Overview: Life Is Work Living systems require energy from outside sources Different organisms have different strategies

Energy harvesting pathways provide great examples of metabolic pathways in general Also they are central to life And they are VERY well-studied (about 125 years) They serve as paradigm for all metabolism

An ecosystem is an open system Energy enters as light and leaves as heat or entropy Energy enters as light Light energy ECOSYSTEM Photosynthesis in chloroplasts and cyanobacteria CO 2 + H 2 O Cellular respiration in mitochondria Organic molecules + O 2 ATP powers most cellular work Heat energy ATP

Concept 9.1: Catabolic pathways The breakdown of organic molecules (sugars) is exergonic and involves several multistep pathways Fermentation is a partial degradation of organic molecules that occurs without O 2 Aerobic respiration complete degradation of organic molecules and requires O 2 Anaerobic respiration is similar to aerobic respiration but requires compounds other than O 2 -used by many types of microbes but has lower energy yield Cellular respiration includes both aerobic and anaerobic respiration but is usually used to refer to aerobic respiration

We trace aerobic respiration by following the path of glucose (although many other substances are also consumed as fuel). C 6 H 12 O O 2  6 CO H 2 O + Energy (ATP + heat)

The movement of electrons during chemical reactions helps to release and move energy stored in organic molecules The electrons are energy carriers (key point) This released energy is ultimately used to synthesize ATP

In electron transfer reactions, one substance loses electrons and another gains them. In an oxidation, a substance loses electrons, or is oxidized In a reduction, a substance gains electrons, or is reduced They must occur together Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions

Example becomes oxidized becomes reduced In an aqueous environment, protons from water follow the electrons. Therefore the reduced form of Y might be written as YH And the reduced form of X as XH

The electron donor is called the reducing agent The electron receptor is called the oxidizing agent During cellular respiration, the fuel (such as glucose) is oxidized, and O 2 is reduced The reactions occur in a series or chain

Electrons carry the energy in these transfer reactions but something is needed to carry the electrons from reaction to reaction Electrons from organic compounds are usually first transferred to NAD +, a coenzyme (other coenzymes can be used as well) As an electron acceptor, NAD + functions as an oxidizing agent during cellular respiration Each NADH (the reduced form of NAD + ) represents stored energy that can be tapped to synthesize ATP or do something else

Oxidation/Reduction of NAD Dehydrogenase Reduction of NAD + Oxidation of NADH 2 e – + 2 H + 2 e – + H + NAD + + 2[H] NADH + H+H+ H+H+ Nicotinamide (oxidized form) Nicotinamide (reduced form) Hydrogen (H) follows electrons in an aqueous environment An enzyme that catalyzes an oxidation is a “dehydrogenase”

NADH passes high energy electrons to the electron transport chain This chain hands off electrons in a series of exergonic steps Finally they reach O 2 which becomes reduced This is the last stop for the electrons so O 2 is called the terminal electron acceptor The energy given off is used to regenerate ATP

There are three important stages: – Glycolysis (breaks down glucose into two molecules of pyruvate) and yields a little ATP by non-oxidative reaction – The Citric acid cycle (completes the breakdown of glucose) and yields a little ATP by non-oxidative reactions – Oxidative phosphorylation (accounts for most of the ATP synthesis) includes electron transport and reduction of oxygen

Mitochondrion Substrate-level phosphorylation ATP Cytosol Glucose Pyruvate Glycolysis Electrons carried via NADH Substrate-level phosphorylation ATP Electrons carried via NADH and FADH 2 Oxidative phosphorylation ATP Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis

Oxidative phosphorylation involves transfer of electrons from reduced coenzymes to oxygen, the terminal electron acceptor. A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation

Enzyme ADP P Substrate Enzyme ATP + Product Substrate Level Phosphorylation Direct transfer of high energy phosphate from substrate to substrate

Concept 9.2: Glycolysis harvests chemical energy by converting glucose to pyruvate Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate Glycolysis occurs in the cytoplasm and has two major phases: – Energy investment phase – Energy payoff phase

Energy investment phase Glucose 2 ADP + 2 P 2 ATPused formed 4 ATP Energy payoff phase 4 ADP + 4 P 2 NAD e – + 4 H + 2 NADH + 2 H + 2 Pyruvate + 2 H 2 O Glucose Net 4 ATP formed – 2 ATP used2 ATP 2 NAD e – + 4 H + 2 NADH + 2 H + Overview of Glycolysis

Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules (except for coenzymes) In the presence of O 2, pyruvate is transported into the mitochondrion First, pyruvate must be converted to acetyl CoA, which links the citric acid cycle to glycolysis

CYTOSOLMITOCHONDRION NAD + NADH+ H Pyruvate Transport protein CO 2 Coenzyme A Acetyl CoA Aerobic Metabolism of Pyruvate CoA is derived from a vitamin- Vitamin B5 or pantothenic acid Acetyl CoA links carbohydrate and fatty acid catabolism (beta oxidation)

The citric acid cycle, also called the Krebs cycle, takes place within the mitochondrial matrix The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH 2 per turn

Pyruvate NAD + NADH + H + Acetyl CoA CO 2 CoA Citric acid cycle FADH 2 FAD CO NAD H + ADP +P i ATP NADH

