Chapter 8: Photosynthesis Mr. Wilmot Part I- Photosynthetic structures And Overview 1. Where it takes place 2. What structures are involved and what are the raw material and products Photosynthesis takes place in the mesophyll cells of the leaf Mesophyll Each Mesophyll Cell contains many Chloroplasts Also, CO2 comes into, and O2 Leaves the cell through the stoma. Water gets to, and Glucose leaves through the Xylem and Phloem Water Glucose CO2 O2 The Chloroplast- Contains Chlorophyll- a light absorbing pigment Chlorophyll is actually located on the membrane of the thylakoid Granum Thylakoid Stroma ___________ _________ ___________ (Stack of Thylakoid) Aqueous Place Inside Chloroplast Chlorophyll and Chloroplasts 1. Light- energy from the sun that travels to earth comes in different wavelengths a. The visible part of this spectrum appears as different colors Pigments- light absorbing molecules that captures energy from the sun Chlorophyll- the plants principle pigment Include Chlorophyll A and Chlorophyll B There are other accessory pigment that augment the primary pigment allowing for more energy to be absorbed Include carotenoids and xanthophylls Chlorophyll absorbs light very well in the blue-violet and red wavelengths • It does not absorb green wavelengths, instead it reflects these wavelengths Chloroplast Anatomy Chloroplasts- site of photosynthesis a. Thylakoids- sac-like photosynthetic membranes • Arranged in stacks called Grana • Contain pigments- chlorophyll b. Stroma- the fluid inside the chloroplast Energy Collection • Light is energy • Pigments absorb light and energy • Light energy is transferred directly to electrons in the chlorophyll molecule • The raising of energy levels of electrons is the driving force of photosynthesis High Energy Electrons Chemically, the high energy electrons produced by chlorophyll require a special carrier • Analogy: Hot potato, oven mitt = special carrier • Electron carriers are the so called oven mitt, transferring these electrons from the chlorophyll to other molecules NADP+: is an electron carrier • Function- accepts and holds two high energy electrons along with a hydrogen ion (H+). • This converts NADP+ to NADPH • Used to help build carbohydrates An Overview of Photosynthesis Light-Dependent Reactions • Equation is a simplified form of photosynthesis; actually requires many steps to get from light to carbohydrates • Actually involves 2 sets of reactions – Light depedendent – Light independent • • • • Light dependent- set of reactions in photosynthesis that use energy from light to produce ATP and NADPH Require light and light absorbing pigments These reactions take place within the thylakoids of chloroplasts Water is need for these reactions as a source of electrons and H+ Oxygen is released as a bi-product Confused? Check this out….. Light phase reactions animation Light Independent Reactions – a.k.a. “The Dark Side”, er Phase a. Plants absorb CO2 from the atmosphere and complete the process of photosynthesis b. ATP and NADPH produced in the “light phase” reactions are used to produce high energy sugars…. with the addition of CO2. Dark Phase Animation c. No light is required d. Reactions occur in the stroma (liquid outside of thylakoid) e. Reliant on the light phase for materials f. Both light and dark phases work together to create energy-rich carbohydrates End of Part 1 The Process of Photosynthesis Light-Dependent Reactions: Generating ATP and NADPH Summary: The light dependent reactions use energy from sunlight to produce oxygen and convert ADP and NADP+ into the energy carriers ATP and NADPH • These carriers provide energy needed to build high-energy sugars from low-energy carbon dioxide The Light-Dependent Reaction Takes Place on the membrane of the Thylakoid Light-Dependent Reactions It happens in two parts called photosystems: 1- Photosystem 2 2- Photosystem 1 Light-Dependent Reactions a. Photosynthesis begins when pigments in photosystem II absorb light, increasing their energy level. Photosystem II Light-Dependent Reactions • These high-energy electrons are passed on to the electron transport chain. Photosystem II High-energy electron Electron carriers Light-Dependent Reactions b. Enzymes on the thylakoid membrane break water molecules into: Photosystem II 2H2O High-energy electron Electron carriers Copyright Pearson Prentice Hall Light-Dependent Reactions – hydrogen ions – oxygen atoms – energized electrons Photosystem II + O2 2H2O High-energy electron Electron carriers Light-Dependent Reactions c. The energized electrons from water replace the high-energy electrons that chlorophyll lost to the electron transport chain. Photosystem II + 2H2O High-energy electron O2 Light-Dependent Reactions d. As plants remove electrons from water, oxygen is left behind and is released into the air. Photosystem II + 2H2O High-energy electron O2 Light-Dependent Reactions e. The hydrogen ions left behind when water is broken apart are released inside the thylakoid membrane. Photosystem II + 2H2O High-energy electron O2 Light-Dependent Reactions f. Energy from the electrons is used to transport H+ ions from the stroma into the inner thylakoid space. Photosystem II Inside Thylakoid + O2 2H2O Outside Thylakoid Light-Dependent Reactions g. High-energy electrons move through the electron transport chain from photosystem II to photosystem