Test your reflexes!!! So all this stuff is about energy??? Who s got the energy to try this one??? Gamers? Check it out.

Transcription

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2 Test your reflexes!!! So all this stuff is about energy??? Who s got the energy to try this one??? Gamers? Check it out.

3 Introduction Living is work. To perform their many tasks, cells require energy from outside sources. In most ecosystems, energy enters as sunlight. Light energy trapped in organic molecules is available to both photosynthetic organisms and others that eat them. Fig. 9.1

4 1. Cellular respiration and fermentation are catabolic, energy-yielding pathways Organic molecules store energy in their arrangement of atoms. Enzymes catalyze the systematic degradation of big organic molecules that are rich in energy to smaller waste products with less energy (catabolic, exergonic, right?) Some of the released energy is used to do work and the rest is dissipated as heat.

5 Cellular respiration is similar to the combustion of gasoline in an automobile engine. The overall process is: Organic compounds + O 2 -> CO 2 + H 2 O + Energy Carbohydrates, fats, and proteins can all be used as the fuel, but it is traditional to start learning with glucose, as it is the most common of these, and proteins are rarely used. C 6 H 12 O 6 + 6O 2 -> 6CO 2 + 6H 2 O + Energy (ATP + heat) The catabolism of glucose is exergonic, delta G = kcal per mole of glucose. Some of this energy is used to produce ATP that will perform cellular work.

6 3. Redox reactions Reactions that result in the transfer of one or more electrons from one reactant to another are redox reactions. Let s see what you remember from your favorite course last year: The loss of electrons is called???????. The addition of electrons is called???????.

7 Oxygen is one of the most potent oxidizing agents (which means that it gets?). An electron loses energy as it shifts from a less electronegative atom to a more electronegative one. A redox reaction that relocates electrons closer to oxygen releases chemical energy that can do work (like phosphorylate an ADP to make ATP).

8 4. Electrons fall from organic molecules to oxygen during cellular respiration In cellular respiration, glucose and other fuel molecules are oxidized, releasing energy. In the summary equation of cellular respiration: C 6 H 12 O 6 + 6O 2 -> 6CO 2 + 6H 2 O Glucose is oxidized, oxygen is reduced, and electrons lose potential energy. Molecules with lots of hydrogen are excellent fuels as their bonds are a source of energetic hilltop electrons that fall down to oxygen, ending up in water.

9 5. The fall of electrons takes a whole bunch of steps. Glucose and other fuels are broken down gradually in a pathway a series of steps, each catalyzed by a specific enzyme. At key steps, hydrogen atoms are stripped from glucose and passed first to a coenzyme like NAD + (nicotinamide adenine dinucleotide). Part of the NAD molecule is niacin, one of the B vitamins listed on your cereal box.

10 This changes the oxidized form, NAD +, to the reduced form, NADH, or NADH 2, or NADH + H + NAD + functions as the oxidizing agent in many of the redox steps during the catabolism of glucose. Fig. 9.4

11 The electrons carried by NADH lose very little of their potential energy in this process. This energy is tapped to synthesize ATP as electrons fall from NADH to oxygen. Again, electrons from molecules like glucose will end up being grabbed by oxygen the oxygen you breathe!!

12 Overview of respiration Respiration occurs in three metabolic stages: glycolysis, the Krebs cycle, and the electron transport chain involving oxidative phosphorylation. Fig. 9.6

13 2.A.2.f.1. Explain the connection between the following in one sentence: Glycolysis, glucose molecules, free energy, ATP, ADP, inorganic phosphate, pyruvate. 2.A.2.f.2. Where does pyruvate go once it s made?

14 2. Glycolysis oxidizes glucose to pyruvate: it occurs in the cytoplasm. During glycolysis, glucose, a six carbon-sugar, is split into two, three-carbon sugars. These smaller sugars are oxidized and rearranged to form two molecules of pyruvate. Each of the ten steps in glycolysis is catalyzed by a specific enzyme. These steps can be divided into two phases: an energy investment phase and an energy payoff phase.

