Cells and Molecules
Cells and Molecules BS 161
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Helen Blick Sr.
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This 13 page Class Notes was uploaded by Helen Blick Sr. on Saturday September 19, 2015. The Class Notes belongs to BS 161 at Michigan State University taught by Donna Koslowsky in Fall. Since its upload, it has received 35 views. For similar materials see /class/207327/bs-161-michigan-state-university in Biological Sciences at Michigan State University.
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Date Created: 09/19/15
Chapter 5 39 The Structure and Function of Macromolecules Overview The Molecules of Life 39 All living things are made up of four main classes of macromolecules carbohydrates lipids proteins and nucleic acids 39 These large macromolecules may consist of thousands of covalently bonded atoms some with mass greater than 100000 daltons 51 Most quotmu 39 39 are nolvmers built from x 39 Three of the four classes of macromoleculesicarbohydrates proteins and nucleic acidsiform chainlike molecules called polymers 0 A polymer is a long molecule consisting of many similar or identical building blocks linked by covalent bonds 0 The repeated units are small molecules called monomers 0 Some of the molecules that serve as monomers have other functions of their own 39 The chemical mechanisms that cells use to make and break polymers are similar for all classes of macromolecules 39 Monomers are connected by covalent bonds that form through the loss of a water molecule This reaction is called a condensation reaction or dehydration reaction 0 When a bond forms between two monomers each monomer contributes part of the water molecule that is lost One monomer provides a hydroxyl group 70H while the other provides a hydrogen atom 7H Cells invest energy to carry out dehydration reactions 0 Dehydration is facilitated by enzymes specialized macromolecules that speed up chemical reactions in cells 39 The covalent bonds that connect monomers in a polymer are disassembled by hydrolysis a reaction that is effectively the reverse of dehydration o In hydrolysis bonds are broken by the addition of water molecules A hydrogen atom attaches to one monomer and a hydroxyl group attaches to the adjacent monomer 52 Carbohydrates serve as fuel and building material 39 Carbohydrates include sugars and their polymers 0 The simplest carbohydrates are monosaccharides or simple sugars o Disaccharides or double sugars consist of two monosaccharides joined by a condensation reaction 0 Polysaccharides are polymers of many monosaccharides Sugars the smallest carbohydrates serve as fuel and a source of carbon 39 Monosaccharides generally have molecular formulas that are some multiple of the unit CHzO o For example glucose has the formula C5H1205 39 Monosaccharides have a carbonyl group gtCO and multiple hydroxyl groups 7 0 Depending on the location of the carbonyl group the sugar is an aldose aldehyde sugar or a ketose ketone sugar 0 Most names for sugars end in ose o Glucose an aldose and fructose a ketose are structural isomers Monosaccharides are also classi ed by the number of carbon atoms in the carbon skeleton o The carbon skeleton of a sugar ranges from three to seven carbons long 0 Glucose and other sixcarbon sugars are hexoses O Fivecarbon sugars are pentoses threecarbon sugars are trioses Although glucose is often drawn with a linear carbon skeleton most sugars including glucose form rings in aqueous solution Monosaccharides particularly glucose are major nutrients for cellular work 0 Cells extract energy from glucose molecules in the process of cellular respiration Simple sugars also function as the raw material for the synthesis of other monomers such as amino acids and fatty acids Two monosaccharides can join with a glycosidic linkage to form a disaccharide via dehydration o Maltose malt sugar is formed by joining two glucose molecules 0 Sucrose table sugar is formed byjoining glucose and fructose Sucrose is the major transport form of sugars in plants 0 Lactose milk sugar is formed by joining glucose and galactose Polysacch arides the polymers of sugars have storage and structural roles Polysaccharides are polymers of hundreds to thousands of monosaccharides joined by glycosidic linkages 0 Some polysaccharides serve for storage and are hydrolyzed as sugars are needed 0 Other polysaccharides serve as building materials for the cell or the whole organism Starch is a storage polysaccharide composed entirely of glucose monomers Plants store surplus glucose as starch granules within plastids including