Living systems require energy from outside sources Overview: Life Is Work Living systems require energy from outside sources Different organisms have different strategies For the Discovery Video Space Plants, go to Animation and Video Files.

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 Organic molecules CO2 + H2O + O2 Cellular respiration in mitochondria Figure 9.2 Energy flow and chemical recycling in ecosystems ATP ATP powers most cellular work Heat energy

We usually use energy harvesting pathways that are catabolic pathways as our examples Chemotrophic pathways

Concept 9.1: Energy-harvesting 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 O2 Aerobic respiration complete degradation of organic molecules and requires O2 Anaerobic respiration is similar to aerobic respiration but requires compounds other than O2 -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

Aerobic respiration is most relevant to humans Discussion Outline Aerobic Respiration Anaerobic Respiration Fermentation

C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy (ATP + heat) We trace aerobic respiration by following the path of the atoms of glucose (although many other substances are also consumed as fuel) as it is used as a source of chemical energy. C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy (ATP + heat)

The movement of electrons during chemical reactions helps to release and move energy stored in organic molecules The electrons carry energy (key point) as they are transferred from one substance to another

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

becomes oxidized becomes reduced 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

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 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 2 e– + 2 H+ 2 e– + H+ NADH H+ Dehydrogenase Reduction of NAD+ NAD+ + 2[H] + H+ Oxidation of NADH Nicotinamide (reduced form) Nicotinamide (oxidized form) Hydrogen (H) follows electrons in an aqueous environment An enzyme that catalyzes an oxidation is a “dehydrogenase” Figure 9.4 NAD+ as an electron shuttle

NADH passes high energy electrons to the electron transport chain This chain hands off electrons in a series of exergonic steps Finally they reach O2 which becomes reduced This is the last stop for the electrons so O2 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 directly by non-oxidative reaction-substrate level phosphorylation The Citric acid cycle (completes the breakdown of glucose) and also yields a little ATP directly reaction-substrate level phosphorylation Oxidative phosphorylation (accounts for most of the ATP synthesis) includes electron transport and reduction of oxygen

Electrons carried via NADH Electrons carried via NADH and FADH2 Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis Citric acid cycle Glucose Pyruvate Mitochondrion Cytosol Figure 9.6 An overview of cellular respiration ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation

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

Substrate Level Phosphorylation Enzyme Enzyme ADP P Substrate + ATP Figure 9.7 Substrate-level phosphorylation Product Direct transfer of high energy phosphate from substrate to substrate

Oxidative phosphorylation involves a membrane and the movement of electrons (and protons)

Glycolysis occurs in the cytoplasm and has two major phases: 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

Overview of Glycolysis Glucose Energy investment phase 2 ADP + 2 P 2 ATP used 4 ADP + 4 P 4 ATP formed Energy payoff phase 2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+ Figure 9.8 The energy input and output of glycolysis 2 Pyruvate + 2 H2O Net Glucose 2 Pyruvate + 2 H2O 4 ATP formed – 2 ATP used 2 ATP 2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+

Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules (except for coenzymes) In the presence of O2 , pyruvate is transported into the mitochondrion First, pyruvate must be converted to acetyl CoA, which links the citric acid cycle to glycolysis

Aerobic Metabolism of Pyruvate CYTOSOL MITOCHONDRION NAD+ NADH + H+ 2 1 3 Acetyl CoA Figure 9.10 Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the citric acid cycle Pyruvate Coenzyme A CO2 Transport protein 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 via Acetyl-CoA, generating 1 ATP, 3 NADH, and 1 FADH2 per turn

Pyruvate CO2 NAD+ CoA NADH + H+ Acetyl CoA CoA CoA Citric acid cycle 2 Figure 9.11 An overview of the citric acid cycle FADH2 3 NAD+ FAD 3 NADH + 3 H+ ADP + P i ATP

