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Energy flow and chemical recycling in ecosystems

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Figs. 9.1 & 9.2

ATP (review)

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ATP hydrolysis provides energy to drive cellular work and energy coupling (review)

from Figs. 8.10 & 8.11

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• Organic molecules (e.g., glucose, glycogen, starch) “store” energy– arrangement of e- in bonds between atoms (review)

• Enzymes catalyze degradation of organic molecules– from energy rich substrates– to simpler waste products with less energy

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Catabolic pathways – ATP production

• Some release energy ĺ work– remainder ĺ dissipated as heat

• Metabolic pathways that release energy stored in complex organic molecules are catabolic (review)

• Fermentation – one catabolic pathway for ATP production– partial degradation of sugars in absence of oxygen (Anaeorbic

metabolism)

• Cellular respiration - a more efficient and widespread catabolic process for ATP production– Oxygen (O2) is a reactant to complete the

breakdown of organic molecules– Most of process occurs in mitochondria

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Cellular Respiration

• Overall process:– Organic compounds + O2 ĺ CO2 + H2O + Energy– Carbohydrates, fats, and proteins can all be used as fuel

• Glucose:– C6H12O6 + 6O2 ĺ 6CO2 + 6H2O + Energy (ATP + heat)

• Catabolism of glucose is exergonic– 'G = - 686 kcal per mole of glucose

Oxidation/Reduction

(redox)

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• Catabolic pathways “relocate” the electrons (e-) stored in food molecules– releases energy for ATP synthesis

Redox reactions release energy when electrons move closer to electronegative atoms

• Redox reactions (oxidation-reduction reactions)– Transfer of one or more electrons from one reactant to

another– Oxidation – loss of e-

– Reduction – addition of e-

• Formation of table salt from sodium and chloride is a redox reaction

• Sodium is oxidized (e- removed; charge changes from 0 to +1)• Chlorine is reduced (e- added; charge changes from 0 to -1)

Redox example: NaCl

• In general:

– X, the electron donor, is the reducing agent and reduces Y– Y, the electron recipient (acceptor), is the oxidizing agent

and oxidizes X

• Redox reactions require both a donor and acceptor

• Redox reactions do not have to involve ionization• Example: nonpolar to polar bonds

– e- move from positions equidistant between two atoms to a position closer to electronegative oxygen atom

– Oxygen is one of the most potent oxidizing agents– e- looses 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

• To reverse the process, energy must be added to pull an electron away from an atom

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• Cellular respiration – energy is released by oxidation of glucose and other fuel molecules

• Summary equation:C6H12O6 + 6O2 ĺ 6CO2 + 6H2O + energy

– Glucose is oxidized– Oxygen is reduced

Redox reactions in cellular respiration

Oxygen is reduced– Electrons lose potential energy

• Cell has a large reservoir of e- associated with hydrogen– Stored in carbohydrates and fats

• Fuels do not spontaneously combine with O2 (burn)– lack activation energy

• Enzymes lower EA (Enzymes and Catalysis lecture)– oxidization reactions controlled

• Glucose and other fuels are broken down in a series of steps, each catalyzed by a specific enzyme

• At key steps, hydrogen atoms are stripped from glucose d d fi t t lik NAD+ ( i ti id

Stepwise energy harvest via NAD+ and the electron transport chain

and passed first to a coenzyme, like NAD+ (nicotinamide adenine dinucleotide…see next slide)

• Dehydrogenase enzymes strip two H atoms from the fuel (e.g., glucose), pass two e- and one H+ to NAD+, and also release one H+

Coenzyme NAD+ as an electron shuttle

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Fig. 9.4

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GLYCOLYSIS

Glycolysis – takes place in the cytoplasm cytoplasm

Glycolysis can take place in the absence of O2

Fig. 9.6

Overview of a metabolic pathway: the energy input and output of GLYCOLYSIS

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Fig. 9.8

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A closer look at glycolysis: energy investment phase

Fig. 9.9Kinases

A closer look at glycolysis: energy investment phase (5 steps)

Fig. 9.9

2 ATP used

A closer look at glycolysis: energy payoff phase

(2) G3P

Fig. 9.9

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A closer look at glycolysis: energy payoff phase (5 steps)

Fig. 9.9

4 ATP (net 2 ATP) and 2 NADH produced

Substrate-level phosphorylation

Example:

Enzyme = Pyruvate Kinase

Phosphoenolpyruvate + ADP ļ pyruvate + ATP

Fig. 9.7

CELLULAR RESPIRATION

Aerobic Metabolism

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Overview of cellular respiration

Fig. 9.6

Oxidation of pyruvate to acetyl CoA - junction between glycolysis and the Krebs cycle

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Fig. 9.10

Closer look at the citric acid cycle

Fig. 9.12

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Summary of citric acid cycle (Krebs cycle)

Fig. 9.11

An overview of cellular respiration

We’ve looked at Glycolysis and the Citric Acid Cycle in detail…So where / how is O2 used in aerobic metabolism????