chloroplasts and withdraw it as needed for energy or carbon 0 Most of the glucose monomers in starch are joined by 174 linkages number 1 carbon to number 4 carbon Animals store glucose in a polysaccharide called glycogen o Glycogen is highly branched like amylopectin o Humans and other vertebrates store a day s supply of glycogen in the liver and muscles Cellulose is a major component of the tough walls of plant cells 0 Plants produce almost 100 billion tons of cellulose per year It is the most abundant organic compound on Earth Like starch cellulose is a polymer of glucose However the glycosidic linkages in these two polymers differ o The linkages are different because glucose has two slightly different ring structures 0 These two ring forms differ in whether the hydroxyl group attached to the number 1 carbon is xed above 5 glucose or below or glucose the plane ofthe ring Starch is a polysaccharide of alpha or glucose monomers Cellulose is a polysaccharide of beta 5 glucose monomers making every other glucose monomer upside down with respect to its neighbors The differing glycosidic linkages in starch and cellulose give the two molecules distinct threedimensional shapes 0 While polymers built with or glucose form helical structures polymers built with B glucose form straight structures 0 The straight structures built with B glucose allow H atoms on one strand to form hydrogen bonds with OH groups on other strands In plant cell walls parallel cellulose molecules held together in this way are grouped into units called micro brils which form strong building materials for plants 0 Cellulose micro brils are important constituents of wood paper and cotton 0 The enzymes that digest starch by hydrolyzing its or linkages cannot hydrolyze the 5 linkages in cellulose o Cellulose in human food passes through the digestive tract and is eliminated in feces as insoluble ber 0 As it travels through the digestive tract cellulose abrades the intestinal walls and stimulates the secretion of mucus which aids in the passage of food 39 Some microbes can digest cellulose to its glucose monomers through the use of cellulase enzymes Many eukaryotic herbivores from cows to termites have symbiotic relationships with cellulosedigesting prokaryotes providing the prokaryote and the host animal access to a rich source of energy 0 Some fungi can also digest cellulose 39 Another important structural polysaccharide is chitin found in the exoskeletons of arthropods including insects spiders and crustaceans o Chitin is similar to cellulose except that it has a nitrogencontaining appendage on each glucose monomer 0 Pure chitin is leathery but can be hardened by the addition of calcium carbonate 0 Chitin also provides structural support for the cell walls of many fungi 53 Lipids are a diverse group of hydrophobic molecules 39 Unlike other macromolecules lipids do not form polymers 39 The unifying feature of lipids is that they have little or no affinity for water because they consist of mostly hydrocarbons which form nonpolar covalent bonds 39 Lipids are highly diverse in form and function Fats store large amounts of energy 39 Although fats are not strictly polymers they are large molecules assembled from smaller molecules via dehydration reactions A fat is constructed from two kinds of smaller molecules glycerol and fatty acids 0 Glycerol is a threecarbon alcohol with a hydroxyl group attached to each carbon 0 A fatty acid consists of a carboxyl group attached to a long carbon skeleton often 16 to 18 carbons long The many nonpolar CiH bonds in the long hydrocarbon skeleton make fats hydrophobic Fats separate from water because the water molecules hydrogenbond to one another and exclude the fats O O In a fat three fatty acids are joined to glycerol by an ester linkage creating a triacylglycerol or triglyceride o The three fatty acids in a fat can be the same or different Fatty acids vary in length number of carbons and in the number and locations of double bon s o If the fatty acid has no carboncarbon double bonds then the molecule is a saturated fatty acid saturated with hydrogens at every possible position 0 If the fatty acid has one or more carboncarbon double bonds formed by the removal of hydrogen atoms from the carbon skeleton then the molecule is an unsaturated fatty acid A saturated fatty acid is a straight chain but an unsaturated fatty acid has a kink wherever there is a cis double bond Fats made from saturated fatty acids are saturated fats Fats made from unsaturated fatty acids are unsaturated fats 0 Most animal fats are saturated 0 Saturated fats are solid at room