ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Ten Outline INTRODUCTION Certain Organisms Photosynthesize Capture energy from sun Build energy-rich food molecules Less Than 1% of the Sun's Energy Is Captured in Photosynthesis fig 10.1 AN EXPERIMENTAL JOURNEY van Helmont's Plant Growth Experiments Weighed tree and soil in pot Plant grew five years, only water added Plant weight gain greater than weight loss of soil Thus determined that plant substance not derived from soil Incorrectly concluded weight gain due to water The Role of Water Experiments by Priestly to determine nature of air Sprig of mint restored air in jar that a burning candle had depleted Mouse could breathe in jar after plant but not before Ingenhousz reproduced experiments Air restored only in presence of sunlight Occurred only with green plant leaves, not roots Proposed that plants split CO2 into carbon and oxygen Carbon and water combined to form carbohydrates Van Niel examined photosynthesis in bacteria Purple sulfur bacteria convert H2S into sulfur, do not release oxygen Proposed H2A is an electron donor, product A comes from splitting H2A Thus O2 from photosynthesis comes from H2O not CO2 Experiments reproduced using radioactive oxygen Carbohydrate typically produced by plants and algae is glucose The Role of Light Blackman's experiments determined that photosynthesis has two-stages Measured effects of changing light intensities and temperature In low light, higher temperature did not accelerate photosynthesis fig 10.2 In strong light, higher temperature did accelerate it Postulated "light" reactions independent of temperature, "dark" reactions independent of light At temperatures above 30% enzymes became denatured Present knowledge First stage requires light, reduces electron carriers, makes ATP from ADP In second stage carriers and ATP reduces C in CO2 and makes glucose Carbon fixation incorporates CO2 carbon into glucose in "dark" reaction Photosynthesis is a redox process Sun energy drives reduction of carrier molecules Reverse to the electron path in oxidative respiration Electrons in respiration loose energy going from sugar to oxygen Mitochondria use released energy to make ATP Electrons in photosynthesis must gain energy going from water to sugar Energy provided by the sun THE BIOPHYSICS OF LIGHT The Photoelectric Effect Intensity of a generated spark was increased in the presence of light Photoelectric effect discovered by Heinrich Hertz Investigated spark generation and electromagnetic (radio) waves Strength intensified by the brightness and wavelength of light Phenomenon explained by Einstein Light consists of units of energy called photons Light blasted electrons from the wire hoop Create positive ions and facilitate passage of current across gap The Energy in Photons Photons possess differing amounts of energy Energy content inversely proportional to the wavelength fig 10.3 Highest energy wavelengths are short wavelength gamma rays Least energetic wavelengths are long wavelength radio waves Energy in visible light Violet has short wavelength and high energy photons Red has long wavelength and low energy photons Ultraviolet Light Sunlight contains short, energetic ultraviolet light Was a probable source of energy in the primitive earth Current earth shielded by the ozone layer Ultraviolet light causes sunburns CAPTURING LIGHT ENERGY IN CHEMICAL BONDS Electrons occupy discrete energy levels while orbiting in their atoms Specific atoms can absorb only certain photons of light Any given molecule has a characteristic absorption spectrum Pigments Defined as molecules that absorb light Carotenoids fig 10.4 Carbon ring linked to chains with alternating double, single bonds Absorb photons over a broad range, not highly efficient Include beta-carotene, vitamin A and retinal Chlorophylls fig 10.5 Absorb photons by excitation like the photoelectric effect Complex ring structure called a porphyrin ring Metal ion within a network of alternating single and double bonds Absorb photons over a narrow range Chlorophyll a absorbs in violet-blue range Chlorophyll b absorbs in the red range Has an absorption spectrum shifted toward green light Is an accessory pigment within the photocenter of plants Wavelength not absorbed by chlorophylls reflected to eyes as green Chlorophyll Is the Primary Light Gathering Pigment in Photosynthesis Englemann attempted to characterize chlorophyll's absorption spectrum fig 10.6 