Presentation on theme: "Photosynthesis: Energy"— Presentation transcript:
1Photosynthesis: Energy CHAPTER 8Photosynthesis: Energyfrom the Sun
2PhotosynthesisBiochemical process in which light energy is converted to chemical energyPhotos = lightsynthesis = to put togetherIn plants, photosynthesis takes place in chloroplasts.It involves many enzyme controlled steps
3Photosynthetic Reactants and Products 6 CO H2O + light C6H12O6 + 6 O2 + 6 H2O
5Photosynthesis Photosynthesis can be divided into two pathways: The light reaction - driven by light energy captured by chlorophyll. Consists of Photosystem I and Photosystem II. It produces ATP and NADPH + H+.The Calvin–Benson cycle - does not use light directly. It uses ATP, NADPH + H+, and CO2 to produce sugars.
6Properties of Light and Pigments Light is the source of energy that drives photosynthesisMolecules that absorb light energy in the visible range are called pigments.
8Properties of Light and Pigments When light and a pigment molecule meet, one of 3 things happenReflection – the light bounces off the moleculeTransmission – the light passes through the moleculeExcitation – the light is absorbed by the molecule. If absorbed, the molecule goes from its ground state to and excited state of higher energyAn electron is boosted to another orbital
9PigmentsWhen a beam of white light shines on an object, and the object appears to be red in color, it is because it has absorbed all other colors from the white light except for the color red.In the case of chlorophyll, plants look green because they absorb green light less effectively than the other colors found in sunlight and reflect the green light not absorb
10Properties of Light and Pigments Different pigment molecules absorb different wavelengths of lightThe particular set of wavelengths that a pigment absorbs is called its absorption spectrumReview Figures 8.7
12Properties of Light and Pigments Chlorophylls are the most important pigments in photosynthesisChlorophyll a is the primary pigment in photosynthesis.Chlorophylls and accessory pigments trap light and transfer energy to a reaction center
14An excited pigment molecule may lose its energy by emitting light of longer wavelength ortransfer the absorbed energy to another pigment molecule as a redox reaction.
15Figure 8.8 Energy Transfer and Electron Transport
16There are two different systems for transport of electrons in photosynthesis. 1. Noncyclic electron transport produces NADPH + H+ and ATP and O2.2. Cyclic electron transport produces only ATP.
17NoncyclicIn noncyclic electron transport, two photosystems are required.Photosystems consist of many chlorophyll molecules and accessory pigments bound to proteins.
18Photosystem IPhotosystem I uses light energy to reduce NADP+ to NADPH + H+.The reaction center contains a chlorophyll a molecule called P700 because it best absorbs light at a wavelength of 700 nm.
19Photosystem IIPhotosystem II uses light energy to split water, producing electrons, protons, and O2.The reaction center contains a chlorophyll a molecule called P680 because it best absorbs light at a wavelength of 680 nm.To keep noncyclic electron transport going, both photosystems must constantly be absorbing light.
20After absorbing light energy: an energized electron leaves the Chl* in the reaction center and participates in a series of redox reactions.the electron flows through a series of carriers in the thylakoid membrane.producing ATP
21Figure 8. 9 Noncyclic Electron Transport Uses Two Photosystems (Part 1)
22Figure 8. 9 Noncyclic Electron Transport Uses Two Photosystems (Part 2)
23Cyclic Electron Transfer Cyclic electron transport produces only ATP.The electron passes from an excited P700 molecule and cycles back to the same P700 molecule.No O2 is released.In cyclic electron flow, photosystem I acts on its own.
24Figure 8.10 Cyclic Electron Transport Traps Light Energy as ATP
25“Z” SchemePhotosystem I & II (P680 & P700) work together to generate ATP and NADPH.This pathway is called the “Z” scheme.Noncyclic
26Noncyclic Electron Flow or Z Scheme In Photosystem II chlorophyll a absorbs light energy to become energized chloropyll a2 electrons are released and caught by the primary electron acceptor.H20 ½ O2 + 2 e- + 2H+
27The electrons pass through a redox chain for chemiosmotic ATP production. The electron transport chain pumps protons across the membrane into the thylakoid space.The protons accumulate establishing a proton concentration gradientATP synthases open and the protons diffuse to generate ATP from ADP.
28Z Scheme Cont’d The electrons are passed to P700 chlorophyll P700 loses electrons to Ferredoxin (Fd)NADP combines with H to form NADPH.NADPH is the source of H used to make C6H12O6
29Figure 8.11 Chloroplasts Form ATP Chemiosmotically
30The Calvin–Benson Cycle The Calvin–Benson cycle makes sugar from CO2ATP and NADPH provide the needed energyThis pathway was elucidated through use of radioactive tracers
31The Calvin–Benson Cycle Three phases:1. Carbon Fixation – RuBP + CO2 6 carbon sugar 3 PG (first stable product)The reaction is catalyzed by rubisco (ribulose bisphosphate carboxylase).2. Series of reactions to produce G3P3. Regeneration of RuBP (7 enzymatic steps)RuBP (ribulose biphosphate) is the initial CO2 acceptor
32The Calvin–Benson Cycle The end product of the cycle is glyceraldehyde 3-phosphate, G3P.There are two fates for the G3P:One-third ends up as starch, which is stored in the chloroplast and serves as a source of glucose.Two-thirds is converted to the disaccharide sucrose, which is transported to other organs.
34RubiscoRubisco is a carboxylase, adding CO2 to RuBP. It can also be an oxygenase, adding O2 to RuBP.These two reactions compete with each other.When RuBP reacts with O2, it cannot react with CO2, which reduces the rate of CO2 fixation.
35PhotorespirationA specialized metabolic pathway in which rubisco reacts with O2 instead of CO2Occurs under stress conditions of hot, dry, bright days when the internal leaf concentration of O2 is greater than CO2 concentration.Glucose production is reduced thereby limiting plant growth
36C3 Plants Most common type of plants on earth. Grow best in temperate zonesIncludes rice, wheat, soybeans, bluegrassOn hot days the stomata close, O2 builds up and photorespiration occurs.The first product is the 3-C molecule of 3PGCO2 + RuBP 3 phosophoglycerate (3 C compound)
38C4 PlantsC4 plants have 2 enzymes (PEP carboxylase & rubisco) for CO2 fixation in 2 different parts of the leaf.PEP carboxylase does not have an affinity for O2 and fixes CO2 even at very low CO2 levels.What is the significance of this fact?C4 plants include sugarcane, corn and other plants that grow in hot, dry climates.
39C4 Plants Cont’d CO2 + PEP carboxylase Oxaloacetate (4 C compound). Occurs in cells near top of leafOxaloacetate diffuses into bundle sheath cells in the interior of the cells.Here oxaloacetate loses a C forming CO2CO2 enters the Calvin-Benson Cycle
40Crassulacean Acid Metabolism (CAM) CAM plants are succulents or water storing plants.Include cacti and pineapplesCAM plants open their stomata only at nightCO2 enters and forms malic acid which is stored as an acid in the vacuoles until morningIn daylight, the CO2 is released from the acid and enters the Calvin Benson Cycle.
41Stomates Stomates close when weather is hot & dry. O2 concentration increases, CO2 concentration decreases.Why?Ribulose requires high concentrations of CO2If sufficient CO2 is unavailable, photorespiration occurs.
42Metabolic Pathways in Plants Both photosynthesis and respiration occurs in plants.Compare photosynthesis and respiration.