Acetyl CoA CoA—SH Citrate H2OH2O Isocitrate NAD + NADH + H + CO2CO2  -Keto- glutarate CoA—SH CO2CO2 NAD + NADH + H + Succinyl CoA CoA—SH P i GTP GDP ADP ATP Succinate FAD FADH 2 Fumarate Citric acid cycle H2OH2O Malate Oxaloacetate NADH +H + NAD Carbon Skeletons Can be made into many substances You do not need to memorize this

Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis Following glycolysis and the citric acid cycle, NADH and FADH 2 carry most of the energy extracted from food in the form of high energy electrons These two electron carriers hand off the electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation

The electron transport chain is in the cristae of the mitochondrion Most of the chain’s components are proteins, which exist in the form of huge multiprotein complexes The carriers alternate reduced and oxidized states as they accept and donate electrons Electrons drop in free energy as they go down the chain and are finally passed to O 2, forming H 2 O

NADH NAD + 2 FADH 2 2 FAD Multiprotein complexes FAD FeS FMN FeS Q  Cyt b   Cyt c 1 Cyt c Cyt a Cyt a 3 IV Free energy (G) relative to O 2 (kcal/mol) (from NADH or FADH 2 ) 0 2 H / 2 O2O2 H2OH2O e–e– e–e– e–e– You do not need to memorize this

Electrons are transferred from NADH or FADH 2 to the electron transport chain Electrons are passed through a number of proteins including cytochromes (each with an iron atom) and iron-sulfur proteins The electron transport chain generates no ATP directly The chain’s function is to break the large free- energy drop from food to O 2 into smaller steps that release energy in manageable amounts

Electron transfer in the electron transport chain causes proteins to pump H + from the mitochondrial matrix to the intermembrane space H + then moves back across the membrane, passing through channels in ATP synthase ATP synthase uses the exergonic flow of H + to drive phosphorylation of ATP This is an example of chemiosmosis, the use of energy in a H + gradient to drive cellular work (Note-this is an important concept)

The energy stored in the electrochemical H + gradient across a membrane connects or couples the redox reactions of the electron transport chain to ATP synthesis The H + gradient is referred to as a proton- motive force, emphasizing its capacity to do work

Protein complex of electron carriers H+H+ H+H+ H+H+ Cyt c Q    VV FADH 2 FAD NAD + NADH (carrying electrons from food) Electron transport chain 2 H / 2 O 2 H2OH2O ADP + P i Chemiosmosis Oxidative phosphorylation H+H+ H+H+ ATP synthase ATP 21

During cellular respiration, most energy flows in this sequence: glucose  NADH  electron transport chain  proton-motive force  ATP Roughly 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP How much ATP is produced?

Maximum per glucose: About 36 or 38 ATP + 2 ATP + about 32 or 34 ATP Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle 2 Acetyl CoA Glycolysis Glucose 2 Pyruvate 2 NADH 6 NADH2 FADH 2 2 NADH CYTOSOL Electron shuttles span membrane or MITOCHONDRION

Concept 9.5: Without oxygen-cells use fermentation or anaerobic respiration to produce ATP Glycolysis can produce ATP with or without O 2 (in aerobic or anaerobic conditions) In the presence of O 2 glycolysis couples with aerobic respiration and uses oxygen as terminal electron to produce ATP In the absence of O 2, glycolysis couples with fermentation or anaerobic respiration to produce ATP

Anaerobic respiration uses an electron transport chain with an electron acceptor other than O 2, for example sulfate, ending with H 2 S not H 2 O Fermentation uses phosphorylation instead of an electron transport chain to generate ATP Note: Fermentation is defined to consist of reactions that regenerate NAD + -these vary from organism to organism

Types of Fermentation alcohol fermentation lactic acid fermentation butyric acid fermentation biohydrogen fermentation

2 ADP + 2 P i 2 ATP GlucoseGlycolysis 2 Pyruvate 2 NADH2 NAD H + CO 2 2 Acetaldehyde 2 Ethanol 2 Alcohol fermentation

Glucose 2 ADP + 2 P i 2 ATP Glycolysis 2 NAD + 2 NADH + 2 H + 2 Pyruvate 2 Lactate Lactic acid fermentation

Fermentation vs. aerobic respiration Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate The processes have different terminal electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O 2 in cellular respiration Cellular respiration produces 38 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule

Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O 2 Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration

Glycolysis in evolution Glycolysis occurs in nearly all organisms Glycolysis probably evolved in ancient prokaryotes before there was oxygen in the atmosphere Later prokaryotes evolved ways to extract more energy from pyruvate by anaerobic respiration Aerobic respiration was last

Proteins Carbohydrates Amino acids Sugars Fats GlycerolFatty acids Glycolysis Glucose Glyceraldehyde-3- Pyruvate P NH 3 Acetyl CoA Citric acid cycle Oxidative phosphorylation

Glucose Glycolysis Fructose-6-phosphate Phosphofructokinase Fructose-1,6-bisphosphate Inhibits AMP Stimulates Inhibits Pyruvate Citrate Acetyl CoA Citric acid cycle Oxidative phosphorylation ATP + – –

NOTE CARD QUESTION What is meant by the term “chemiosmotic coupling mechanism”