Energy in food is stored as carbohydrates (such as glucose), proteins & fats. Before that energy can be used by cells, it must be released and transferred to ATP.

Aerobic Cellular Respiration: the process that releases energy by breaking down food (glucose) molecules in the presence of oxygen. Formula: C6H12O6 + 6O2 → 6CO2 + 6H2O +~ 36 ATP Fermentation: the partial breakdown of glucose without oxygen. It only releases a small amount of ATP. Glycolysis: the first step of breaking down glucose—it splits glucose (6C) into 2 pyruvic acid molecules (3C each)

The transfer of electrons during chemical reactions releases energy stored in organic compounds such as glucose. Oxidation-reduction reactions are those that involve the transfer of an electron from one substance to another.

In cellular respiration, glucose is broken down in a series of steps. As it is broken down, electrons from glucose are transferred to NAD+, a coenzyme When it receives the electrons, it is converted to NADH. NADH represents stored energy that can be used to make ATP

NADH passes the electrons to the electron transport chain, a series of proteins embedded in the inner membrane of the mitochondria. The electrons (and the energy they carry) are transferred from one protein to the next in a series of steps.

Cellular respiration has three stages: Glycolysis (breaks down glucose into two molecules of pyruvate) The Citric Acid cycle/Kreb’s Cycle (completes the breakdown of glucose) Electron Transport Chain and Oxidative phosphorylation (accounts for most of the ATP synthesis) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

An Overview of Cellular Respiration–Part 1 Electrons carried via NADH Glycolysis Glucose Pyruvate Cytosol Figure 9.6 An overview of cellular respiration ATP Substrate-level phosphorylation

An Overview of Cellular Respiration—Part 2 Electrons carried via NADH Electrons carried via NADH and FADH2 Glycolysis Citric acid cycle Glucose Pyruvate Mitochondrion Cytosol Figure 9.6 An overview of cellular respiration ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation

An Overview of Cellular Respiration—Part 3 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

Substrate-level phosphorylation: Phosphate is added to ADP to make ATP by using an enzyme: Oxidative phosphorylation: Phosphate is added to ADP to make ATP by ATP Synthase—a protein embedded in the mitochondria membrane (requires O2) WAY MORE EFFICIENT!! PRODUCES LOTS MORE ATP!

“Glyco”=sugar; “lysis”=to split In this first series of reactions, glucose (C6) is split into two molecules of pyruvic acid (C3). This occurs in the cytoplasm of cells and does not require oxygen. This releases only 2 ATP molecules, not enough for most living organisms. http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter25/animation__how_glycolysis_works.html

Glycolysis Energy investment phase Glucose 2 ADP + 2 P 2 ATP used Energy payoff phase 4 ADP + 4 P 4 ATP formed 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+

The Citric Acid Cycle (also called the Kreb’s Cycle) completes the breakdown of pyruvate and the release of energy from glucose. It occurs in the matrix of the mitochondria.

In the presence of oxygen, pyruvate enters the mitochondria. Before the pyruvate can enter the Citric Acid Cycle, however, it must be converted to Acetyl Co-A. Some energy is released and NADH is formed.

Converting Pyruvate to Acetyl CoA: 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

The Acetyl Co-A enters the Citric Acid Cycle in the matrix of the mitochondria. The Citric Acid cycle breaks down the Acetyl Co-A in a series of steps, releasing CO2 It produces 1 ATP, 3 NADH, and 1 FADH2 per turn.

The Citric Acid Cycle The Citric Acid cycle (also called the Krebs Cycle) has eight steps, each catalyzed by a specific enzyme The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate (Citric Acid). The next seven steps decompose the citrate (Citric Acid) back to oxaloacetate, making the process a cycle Oxaloacetate + Acetyl CoA Citric Acid

The Citric Acid Cycle: Acetyl CoA Oxaloacetate Malate Citrate CoA—SH NADH +H+ 1 H2O NAD+ 8 Oxaloacetate 2 Malate Citrate 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

Each Citric Acid Cycle only produces 1 ATP molecule Each Citric Acid Cycle only produces 1 ATP molecule. The rest of the energy from pyruvate is in the NADH and FADH2. The NADH and FADH2 produced by the Citric Acid cycle relay electrons extracted from food to the electron transport chain.

The electron transport chain is in the cristae of the mitochondrion Most of the chain’s components are proteins, which exist in 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 Oxygen is the final electron acceptor. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Electron Transport Chain 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 10 2 e– (from NADH or FADH2) 2 H+ + 1/2 O2 H2O

Electrons are transferred from NADH or FADH2 to the electron transport chain Electrons are passed through a number of proteins to O2 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 Animation: http://sp.uconn.edu/~terry/images/anim/ETS.html

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

Chemiosmosis couples the electron transport chain to ATP synthesis 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

During cellular respiration, most energy flows in this sequence: glucose  NADH  electron transport chain  proton-motive force  ATP About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP + 6 O2 6CO2 + 6H2O + 38 ATP

ATP Yield per molecule of glucose at each stage of cellular respiration: 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: