Unit 3 Cell Energetics Ch 8 9 and 10 Enzymes Cellular Respiration and Photosynthesis notes PDF

Unit 3: Cell Energetics Chapters 8, 9 & 10  Chapter 8: “Metabolism” Metabolism: All of an organism’s chemical processes Metabolic Pathways: Step-by-step series of enzyme-catalyzed reactions. Two main types: Catabolic: Release energy by breaking down complex molecules. (Ex...

Unit 3: Cell Energetics Chapters 8, 9 & 10  Chapter 8: “Metabolism” Metabolism: All of an organism’s chemical processes Metabolic Pathways: Step-by-step series of enzyme-catalyzed reactions. Two main types: Catabolic: Release energy by breaking down complex molecules. (Ex: cellular respiration) Anabolic: Consume energy to build complex molecules. (Ex: protein synthesis) CATabolic: Break Down Anabolic: Build Up Release energy Consume energy  ENERGY: BASIC PRINCIPLES Energy: Ability to do work (move matter against opposing forces like gravity) Cells cannot make or recycle energy. What is the ultimate source of their energy? The Sun Forms of Energy: Kinetic: energy of motion Potential: stored energy; based on position or arrangement (chemical) Energy Transformations: Energy can be converted from one form to another. Two laws of thermodynamics: First Law: Energy can be transformed but not created or destroyed. (Also called the Law of Conservation of Energy)  Second Law: Every energy transformation increases the entropy (chaos, disorder) of the universe. In other words: as energy is transformed, much of it is converted to heat, a random (low quality) state of energy Organisms must use energy to maintain organization in a random universe. Free Energy: Amount of energy available to do work for cells. Determines whether a reaction will occur spontaneously. Cells do 3 basic types of work: 1. Mechanical: movement (muscles, cilia...) 2. Transport: pumping substances across membranes 3. Chemical: exergonic vs. endergonic reactions ATP (Adenosine Triphosphate): Immediate source of energy to drive cell work. Made of adenine bonded to ribose bonded to 3 phosphate groups. Unstable triphosphate tail, broken by hydrolysis ATP + H2O ADP + Pi + energy (ADP = Diphosphate) Cells can renew ATP by phosphorylating the ADP again. Catabolic (exergonic) reactions provide the energy to re-make ATP and Anabolic (endergonic) reactions break down the ATP. These reactions are paired!  Activation Energy: Amount of energy needed to start a chemical reaction. Affects speed of reactions. (∆G = change in energy. ∆G is negative for exergonic reactions) Different reactions require different amounts of energy The Energy must produce 1 or both of the following effects: Increase collision rates of reactants Agitate the internal structure of a reactant enough to permit disruption of chemical bonds Ways to achieve the above effects: Raise temperature Increase concentration of reactants Increase pressure on reactants Add a catalyst  Catalysts: Chemical agents that accelerate a reaction without being permanently changed in the process. activation energy They function by lowering the amount of ______________ needed. Catalysts cannot speed up a reaction that could not take place on its own. Characteristics of Biological Catalysts: Called: enzymes All are: proteins Highly specific for a given reaction. Due to: shape of active site Many can catalyze reversible reactions in either direction. AffectedpH, by:temperature, conc. of reactants and produc ionic conc., and enzyme inhibitors (poisons) Names often end in “-ase”. Examples: https://www.youtube.com/watch?v ligase, catalase =qgVFkRn8f10  Specificity of Enzymes: substrate The ___________ active site binds to the enzyme’s _______ Active site = pocket or groove on enzyme, formed with only a few amino acids, the shape conforms to the substrate Two Theories Explain Enzyme Action: Lock & Key (wrong) Induced Fit Actions of Enzyme / Substrate Complex: Substrate binds active site of enzyme. Induced fit of active site: distorts substrate’s bonds Active site provides micro-environment: conducive to the chemical reaction Amino acid side chains: may participate in reaction Induced Fit Example Unbound Enzyme Bound Enzyme This is a molecular model of the This is carboxypeptidase A with a unbound carboxypeptidase A. The substrate (turquoise) bound in the active colored atoms show the approximate volume site. The active site is in the induced and shape of the active site. Note