15 In the energy investment phase, ATP provides activation energy by phosphorylating glucose. This requires 2 ATP per glucose. In the energy payoff phase, ATP is produced by substrate-level phosphorylation and NAD + is reduced to NADH. 4 ATP (gross) and 2 NADH are produced per glucose. Watch Fig. 9.8

16 Fig. 9.9a

17 Fig. 9.9b

18 The net yield from glycolysis is 2 ATP (made 4, used 2) and 2 NADH per glucose. No CO 2 is produced during glycolysis. Glycolysis occurs whether O 2 is present or not, therefore these reactions are said to be anaerobic. If enough dissolved O 2 is present in the cell, pyruvate moves into the mitochondrion to the Krebs cycle and the energy stored in NADH can be converted to ATP by the electron transport system and oxidative phosphorylation.

19 3. The Krebs cycle completes the energy-yielding oxidation of organic molecules: a closer look More than three quarters of the original energy in the glucose molecule we started with is still present in two molecules of pyruvate. The rest is in the 2 ATP s and 2 NADH s produced, and some has been lost as heat. If oxygen is present, pyruvate enters the mitochondrion where enzymes of the Krebs cycle complete its oxidation into carbon dioxide.

20 As pyruvate enters the mitochondrion, a multienzyme complex modifies it to acetyl CoA which enters the Krebs cycle in the matrix. A carboxyl group is removed as CO 2, causing this reaction to be called oxidative decarboxylation. A pair of electrons is transferred from the remaining two-carbon fragment to NAD + to form NADH. The oxidized fragment, acetate, combines with coenzyme A to form acetyl CoA. Fig. 9.10

21 The Krebs cycle is named after Hans Krebs, who was largely responsible for elucidating its pathways in the 1930 s. Watch here This cycle begins when 2C acetate (vinegar) from acetyl CoA combines with 4C oxaloacetate to form 6C citrate (citric acid, like in orange juice). Ultimately, after several reactions, the oxaloacetate is recycled and the acetate is broken down to CO 2. Each cycle produces one ATP by substrate-level phosphorylation, three NADH, and one FADH 2 (another electron carrier) per acetyl CoA. Since our original glucose gave rise to 2 acetyl CoA s, two Krebs cycle sets of reactions will result.

22 2.A.2.f.3. Explain the connection between the following in one sentence: Krebs cycle, carbon dioxide, organic intermediates, ATP, ADP, inorganic phosphate, substrate level phosphorylation, coenzymes. 2.A.2.f.4. Explain the role of NADH and FADH2.

23 The Krebs cycle consists of eight steps. These rxns occur in the matrix, the fluid inside the inner membrane. The presence of the enzymes necessary for the reactions in the matrix is an example of how structure is related to function. Fig. 9.11

24 The conversion of pyruvate and the Krebs cycle produces large quantities of electron carriers. Remember, the electrons were originally part of the glucose molecule we started with. Fig. 9.12

25 2.A.1.c. Explain how energy-related pathways in biological systems are sequential and may be entered at multiple points in the pathway by using one of the examples below: Krebs cycle, Glycolysis, Calvin cycle, or Fermentation

26 4. The electron transport chain (ETC) produces most of the ATP Only 4 of 38 ATP ultimately produced by the aerobic breakdown of one glucose molecule are derived from substrate-level phosphorylation. The vast majority of the ATP comes from the energy in the electrons carried by NADH (and FADH 2 ). The energy in these electrons is used in the electron transport system to power ATP synthesis. Let s watch here.

27 Because it is folded, thousands of copies of the electron transport chain are found in the extensive surface of the cristae, the inner membrane of the mitochondrion. This is another classic example of how structure is related to function in the mitochondrion. Pay attention for another in a few minutes. Most components of the chain are proteins that are bound with prosthetic groups that can alternate between reduced and oxidized states as they accept and donate electrons. Many are of a type called cytochromes.