Can be made into many substances Acetyl CoA CoA—SH NADH +H+ 1 H2O NAD+ 8 Oxaloacetate 2 Can be made into many substances Malate Citrate Carbon Skeletons Isocitrate NAD+ Citric acid cycle NADH 3 7 + H+ H2O CO2 Fumarate CoA—SH -Keto- glutarate Figure 9.12 A closer look at the citric acid cycle 4 6 CoA—SH FADH2 5 CO2 NAD+ FAD Succinate P NADH i GTP GDP Succinyl CoA + H+ ADP ATP 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 FADH2 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 For the Cell Biology Video ATP Synthase 3D Structure — Side View, go to Animation and Video Files. For the Cell Biology Video ATP Synthase 3D Structure — Top View, go to Animation and Video Files.

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 O2, forming H2O

You do not need to memorize this NADH 50 2 e– NAD+ FADH2 2 e– FAD Multiprotein complexes  40 FMN FAD Fe•S  Fe•S Q  Cyt b Fe•S 30 Cyt c1 IV Free energy (G) relative to O2 (kcal/mol) Cyt c Cyt a Cyt a3 20 Figure 9.13 Free-energy change during electron transport 2 e– 10 (from NADH or FADH2) 2 H+ + 1/2 O2 H2O You do not need to memorize this

Electrons are transferred from NADH or FADH2 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 O2 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

Electron transport chain 2 Chemiosmosis Protein complex of electron carriers Cyt c V Q   ATP synthase  2 H+ + 1/2O2 H2O FADH2 FAD NADH NAD+ Figure 9.16 Chemiosmosis couples the electron transport chain to ATP synthesis ADP + P ATP i (carrying electrons from food) H+ 1 Electron transport chain 2 Chemiosmosis Oxidative phosphorylation

How much ATP is produced? 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

CYTOSOL Electron shuttles span membrane MITOCHONDRION 2 NADH or 2 FADH2 2 NADH 2 NADH 6 NADH 2 FADH2 Glycolysis Oxidative phosphorylation: electron transport and chemiosmosis 2 Pyruvate 2 Acetyl CoA Citric acid cycle Glucose + 2 ATP + 2 ATP + about 32 or 34 ATP Figure 9.17 ATP yield per molecule of glucose at each stage of cellular respiration About 36 or 38 ATP Maximum per glucose:

Concept 9.5: Without oxygen-cells use fermentation or anaerobic respiration to produce ATP Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions) In the presence of O2 glycolysis couples with aerobic respiration and uses oxygen as terminal electron to produce ATP In the absence of O2, glycolysis couples with fermentation or anaerobic respiration to produce ATP

Anaerobic respiration uses an electron transport chain with an electron acceptor other than O2, for example sulfate, ending with H2S not H2O 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

Alcohol fermentation 2 ADP + 2 P 2 ATP Glucose Glycolysis 2 Pyruvate 2 NADH 2 CO2 + 2 H+ Figure 9.18a Fermentation 2 Acetaldehyde 2 Ethanol Alcohol fermentation

Lactic acid fermentation 2 ADP + 2 P 2 ATP i Glucose Glycolysis 2 NAD+ 2 NADH + 2 H+ 2 Pyruvate Figure 9.18b Fermentation Lactic acid fermentation 2 Lactate

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 O2 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 O2 Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration

Citric acid cycle Oxidative phosphorylation Proteins Carbohydrates Fats Amino acids Sugars Glycerol Fatty acids Glycolysis Glucose Glyceraldehyde-3- P NH3 Pyruvate Acetyl CoA Figure 9.20 The catabolism of various molecules from food Citric acid cycle Oxidative phosphorylation

Figure 9.21 The control of cellular respiration Glucose AMP Glycolysis Fructose-6-phosphate Stimulates + Phosphofructokinase – – Fructose-1,6-bisphosphate Inhibits Inhibits Pyruvate ATP Citrate Acetyl CoA Figure 9.21 The control of cellular respiration Citric acid cycle Oxidative phosphorylation