the zinc ion conformation. The same three amino acids (magenta) in the pocket of the active site. (Arg 145, Tyr 248, and Glu 270) are labeled to Three amino acids located near the active site demonstrate the shape change. (Arg 145, Tyr 248, and Glu 270) are labeled. Cofactors: Small molecules required for proper enzyme activity (NOT proteins). Inorganic Cofactors: metal atoms of zinc, copper, or iron Organic Cofactors: called coenzymes, most are vitamins Proper enzyme functioning requires that you get your vitamins and minerals, one way: Or another…  Enzyme Inhibitors: Chemicals that slow or stop enzyme activity. Irreversible (covalent bond) Or reversible (hydrogen bond) Competitive Inhibitors: resemble substrate, can bond to active site Noncompetitive Inhibitors: bond to another partmay be metabolic of enzyme poison & warp active(DDT), site many antibiotics are also noncompetitive inhibitors. Allosteric Regulation of Enzymes: Allosteric site: specific receptor site other than active site Usually only present on enzymes with 2 or more polypeptide chains. Have 2 conformations: active and inactive Binding of activator stabilizes active shape Enzyme activity changes in response to relative activators proportions of ________________ and inhibitors ________________. ←Allosteric site Chapter 9: Cellular Respiration Cells require energy to do from outside work: sources Remember: Matter (C-H-O) is ____________ recycled Energy is a one-way flow: Enters as __________ sunlight heat → leaves as ______ Energy in food is contained in bonds. When bonds break (________ catabolic reaction), energy is released but not used to do work directly. Instead, what it is the energy stored as? ATP Phosphorylation: Transfer of phosphate group(s) from ATP to other molecules. Causes the molecule to undergo a change that performs work. ATP must be re-made: _______________ Cellular Respiration powers the synthesis of ATP. Respiration is an Oxidation-Reduction Process ∙Chemical reaction involving the transfer of 1 or more ___________ electrons between reactants Also called: Redox ∙Oxidation = Loss of electrons (lowers energy) X = reducing agent, Y = oxidizing agent ∙Reduction = Gain of electrons (raises energy) Xe- + Y → X + Ye- C6H12O6 + 6O2 → 6CO2 + 6H2O ∙Oxygen is a good oxidizing agent because: It’s very electronegative (attracts electrons) ∙Electrons lose potential energy when shifted to a more electronegative atom. (releases energy) ∙Cellular respiration releases: 686 Kcal/mole glucose That’s a LOT!  Respiration is a Step-by-Step Process Energy is released as electrons (on hydrogens) glucose to ________ are transferred from ________ oxygen H transfer requires many steps. Each step catalyzed by a co-enzyme (accepts H’s): NAD+ (nicotinamide adenine dinucleotide) ∙NAD+ also called: oxidizing agent / e- acceptor  How NAD+ traps electrons from glucose and other foods: Enzymes called Dehydrogenases remove 2 sugar hydrogen atoms from the substrate ______. ∙Two hydrogens = 2 protons & 2 electrons ∙NAD+ bonds 1 proton & 2 electrons to be reduced to NADH Remaining proton is released to the surrounding solution as: Hydrogen ion (H+) ∙NADH stores the electrons’ energy until it can be released through an: electron transport chain Overview of Cellular Respiration: Can you write in the reactants and products? C6H12O6 + 6O2 → 6CO2 + 6H2O+ Energy (ATP) Stage Location Purpose Glycolysis Oxidation of glucose cytosol to pyruvate to gain 2 ATP Krebs Cycle Oxidation of Mitochondrial pyruvate (acetyl- matrix CoA) down to CO2 to Electron Transport gain 2 ATP Chain and Oxidative Inner Extract energy from Phosphorylation membrane of electron and H+ mitochondria flow to indirectly make up to 34 ATP 2 Different ways to make ATP Substrate level Phosphorylation (during glycolysis & Krebs). Small Amount of ATP is made when a phosphate group is transferred DIRECTLY from substrate to ADP. Oxidative Phosphorylation (at the end of the process). Large Amount of ATP made when a phosphate group is transferred INDIRECTLY to ADP after oxygen, ETC, and proton-motive forces are used.  