28 Significant Release of Potential Energy As they pass down the electron transport chain, electrons drop in free energy, much like our favorite geometry teacher winning the mud pit high dive competition at the Gormley family reunion,

29 Electrons carried by NADH are transferred to the first molecule in the electron transport chain, flavoprotein. The electrons continue along the chain, which includes several cytochrome proteins and one lipid carrier. The electrons carried by FADH 2 have lower free energy and are added to a later point in the chain. Fig. 9.13

30 Electrons from NADH or FADH 2 ultimately pass to oxygen, the so-called final electron acceptor, causing this to be called aerobic respiration. The electron transport chain generates no ATP directly. The movement of electrons along the chain does contribute to a process called chemiosmosis, which leads to ATP synthesis by oxidative phosphorylation (or ox-phos as it s referred to in small talk at biologists cocktail parties). Here s how it works:

31 A proton gradient is produced by the movement of electrons along the electron transport chain, because several chain molecules can use the exergonic flow of electrons to pump H + from the matrix to the intermembrane space. The gradient produced involves more protons in the intermembrane space than in the matrix. Since this space is small, it is easy to build up a steep gradient, another example of which theme? This gradient represents a form of potential energy, very similar to the one in a flashlight battery. This concentration of H + is the proton-motive force.

32 Fig. 9.15

33 A protein complex, ATP synthase, in the cristae actually makes ATP from ADP and P i. The energy of the proton gradient is used as the source of power to do the work of ATP synthesis. How about another look at the animation? Fig. 9.14

34 The ATP synthase molecules are the only place that will allow H + to diffuse back to the matrix. This exergonic flow of H + through the protein complex is used by the enzyme to generate ATP. This coupling of the redox reactions of the electron transport chain to ATP synthesis is called chemiosmosis. The energy from glucose electrons was used to move protons across a membrane (uphill, so to speak), and when they passively flowed back across the membrane (downhill), their energy was used to do the work of making ATP. Cool, eh? Let s rap it up.

35 2.A.2.g. Briefly explain how the ETC establish an electrochemical gradient across membranes. 2.A.2.g.1. Identify where Electron transport chain reactions occur. 2.A.2.g.2. Contrast the ETC in cellular respiration and photosynthesis. 2.A.2.g.3. Describe the formation of a proton gradient in prokaryotes and eukaryotes. 2.A.2.g.4. Explain what generates ATP from ADP and inorganic phosphate. 2.A.2.g.5. In cellular respiration, explain how decoupling oxidative phosphorylation from electron transport is involved in thermoregulation.

36 Let s review a theme thing before we go on to something else The mitochondrion is a classic example of how structure is related to function on the cellular level. Describe the three aspects of its structure that are examples of this theme and explain how they contribute to efficient function. How do prokaryotic cells accomplish this? And connect this to this endosymbiosis thing

37 Use representations and models to analyze how cooperative interactions within organisms promote efficiency in the use of energy and matter. [LO 4.18, SP 1.4] Use representations to pose scientific questions about what mechanisms and structural features allow organisms to capture, store, and use free energy. [LO 2.4, SP 1.4, SP 3.1]

38 5. Didn t you always want to know how many ATP s the breakdown of one glucose molecule generates? During respiration, most energy flows from glucose -> NADH -> electron transport chain -> protonmotive force (gradient) -> ATP. Considering the fate of carbon, one six-carbon glucose molecule is oxidized to six CO 2 molecules. Some ATP is produced by substrate-level phosphorylation during glycolysis and the Krebs cycle, but most comes from oxidative phosphorylation involving the ETC.

