Light-Dependent Reactions Gen Bio 1 PDF

GENERAL October 2024 BIOLOGY 1 LIGHT DEPENDENT REACTIONS By: R. PALMES and CAMANAY Grade 12 STEM B LIGHT DEPENDENT REACTIONS Table of Content Introduction to Light Dependent The Five Stages of Light Dependent I Reactions...

GENERAL October 2024 BIOLOGY 1 LIGHT DEPENDENT REACTIONS By: R. PALMES and CAMANAY Grade 12 STEM B LIGHT DEPENDENT REACTIONS Table of Content Introduction to Light Dependent The Five Stages of Light Dependent I Reactions V Reactions Importance of Light Dependent Factors Affecting Light Dependent II Reactions in Photosynthesis VI Processes Discussion of Photosystem II (PSII) and III Photosystem I (PSI) VII Assessment (Quiz) Overview of Light Dependent Reaction IV Processes GENERAL LIGHT DEPENDENT REACTIONS I. Introduction to Light Dependent Reactions The Light-Dependent Reactions (LDRs) is a the initial stage of photosynthesis. It is a photochemical reaction taking place in the thylakoid membranes of chloroplasts, where light energy is transformed into adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). GENERAL LIGHT DEPENDENT REACTIONS I. Introduction to Light Dependent Reactions NADPH ATP GENERAL LIGHT DEPENDENT REACTIONS II. Importance of Light Dependent Reactions in Photosynthesis The Light Dependent Reactions in photosynthesis are crucial because they capture light energy to produce ATP and NADPH. These energy carriers are then used in the Calvin Cycle to convert carbon dioxide into glucose, which fuels the plant's growth and metabolism. Without these reactions, plants couldn't produce the energy they need to survive. GENERAL LIGHT DEPENDENT REACTIONS III. Discussion of Photosystem II (PSII) and Photosystem I (PSI) Photosystem II (PSII) PSII is the first protein-pigment complex involved in the light- dependent reactions. It captures light energy and uses it to split water molecules (photolysis), releasing oxygen, protons, and electrons. The electrons are excited to a higher energy level and transferred through the electron transport chain to create a proton gradient, which drives ATP synthesis. PSII plays a critical role in initiating the flow of electrons and producing the oxygen we breathe. GENERAL LIGHT DEPENDENT REACTIONS III. Discussion of Photosystem II (PSII) and Photosystem I (PSI) Photosystem I (PSI) PSI is the second photosystem in the sequence, receiving electrons from the electron transport chain. It absorbs additional light energy to re-energize these electrons, which are then used to reduce NADP⁺ to NADPH. This energy-rich molecule, along with ATP from PSII, is essential for the Calvin Cycle, where carbon dioxide is converted into glucose. PSI ensures the production of NADPH, completing the electron transfer process necessary for photosynthesis. GENERAL LIGHT DEPENDENT REACTIONSIII. Overview of Light Dependent Reaction Processes Light Absorption Electron Transfer Electron Transport and and and Energy Transfer Water Splitting Proton Pumping NADPH Formation ATP Synthesis and via Chemiosmosis Refuction GENERAL LIGHT DEPENDENT REACTIONS V. THE FIVE STAGES OF LIGHT DEPENDENT GENERAL LIGHT DEPENDENT REACTIONS 1. Light Absorption and Energy Transfer Pigments absorb light energy - In this stage, chlorophyll and other pigments in Photosystem II (PSII) absorb sunlight. The absorbed energy excites electrons in the pigments, raising them to higher energy levels. This energy is transferred to the reaction center of PSII, where it boosts specific electrons to begin the electron transport chain. GENERAL LIGHT DEPENDENT REACTIONS 1. Light Absorption and Energy Transfer TYPES OF PIGMENTS CHLOROPHYLL CAROTENOIDS PHYCOBILIPROTEINS (Blue and Red) (Yellow and Orange ) (Red and Blue) GENERAL LIGHT DEPENDENT REACTIONS 2. Electron Transport and Water Splitting Light-driven water splitting - The excited electrons in PSII are passed to an electron carrier, leaving a deficit of electrons in the photosystem. To replace them, water molecules are split (photolysis), releasing oxygen gas, protons (H⁺), and electrons. This process ensures a continuous supply of electrons and contributes to the oxygen we breathe. GENERAL LIGHT DEPENDENT REACTIONS 3. Electron Transport and Proton Pumping Energy transferred to reaction centers - As electrons move down the electron transport chain, they pass through a series of protein complexes, including the cytochrome complex. The energy from these electrons is used to pump protons (H⁺) from the stroma into the thylakoid lumen, creating a proton gradient across the thylakoid membrane. GENERAL LIGHT DEPENDENT REACTIONS 4. ATP Synthesis via Chemiosmosis The proton gradient drives ATP synthesis through ATP synthase, which generates ATP from ADP and inorganic phosphate (Pi) via chemiosmosis. - The proton gradient built up in the thylakoid lumen creates a difference in concentration and charge. Protons flow back into the stroma through ATP synthase, a