HOW CELLS RELEASE STORED ENERGYChapter 7
Energy-releasing PathwaysSun producers consumers (heterotrophs)Producers harvest the suns energy to make glucoseConsumers eat the producers (and other consumers) to obtain energy (glucose)Consumers break down glucose to convert its energy to ATP using aerobic (with O2) or anaerobic (without O2) respiration
The chemical equation for aerobic respiration shows its relationship to photosynthesisC6H12O6 + 6O2 6CO2 + 6H2O + ATPSee the small figure on page 120 showing the relationship between photosynthesis and aerobic respiration
Energy-releasing PathwaysBoth aerobic and anaerobic respiration start with glycolysis in the cytoplasmProduces a small amount of ATP
Aerobic respiration continues in the mitochondria with the Krebs cycle and electron transfer phosphorylation (This is the same as electron transfer chain. The book uses phosphorylation, I will refer to it as the electron transfer chain.)Produces a large amount of ATP
Fig. 7-2b, p.108
Fig. 7-3, p.109
QuestionsWhat are heterotrophs?Which type of respiration occurs in the presence of oxygen?What metabolic pathway is used by both aerobic and anaerobic respiration?Where does glycolysis occur?Where does the Krebs cycle occur?Which type of respiration produces the most ATP?
GlycolysisGlycolysis is a metabolic pathway that occurs in the cytoplasmGlucose is broken down through a series of intermediates to two pyruvate moleculesStudy unit 7.3 and figure 7.5The following slide highlights some important steps of the pathway
GlycolysisTwo ATP molecules are used to energize the rearrangement of glucose into two 3-carbon molecules called PGALFor glycolysis you have to spend some energy to earn some energyBoth PGAL molecules are rearranged through several intermediatesElectrons are stripped from PGAL producing NADHATP is produced when phosphate groups are transferred to ADP (phosphorylation)The final product is two pyruvate molecules
GlycolysisResults per glucose molecule2 ATP totalUsed 2 ATPMade 4 ATP2 NADH moleculesCo-enzymes carrying electrons and H+Will be shuttled to the electron transport chain2 pyruvate molecules
QuestionsWhat starting molecule is broken down during glycolysis?How much energy is required to get glycolysis started?After the input of energy what 3-carbon intermediate is formed?What is the final product of glycolysis?How much ATP is generated (gross and net)?How much NADH is generated?
Krebs CycleThe Krebs cycle is a cyclic metabolic pathway that occurs in the mitochondriaPyruvate is rearranged to form Acetyl-CoA which then enters the Krebs cycle where it is broken down in a series of steps to CO2Study unit 7.4 and figure 7.7The following slide highlights some important steps of the pathway
Krebs CyclePyruvate enters the mitochondrion (remember there are two pyruvate molecules for every glucose molecule)Pyruvate is changed to Acetyl-CoA by the following reactionsElectrons and H+ are stripped to make NADHCO2 is releasedCo-enzyme A is attachedAcetyl-CoA combines with oxaloacetate to form citrateSeveral rearrangements and intermediates result inTwo more CO2 molecules are released Three more NADH (and one FADH2, which is another electron carrying co-enzyme) form by stripping electrons and H+ from the intermediatesOne molecule of ATP is formedUltimately oxaloacetate is reformed to start the cycle again
Fig. 7-6a, p.113
Krebs CycleResults per glucose molecule (remember there are two pyruvates produced per molecule of glucose)Six CO2 molecules releasedAccounts for all six carbons found in glucose C6H12O6Eight NADH plus two FADH2 moleculesCo-enzymes carrying electrons and H+Will be shuttled to the electron transport chain Two ATP molecules
QuestionsWhere does the Krebs cycle occur?Prior to starting the Krebs cycle pyruvate is changed to what molecule?Acetyl-CoA is bound to ______ to form _____.During the Krebs cycle electrons and hydrogen atoms are stripped and carried by what two co-enzymes?How many ATP molecules are formed (per one glucose)?How many NADH molecules are formed (per one glucose)?