39 Each NADH from the Krebs cycle and the conversion of pyruvate contributes enough energy to generate a maximum of 3 ATP s. The NADH from glycolysis may also yield 3 ATP. Each FADH 2 from the Krebs cycle can be used to generate about 2ATP s. In some eukaryotic cells, NADH produced in the cytosol by glycolysis may be worth only 2 ATP. The electrons must be shuttled to the mitochondrion. In some shuttle systems, the electrons are passed to NAD +, in others the electrons are passed to FAD.

40 Assuming the most energy-efficient shuttle of NADH from glycolysis, a maximum yield of 34 ATP is produced by oxidative phosphorylation. This plus the 4 ATP from substrate-level phosphorylation gives a bottom line of 38 ATP. This maximum figure does not consider other uses of the proton-motive force, or the leaking of protons through other gaps in the membrane, so, as always, efficiency is not 100%, so 38 ATP s per glucose would be the absolute max.

41 Fig. 9.16

42 How efficient is respiration in generating ATP? Complete oxidation of glucose releases 686 kcal per mole. Formation of each ATP requires at least 7.3 kcal/mole. Efficiency of respiration is 7.3 kcal/mole x 38 ATP/glucose/686 kcal/mole glucose = 40%. The other approximately 60% is lost as heat. Cellular respiration is remarkably efficient in energy conversion. Your car gets less than 40% efficiency from the gas it burns.

43 Sometimes there is just not enough oxygen Oxidation refers to the loss of electrons to any electron acceptor, not just to oxygen. In glycolysis, glucose is oxidized to two pyruvate molecules with NAD + as the oxidizing agent, not O 2. Some energy from this oxidation produces 2 ATP (net). If oxygen is present, additional ATP can be generated when NADH delivers its electrons to the electron transport chain. But glycolysis generates 2 ATP whether oxygen is present (aerobic) or not (anaerobic).

44 Anaerobic catabolism of sugars can occur by fermentation. Glycolysis doesn t need oxygen, but it does need NAD +. If the NAD + pool is exhausted, glycolysis shuts down. Under aerobic conditions, NADH transfers its electrons to the electron transport chain, recycling NAD + to keep glycolysis and the Krebs cycle going. Under anaerobic conditions, various fermentation pathways generate ATP by glycolysis and recycle NAD + by transferring electrons from NADH to pyruvate. All fermentation occurs in the cytoplasm.

45 In alcohol fermentation, pyruvate is converted to ethanol in two steps. First, pyruvate is converted to a two-carbon compound, acetaldehyde, by the removal of CO 2. Second, acetaldehyde is reduced by NADH to ethanol. Alcohol fermentation by yeast is used in baking and winemaking. The acetaldehyde can give you a headache, like this young man Fig. 9.17a

46 See what happens when your fraternity brothers catch you in the kitchen after one too many

47 How about an early Christmas tune?

48 During lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate ( 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 O 2 is scarce. The waste product, lactate, may cause muscle fatigue (or does it?) but ultimately it is converted back to pyruvate in the liver. Fig. 9.17b

49 Some organisms (facultative anaerobes), 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 routes. Fig. 9.18

50 The oldest bacterial fossils are over 3.5 billion years old, appearing long before appreciable quantities of O 2 accumulated in the atmosphere. Therefore, the first prokaryotes may have generated ATP exclusively from glycolysis. The fact that glycolysis is also the most widespread metabolic pathway and occurs in the cytosol without membrane-enclosed organelles, suggests that glycolysis evolved early in the history of life.

51 Construct explanations of the mechanisms and structural features of cells that allow organisms to capture, store, or use free energy. [LO 2.5, SP 6.2] Describe specific examples of conserved core biological processes and features shared by all domains or within one domain of life, and how these shared, conserved core processes and features support the concept of common ancestry for all organisms. [LO 1.15, SP 7.2]

52 2. Glycolysis and the Krebs cycle connect to many other metabolic pathways Glycolysis can accept a wide range of carbohydrates. Polysaccharides, like starch or glycogen, can be hydrolyzed to glucose monomers that enter glycolysis. Other hexose sugars, like galactose and fructose, can also be modified to undergo glycolysis. Proteins and fats can also enter the same respiratory pathways used by carbohydrates.