Arranged alga across a miniature spectrum on a microscope slide Used aerobic bacteria to assess rate of oxygen production Most bacteria accumulated in red and violet-blue regions Users include plants, algae and most photosynthetic bacteria Do not use retinal pigment because of its low efficiency Chlorophyll absorbs in a narrow range, but with great efficiency HOW LIGHT DRIVES CHEMISTRY: THE LIGHT REACTIONS Absorbing Light Energy Light reactions occur on photosynthetic membranes fig 10.7 Photosynthesis occurs on cell membranes in bacteria In plants and algae, photosynthesis occurs in chloroplasts Evolutionary descendants of photosynthetic bacteria Photosynthetic membranes located within the chloroplasts Light reactions occur in three stages Primary photoelectric event Photon of light captured by a pigment Electron within the pigment is excited Excited electron shuttled along electron-carrier molecules Carrier molecules embedded within photosynthetic membrane Proton-pumping channel transports proton across membrane Electron induces event and is passed to an acceptor Passage of protons drives chemiosmotic synthesis of ATP Evolution of the Photocenter Light is captured by network of pigments called the photocenter fig 10.8 Arrangement permits channeling of energy to a central point Collects energy very efficiently Photocenter focuses energy on reaction center chlorophyll (P700 of photosystem I in plants) Passes energy to primary electron acceptor - ferredoxin? Chlorophyll passes only energy to adjacent molecule; its electron returns to lower energy level Excited electrons do not physically pass from pigment to pigment Analogy: cue ball hitting other balls at break, only end ones move Photosystem protein matrix holds pigment in optimal orientation Bacterial Light Reactions Sulfur bacteria Evolved photosynthetic units three billion years ago Photon absorption transmits electron from P pigment to ferredoxin Electron is accompanied by proton, a hydrogen atom Sulfur bacteria extract proton from H2S, sulfur by-product Other organisms extract proton from H2O, oxygen by-product Ejection of an electron from P leaves it one electron short Bacteria channel electron back via electron-transport system Passage drives a proton pump, chemiosmotically generates an ATP Overall process called cyclic photophosphorylation fig 10.9 Process is not a true circle Returned electron is not same one that left, but has same energy Process is the fundamental component of photosynthesis Limitations of cyclic photophosphorylation Geared only towards energy production Does not provide for biosynthesis Ultimate point of photosynthesis is to generate carbon compounds Sugars are more reduced than CO2, have more hydrogen atoms Bacteria inefficiently scavenge hydrogens from other sources The Advent of Photosystem II Other bacteria evolved an improved version of the photocenter Solved the reducing power problem New process grafted on to original photosynthetic process New process used chlorophyll a Originated with the evolution of cyanobacteria Second system called photosystem II Molecules of chlorophyll a are arranged with a different geometry More of shorter wavelengths are absorbed than in earlier process In plants, the earlier process is called photosystem I Absorption peak of pigment is 680 nanometers, called P680 How the Two Photosystems Work Together In Plants and Algae Plants, green algae and cyanobacteria possess a two-stage photocenter fig 10.10 Photosystem II acts first Excited electron is donated to an electron transport chain Passes electron on to photosystem I Each electron drives proton pump, chemiosmotically generates ATP fig 10.11 Excited electron absorbed by photosystem I Photosystem I now absorbs a photon Electron goes to primary electron acceptor generating reducing power Acceptor contributes two electrons to reduce nicotine adenine dinucleotide phosphate (NADP+) to NADPH Different carriers prevent cross flow of electrons between photosynthesis and oxidative respiration Energy from photosystem II, first photoevent, generates ATP Energy from photosystem I, second event, generates reducing power The Formation of Oxygen Gas Electron obtained from another source to replace that lost from P680 P680 becomes a strong oxidant (electron-seeker) Obtains electron from a protein called Z Removal makes Z a strong electron-acceptor Z obtains electrons from water Z catalyzes reactions that split water into OH- and