temperature 0 Plant and fish fats are liquid at room temperature and are known as oils The kinks caused by the cis double bonds prevent the molecules from packing tightly enough to solidify at room temperature 0 The phrase hydrogenated vegetable oils on food labels means that unsaturated fats have been synthetically converted to saturated fats by the addition of hydrogen I Peanut butter and margarine are hydrogenated to prevent lipids from separating out as oil 0 A diet rich in saturated fats may contribute to cardiovascular disease atherosclerosis through plaque deposits 0 The process of hydrogenating vegetable oils produces saturated fats and also unsaturated fats with trans double bonds These trans fat molecules contribute more than saturated fats to atherosclerosis The major function of fats is energy storage 0 A gram of fat stores more than twice as much energy as a gram of a polysaccharide such as starch Because plants are immobile they can function with bulky energy storage in the form of starch Plants use oils when dispersal and compact storage are important as in seeds Animals must carry their energy stores with them so they benefit from having a more compact fuel reservoir of fat 0 O Phospholipids are major components of cell membranes Phospholipids have two fatty acids attached to glycerol and a phosphate group at the third position 0 The phosphate group carries a negative charge 0 Additional smaller groups usually charged or polar may be attached to the phosphate group to form a variety of phospholipids The interaction of phospholipids with water is complex 0 The fatty acid tails are hydrophobic but the phosphate group and its attachments form a hydrophilic head 0 When phospholipids are added to water they form assemblages with the hydrophobic tails pointing toward the interior Phospholipids are arranged as a bilayer at the surface of a cell 0 The hydrophilic heads are on the outside of the bilayer in contact with the aqueous solution and the hydrophobic tails point toward the interior of the bilayer o The phospholipid bilayer forms a barrier between the cell and the external env1ronment o Phospholipids are the major component of all cell membranes Steroids include cholesterol and certain hormones Steroids are lipids with a carbon skeleton consisting of four fused rings Different steroids are created by varying the functional groups attached to the rings Cholesterol an important steroid is a component in animal cell membranes Cholesterol is the precursor from which all other steroids are synthesized 0 Many of these other steroids are hormones including the vertebrate sex hormon Although cholesterol is an essential molecule in animals high levels of cholesterol in the blood may contribute to cardiovascular disease 0 Both saturated fats and trans fats exert their negative impact on health by affecting cholesterol levels 54 Proteins have many structures and a Wide range of functions Proteins account for more than 50 of the dry mass of most cells They are instrumental in almost everything an organism does 0 Protein functions include structural support storage transport cellular signaling movement and defense against foreign substances Most important protein enzymes function as catalysts in cells regulating metabolism by selectively accelerating certain chemical reactions without being consumed O Humans have tens of thousands of different proteins each with a specific structure and function Proteins are the most structurally complex molecules known 0 Each type of protein has a complex threedimensional shape All protein polymers are constructed from the same 20 amino acid monomers Polymers of proteins are called polypeptides A protein consists of one or more polypeptides folded and coiled into a speci c conformation Amino acids are the monomers from which proteins are constructed Amino acids are organic molecules with both carboxyl and amino grou s At the center of an amino acid is an asymmetric carbon atom called the alpha 0 carbon Four components are attached to the a carbon a hydrogen atom a carboxyl group an amino group and a variable R group or side chain 0 Different R groups characterize the 20 different amino acids 0 An R group may be as simple as a hydrogen atom as in the amino acid glycine or it may be a carbon skeleton with various functional groups attached as in glutamine The physical and chemical properties of the R group determine the unique characteristics of a particular amino acid One group of amino acids has nonpolar R groups which are hydrophobic 0 Another group of amino acids has polar R groups which are hydrophilic o A third