GLYCOLYSIS: “Splitting of sugar” ∙10 Step process ∙No CO2 released as Glucose is oxidized to ______ pyruvate ∙Reactions occur in two phases: ∙Energy Investing: 2 ATP’s used Energy Yielding: (Per Glucose) 4 ATP (2 ATP NET GAIN!!!) 2 NADH  Energy Investment Phase: ∙Step1)Glucose is phosphorylated by: using 1 ATP Energizes glucose: makes it more reactive. Electric charge of PO4 traps glucose in cell – WHY? Membrane blocks charged particles Step 2) Glucose-6-phosphate is rearranged and converted to Fructose 6-phosphate. Step 3) Another ATP is added: Sugar now has phosphate groups at each end – it’s ready to be split Step 4) Sugar splits into two 3-C sugars: PGAL (glyceraldehyde phosphate) and its isomer. Step 5) An enzyme catalyzes the conversion of the isomer to PGAL because the next enzyme in glycolysis is unreceptive to the isomer. Remember: PGAL = G3P (3 Carbon sugar) Energy Yielding Phase: (Remember there are 2 PGAL’s per glucose) Step 6) Two reactions by one enzyme cause Sugar to oxidize (e- and H+ go to NAD) to: Form NADH -- exergonic ∙Phosphate group to attach to the oxidized substrate (came from inorganic phoshate) Step 7) Phosphate group just added is removed and joined to ADP (substrate level phosphorylation): remaining 3-carbon compouind is an acid (not sugar) By now, 2 ATP’s per glucose are made (ZERO net gain)  Step 8) An Enzyme relocates the remaining phosphate group in order to prepare the substrate for the next reaction. Step 9) An enzyme forms a double bond in the substrate by removing H2O: Makes Phosphoenolpyruvate: (PEP) Step 10)PEP transfers a phosphate group to ADP to make ATP, remaining substance is pyruvate.What is this process called? Substrate-level phosphorylation PER GLUCOSE, this step makes ___2  GLYCOLYSIS SUMMARY: Glucose _________________ 2 NAD+ + _____________ + _____________ 2 ATP → 2NADH +2_______________ ___________ ATP Net Gain! + _________________ 2 Pyruvates At the End of glycolysis: ∙Much of the glucose energy is still left in the pyruvates ∙Without oxygen, this energy is WASTED (all the cell can do is fermentation). ∙With oxygen, __________ pyruvate is transported into the mitochondria ____________ where _____________ oxidation is completed. Pyruvate “Shuttle” (Between Glycolysis & Krebs) As soon as pyruvate enters mitochondria, one carboxyl group (-COOH) is removed as: CO2 (Low energy -- released) and NADH (Stored energy) ∙Remaining 2-C acetyl group joins coenzyme A (CoA) to make it very reactive. Called: Acetyl CoA  KREBS CYCLE: Also called Citric acid cycle; Carbon pathway ∙Discovered by Sir Hans Krebs ∙Occurs in: mitochondrial matrix (M-Compartment) ∙Completes __________ oxidation of organic fuel (food). ∙Removes carboxyl groups as ____ CO2and _______NADH ∙Regenerates oxaloacetate: to re-do the cycle! ∙8 steps, controlled by enzymes Net Products per acetyl CoA (double numbers for “per glucose”): 3 NADH,1 FADH 1 ATP, 2 CO 2, 2  KREBS CYCLE: Step 1) Unstable acetyl CoA bond breaks, CoA is released, Acetyl group bonds to a 4-C oxaloacetate to form a 6-C Citrate Step 2) Water is removed, another is added back to convert citrate to its isomer: isocitrate Step 3) Isocitrate loses CO2 and remaining 5-C is oxidized to reduce:NAD+ to NADH Step 4) Multienzyme complex catalyzes: CO2 oxidation of remaining 4-C Removal of ______, reduce NAD+ to ______, compound to _______ NADH and attachment of CoA with a high energy bond to form succinyl CoA. Step 5) Phosphate group is transferred to GDP (→GTP) and then to ADP (→ATP). This is: Substrate-level phos. The remaining 4-C compound is succinate. Step 6) Succinate is oxidized into fumarate as: 2 Hs are transferred to FAD (→ FADH2) Step 7) Water is added to fumarate which rearranges its bonds to become Malate. Step 8) Malate is oxidized into oxaloacetate as: 1 H is transferred to NAD+ (→ NADH) Why is Step 8 “way cool”? oxaloacetate is re-generated to start  ***Don’t Forget: For each glucose, there are 2 “turns” of the Krebs cycle: 2 ____ATP produced by substrate ______________________ level phosphorylation Plus many high energy electrons on: 6 NADH and 2 FADH2 ELECTRON TRANSPORT AND OXIDATIVE PHOSPHORYLATION: Electron transport chain is made of electron carrier molecules embedded in the inner mitochondrial membrane. Each successive carrier has a higher electronegativity than the carrier before it so: energy is released as e- transfers. (Stored as potential energy – ATP is NOT made directly!) Most of the carrier molecules are proteins tightly bound to (nonprotein) cofactors which alternate between reduced and oxidized states as they accept and donate e-. Electron transport chain: NADH transfers e- to the first transport molecule in the membrane. The electron is then passed down the chain to the final electron acceptor: oxygen (1/2 O2) *FADH2 adds its electrons later in the chain so they: produce less ATP.  