H+ OH- collected to form water and oxygen H+ (protons) are transported across the membrane Augments proton gradient from electrons passing to photosystem I Organisms that use only photosystem I utilize ATP to make NADPH Comparing Plant and Bacterial Light Reactions Removal of electrons from pigment provides energy P700 provides enough to extract hydrogen from H2S but not H2O P680 provides enough to extract hydrogen from H2O Cyanobacteria, algae and plants use the double P680/P700 system Electrons and associated hydrogens must be extracted from water Oxygen continuously produced as a result HOW THE PRODUCTS OF THE LIGHT REACTIONS ARE USED TO BUILD ORGANIC MOLECULES FROM CO2 Light Independent Reactions Comprise Dark Reactions of Photosynthesis ATP generated in light reaction used to build sugars Atmospheric CO2 is reduced during carbon fixation The Calvin Cycle Ribulose 1,5 bisphosphate (RuBP) is a five-carbon molecule Produced by reassembling intermediates of glycolysis Fructose-6-phosphate (F6P) + glyceraldehyde-3-phosphate (G3P) Dark reactions are cyclic in nature At beginning of cycle, CO2 is bound to RuBP Six-carbon molecule splits to form two phosphoglycerates (PGA) fig 10.12 Process called C3 photosynthesis PGA converted to glyceraldehyde phosphate molecules Some are used to reconstitute RuBP, others assembled into sugars) fig 10.13 At each turn of the cycle one CO2 is added Takes six turns to produce a six-carbon sugar like glucose THE CHLOROPLAST AS A PHOTOSYNTHETIC MACHINE In Eukaryotes, Photosynthesis Occurs in the Chloroplasts fig 10.14 Internal membranes organized into flattened sacs called thylakoids Numerous thylakoids stacked in arrangements called grana fig 10.15 Photosynthetic pigments bound to membranes in thylakoids Architecture of the Chloroplast Membrane is impermeable to most molecules and protons Proton transit occurs through transmembrane channels Exit of protons from interior is driven by diffusion Occurs at ATP-synthesizing proton channels Channels are knobs on external surface of thylakoid membrane ATP released into surrounding fluid within chloroplast, the stroma Stroma contains enzymes of the Calvin cycle fig 10.16 Catalyze reactions that fix carbon and use ATP and NADPH Thylakoid membrane pumps protons from stroma to its interior ATP produced on stroma side as H+ pass back through membrane fig 10.17 PHOTOSYNTHESIS IS NOT PERFECT Evolution Favors Workable, Not Always Optimal Solutions RuBP carboxylase (rubisco) secondarily interferes with Calvin cycle Initiates oxidation of RuBP CO2 is released without the production of ATP or NADPH Process called photorespiration, acts to undo photosynthesis Both reactions occur at the same active site Decarboxylation reaction of photorespiration requires oxygen Little photorespiration occurred prior to the O2 atmosphere C3 plants lose one fourth to one half of their fixed carbon in this way Loss is related to increased temperature Oxidation of RuBP increases more than its photosynthesis Tropical plants adapted to counteract this problem The C4 Pathway Include grasses and other plants Called C4 pathway since first product is a four-carbon molecule Concentrate CO2 by carboxylating phosphoenolpyruvate (PEP) fig 10.18 Resulting four-carbon oxaloacetate converted to malate Malate conveyed to bundle-sheath cells, impermeable to CO2 fig 10.19 Malate decarboxylated to pyruvate, releasing CO2 in the cell Pyruvate returns to leaf cell, changed back to phosphoenolpyruvate Requires two high energy bonds, ATP becomes AMP C4 plants are found in hot climates fig 10.20 Process uses 30 ATP, normal photosynthesis uses 18 ATP Saves the loss of fixed carbon as occurs in C3 plants C4 plants also use C3 photosynthesis The Crassulacean Acid Pathway Crassulacean acid metabolism (CAM) also used by plants in hot climates Succulents open their stomata at night and close them during the day Reduces photorespiration by reducing CO2 available Also utilizes both C3 and C4 pathways C4 pathway at night, C3 pathway in the same cells in the daytime C4 plants use different locations for C3 and C4 photosynthesis fig 10.21 A LOOK BACK A Cell's Metabolism Indicates Its Evolutionary Past Modern Plant Two-Stage Photocenters Explain Evolution of Photosynthesis fig 10.22 Second stage evolved in anaerobic bacteria millions of years earlier Calvin cycle uses part of the glycolytic process in reverse Chlorophyll pigments are slightly modified bacterial pigments