One last look at the light reaction - Georgetown High School



Pathways that Harvest and Store Chemical EnergyKey PrinciplesLiving is workAlmost all the energy to do work ultimately comes from the sunSome organisms can store the energy from the sun in an anabolic reaction called photosynthesisEnergy to do work is stored in organic compoundsCells degrade organic compounds to get the energy they need for liveThis catabolic reaction is called cellular respirationFive principles governing metabolic pathways:Chemical transformations occur in a series of intermediate reactions that form a metabolic pathway.Each reaction is catalyzed by a specific enzyme.Most metabolic pathways are similar in all organisms.Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy MetabolismIn eukaryotes, many metabolic pathways occur inside specific organelles.Each metabolic pathway is controlled by enzymes that can be inhibited or activated.Figure 6.1 The Concept of Coupling Reactions We’re going to start with cellular respiration: So what’s the point of cellular respirationALL living things need ATPSome need more than othersCellular respiration allows a living thing to efficiently harvest a lot of ATP (up to 38) from one sugar moleculeKey DefinitionsFermentation – sugar is partially broken up to produce some energy. This takes place without oxygen presentCellular Respiration – oxygen is used to completely consume sugar molecules. CR extracts much more energy than fermentation and occurs much more often (more efficient therefore used more in nature)An overviewLight energy goes into the ecosystem. LE is used by photosynthesizers to make organic moleculesCR in all organisms break down those organic molecules to make ATP (and heat)Cellular RespirationOrganic + Oxygen Carbon + Water + Energycompounds dioxideORC6H12O6 + 6 O2 6 CO2 + 6H2O + Energy (ATP + heat)Another way to look at itYou need energy in a small and usable form for the cells to be able to utilize it just like you need money in a small and usable form in order to utilize a vending machine.Your cells use the mitochondria in a process that changes the large bills (glucose) into coins (ATP) that can be used. Cellular Respiration is that process.Another way to look at itAnother way to look at itAnother way to look at itRedox ReactionsThe relocation of electrons releases the energy stored in food moleculesThis movement of electrons occurs in oxidation-reduction reactions. (Redox for short)In a redox reaction, there are 2 reactants …one acts as an electron acceptor and one acts as an electron donor.OxygenOxygen is a powerful oxidizing agent. (It can really pull the electrons in.)Relocating the electrons in a reaction will release energy that can be put to workThink of the burning of fuel…it’s a redox reaction that uses oxygen to pull electrons away from the fuel. The result is carbon dioxide, water and lots of energy. This is why fire is hot and requires oxygen in order to progress. Electrons “fall” from organic molecules to oxygen during cellular respirationIn cellular respiration, glucose is oxidized, releasing energy.In the summary equation of cellular respiration: C6H12O6 + 6O2 6CO2 + 6H2OGlucose is oxidized, oxygen is reduced, and electrons loose potential energy.Molecules containing hydrogen are excellent fuels because their bonds are a source of “hilltop” electrons that “fall” closer to oxygen.However, these fuels do not spontaneously combine with O2 because they lack the activation energy.Enzymes lower the barrier of activation energy, allowing these fuels to be oxidized slowly.Things Exploding…glad it’s not usIf we oxidized glucose in one step it would literally be like putting a match to gasoline. The reaction would be uncontrolled and release all the energy all at once and we’d explode. So it occurs in a step wise fashion using a coenzyme to temporarily shuttle the electrons.Nicotinamide adenine dinucleotide (NAD+)NAD+ is a coenxyme that works as an oxidizing agent (it’s reduced)As NAD+ is reduced, it becomes NADHThe process of cellular respirationThree stages of Cellular Respiration (CR)GlycolysisThe Krebs cycleThe electron transport chain and oxidative phosphorylationGlycolysisLiterally means “splitting of sugar”Glucose has 6 carbonsGlycolysis makes that 6 carbon sugar into two 3-carbon molecules.It takes 10 steps. Glycolysis happens in the cytoplasmThis step needs no oxygen. This step will take place whether oxygen is present or not.No CO2 is formedThe process nets 2 ATPIt also produces 2 NADH and 2 Pyruvate that can be made into ATP during the next 2 steps of cellular respiration provided oxygen is presentMoving OnWe’ve gotten less than 25% of the energy from glucose at this point. The rest is stored in the pyruvate and some can be extracted during the next step – Kreb’s cycleTo prepare for Kreb’s cycle, a