53 Proteins must first be digested to individual amino acids. Amino acids that will be catabolized must have their amino groups removed via deamination. The nitrogenous waste is excreted as ammonia, urea, or another waste product. The carbon skeletons are modified by enzymes and enter as intermediaries into glycolysis or the Krebs cycle depending on their structure. Usually, a cell will only use protein for energy if it is very low on carbs and fats, such as in starvation situations.

54 The cell can also use fats to make ATP. Fats must be digested to glycerol and fatty acids. Glycerol can be converted to glyceraldehyde phosphate, an intermediate of glycolysis. The rich energy of fatty acids is accessed as fatty acids are split into two-carbon fragments of acetic acid. These molecules enter the Krebs cycle as acetyl CoA. In fact, a gram of fat will generate twice as much ATP as a gram of carbohydrate via aerobic respiration, and the greater percentage of the ATP s produced by your cells at rest come from the fats in your last meal.

55 The daily cycle looks like this: A meal is digested, lipid fragments and simple sugars get into cells and are used to make ATP. Extra glucose is packed into glycogen in liver and muscle cells for later use. When they are full, extra fats and sugars are converted to stored fats in fat cells. As you run low on ATP s, your body first breaks down the glycogen to free up glucoses, then when those run low the stored fats start getting broken down. This usually starts occurring after 20 minutes or so of activity.

56 Carbohydrates, fats, and proteins can all be catabolized through the same pathways. Fig. 9.19

57 Not all the organic molecules of food are completely oxidized to make ATP. Intermediaries in glycolysis and the Krebs cycle can be used to make needed compounds. For example, a human cell can synthesize about half the 20 different amino acids by modifying compounds from the Krebs cycle. Glucose can be synthesized from pyruvate and fatty acids from acetyl CoA.

58 3. Feedback mechanisms control cellular respiration Basic principles of supply and demand regulate the metabolic economy. It is an adaptive advantage if a cell only make substances such as ATP when it needs them. If ATP levels drop, catabolism speeds up to produce more ATP; when it is high, catabolism slows. Here s how:

59 Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway. One strategic point occurs in the third step of glycolysis, catalyzed by phosphofructokinase. Fig. 9.20

60 Allosteric regulation of phosphofructokinase sets the pace of respiration. This enzyme is allosterically inhibited by ATP and stimulated by AMP (derived from ADP). It responds to shifts in balance between production and degradation of ATP: ATP <-> ADP + P i <-> AMP + P i. Thus, when ATP levels are high, inhibition of this enzyme slows glycolysis. When ATP levels drop and ADP and AMP levels rise, the enzyme is active again and glycolysis speeds up. This is a classic example of a negative feedback loop.

61 Citrate, the first product of the Krebs cycle, is also an inhibitor of phosphofructokinase. This synchronizes the rate of glycolysis and the Krebs cycle. Also, if intermediaries from the Krebs cycle are diverted to other uses (e.g., amino acid synthesis), glycolysis speeds up to replace these molecules. Metabolic balance is augmented by the control of other enzymes at other key locations in glycolysis and the Krebs cycle. Cells are thrifty, expedient, and responsive in their metabolism.

62 Hollywood often has a tough time with science Tell me what you think of this bit of testimony from an expert witness. In case you haven t seen the movie, here s the famous ending. Now how about that Q10 thing?

63 Q8 The net annual primary productivity of a particular wetland ecosystem is found to be 8,000 kcal/m2. If respiration by the aquatic producers is 12,000 kcal/m2per year, what is the gross annual primary productivity for this ecosystem, in kcal/m2 per year? Round to the nearest whole number.

64 Q8 NPP=GPP-R 8,000 = GPP 12,000 8, ,000= GPP 20,000=GPP