As molecular O2 is reduced, it also picks up 2 protons to form water. For every __________, 2 NADH’s one ______O2 is reduced to 2 H 2O ___________ molecules. The Electron transport chain does NOT produce ATP directly. It generates a _________ proton gradient across the inner mitochondrial membrane, which stores _______________ potential energy that can be used to ___________ ______________ phosphorylate ADP. Chemiosmosis: Proposed by Peter Mitchell, 1961 Explains how a H+ gradient produced during E.T.C. powers the synthesis of ATP. Site: inner mito. memb. where ATP synthase is embedded. (enzyme – helps make ATP) Hydrogens (from NADH and FADH2) are separated into H+ and e- & only the electrons are transferred through the chain. Exergonic flow of electrons pumps H+: from matrix to the intermembrane space H+ gradient = proton-motive force (can do work) It is an electrochemical gradient: (H+ conc. and charge conc.) ONLY H+ diffuses back through the membrane: through ATP synthase ATP synthase uses H+ gradient “current” to: phosphorylate ADP and make ATP. The mechanism by which this occurs is not fully known (Perhaps YOU will discover it one day!!!) Respiratory Poisons: Inhibit cellular respiration by disrupting chemiosmosis. Cyanide: blocks e- flow to O2 during E.T.C. inhibits ATP synthase Oligomycin (antibiotic): (in bacteria) Dinitrophenol (DNP): called “uncoupler”, makes lipid bilayer leaky to H+, E.T.C. energy turns to heat, cell “burns up” Uses of Chemiosmosis: Cellular respiration: oxidative phosphorylation Photosynthesis: photophosphorylation, (light energy drives the e- flow) Bacteria: (no mitochondria),create H+ gradients across plasma membranes (pump across memb) ATP Gain during all Steps of Cellular Respiration: ATP made by direct substrate level phosphorylation: Glycolysis = 2 ATP (net) Krebs cycle =2 ATP ATP made when chemiosmosis couples E.T.C. to oxidative phosphorylation: Maximum= 34 ATP Some energy is lost:as heat Eukaryotic cells are about 40-60% efficient (compared to cars – only 25%) Oxygen requirements: All organisms fit into one of three categories: Strict Aerobes: require oxygen (us) Strict Anaerobes: poisoned by oxygen, use respiration – sulfates or nitrates receive e- instead of O2 (some bacteria) Facultative Anaerobes: Use O2 if available  Anaerobic Respiration = Fermentation (See Figure 9.17, p. 175!) Starts with glycolysis:occurs in cytosol, no O2 needed, 2 ATP gained Steps after glycolysis do NOT yield ATP: only purpose is to recycle NAD+ Two main types of fermentation: ALCOHOL FERMENTATION: In Yeast NADH NAD+ 2 pyruvate --------------------> 2 ethanol + 2 CO2 LACTIC ACID FERMENTATION: Human Muscle, fungi, bacteria – (cheese & yogurt) NADH NAD+ 2 pyruvate --------------------> 2 lactic acid ↓ to the liver converted back to 2 pyruvate (wasting energy) Catabolism of other “chow”: Carbohydrates: hydrolyzed to glucose Proteins: hydrolyzed to amino acids, deamination, enter as pyruvates or later glycolysis Lipids: glycerol enters ___________ (as glyceraldehyde), fatty acids enter at acetyl CoA ____________ REMEMBER: Not all food is used for ATP, some provides carbon skeletons! Control of Respiration: Krebs cycle is controlled by ____________________. feedback inhibition Citrate and ATP can both inhibit phosphofructokinase (glycolysis enzyme). ADP activates it. What type of enzyme is it? Allosteric  Anaerobic Aerobic Stages Locations Reactants Products Role of NAD+/NADH Energy Yield  Anaerobic Aerobic Stages Glycolysis Glycolysis Fermentation Kreb’s Cycle Electron Transport Chain Location(s) Cytoplasm Cytoplasm & Mitochondria Reactants Glucose Glucose Oxygen Products Ethanol & CO2 CO2 & H2O or Lactic Acid (Muscle Cells) Role of NAD+/NADH “Recycled” NAD+ is reduced (gains e-) to NADH. NADH carries electrons to ETC Energy Yield Low (2 ATP) High (36 ATP) Chapter 10: Photosynthesis Organisms need organic compounds for energy and carbon skeletons Two types of organisms:heterotrophs – get organic compounds from other organisms autotrophs – make their own organic comp. Two types of autotrophs (“self-feeders”): Chemoautotrophs: obtain energy by oxidizing inorganic comps w/out light (rare, bacteria). Photoautotrophs: use light energy to produce organic compounds. Photoautotrophs