Lec 6 Photosynthesis PDF

BIO 11 UNIFYING CONCEPTS OF BIOLOGY PHOTOSYN- THESIS LECTURE 6 Photosynthesis and Cellular Respiration Provide Energy for Life Photosynthesis takes place in some prokaryotes and in the chloroplasts of plants and al...

BIO 11 UNIFYING CONCEPTS OF BIOLOGY PHOTOSYN- THESIS LECTURE 6 Photosynthesis and Cellular Respiration Provide Energy for Life Photosynthesis takes place in some prokaryotes and in the chloroplasts of plants and algae. Cellular respiration in many prokaryotes and in the mitochondria of almost all eukaryotes - in the cells of plants, animals, fungi, and protists. Photosynthesis converts light energy to the chemical energy of food Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria The structural organization of these cells allows for the chemical reactions of photosynthesis Chloroplasts: The Sites of Photosynthesis in Plants Leaves are the major locations of photosynthesis Their green color is from chlorophyll, the green pigment within chloroplasts Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf Each mesophyll cell contains 30–40 chloroplasts Carbon dioxide enters and oxygen exits the leaf through microscopic pores called stomata The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana Chloroplasts also contain stroma, a dense interior fluid Tracking Atoms Through Photosynthesis: Scientific Inquiry Photosynthesis is a complex series of reactions that can be summarized as the following equation: Phosynthesis as a Redox Process Photosynthesis reverses the direction of electron flow compared to cellular respiration Photosynthesis is a redox process in which water is oxidized and carbon dioxide is reduced Photosynthesis is an endergonic process; the energy boost is provided by light The Two Stages of Photosynthesis: A Preview Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) The Nature of Sunlight Light is a form of electromagnetic energy, also called electromagnetic radiation Wavelength is the distance between crests of waves Wavelength determines the type of electromagnetic energy The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors we can see Light also behaves as though it consists of discrete particles, called photons Photosynthetic Pigments: The Light Receptors Pigments are substances that absorb visible light Different pigments absorb different wavelengths Wavelengths that are not absorbed are reflected or transmitted A spectrophotometer measures a pigment’s ability to absorb various wavelengths This machine sends light through pigments and measures the fraction of light transmitted at each wavelength An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. Engelmann In his experiment, he exposed different segments of a filamentous alga to different wavelengths Areas receiving wavelengths favorable to photosynthesis produced excess oxygen He used the growth of aerobic bacteria clustered along the alga as a measure of oxygen production Chlorophyll a is the main photosynthetic pigment Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll Excitation of Chlorophyll by Light Fluorescence, an afterglow, occurs when photons are given off after excited electrons fall back to ground state If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat Reaction-Center Complexes Associated with Light-Harvesting Complexes A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center A primary electron acceptor in the reaction center accepts excited electrons and is reduced as a result Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions Linear Electric Flow Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy Cylic Electron Flow Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH No oxygen is released Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle A Comparison of Chemiosmosis in Chloroplasts and Mitochondria Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP Calvin cycle uses ATP and NADPH to reduce carbon dioxide to sugar Carbon enters the cycle as carbon dioxide and leaves as a sugar named glyceraldehyde 3-phospate (G3P) For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of carbon dioxide The Calvin cycle has three phases Carbon fixation (catalyzed by rubisco) Reduction Regeneration of the carbon dioxide acceptor (RuBP) Photorespiration: An Evolutionary Relic? In most plants (C3 plants), initial fixation of carbon dioxide, via rubisco, forms a three-carbon compound (3-phosphoglycerate) In photorespiration, rubisco adds oxygen instead of carbon dioxide in the Calvin cycle, producing a two-carbon compound Photorespiration consumes oxygen and organic fuel and releases carbon dioxide without producing ATP or sugar In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle C4 Plants C4 plants minimize the cost of photorespiration by incorporating carbon dioxide into four-carbon compounds in mesophyll cells This step requires the enzyme PEP carboxylase PEP carboxylase has a higher affinity for carbon dioxide than rubisco does; it can fix carbon dioxide even when concentrations are low These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle CAM Plants Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon CAM plants open their stomata at night, incorporating carbon dioxide into organic acids Stomata close during the day, and carbon dioxide is released from organic acids and used in the Calvin cycle The Importance of Photosynthesis: A Summary The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits In addition to food production, photosynthesis produces the oxygen in our atmosphere END OF DISCUSSION