group of amino acids has functional groups that are charged ionized at cellular pH Some acidic R groups have negative charge due to the presence of a carboxyl 0 group Basic R groups have amino groups with positive charge All amino acids have carboxyl and amino groups The terms acidic and basic in this context refer only to these groups in the R groups 00 Amino acids are joined together when a dehydration reaction removes a hydroxyl group from the carboxyl end of one amino acid and a hydrogen atom from the amino group of another 0 The resulting covalent bond is called a peptide bond Repeating the process over and over creates a polypeptide chain 0 At one end is an amino acid with a free amino group the N terminus and at the other end is an amino acid with a free carboxyl group the Cterminus Polypeptides range in size from a few monomers to thousands Each polypeptide has a unique linear sequence of amino acids Protein conformation determines protein function I A functional protein consists of one or more polypeptides that have been twisted folded and coiled into a unique sha e It is the order of amino acids that determines the threedimensional structure of the protein A protein s specific structure determines its function In almost every case the function of a protein depends on its ability to recognize and bind to some other molecule 0 For example an antibody binds to a particular foreign substance 0 An enzyme recognizes and binds to a specific substrate facilitating a chemical reaction The function of a protein is an emergent property resulting from its specific molecular r er Three levels of structure rimary secondary and tertiar structures organize the fo ding Within a single polypepti e o Quaternary structure arises when two or more polypeptides join to form a protein The primary structure of a protein is its unique sequence of amino acids y structure results from hvdrogen bonds between the repeating of the polypeptide backbone o The weakly positive hydrogen atom attached to the nitrogen atom has an af nity for the oxygen atom of a nearby peptide bond 0 Each hydrogen bond is weak but the sum of many hydrogen bonds stabilizes the structure of part of the protein One secondary structure is the a helix a delicate coil held together by hydrogen bonding between every fourth amino acid 0 Some brous proteins such as ockeratin the structural protein of hair have the or helix formation over most of their length The other main type of secondary structure is the 5 pleated sheet 0 In this structure two or more regions of the polypeptide chain lying side by side are connected by hydrogen bonds between parts of the two parallel polypeptide backbones O Pleated sheets are found in many globular proteins and they dominate some brous proteins including the silk protein of a spider s web Tertiary structure is determined by interactions among various R groups 0 These interactions include hydrogen bonds between polar andor charged areas ionic bonds between charged R groups and hydrophobic interactions and van der Waals interactions among hydrophobic R groups Although these three interactions are relatively weak their cumulative effect helps give the protein a unique s ape Strong covalent bonds called disul de bridges that form between the sulfhydryl groups SH of two cysteine monomers act to rivet parts of the protein together 0 O Quaternary structure results from the aggregation of two or more polypeptide subunits o Collagen is a brous protein made up of three polypeptides that are supercoiled into a larger triple helix I The helix provides structural strength for collagen s role in connective tissue 0 Hemoglobin is a globular protein with quaternary structure I Hemoglobin consists of four polypeptide subunits two or and two 5 chains I Both types of subunits consist of primarily ochelical secondary structure I Each subunit has a nonpeptide heme component with an iron atom that binds oxygen Protein structure also depends on the physical and chemical conditions of the protein s environment 0 Alterations in pH salt concentration temperature or other factors can unravel or denature a protein 0 These forces disrupt the hydrogen bonds ionic bonds and disulfide bridges that maintain the protein s shape 0 Because it is misshapen a denatured protein is biologically inactive Most proteins become denatured if they are transferred from an aqueous environment to an organic solvent 0 The polypeptide chain refolds so that its hydrophobic regions face outward toward the solvent Other denaturation agents include chemicals that disrupt the hydrogen bonds ionic bonds and disulfide bridges