Fig. 9.6

An introduction to electron transport chains (ETCs)

Fig. 9.5

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'G during electron transport in electron transport chain

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Chemiosmosis couples the electron transport chain to ATP synthesis

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ATP synthase, a molecular mill

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Fig. 9.14

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ATP synthase model

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Each molecule of glucose yields many ATP molecules during cellular respiration

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Each molecule of glucose yields many ATP molecules during cellular respiration:KDW�LI�QR�2�"

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ANAEROBIC METABOLISM:

Fermentation

Pyruvate as a key juncture in catabolism

Fig. 9.18

Fermentation by facultative anaerobes (e.g., yeast)

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Fermentation in muscle

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CONNECTIONS TO OTHER METABOLIC PATHWAYS

Catabolism of various molecules from food

Fig. 9.19

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Control of cellular respiration: inhibition by negative feedback (also activators)

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Fig. 9.20

Energy flow and chemical recycling in ecosystems

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Figs. 9.1 & 9.2

BSC 2010 Chase

Oxidation and reduction Motivation: In the next severa; lectures, we will go over the primary ways in which cells obtain energy (synthesize ATP!) that is necessary for maintaining life. In the first lecture, we focus on the common theme of oxidation-reduction reactions in these processes. Objectives: ¾ Define oxidation and reduction ¾ Explain how redox reactions are involved in energy exchanges ¾ Write the summary equation for cellular respiration. Write the specific chemical

equation for the degradation of glucose. ¾ Describe the role of NAD+ during respiration

Photosynthesis/chloroplast Fermentation Cellular respiration/Mitochondrion Oxidation Oxidizing agent Reduction Reducing agent Redox Coenzyme Nicotinamide adenine dinucleotide (NAD+/NADH) Dehydragenases (a class of enzymes)

BSC 2010 Chase

Aerobic Respiration Motivation: Aerobic respiration, most of which occurs in mitochondria, is the most efficient metabolic pathway for ATP production. Objectives: ¾ Name the three stages of cellular respiration and state the region of the eukaryotic cell

where each stage occurs. ¾ Describe how the carbon skeleton of glucose changes as it proceeds through glycolysis. ¾ Explain why ATP is required for the preparatory steps of glycolysis. ¾ Identify where substrate-level phosphorylation and the reduction of NAD+ occur in

glycolysis. ¾ Describe where pyruvate is oxidized to acetyl CoA, what molecules are produced, and

how this process links glycolysis to the citric acid cycle. ¾ List the products of the citric acid cycle. Explain why it is called a cycle. ¾ Distinguish between substrate level phosphorylation and oxidative phosphorylation. ¾ Explain where and how the respiratory electron transport chain creates a proton

gradient. ¾ Describe the structure and function of the four subunits of ATP synthase. ¾ Summarize the net ATP yield from the oxidation of a glucose molecule (include

coenzyme production during glycolysis and cellular respiration). Oxidative phosphorylation Substrate-level phosphorylation Kinase (a class of enzymes) Glycolysis (10 steps) Energy investment and payoff phases Intermediates Pyruvate Citric acid cycle, also known as the Krebs cycle (8 steps) Acetyl coenzyme A (acetyl CoA) Intermediates

BSC 2010 Chase

Electron transport chain NADH/FADH2

Cytochromes O2

Chemiosmosis Proton-motive force (electrochemical gradient for H+) ATP synthase Rotor Stator Rod (stalk) Knob (catalytic sites for ADP phosphorylation)

BSC 2010 Chase

Anaerobic metabolism Motivation: Anaerobic respiration is a metabolic pathway for ATP production in the absence of oxygen. It is commercially important, and was significant in early evolution because oxygen was not a significant part of the earth’s atmosphere. Objectives: ¾ State the basic function of fermentation. ¾ Compare what happens to pyruvate in alcohol fermentation and lactic acid fermentation. ¾ Compare the processes of fermentation and cellular respiration. ¾ Describe the evidence that that suggests that glycolysis is an ancient metabolic

pathway. (Connecting glycolysis and the citric acid cycle to other metabolic pathways) ¾ Describe how food molecules other than glucose can be oxidized to make ATP. ¾ Explain how glycolysis and the citric acid cycle can contribute to anabolic pathways. ¾ Explain how ATP production is controlled by the cell and describe the role that the

allosteric enzyme phosphofructokinase plays in the process. Alcohol fermentation Acetaldehyde Ethanol Lactic acid fermentation Lactate Facultative anaerobes