Photosynthesis Reactions Lecture 17 PDF

Photosynthesis Reactions Lecture 17 Spring 2024 Light Pigments absorb light - visible Violet/Blue - shorter wavelengths - higher energy Red - longer wavelengths - lower energy Plants - Chlorophyll a, Chlorophyll b, Carotenoids Absorb wide range of wavelengths Convert...

Photosynthesis Reactions Lecture 17 Spring 2024 Light Pigments absorb light - visible Violet/Blue - shorter wavelengths - higher energy Red - longer wavelengths - lower energy Plants - Chlorophyll a, Chlorophyll b, Carotenoids Absorb wide range of wavelengths Convert light energy into chemical energy  Pigments The plant pigments are found in chloroplasts on the membranes of the thylakoids. Chlorophyll a: Absorbs blue-violet (reflects blue-green). Main photosynthetic pigment. Chlorophyll b: Absorbs red-blue (reflects yellow-green). Accessory Pigment. Carotene: Absorbs blue-purple (reflects orange). Accessory Pigment. Xanthophyll: Absorbs blue-purple (reflects yellow). Accessory Pigment. Accessory pigments: absorb other colors of light that chlorophyll a can't absorb. They help boost energy absorption. Accessory Pigments  Structure The molecule chlorophyll a has a specific shape. This shape causes wavelengths of light that we see as a dark bluish green to be reflected back. Change the shape of that molecule by adding only two atoms, making it chlorophyll b, and the light that is reflected back is now less blue and more yellow. Chlorophyll Photosystems - pigment molecules (chlorophyll) & proteins Membranes of thylakoids - absorb photons Excites e- in chlorophyll - chlorophyll donates e- H2O is split to replace 2 e- Chlorophyll regenerated O2 to environment, H+ to thylakoid space Process Sugar Steps (Light) Light-Dependent Light-Dependent Light-dependent reactions require sunlight. In the light-dependent reactions, energy from sunlight is absorbed by chlorophyll and converted into stored chemical energy, in the form of the electron carrier molecule NADPH (nicotinamide adenine dinucleotide phosphate) and the energy currency molecule ATP (adenosine triphosphate). The light-dependent reactions take place in the thylakoid membranes in the granum (stack of thylakoids), within the chloroplast. Photosystem Energy - Reaction Center (chlorophyll a) Transfer Energy - Primary Electron Acceptor Antenna Pigments chlorophyll b carotenoids xanthophyll Electron Transport Reaction Center Photosystem I: P700 Photosystem II: P680 Electrons passed along electron acceptors Final = NADP+ è NADPH Our old friend ATP Synthase e- PSII - transfers e- to proteins in membrane - ETC e- energy moves H+ from stroma to thylakoid space After energy is used e- accepted by pigment molecule in PSI Electrochemical gradient - H+ create charge H+ - move down gradient through ATP Synthase  Summary To replace the electron in the reaction center, a molecule of water is split. This splitting releases an electron and results in the formation of oxygen (O2) and hydrogen ions (H+) in the thylakoid space. Technically, each breaking of a water molecule releases a pair of electrons, and therefore can replace two donated electrons. The replacing of the electron enables the reaction center to respond to another photon. The oxygen molecules produced as byproducts find their way to the surrounding environment. The hydrogen ions play critical roles in the remainder of the light-dependent reactions. Thylakoid Membrane Steps (Calvin) OF LIGHT-DEPENDENT REACTIONS (ATP, NADPH) Calvin Cycle Calvin Cycle - Light-Independent - Dark Rxn CO2 enters through stomata diffuses into stroma Uses energy from light reactions ATP & NADPH  Light-Independent In the light-independent reactions or Calvin cycle, the energized electrons from the light-dependent reactions provide the energy to form carbohydrates from carbon dioxide molecules. Although the light-independent reactions do not use light as a reactant (and as a result can take place at day or night), they require the products of the light-dependent reactions to function. The light-independent molecules depend on the energy carrier molecules, ATP and NADPH, to drive the construction of new carbohydrate molecules. After the energy is transferred, the energy carrier molecules return to the light-dependent reactions to obtain more energized electrons. Light-Independent Calvin Cycle Fixation, reduction, regeneration Enzyme - RuBisCO Molecule - ribulose bisphosphate (RuBP)  Photosystems Two types of photosystems are embedded in the thylakoid membrane: photosystem II ( PSII) and photosystem I (PSI). Each photosystem plays a key role in capturing the energy from sunlight by exciting electrons. Photosystems consist of a light-harvesting complex and a reaction center. Pigments in the light-harvesting complex pass light energy to two special chlorophyll a molecules in the reaction center. The light excites an electron from the chlorophyll a pair, which passes to the primary electron acceptor. The excited electron must then be replaced. In photosystem II, the electron comes from the splitting of water, which releases oxygen as a waste product. In photosystem I, the electron comes from the chloroplast electron transport chain. Photosystems Concept Check Describe light, pigments, chlorophyll (properties, function) Compare the components of the light-dependent and light- independent reactions Describe a photosystem and the role it plays in photosynthesis Explain the role of chemiosmosis in photosynthesis Describe inputs and outputs of the light-dependent vs light- independent reactions.