that maintain a protein s shape Denaturation can also be caused by heat which disrupts the weak interactions that stabilize conformation o This explains why extremely high fevers can be fatal Proteins in the blood become denatured by the high body temperatures Some but not all proteins can return to their functional shape after denaturation o This suggests that the information for building a specific shape is intrinsic to the protein s primary structure 0 In the crowded environment inside a cell specific proteins may assist with the folding of other proteins Biochemists now know the amino acid sequences of more than 12 million proteins and the threedimensional shapes of about 8500 proteins 0 Nevertheless it is still difficult to predict the shape of a protein from its primary structure alone Most proteins appear to undergo several intermediate stages before reaching their mature structure The folding of many proteins is assisted by chaperonins or chaperone proteins 0 Chaperonins do not specify the final structure of a polypeptide but rather work to segregate and protect the polypeptide while it folds spontaneously 0 Molecular systems in the cell interact with chaperonins marking incorrectly folded proteins for refolding or for destruction Accumulation of incorrectly folded polypeptides is associated with many diseases including Alzeimer s disease and Parkinson s disease 55 Nucleic acids store and transmit hereditary information The amino acid sequence of a polypeptide is programmed by a unit of inheritance known as a gene A gene consists of DNA a polymer known as a nucleic acid There are two types of nucleic acids RNA and DNA The two types of nucleic acids are ribonucleic acid RNA and deoxyribonucleic acid DNA RNA and DNA are the molecules that enable living organisms to reproduce their complex components from generation to generation 0 DNA provides directions for its own replication 0 DNA also directs RNA synthesis and through RNA controls protein synthesis A nucleic acid strand is a polymer of nucleotides Nucleic acids are polymers made of nucleotide monomers organized as polynucleotides Each nucleotide consists of three parts a nitrogenous base a pentose sugar and a phosphate group The nitrogenous bases are rings of carbon and nitrogen that come in two types purines and pyrimidines o Pyrimidines have a single sixmembered ring of carbon and nitrogen atoms I There are three different pyrimidines cytosine C thymine T and uracil U 39 Thymine is found only in DNA and uracil is found only in RNA 0 Purines have a sixmembered ring joined to a vemembered ring I The two purines are adenine A and guanine G The pentose joined to the nitrogenous base is ribose in nucleotides of RNA and deoxyribose in DNA 0 The only difference between the sugars is the lack of an oxygen atom on carbon 2 in deoxyribose 0 Because the atoms in both the nitrogenous base and the sugar are numbered the sugar atoms are distinguished by a prime after the number 0 Thus the second carbon in the sugar ring is the 2 2 prime carbon and the carbon that sticks up from the ring is the 5 carbon 0 The combination of a pentose and a nitrogenous base is a nucleoside The addition of a phosphate group creates a nucleoside monophosphate or nucleotide Polynucleotides are synthesized when adjacent nucleotides are joined by covalent bonds called phosphodiester linkages that form between the 70H group on the 3 of one nucleotide and the phosphate on the 5 carbon of the next 0 This process creates a repeating backbone of sugarphosphate units with H J 39 quot ofthe quot bases The two free ends of the polymer are distinct 0 One end has a phosphate attached to a 5 carbon this is the 5 end 0 The other end has a hydroxyl group on a 3 carbon this is the 3 end Inheritance is based on replication of the DNA double helix An RNA molecule is a single polynucleotide chain DNA molecules have two polynucleotide strands that spiral around an imaginary axis to form a double 11th The sugarphosphate backbones of the two polynucleotides are on the outside of the helix 0 The two backbones run in opposite 5 9 3 directions from each other an arrangement referred to as antiparallel The two polynucleotides or strands are held together by hydrogen bonds between the paired bases and by van der Waals interactions between the stacked bases 0 Adenine A always pairs with thymine T and guanine G with cytosine C Chapter 10 Review Photosynthesis I Overview of the Process A Life on Earth is solar powered 1 Photosynthetic organisms can capture light energy from the sun and convert it to chemical energy stored in sugars and other organic