couple of things must happen to the pyruvatePyruvate Acetyl CoAThe pyruvate is pulled into the mitochondria (remember that glycolysis occurs in the cytoplasm) Pyruvate’s carboxyl’s group is removed and given off as CO2NAD NADH through oxidation to produce an acetate molecule.Coenzyme A is attached to make Acetyl Coenxyme A or Acetyl CoAPyruvate Oxidation It produces an acetyl CoA for every pyruvateIt also produces one NADH for every pyruvate It produces carbon dioxide as a waste.TOTALS so farNADH = 42 from glycolysis2 from pyruvate oxidationATP = 22 from glycolysis CO2 = 22 from pyruvate oxydationKreb’s Cycle8 steps, each using a specific enzyme Kreb’s cycleKreb’s cycle takes place in the matrix of the mitochondriaThe cycle turns 2 times for every glucose molecule.NumbersFor you accounting folks, here’s where we are:10 NADH (2 from glycolysis, 2 from pre-Kreb’s step, 6 from Kreb’s cycle)2 FADH2 (from Kreb’s)4 ATP (2 from glycolysis and 2 from Krebs)6 CO2 (2 from pre-Kreb’s step, 4 from Kreb’s cycle)TOTALS so farNADH = 102 from glycolysis2 from pyruvate oxidation6 from Kreb’s cycleFADH2 = 22 from Kreb’s cycleATP = 42 from glycolysis 2 from Kreb’s cycleCO2 = 6 2 from pyruvate oxydation4 from Kreb’s cycleElectron Transport Figure 6.12 Electron Transport and ATP Synthesis in MitochondriaOnly 4 ATP molecules are produced as a result of glycolysis and Kreb’s cycle.Quite a bit more ATP comes from the energy in the electrons carried by NADH (and FADH2).The energy in these electrons is used in the electron transport system to power ATP synthesis.Inner Membrane of MitochondriaThousands of copies of the electron transport chain are found in the cristae (inner membrane of the mitochondrion)Most components of the chain are proteins that can alternate between reduced and oxidized states as they accept and donate electrons. Electrons drop in free energy as they pass down the electron transport chain. This is because the electrons are repositioned closer and closer to oxygen.Beginning of the Electron Transport ChainElectrons carried by NADH are transferred to the first molecule in the electron transport chain, flavoprotein.The electrons continue along the chain that includes various proteins and one lipid carrier.The electrons carried by FADH2 have lower free energy and are added to a later point in the chainFunction of the ETCIts function is to break the large free energy drop from food to oxygen into a series of smaller steps that release energy in manageable amounts. It uses these small releases of energy to pump H ions out of the matrix and into the innermembrane spaceThe electron transport chain generates no ATP directly. Instead ATP production takes place immediately after ETC. This step is often called oxidative phosphorylation. Since it always happens immediately after the ETC they are often referred together.ATP ProductionA protein gradient is produced by the movement of electrons along the electron transport chain.The ETC uses the exergonic flow of electrons to pump H+ from the matrix to the intermembrane space.This concentration of H+ is the proton-motive force.Electron Transport ChainETC breaks the fall of electrons to oxygen into several steps each releasing a small amount of energy each step. That energy is transformed into ATP.Think of the ETC as a tall hill, entering at the top is NADH.Electrons are passed from carrier to carrier. At the bottom oxygen catches the electrons to make water. What keeps the electrons moving is that each carrier is more electronegative than the one before it…thus attracting the electronsBut where’s the ATP?A protein complex, ATP synthase, in the cristae are the only place that will allow H+ to diffuse back to the matrix.As the H+ flows back into the matrix ATP synthase generates ATP from ADP and Pi.So ATP synthase is an enzyme that uses the proton motive force to make ATP.ChemiosmosisChemiosmosis is a scary sounding word that refers to a process that uses energy stored in the form of an H+ gradient across a membrane to drive cellular work.Cellular Respiration uses Chemiosmosis to make ATP at the end of the ETC. 1. Fermentation enables some cells to produce ATP without the help of oxygenGlycolysis generates 2 ATP whether oxygen is present (aerobic) or not (anaerobic).Fermentation can generate ATP from glucose as long as there is a supply of NAD+ to accept electrons.Alcohol FermentationLactic Acid FermentationDuring lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate (ionized form of lactic acid). Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt.Muscle cells switch from aerobic respiration to lactic acid fermentation to generate ATP when O2 is scarce.The waste product, lactate, may cause muscle fatigue, but ultimately it is converted back to pyruvate in the liver.Facultative AnaerobesSome organisms including yeast and many bacteria, can survive using either fermentation or respiration.At a cellular level, human muscle cells can behave as facultative anaerobes, but nerve cells cannot.For facultative anaerobes, pyruvate is a fork in the metabolic road that leads to two alternative routesFermentation vs. cellular respirationAlike – both use glycolysisBoth use NAD as an acceptorBoth produce ATPDifferentFermentation – NAD is regenerated using an organic molecule; CR – NAD is regenerated by oxygenCR – WAY more ATP is generatedEvolution of Metabolic PathwaysGlycolysis is a metabolic heirloom. Believed to be the most ancient way to achieve ADP phosphorylation (to make ATP) because:Location is cytosol pre-dates need for membrane-enclosed structuresTaking place in anaerobic conditions - much like what early atmosphere was before photosynthesizing bacteria were present.Versatility of Metabolic PathwaysGlucose is not the only energy-containing molecule that can serve as a reactant in cellular respiration. Figure 6.14 sums up the way that the other major energy-containing macromolecules enter the cellular respiration equation.Enzymatic Control of Metabolic PathwaysCells use negative feedback to control ATP productionOne of the most important steps takes place in glycolysis and uses an enzyme called phosphofrucokinase. This enzyme makes a product that is then irreversibly committed to the glycolytic pathway. So, this is the point of no return…we have to control this enzyme to control the overall speed of cellular respiration ATP noncompetitively inhibits phosphofructokinaseSo does citrate.AMP stimulates phosphofrutokinase.Photosynthesis Where – Takes place in the chloroplast. Mostly in leaf cells, mostly in the mesophyll (middle layer of leaf)Overall reaction:6CO2 + 6H2O C6H12O6 + 6O2Carbon dioxide comes in and Oxygen goes out through the stoma. (stomata = plural)Water is absorbed from the rootsPhotosynthesis is has 2 partsThe light reactions convert solar energy to chemical energy.The Calvin cycle takes CO2 from the atmosphere incorporates it into an organic molecule and uses energy from the light reaction to reduce the new carbon molecule into glucose.Let’s take a closer look at chloroplastsA typical mesophyll cell has 30-40 chloroplasts Each chloroplast has two membranes around a central space, called the stroma.In the stroma are membranous sacs, called thylakoids.These have an internal space filled with water called the thylakoid lumen or thylakoid space.Thylakoids may be stacked into columns called grana.Light ReactionWhere – thylakoid membranesWhat – Takes sunlight and uses it to split water. What results – The electrons from water are given to an electron acceptor called NADP to make NADPH. Also, an ATP molecule is made. This is called photophosphorylation (wow!)The oxygen (from the water) is given off.What next – The Calvin cycle uses the NADPH and ATP.NADP?? Are you trying to confuse meRemember NAD and NADH from cellular respiration…well, NADP is close enough to confuse you.NADP is very much like NAD, it just has an extra phosphate groupFunctions like NAD as well in that it is an electron acceptor. NADP becomes NADPH as it is reduced.(Just remember that the one with the P is in photosynthesis. )When light meets matter, it may be reflected, transmitted, or absorbed.Different pigments absorb photons of different wavelengths.A leaf looks green because chlorophyll, the dominant pigment, absorbs red and blue light, while transmitting and reflecting green light. Light is a form of electromagnetic radiation, which travels as a wave but also behaves as particles (photons).Photons can be absorbed by a molecule, adding energy to the molecule—it moves to an excited state.The light reaction can perform work with those wavelengths of light that are absorbed.In the thylakoid are several pigments that differ in their absorption spectrum.Chlorophyll a, the dominant pigment, absorbs best in the red and blue wavelengths, and least in the green. Other pigments with different structures have different absorption spectra.Absorption of lightWhen a molecule absorbs a photon, one of that molecule’s electrons is elevated to an orbital with more potential energy.The electron moves from its ground state to an excited state.It will fall back down to it’s ground state very quickly (one billionth of a second). When it falls to ground state it will give off heat and sometimes light.In chlorophyll it is prevented from falling down to ground state by having an electron acceptor available to take the electron.Isolated ChlorophyllAn isolated chlorophyll molecule ( not a part of a chloroplast) when hit with a photon will experience the electron moving to an excited state and then as the electron moves back to ground state, the chlorophyll will glow red and release heatSo instead of isolated…Chlorophyll is