Electron Transfer Chain(aka: electron transfer phosphorylation)Electron transfer is an energy producing process that occurs over the inner mitochondrial membraneStudy unit 7.5 and figure 7.8The following slides highlight some important steps of the transfers
Electron Transfer Chain(aka: electron transfer phosphorylation)NADH and FADH2 carry the electrons and H+ to the electron transfer chainAs the electrons move through the chain they release small amounts of energy allowing the transfer chain to shuttle H+ over the membrane
Fig. 7-7b, p.114
Electron Transfer Chain(aka: electron transfer phosphorylation)An H+ gradient forms in the outer mitochondrial compartment (in between the two membranes)The resulting gradient propels H+ across the mitochondrial membrane through ATP synthasesThe flow has enough force to cause the synthases to attach phosphate to ADP, forming ATPThe process is called chemiosmosis or H+ electrochemical gradient
Fig. 7-7b, p.114
Electron Transfer Chain(aka: electron transfer phosphorylation)Once the electrons have moved through the electron transfer chain, they are accepted by O2 which is the terminal electron acceptorO2 combines the electrons and H+ to form waterO2 + H+ + electrons H2O
Fig. 7-7b, p.114
Electron Transfer Chain(aka: electron transfer phosphorylation)Results per glucose molecule32 molecules of ATP!Depending on the needs of the cell the amounts can fluctuateShifting concentrations of reactant, intermediates and productsShuttling mechanisms for moving NADH may use some ATP
QuestionsWhere is the electron transfer chain?What carries the electrons to the chain?What happens when energy is released by electrons moving through the electron transfer chain?The build up of a H+ concentration gradient provides the force to cause what important event?For aerobic respiration what is the terminal electron acceptor?How many molecules of ATP can be formed by the electron transfer chain (per glucose molecule)?
Energy-releasing PathwaysTotal possible for one molecule of glucose (cells dont always harvest this much as they may use the intermediates in other processes)Glycolysis = 2 ATPKrebs= 2 ATPElectron transfer = 32 ATPTotal = 36 ATP
See figure 7.9 for a summary of aerobic respiration
Fig. 7-8, p.115
Energy-releasing PathwaysAnaerobic respiration uses glycolysis, but due to the lack of oxygen Krebs and electron transfer are not usedInstead of oxygen being the terminal electron acceptor, an organic substance is used for NADH to donate the electrons (NADH must be recycled to NAD+ to be used in glycolysis again)Alcoholic fermentation (fig 7.10)Pyruvate is converted to acetaldehyde which accepts electrons from NADH producing ethanol and CO2Used to produce yeast breads and alcoholic beveragesLactate fermentation (figure 7.11)Pyruvate accepts electrons from NADH producing lactateMuscle cells use this pathway when they are not receiving enough O2Anaerobic pathways are referred to as fermentation pathways
Fig. 7-9b, p.116
Fig. 7-10a, p.117
Fig. 7-10b, p.117
Fig. 7-10c, p.117
Fig. 7-9c, p.116
Fig. 7-11, p.117
Alternative Energy SourcesGlucose is a type of carbohydrateCarbohydrates are an important part of the diet to provide energyProteins and lipids (fats) can also be used for energyThe molecules are broken down to form PGAL or Krebs cycle intermediatesThey can then be used to produce ATPSee figure 7.12
Fig. 7-12, p.119
QuestionsAnaerobic respiration can also be called _____.During anaerobic respiration what type of molecule becomes the electron accepter for NADH?What pathway is used by yeast in bread making?What pathway can be used by some muscle fibers?What other molecules can be used for energy?What are alternate energy sources generally broken down to?
SummaryAerobic respirationGlycolysisKrebsElectron transfer chainAnaerobic respiration/fermentationEthanolLactic acidAlternate energy sources