include: plants, algae, some protists Humans rely on photoautotrophs for: food & oxygen Photosynthesis: Metabolic process which transforms light _____________ energy trapped by chloroplasts ____________ into ____________ chemical bond energy sugars stored in _____________ and other organic molecules. Makes energy-rich organic molecules from energy-poor molecules: CO2 and H2 O CO2 light Uses _________ as a carbon source and ________ as the energy source. LEAVES: All green plant parts have chloroplasts, but leaves are the main organs of photosynthesis in most plants. Leaf layers: Top to bottom ∙Cutin: wax layer (also called cuticle) ∙Upper epidermis: protects ∙Palisade layer: chloroplasts ∙Spongy layer: contains chloroplasts and air spaces ∙Lower epidermis: protects, contains stomata (openings) which are controlled by guard cells (w/ chloroplasts) Mesophyll: The area including the palisade and spongy layers. Veins are incorporated here. CHLOROPLAST PARTS: Enclosed by: double membrane Thylakoid: membrane pouches w/ chlorophyll (May be in grana stacks) Stroma: fluid surrounding thylakoids Photosynthetic Prokaryotes No chloroplasts (DUH!) Chlorophyll is built into plasma membrane or vesicle membrane Cyanobacteria have stacks of vesicle membranes: similar to grana Photosynthesis Overview: 6CO2 + 12H2O + Light Energy → C6H12O6 + 6O2 + 6H2O Water appears on both sides because it is newly formed during the process. To simplify, show only net change in water: 6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2 What does this remind you of??? It’s the opposite of respiration Now, reduce the formula to its simplest form: CO2 + H2O → [CH2O] + O2 Remember the C,H,O ratio for a sugar? 1:2:1 Photosynthesis is basically building a sugar: one carbon at a time! Splitting Water: O2 released came from the ________ People used to think the ______ CO2 plants take in. H 2O C.B. van Neil (1930’s) predicted the O2 came from ______ He discovered this while studying bacteria that use H2S instead of water to get their H’s. These bacteria release sulfur as waste. van Neil concluded: H2O is split to get H for photosynthesis. Later support: oxygen-18 tracer shows oxygen from H O 2 is released as O2 Mass Spectrometry equipment  Respiration Photosynthesis Electrons from sugar are Electrons from water are transferred to oxygen transferred to CO2 forming water forming sugar Sugar is oxidized CO2 is reduced into sugar Exergonic Endergonic Photosynthesis occurs in Two Stages: ∙ Light Reactions: convert solar energy to chemical energy (ATP and NADPH) Site: Thylakoid membranes ∙ Calvin Cycle (Light Independent Reactions): carbon fixation reactions reduce CO2 to carbohydrate using ATP & NADPH Site: Stroma fluid Photosynthetic Pigments: As light meets matter, it can be reflected, transmitted, or absorbed. Pigments: absorb visible light Different pigments absorb light of different wavelengths: each pigment has an “absorption spectrum” The wavelengths that are absorbed: “disappear.” The colors we see are those reflected! Spectrophotometer: measures the ability of a pigment to absorb various wavelengths. Chlorophyll a: Main pigment in chloroplasts (blue-green), temp. dependent Accessory pigments: Absorb light & transfer the energy to chlorophyll a. Chlorophyll b: yellow-green Carotenoids (family of pigments): various shades of yellow and orange Why have several pigments??? To absorb different (more) colors of light  Photooxidation of Chlorophyll When pigments absorb photons of light energy, Colors of absorbed light disappear from spectrum, but energy cannot disappear. electrons from its lowest energy Photon boosts one of the pigment molecule’s ___________ (ground state) state _______________________ to an orbital of higher potential energy (excited state) __________________________ Excited state is unstable: Without intervention, electron would fall back and release energy (as heat or fluorescence) Thylakoid membranes contain electron acceptors which trap the excited electrons before they can return to ground state. oxidized Chlorophyll is _____________ reduced while the electron acceptor is _______________. Photosystem Assembly: In thylakoid, 3 parts: Antenna complex = 100’s of pigment molecules Reaction center = only pair of chlorophyll a molecules which can donate e- to the e- acceptor Primary e- acceptor TWO TYPES OF PHOTOSYSTEMS: Differ in their location