protein complex that harnesses this flow to synthesize ATP from ADP and inorganic phosphate (Pi). This process is called chemiosmosis. GENERAL LIGHT DEPENDENT REACTIONS 5. NADPH Formation and Reduction Proton gradient drives ATP synthesis - After electrons have passed through Photosystem I (PSI) and absorbed more light energy, they are transferred to NADP⁺ along with protons to form NADPH. This high- energy molecule, along with ATP, is used in the Calvin Cycle to convert carbon dioxide into glucose, completing the light-dependent reactions. GENERAL LIGHT DEPENDENT REACTIONS VI. FACTORS AFFECTING LIGHT DEPENDENT GENERAL LIGHT DEPENDENT REACTIONS 1. Light Intensity Higher light intensity increases the energy available for exciting electrons in PSII and PSI, leading to higher rates of ATP and NADPH production. However, if light intensity becomes too high, it can damage pigments and proteins in the photosystems. GENERAL LIGHT DEPENDENT REACTIONS 2. Wavelength of Light Different pigments absorb specific wavelengths of light more effectively. Chlorophyll absorbs red and blue light best, while green light is less effective. The efficiency of the light-dependent reactions depends on the availability of optimal wavelengths. GENERAL LIGHT DEPENDENT REACTIONS 3. Temperature Temperature affects the activity of enzymes involved in electron transport and ATP synthesis. Moderate temperatures enhance these reactions, but extreme temperatures (too high or too low) can denature enzymes, slowing or halting the process. GENERAL LIGHT DEPENDENT REACTIONS 4. Carbon Dioxide (CO₂) Concentration Although CO₂ directly affects the Calvin Cycle, its availability indirectly influences light -dependent reactions. When CO₂ is limited, the Calvin Cycle slows down, reducing the demand for ATP and NADPH and, in turn, decreasing the rate of light-dependent reactions. GENERAL LIGHT DEPENDENT REACTIONS 5. Water Availability Water is crucial for photolysis (splitting of water) in PSII, which provides electrons to replace those lost during electron transport. A lack of water reduces photolysis, limiting electron flow and oxygen release, thereby slowing the entire photosynthetic process. GENERAL LIGHT DEPENDENT REACTIONS 6. Chlorophyll Concentration A higher concentration of chlorophyll increases the plant’s ability to absorb light energy, enhancing the efficiency of light- dependent reactions. Conversely, low chlorophyll levels (due to nutrient deficiencies or stress) reduce photosynthetic efficiency. GENERAL LIGHT DEPENDENT REACTIONS 7. Presence of Inhibitors or Pollutants Chemicals like herbicides or pollutants can disrupt photosynthesis by inhibiting electron transport or damaging photosystems. For example, certain herbicides block the electron flow, preventing ATP and NADPH formation, which can impair plant growth. GENERAL LIGHT DEPENDENT REACTIONS VII. ASSESSMENT (QUIZ) GENERAL TEST ON LIGHT DEPENDENT I. Multiple Choice (1-5) REACTIONS 1. What is the primary function of the light-dependent reactions in photosynthesis? a) To produce glucose b) To convert light energy into chemical energy (ATP and NADPH) c) To absorb carbon dioxide d) To release oxygen 2. Which photosystem is involved first in the light-dependent reactions? a) Photosystem I b) Photosystem II c) Photosystem III d) Both Photosystems I and II simultaneously 3. What molecule is split during the light-dependent reactions to provide electrons? a) Glucose b) Carbon dioxide c) Water d) Oxygen 4. What process synthesizes ATP during the light-dependent reactions? a) Photolysis b) Chemiosmosis c) Glycolysis d) Fermentation 5. Which pigments are primarily involved in absorbing light for photosynthesis? a) Carotenoids b) Chlorophyll c) Phycobiliproteins d) All of the above II. True or False (6-10) 6. The light-dependent reactions occur in the stroma of chloroplasts. 7. Photosystem I absorbs light energy to re-energize electrons before they are used to form NADPH. 8. The light-dependent reactions release carbon dioxide a a by-product. 9. Higher light intensity always increases the rate of light-dependent reactions without limit. 10. Chlorophyll is responsible for absorbing green light, which is most effective for photosynthesis. III. Identification (11-15) 11. Identify the process that converts light energy into chemical energy during the light-dependent reactions. Answer: _______________________________ 12. Name the high-energy molecule formed when electrons from Photosystem I are transferred to NADP⁺. Answer: _______________________________ 13. What term describes the process of splitting water molecules to release electrons in Photosystem II? Answer: _______________________ 14. Identify the structure in chloroplasts where ATP synthesis occurs during the light-dependent reactions. Answer: ________________________ 15. Give two (2) factors affecting light dependent processes. Answers: _____________ ______________