molecules a Photosynthetic organisms are Autotrophs they can produce organic molecules from C02 and other inorganic raw materials obtained from the environment Autotrophs are the producers of the biosphere b Hetemtmphs live on organic compounds produced by other organisms II An Overview of Photosynthesis A What is photosynthesis 1 The process by which autotrophs eg plants convert sunlight into chemical energy 2 Overall reaction 6C02 12 H20 light 9 CaHian 602 6HzO B How does photosynthesis work 1 ExperiJnents using isolated chloroplasts and heavy isotopes of oxygen confirmed that the oxygen atoms released during photosynthesis come from water 2 Melvin Calvin found that radioactively labeled carbon dioxide is incorporated into carbohydrates even in the dark 3 Therefore photosynthesis consists of two independent reactions a The light reactions split water mallte ATP and reduce electron carriers NADPH b The lightindependent reactions use the energy in ATP and NADPH to x COz into carbohydrates Fig 101 C Where does photosynthesis occur 1 Photosynthesis occurs in leaf cells in organelles called chloroplasts Fig 102a 2 Chloroplasts arise from embryonic structures called proplastids Box 101 Fig 103 3 Structure of a chloroplast a Chloroplasts have two organelle membranes Fig 102b b The inside of the chloroplast is lled with membrane sacs called thylalltoids 1 A stack of thylalltoids is a granum plural grana Fig 102b 2 The uidfilled space around the grana is the stroma c Thylalltoids have a considerable amount of the bluegreen absorbing pigment chlorophyll in their membranes H How Does Chlorophyll Capture Light Energy A Light is full of electromagnetic energy 1 Light behaves like a wave a Visible light ranges in wavelength from 380 to 750 nm Fig 104 b Light with shorter wavelength has more energy than that of longer wavelength 2 Light behaves like a particle a Light is packaged in discrete units called photons b Each photon of a particular wavelength has a speci c amount of energy Pigments are molecules that can absorb the electromagnetic energy of light 1 When a photon of light hits a pigment one of three things can happen a The photon can be absorbed b The photon can be transmitted c The photon can be re ected 2 Most pigments absorb speci c wavelengths of light 3 The wavelengths that are not absorbed by a pigment are transmitted or re ected thus giving the pigment that color a Biologists can plot absorption spectrums that illustrate which wavelengths a given pigment will absorb and re ect Fig 107a 1 Chlorophyll a and b absorb red and blue wavelengths and they tranth green 2 Carotenoids absorb blue and green and they tranth yellow orange or red 4 Which wavelengths drive photosynthesis a Experiments using photosynthetic algae allowed biologists to plot an action spectrum b An action spectrum shows which wavelengths are effective at facilitating photo synthesis c Action spectra show that blue and red wavelengths are the most effective at driving photosynthesis Fig 107b The Role of Accessory Pigments 1 Carotenoids xanthophylls are common accessory pigments found in chloroplasts Fig 108a 2 They are responsible for leaf color in the autumn when chlorophyll begins to degenerate 3 They absorb photons belonging to the blue and green part of the spectrum and pass the energy on to chlorophyll thus functioning to extend the range of wavelengths that can drive photosynthesis 4 They also function to protect chlorophyll from free radical damage What does chlorophyll look like Fig 108b 1 A long isoprene tail anchors the molecule in the thylakoid membrane 2 A head with a large ring structure has a magnesium Mg atom in the middle What happens when a photon is absorbed by chlorophyll 1 When a photon hits chlorophyll the energy is transferred to an electron in the head region 2 This energy causes that electron to move to a higher orbital one with greater potential energy because it is farther from the atom s nucleus Fig 109 3 Once the electron is in the higher orbital there are several ways it can return to its resting state a If the electron sianly falls back to its original position the excess energy is released as light uorescence and heat Fig 1010 b The energy can be transferred to other chlorophyll molecules in the antenna complex Fig 1012a 1 When the energy is passed from one chlorophyll molecule to the next an electron is excited in response 2 As energy is transmitted the original electron falls back into its original orbital 3 The antenna complex acts to transmit the energy from speci c wavelengths of light to the receiver a special chlorophyll molecule called the reaction