relative to specific proteins and e- acceptors. Photosystem I: Absorbs far red light best (700nm) Photosystem II: Absorbs red best (680nm) Two routes for Electron Flow: Once excited, electrons flow: Cyclic: Photosystem I only Non-Cyclic: Photosystems I and II During the light reactions, there are two possible routes for electron flow Cyclic Electron Flow: Simpler pathway, involves only photosystem I: generates only ATP (no O2 or NADPH) Pigments absorb energy and channel it to: the P700 reaction center P700 chlorophyll a’s electrons become excited, leave the molecule and are trapped by: a primary e- acceptor Electrons are passed along an ETC until returned to their ground state in P700: They CYCLE back to their start point. What happens to the energy released by ETC? It: Pumps H+ ions (into thylakoid from stroma) Creates proton-motive force H+ flow through ATP synthase in thylakoid membrane ATP is made This type of ATP production is called: cyclic photophosphorylation  NonCyclic Electron Flow: Involves both photosystem I and photosystem II (SAME) Pigments absorb energy and channel it to: the P700 reaction center (SAME) P700 chlorophyll a’s electrons become excited, leave the molecule and are trapped by: primary e- acceptor Electrons are passed to NADP+ with H+ from water = NADPH (High energy e-) The e- that left the chlorophyll a must be replaced with photosystem II electrons Light energy absorbed by P680 excites e- which are passed to the same ETC as cyclic electron flow until they reach P700 and replace the missing electrons: the ETC makes ATP (same as cyclic flow) The actual ATP production is the same as cyclic, but is called: noncyclic photophosphorylation The P680 e- are replaced by e- from: water WATER was split into: H+ (joined NADP+) e- (replaces P680 e-) O (joins another O to be released as O2) Cyclic and Non-cyclic Electron Flow: Why have both??? ATP Cavin cycle requires more ________ NADPH than ____________ noncyclic BUT ________________ equal flow makes roughly _________ ATP amounts. Cyclic flow makes the extra ___________ needed. ATP Synthesis: Respiration vs. Photosynthesis Both use chemiosmosis ATP synthase and many e- carriers are similar. Energy source is different: food vs. light Proton (pH) gradient: experiments show that chemiosmosis occurs in both Calvin Cycle: Uses the ATP and NADPH made during light reactions to reduce carbon dioxide to sugar. (also called Light Independent Reactions) Actual product = G3P (glyceraldehyde 3-phosphate) -- 3-C sugar also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as GADP, GAP or PGAL Cyclic because entry compound RuBP is re-generated. 3 CO2 must go through the cycle to get one sugar – WHY? Each CO2 provides one Carbon and G3P has 3 In addition to 3 CO2: 9 ATP and 6 NADPH are used Step 1: CO2 bonds to RuBP (5-carbon) with help from RuBP carboxylase: (Rubisco) enzyme -- very abundant protein in plants Step 2: The unstable 6-carbon compound then splits Step 3: Each 3-carbon piece receives: a Phos. group from ATP Step 4: NADPH adds e- pair to each piece to reduce it to a 3-carbon sugar (G3P)  3 Recycling Steps: For every ______ 3 CO ’s enter, “turns of the cycle” ______ 2 ______ 1 G3P is gained for use and ______ 3 RuBP’s are re-made. Products of Photosynthesis: Can be used for energy or carbon skeletons Sugars typically leave the leaf as: sucrose Most abundant organic molecule in plants: cellulose (structures) Energy storage in plants = starch in amyloplasts, roots, tubers, and fruits PHOTORESPIRATION: A process which reduces the sugar yield of photosynthesis On hot, dry days: plants close stomata to conserve water Oxygen builds up in leaves and CO2 decreases. O2 competes with CO2 for the active site on Rubisco (enzyme) O2 CO2 When ______ is bonded in place of __________, the 2-carbon product is CO2 oxidized to _______ H2O (in peroxisomes) and ________ This is bad because RuBP (organic matter) is taken out of the Calvin cycle. Believed to be: an evolutionary relic (There was no O2 on early Earth when this process evolved.) Methods of Minimizing Photorespiration: C4 Plants: Unlike typical (C3) Plants, C4 Plants use PEP: PEP binds CO2 in mesophyll (fixes into 4-C malate) and shuttles it to rubisco. PEP cannot bond to O2  CAM Plants (Crassulacean Acid Metabolism): Open stomates at night, take in CO2 and store the carbons and oxygens bonded into organic acids until morning.