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BIOLS 102 Dr. Tariq Alalwan Biology, 10e Sylvia S. Mader Lectures by Tariq Alalwan, Ph.D. Learning Objectives List the types of organisms that carry on photosynthesis. Identify the structure of a chloroplast and explain how its components interact and facilitate the process of it t i t t d f ilit t th f photosynthesis. Show that photosynthesis is a redox reaction that produces a carbohydrate and releases O2. Describe what happens to an electron in a biological molecule such as chlorophyll when a photon of light energy is absorbed. Learning Objectives (cont.) Explain why leaves are green, with reference to the electromagnetic spectrum. Describe how the thylakoid membrane is organized to produce ATP. Describe the three phases of the Calvin cycle, and indicate when ATP and/or NADPH are involved. Compare and contrast three modes of photosynthesis and tell how each is adapted to a particular environment. Chapter 7: Photosynthesis 1 BIOLS 102 Dr. Tariq Alalwan Photosynthetic Organisms Organisms cannot live without energy Almost all the energy that organisms used comes from the solar energy Photosynthesis: Ph t th i A process that captures solar energy and transforms it into chemical energy (carbohydrate) Occurrs in higher plants, algae, mosses, photosynthetic protists and some bacteria (cyanobacteria) Photosynthetic Organisms Autotrophs – organisms having the ability to synthesize carbohydrates Heterotrophs – organisms that must take in preformed organic molecules Both autotrophs & heterotrophs use carbohydrates produced by photosynthesis Flowering Plants as Photosynthesis Photosynthesis takes place in the green portions of plants Leaf of flowering plant contains mesophyll tissue with chloroplasts p Specialized to carry on photosynthesis CO2 enters leaf through stomata Diffuses into chloroplasts in mesophyll cells In the stroma, CO2 is combined with H2O to form C6H12O6 (sugar) Thylakoid membrane contains chlorophyll , which absorbs solar energy drives photosynthesis Chapter 7: Photosynthesis 2 BIOLS 102 Dr. Tariq Alalwan The Process of Photosynthesis The process of photosynthesis is an example of a redox reaction (i.e. movement of electrons from one molecule to another) g g p y y g In living things, electrons are accompanied by hydrogen ions (H+ + e) Oxidation is the loss of hydrogen atoms; Reduction is the gain of hydrogen atoms The Process of Photosynthesis (cont.) CO2 reduction requires energy and hydrogen atoms Solar energy will be used to generate ATP needed to reduce CO2 to sugar Electrons needed to reduce CO2 will be carried by a coenzyme (NADP+) NADP+ is the active redox coenzyme of photosynthesis When NADP+ is reduced, it accepts 2 e‐ and H+ When NADP+ is oxidized, it gives up its electrons Chapter 7: Photosynthesis 3 BIOLS 102 Dr. Tariq Alalwan Photosynthetic Reactions In 1930, van Niel showed that O2 given off by photosynthesis comes from water and not from CO2 Researchers later confirmed using the isotope oxygen (18O) that O2 given off from photosynthesis comes from water not CO2 When water splits, O2 is released and the hydrogen atoms (2 e‐ + H+) are taken up by NADPH NADH later reduces CO2 to a carbohydrate Plants as Solar Energy Converters Higher energy wavelengths are screened out by the ozone layer in the upper atmosphere Lower energy wavelengths are screened out by water vapor and CO d CO2 Photosynthetic pigments use primarily the visible light portion of the electromagnetic spectrum Pigments found in chlorophyll absorb various portions of visible light; this is called their absorption spectrum Graph showing relative absorption of the various colors of the rainbow Photosynthetic Pigments Chlorophylls a and b Main pigment that initiate light‐dependent reactions of photosynthesis Absorb violet, blue and red light best Very little green light is absorbed; most is reflected (this is why leaves appear green) Carotenoids Yellow and orange accessory photosynthetic pigments Absorb light in violet, blue, and green regions Chapter 7: Photosynthesis 4 BIOLS 102 Dr. Tariq Alalwan Absorption and Action Spectrum Spectrophotometer An instrument which measures the amount of light of a specified wavelength which passes through a sample Amount of light absorbed at each wavelength is plotted on a graph, resulting in an absorption spectrum Phosynthetic Pigments and Photosynthesis Two Sets of Reactions Light Reactions: Take place only in the presence of light Chlorophyll present in thylakoid membranes absorbs solar energy energizes electrons Electrons move down electron transport chain Pumps H+ into thylakoids Used to make ATP out of ADP and NADPH out of NADP+ Calvin Cycle Reactions: In the stroma, CO2 is reduced to a carbohydrate Reduction requires the ATP and NADPH produced in the light reactions Chapter 7: Photosynthesis 5 BIOLS 102 Dr. Tariq Alalwan Photosynthesis Overview Light Reactions The light reactions utilize two light‐gathering units (i.e., photosystems): Photosystem I (PSI) Photosystem II (PSII) Ph t t II (PSII) A photosystem unit consists of Pigment complex (molecules of chlorophyll a, chlorophyll b and the carotenoids) Electron acceptor molecules that help collect solar energy like an antenna Occur in the thylakoid membranes Noncyclic Electron Pathway Uses two photosystems, PS I and PS II Energy enters the system when PS II becomes excited by light Causes high‐energy electrons (e Causes high energy