️

Glycolysis

• Energy Yield and Intermediates:

• Occurs in the cytosol and is the first stage of cellular respiration.

• Breaks down glucose into two molecules of pyruvate, generating a small amount of ATP and NADH in the process. Reference: NCERT Biology Class 11 Chapter 14 Respiration in Plants

• Regulation of Glycolysis:

• Regulated by various enzymes, including phosphofructokinase and pyruvate kinase.

• Controlled by feedback inhibition and hormonal signals. Reference: NCERT Biology Class 11 Chapter 14 Respiration in Plants

Krebs’s Cycle (Citric Acid Cycle)

• Reactions of the Krebs’ Cycle:

• A series of chemical reactions that take place in the mitochondria.

• Pyruvate from glycolysis enters the cycle and undergoes a series of oxidative reactions, generating NADH, FADH2, ATP, and CO2. Reference: NCERT Biology Class 11 Chapter 14 Respiration in Plants

• Energy Yield and Intermediates:

• Produces a significant amount of ATP, NADH, and FADH2, which are used in the electron transport chain.

• Supplies intermediates for other metabolic pathways. Reference: NCERT Biology Class 11 Chapter 14 Respiration in Plants

Electron Transport Chain (ETC)

• Components of ETC:

• Consists of a series of electron carriers, including proteins, coenzymes, and cytochromes, located in the inner mitochondrial membrane.

• Includes Complexes I-IV or NADH-CoQ oxidoreductase, succinate-CoQ oxidoreductase, CoQ-cytochrome c oxidoreductase, and cytochrome c oxidase. Reference: NCERT Biology Class 11 Chapter 14 Respiration in Plants

• Mechanism of Electron Transport and ATP Synthesis:

• Electrons pass through the electron carriers, losing energy that is used to pump hydrogen ions (H+) across the mitochondrial membrane, generating a proton gradient.

• The flow of H+ ions back through ATP synthase drives the synthesis of ATP. Reference: NCERT Biology Class 11 Chapter 14 Respiration in Plants

• Inhibitors of ETC:

• Certain substances, such as cyanide and carbon monoxide, can inhibit the electron transport chain, blocking ATP synthesis. Reference: NCERT Biology Class 11 Chapter 14 Respiration in Plants

Oxidative Phosphorylation

• ATP Synthesis through Chemiosmosis:

• Process by which ATP is synthesized during cellular respiration.

• As hydrogen ions (H+) pumped during electron transport flow back through ATP synthase enzyme, the movement of H+ ions drives the conformational changes in the enzyme, leading to the formation of ATP. Reference: NCERT Biology Class 11 Chapter 14 Respiration in Plants

• Uncoupling of Oxidative Phosphorylation:

• Process in which the electron transport chain is functional, but ATP synthesis is not linked to it due to the presence of uncouplers.

• Leads to the release of energy as heat rather than ATP production. Reference: NCERT Biology Class 11 Chapter 14 Respiration in Plants

Amphibolic Pathways

• Gluconeogenesis:

• Biosynthetic pathway that converts non-carbohydrate molecules (such as lactate, pyruvate, and amino acids) into glucose.

• Plays a crucial role in maintaining blood glucose levels during fasting and starvation. Reference: NCERT Biology Class 12 Chapter 15 Respiration in Plants

• Glycogen Metabolism:

• Process involving the breakdown (glycogenolysis) and synthesis (glycogenesis) of glycogen, a storage form of glucose in animals and humans.

• Glycogenolysis occurs during periods of increased energy demand, while glycogenesis occurs when glucose is abundant. Reference: NCERT Biology Class 12 Chapter 15 Respiration in Plants

• Pentose Phosphate Pathway (PPP):

• Alternative pathway for glucose metabolism that generates pentose sugars (5-carbon sugars) and NADPH.

• Provides precursors for nucleotide synthesis and helps in the detoxification of harmful substances. Reference: NCERT Biology Class 12 Chapter 15 Respiration in Plants

Plant Respiration

• Glycolysis and TCA Cycle in Plants:

• Similar to cellular respiration in animals, plants undergo glycolysis in the cytosol and the TCA cycle in the mitochondria to generate energy.

• However, there are some differences, such as the presence of alternative pathways in plants. Reference: NCERT Biology Class 12 Chapter 15 Respiration in Plants

• Alternative Oxidase Pathway:

• An alternative pathway in plant mitochondria that allows the transfer of electrons from ubiquinone to oxygen, bypassing some components of the electron transport chain.

• Helps in maintaining redox balance and heat production in plants. Reference: NCERT Biology Class 12 Chapter 15 Respiration in Plants

• Respiratory Quotient (RQ) and its Significance:

• RQ is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed during respiration.

• Provides information about the type of respiratory substrate being utilized (e.g., RQ = 1 for carbohydrates and RQ = 0.7 for fats). Reference: NCERT Biology Class 12 Chapter 15 Respiration in Plants

Regulation of Respiration

• Pasteur Effect:

• Phenomenon where the rate of respiration increases under aerobic conditions compared to anaerobic conditions, even when the glucose concentration is the same.

• Occurs due to the efficient energy production through aerobic respiration. Reference: NCERT Biology Class 12 Chapter 15 Respiration in Plants

• Crabtree Effect:

• Phenomenon where the rate of respiration increases even under aerobic conditions when glucose concentration is high (fermentation starts even in the presence of oxygen).

• Occurs in certain yeast and tumor cells. Reference: NCERT Biology Class 12 Chapter 15 Respiration in Plants

• Hormonal Regulation of Respiration:

• Hormones such as insulin and glucagon can regulate the rate of cellular respiration.

• Insulin promotes glucose utilization and inhibits the breakdown of fats, leading to decreased respiration rates.

• Glucagon has the opposite effects. Reference: NCERT Biology Class 12 Chapter 15 Respiration in Plants