I. Photosystem II + O2 2H2O Photosystem I Light-Dependent Reactions h. Pigments in photosystem I use energy from light to re-energize the electrons. + O2 2H2O Photosystem I Light-Dependent Reactions i. NADP+ then picks up these high-energy electrons, along with H+ ions, and becomes NADPH. + O2 2H2O 2 NADP+ 2 2 NADPH Light-Dependent Reactions j. As electrons are passed from chlorophyll to NADP+, more H+ ions are pumped across the membrane. + O2 2H2O 2 NADP+ 2 2 NADPH Light-Dependent Reactions k. Soon, the inside of the membrane fills up with positively charged hydrogen ions, which makes the outside of the membrane negatively charged. + O2 2H2O 2 NADP+ 2 2 NADPH Light-Dependent Reactions •The difference in charges across the membrane provides the energy to make ATP. + O2 2H2O 2 NADP+ 2 2 NADPH Light-Dependent Reactions •H+ ions cannot cross the membrane directly. ATP synthase + O2 2H2O 2 NADP+ 2 2 NADPH Light-Dependent Reactions •The cell membrane contains a protein called ATP synthase that allows H+ ions to pass through it. ATP synthase + O2 2H2O 2 NADP+ 2 2 NADPH Light-Dependent Reactions l. As H+ ions pass through ATP synthase, the protein rotates. ATP synthase + O2 2H2O 2 NADP+ 2 2 NADPH Light-Dependent Reactions m. As it rotates, ATP synthase binds ADP and a phosphate group together to produce ATP. ATP synthase + O2 2H2O 2 NADP+ 2 ADP 2 NADPH Light-Dependent Reactions •Because of this system, light-dependent electron transport produces not only high-energy electrons but ATP as well. ATP synthase + O2 2H2O 2 NADP+ 2 ADP 2 NADPH ATP synthase + O2 2H2O 2 NADP+ 2 ADP 2 NADPH End of Part 2 B. The Calvin Cycle • Also called the light independent (aka Calvin cycle, aka “dark phase”)reactions • ATP and NADPH formed by the lightdependent reactions contain an abundance of chemical energy, but they are not stable enough to store that energy for more than a few minutes. • 3. During the Calvin cycle plants use the energy that ATP and NADPH contain to build high-energy compounds that can be stored for a long time. The Calvin Cycle a. Six carbon dioxide molecules enter the cycle from the atmosphere and combine with six 5carbon molecules. CO2 Enters the Cycle The Calvin Cycle b. The result is twelve 3-carbon molecules, which are then converted into higherenergy forms. • The energy for this conversion comes from ATP and high-energy electrons from NADPH. Energy Input 12 12 ADP 12 NADPH 12 NADP+ The Calvin Cycle c. Two of twelve 3-carbon molecules are removed from the cycle. Energy Input 12 12 ADP 12 NADPH 12 NADP+ • The 2 removed molecules are used to produce sugars, lipids, amino acids and other compounds. 12 12 ADP 12 NADPH 12 NADP+ 6-Carbon sugar produced Sugars and other compounds d. The 10 remaining 3-carbon molecules are converted back into six 5-carbon molecules, which are used to begin the next cycle. 12 12 ADP 6 ADP 12 NADPH 6 12 NADP+ 5-Carbon Molecules Regenerated Sugars and other compounds The Calvin Cycle – The two sets of photosynthetic reactions work together. a. The light-dependent reactions trap sunlight energy in chemical form. b. The light-independent reactions use that chemical energy to produce stable, highenergy sugars from carbon dioxide and water. C. Factors Affecting Photosynthesis 1. Temperature, Light, and Water a. Temperature – Among most important factors that affect photosynthesis are temperature, light intensity, and availability of water. – Reactions made possible by enzymes that function best b/w 0oC and 35oC – Above or below these temps could slow down rate of photosynthesis – Low temps, may stop entirely b. Light – Intensity of light affects rate of photosynthesis – High light intensity increases rate of photosynthesis – After intensity reaches a certain level, plant reaches maximum rate of photosynthesis c. Water – Water is one of raw materials of photosynthesis – Shortage of water can slow or stop photosynthesis – Water loss can damage plant tissues – Plants that live in dry conditions often have waxy coatings on leaves to reduce water loss – May also have biochemical adaptations that make photosynthesis more efficient under dry conditions 2. Photosynthesis Under Extreme Conditions a. To conserve water: plants in bright, hot conditions close small openings in leaves that normally admit carbon dioxide – Keeps plants from drying out – Causes carbon dioxide w/in leaves to fall to very low levels – Photosynthesis slows down or stops 3. C4 Photosynthesis Animation a. Specialized chemical pathway that allows them to capture very low levels of carbon dioxide to pass it on to Calvin cycle • 1st compound formed in pathway contains 4 carbon atoms c. Enables photosynthesis to keep working under intense light and high temperatures d. Requires extra energy in form of ATP to function e. Examples: corn, sugar cane, sorghum 4. CAM Plants a. Carbon dioxide becomes incorporated into organic acids during photosynthesis – Process called Crassulacean Acid Metabolism b. Admit air into leaves only at night c. In cool darkness, carbon dioxide is combined w/ existing molecules to produce organic acids, “trapping” carbon w/in leaves d. During daytime, leaves tightly sealed to prevent loss of water compounds release carbon dioxide enable carbohydrate production • Examples: pineapple, cacti, “ice plants” (near freeways along west coast to retard brush fires and prevent erosion)