electrons (e‐)to leave the reaction center chlorophyll a molecule Electron travels down electron transport chain to PS I PS II takes replacement electron from H2O, which splits, releasing O2 and H+ ions; H2O 2 H+ + 2 e‐ + ½ O2 H+ ions temporarily stay within the thylakoid space and contribute to a H+ ion gradient Which causes ATP production, how? Chapter 7: Photosynthesis 6 BIOLS 102 Dr. Tariq Alalwan Noncyclic Electron Pathway (cont.) Low‐energy electrons leaving the electron transport system enter PS I PS I captures light energy and ejects the energized electron (e‐) from the reaction center chlorophyll a Transferred permanently by electron acceptors to a molecule of NADP+ Causes NADPH production; NADP+ + 2 e‐ + H+ NADPH NADPH and ATP produced are used by the Calvin cycle reaction or light‐independent (dark) reaction enzymes in the stroma Organization of the Thylakoid Membrane PS II: Consists of a pigment complex and electron‐acceptor molecules Adjacent to an enzyme that oxidizes water Oxygen is released as a gas (by‐product) Electron transport chain (ETC): Consists of Pq (plastoquinone) and cytochrome complexes Carries electrons from PS II to PS I Pumps H+ from the stroma into thylakoid space Chapter 7: Photosynthesis 7 BIOLS 102 Dr. Tariq Alalwan Organization of the Thylakoid Membrane PS I: Consists of a pigment complex and electron acceptor molecules Adjacent to enzyme that reduces NADP+ to NADPH ATP synthase complex: Has a channel for H+ flow Which drives ATP synthase to join ADP and Pi Organization of a Thylakoid ATP Production Thylakoid space acts as a reservoir for H+ ions Each time water is oxidized, two H+ remain in the thylakoid space Electrons yield energy Used to pump H+ across thylakoid membrane Move from stroma into the thylakoid space Flow of H+ back across thylakoid membrane Energizes ATP synthase enzyme thus producing ATP from ADP + Pi This method of producing ATP is called chemiosmosis as it is tied to an electrochemical H+ gradient Chapter 7: Photosynthesis 8 BIOLS 102 Dr. Tariq Alalwan Calvin Cycle Reactions A series of reactions producing carbohydrates; these reactions follow the light reactions The cycle is named for Melvin Calvin who used a radioactive isotope of carbon to trace the reactions Known as C3 photosynthesis Involves three phases: Carbon dioxide fixation Carbon dioxide reduction Regeneration of RuBP Fixation of Carbon Dioxide The first step of the Calvin cycle Atmospheric CO2 enters the stroma of the chloroplast via the stomata of the leaves CO2 is attached to 5‐carbon RuBP molecule Results in an intermediate 6‐carbon molecule This splits into two 3‐carbon molecules (3PG) Reaction is accelerated by RuBP Carboxylase (Rubisco) Comprises 20–50% of the protein content of chloroplasts, why? CO2 now “fixed” because it is part of a carbohydrate Reduction of Carbon Dioxide ATP phosphorylates each 3PG molecule and creates 1,3‐ bisphosphoglycerate (BPG) BPG is then reduced by NADPH to glyceraldehyde‐3‐ phosphate (G3P) NADPH and some ATP used for the reduction reactions are produced in the light reactions Chapter 7: Photosynthesis 9 BIOLS 102 Dr. Tariq Alalwan Regeneration of RuBP RuBP used in CO2 fixation must be replaced For every three turns of Calvin Cycle, Five G3P molecules are used To remake three RuBP molecules 5 X 3 (carbons in G3P) = 3 X 5 (carbons in RuBP) The Calvin Cycle C3 Plants C3 plants include more than 95% percent of the plant species on earth (e.g. trees) Called C3 because the CO2 is first incorporated into a 3‐ p carbon compound CO2 + RUBP Rubisco 2 3PG In hot dry conditions, the photosynthetic efficiency of C3 plants suffers because of photorespiration Stomata must close to avoid wilting CO2 decreases and O2 increases O2 starts combining with RuBP instead of CO2 Chapter 7: Photosynthesis 10 BIOLS 102 Dr. Tariq Alalwan C4 Photosynthesis Uses another method of CO2 uptake forming a 4‐ carbon molecule instead of the two 3‐carbon molecules In C4 plants Bundle sheath cells as well as the mesophyll cells contain chloroplasts The enzyme PEP carboxylase (PEPCase) fixes CO2 to PEP (a C3 molecule) oxaloacetate (a C4 molecule) Oxaloacetate is reduced to malate, which is pumped into the bundle sheath cells Here the 4‐carbon molecule is broken down into CO2, which enters the Calvin cycle to form sugars & starch Chloroplast distribution in C4 vs. C3 Plants CO2 Fixation in C4 vs. C3 Plants In hot and dry climates C4 plants avoid photorespiration as PEP in mesophyll cells does not bind to O O2 Net productivity of C4 is about 2‐3 times of C3 plants In cool, moist conditions, CO2 fixation in C3 plants is three times more efficient than in C4 Chapter 7: Photosynthesis 11 BIOLS 102 Dr. Tariq Alalwan CAM Photosynthesis Called CAM (Crassulacean‐acid metabolism) after the plant family in which it was first found CAM plants partition carbon fixation by time Crassulacean‐Acid Metabolism C l A id M b li During the night through their open stomata CAM plants use PEPCase to fix CO2 Forms C4 molecules (malate) Stored in large vacuoles in mesophyll cells CAM Photosynthesis (cont.) During daylight NADPH and ATP are available from the light‐dependent reactions Stomata closed for water l df conservation but CO2 cannot enter photosynthesizing tissues C4 formed at night is broken down to CO2 during the day, which enters the Calvin cycle Photosynthesis in a CAM plant is minimal, due to limited amount of CO2 fixed at night Chapter 7: Photosynthesis 12