center 4 The antenna complex and the reaction center form a photosystem c What happens when an electron in the reaction center is excited 1 The excited electron is passed from the reaction center to a molecule that acts as an electron acceptor Fig 1012b 2 This redox reaction is the point at which electromagnetic energy is converted to chemical energy 111 The Light Reactions Harvesting the Sun s Energy and Capturing It in ATP and Electron Carriers A What is the nature of the photosynthetic reaction center and how does it work 1 Plants and algae have two distinct types of reaction centers a The two reaction centers were named photosysteml and II b When both systems are active photosynthesis runs at its peak efficiency B How Does Photosystem 11 Work Fig 1014 1 2 7 8 An electron in a chlorophyll P680 molecule is excited A molecule called pheophytin takes the electron from chlorophyll a Pheophytin is structurally similar to chlorophyll but lacks a magnesium atom in its head region b Pheophytin functions as an electron acceptor The electron is then passed from pheophytin through an electron transport chain a Similar to the electron transport chain found in mitochondria b Contains several qujnones and cytochromes c One particular qujnone plastoqujnone is not anchored to a protein and can shuttle electrons from pheophytin to the rest of the electron transport chain As plastoquinone passes the electron it also pumps protons into the interior of the thylakoid creating a proton gradient inside the thylakoid The protons can diffuse out of the thylakoid only through ATP synthase a ATP synthase uses the protonmotive force to synthesize ATP b The production of ATP by this process is called photophosphorylation The original chlorophyll molecule is oxidized after its excited electron was taken away Water is split by photosystem II and the electrons from water are used to reduce chlorophyll P680 Oxygen is released when water is oxidized C How Does Photosystem I Work Fig 1015 1 Heliobacteria produce NADPH when exposed to sunlight D Ho 1 2 In heliobacteria when the reaction center of photosystem II absorbs a photon of light the excited electron is eventually passed to ferredoxin An enzyme called ferredoxinNADP oxidoreductase transfers the electron and a proton to NADP to form NADPH w Do Photosystems II and I Interact Fig 1016 Robin Hill and Fay Bendall proposed that photosystem I and II interact in a speci c way The model they proposed is called the Z scheme Fig 1016 a When a photon of light is absorbed by chlorophyll P580 the excited electron is passed to pheophytin Pheophytin donates the electron to an electron transport chain Using the energy released by the reductionoxidation redox reactions occurring in the electron transfer chain plastoquinone pumps protons into the interior of the thylakoid ATP synthase uses the protonmotive force created by plastoquinone to generate ATP A small protein called plastocyanin accepts the electron from the electron transport chain and transfers it to a chlorophyll P700 molecule in photosystem I When a photon of light is absorbed by chlorophyll P700 in photosystem I the excited electron is passed to ferredoxin An enzyme called ferredoxinNADP oxidoreductase transfers the electron and a proton to NADPt to form NADPH Cyclic Photophosphorylation Fig 1017 b C d quot5 g a Intermittently photosystem I donates an electron to the electron transport chain of photosystem II b This results in the production of additional ATP but no NADPH c This occurs when extra ATP is needed Photosystems I and II are distributed differentially in individual grana Fig 1018 IV The Calvin Cycle Using the Energy in ATP and NADH to Reduce Carbon Dioxide A Ho 1 2 B 1 2 W does a plant cell use ATP and NADPH to reduce carbon dioxide to sugar To isolate the molecules produced as carbon dioxide is reduced to sugar Melvin Calvin fed radioactively labeled carbon dioxide to green algae Green algae were fed a pulse of 14C02 After a speci ed amount of time the cells were killed ground up into a crude extract and paper chromatography was used to isolate individual molecules at different time intervals Fig 1019 Results Immediately after the pulse a threecarbon compound called 3 phosphoglycerate 3PG containing 14C was isolated Conclusion Carbon dioxide is combined with a compound to form 3 phosphoglycerate What compound reacts with carbon dioxide to produce 3PG When no 2carbon compound in the system could be found Calvin proposed that a 5carbon compound reacts with carbon dioxide and then splits in two An overview Fig 1020
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