a contained within a photosystem found on the membrane of the thylakoid.PhotosystemsThere are two photosystems. See top of page 184 and fig. 10.11 on page 185 to see a photosystem. Here’s what they are – a complex of proteins that includes a few hundred pigment molecules. One of the pigment molecules is located in the reaction center along with a primary electron acceptorHere’s how they work – one pigment molecule absorbs a photon. Excited electron is passed from pigment molecule to pigment molecule, until it reaches the reaction center.At that point to electron is passed to a primary electron acceptor (in the reaction center.)Because the electron is captured by the primary acceptor, it leaves an electron “hole” in the pigment molecule in the reaction center that must be filled Let’s walk through the light reaction occurs:Step 1: Photosystem II absorbs light, it bounces until it reaches the reaction center and is passed to primary acceptor. Step 2 The “hole” left by the electron leaving must be filled. Water is split to use the electrons from hydrogen. The oxygen is given off as a waste product.light reaction continued:Step 3: the electron passes from the primary electron acceptor at photosystem II to photosystem I via an electron transport chain. Step 4: As the electron falls down the ETC, the thylakoid membrane is used to synthesize ATP (remember, pumping H ions out and letting them back through an ATP synthase molecule) Step 5: Meanwhile Photosystem I has also absorbed a photon and passed an excited electron around until it reaches the pigment molecule at the reaction center. It is passed to a primary electron acceptor and the electron “hole” in the pigment molecule is filled by the electron from the ETC. (from photosystem II)Step 6: The primary electron acceptor from photosystem I passes the electrons to a second ETC. The ultimate electron acceptor is NADP and it becomes NADPH.Steps 1 – 6 : This is called noncyclic electron flow because it requires light and water to start it each time. The ATP made is called noncyclic photophosphorylationWhen the cell has too many NADPH, it will switch to cyclic electron flow which just uses photosystem I and only generates ATP. (Makes no NADPH, no Oxygen and uses no water). It still requires light. . This is called cyclic photophosphorylationNoncyclic CyclicOne last look at the light reactionRequires lightMakes oxygenUses waterMake ATP from ADP and inorganic PMakes NADPH from NADPTakes place in the thylakoid membraneCalvin CycleWhere – in the stromaWhat – Like the Krebs cycle; has 3 major parts: carbon fixation, reductions and regeneration of CO2 acceptor.Input– 3 molecules of CO2, 6NADPH, & 9ATP Output – 1 G3P (3-carbon sugar)G3P is later combined with another G3P to make glucose…but that’s not part of the Calvin cyclePhase 1: Carbon Fixation3 molecules of carbon dioxide are attached to RuBP. It uses rubisco as the enzyme for this reaction.Phase 2: ReductionPhosphate from ATP is added to the moleculeNADPH reduces the resulting molecule to make G3PEvery 3 molecules of carbon dioxide with the RuBP will make 6 G3P. 5 of them must be recycled to make RuBP. The other one leaves Calvin cycle as the product.Phase 3: Regeneration of the CO2 acceptorThe 5 G3P that are not given off are reorganized using ATP to result in 3 RuBP.Count the Carbons!!Start with 15 (three 5-carbon molecules RuBP) Add 3 from atmosphere (3 CO2)After Reductions you have 18 (six 3-carbon molecules) (6 G3P) 3 (one 3-carbon) goes to make ? glucose and 15 (five 3-carbon) get reshuffled make three 5-carbon (15)?Adaptations for hot, dry climatesAlternative mechanisms of carbon fixationThese alternatives are metabolic adaptations and have evolved in hot, arid climates.The problem: CO2 comes in through the stoma Stomata must close during the hot, dry part of the day to prevent water loss from a plant.When the stomata close, the plant not only can’t get the CO2 that it needs for photosynthesis but the oxygen builds up. PhotorespirationNormally, the initial fixation of carbon occurs via rubisco, the Calvin cycle enzyme that adds CO2 to RuBP.This occurs in most plants, called C3 plants because they first organic product of carbon fixation is a three-carbon compound. However, rubisco will bind to O2 just as readily as CO2. The product is waste (actually gets broken down by peroxisomes Neither oxygen, carbohydrates nor ATP and NADPH are produced. This is called photorespiration. Preventing PhotorespirationTwo methods: C4 plants and CAM plantsC4 plants CAM plants The stoma are only opened at night. CO2 is picked up at that time and stored in the form of an organic acid or stored in a separate location within the plantLater (during the day) the CO2 is then incorporated into the Calvin cycle. ................
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