Cellular Respiration in DETAIL H. Biology

The Stages of Cellular Respiration Respiration is a cumulative process of 3 metabolic stages 1. Glycolysis 2. Kreb’s Cycle (The citric acid cycle) 3. Electron Transport Chain (Oxidative phosphorylation)

Stage #1: Glycolysis Glycolysis produces energy by oxidizing glucose  pyruvate Glycolysis  Means “splitting of sugar”  Breaks down glucose into pyruvate cytoplasm  Occurs in the cytoplasm of the cell 1 glucose breaks down  4 ATP made + 2 pyruvate molecules (net gain of 2 ATP…NOT 4 ATP)

Glycolysis Main Goal of Glycolysis is to turn glucose into two pyruvate: - Series of 10 steps - Produces a net gain of 2 ATP and 2 NADH (e- carriers) - From here it can go to the Krebs cycle (aerobic respiration) or to Fermentation (anaerobic) - Glycolysis is anaerobic - Occurs in the cytoplasm Main Goal of Glycolysis is to turn glucose into two pyruvate: - Series of 10 steps - Produces a net gain of 2 ATP and 2 NADH (e- carriers) - From here it can go to the Krebs cycle (aerobic respiration) or to Fermentation (anaerobic) - Glycolysis is anaerobic - Occurs in the cytoplasm Overall: Glucose → 2 Pyruvate; net gain 2 ATP and 2 NADH

Intermediate step Pyruvate (made in the cytosol via glycolysis) diffuses into the mitochondria. As it diffuses is, it produces one molecule of CO 2  loses a carbon (goes from 3C to 2C). This new 2C molecule is acetyl CoA. Acetyl CoA is what enters into the Krebs cycle.

GlycolysisNET Glucose  2 pyruvate (pyruvic acid) + 2 H 2 O Glucose  2 pyruvate (pyruvic acid) + 2 H 2 O 4 ATP formed – 2 ATP used  2 ATP GAIN 4 ATP formed – 2 ATP used  2 ATP GAIN  substrate-level phosphorylation used 2 NAD+ + 4e- + 4H+  2 NADH + 2H+ 2 NAD+ + 4e- + 4H+  2 NADH + 2H+ **Glycolysis can proceed WITHOUT O 2

Glycolysis consists of two major phases 1. Energy investment phase (endergonic= uses 2 ATP) 2. Energy payoff phase (exogonic = makes 4 ATP) Glycolysis Citric acid cycle Oxidative phosphorylation ATP 2 ATP 4 ATP used formed 1 Glucose 2 ADP + 2 P 4 ADP + 4 P 2 NAD e H + 2 NADH + 2 H + 2 Pyruvate + 2 H 2 O Energy investment phase Energy payoff phase Glucose 2 Pyruvate + 2 H 2 O 4 ATP formed – 2 ATP used 2 ATP 2 NAD e – + 4 H + 2 NADH + 2 H +

Stage #2: The Kreb’s Cycle The citric acid cycle – Takes place in the matrix of the mitochondrion **NEEDS O 2 TO PROCEED (unlike glycolysis)

Stage #1 ½ : The Citric Acid Cycle Before the citric acid cycle can begin – Pyruvate must first be converted to acetyl CoA, which links the citric acid cycle to glycolysis CYTOSOL MITOCHONDRION NADH + H + NAD CO 2 Coenzyme A (a vitamin) Pyruvate Acetyle CoA S CoA C CH 3 O Transport protein O–O– O O C C CH 3 Figure 9.10 Uses active transport Diffuses out of cell Acetyl group= unstable

The Kreb’s Cycle Also called the “Citric Acid cycle” NAD+ and FAD (both are coenzymes) = electron “carriers”; proton acceptors – They are reduced and carry e-’s from Citric cycle to ETC – Dehydrogenase catalyzes hydrogen transfer reaction

NAD+ and FAD Oxidized Form Reduced Form NAD+ NADH (2 e-, 1 H) FAD FADH 2 (4 e-, 2 H) THINK: FADH 2 come into play in the 2 nd stage of cellular respiration; it is also the 2 nd electron carrier

An overview of the citric acid cycle (this occurs for EACH pyruvate molecule) ATP 2 CO 2 3 NAD + 3 NADH + 3 H + ADP + P i FAD FADH 2 Citric acid cycle CoA Acetyle CoA NADH + 3 H + CoA CO 2 Pyruvate (from glycolysis, 2 molecules per glucose) ATP Glycolysis Citric acid cycle Oxidative phosphorylation Figure 9.11

Krebs/ citric acid cycle Main Function of the Krebs → to make electron carriers (NADH and FADH 2 ) to send to the ETC Series of 8 steps; Occurs in the mitochondrial matrix So…1 glucose produces: 2 ATP 6 NADH 2 FADH 2 (remember: 1 glucose = 2 pyruvates) So…1 glucose produces: 2 ATP 6 NADH 2 FADH 2 (remember: 1 glucose = 2 pyruvates) Acetyl CoA (2C) enters the Krebs and combines with another molecule (4C) to form citric acid (hence citric acid cycle) Electron Carriers

Krebs → Makes 1 ATP, 3 NADH, and 1 FADH 2 per turn You do NOT need to memorize this!

Figure 9.12 Acetyl CoA NADH Oxaloacetate Citrate Malate Fumarate Succinate Succinyl CoA  -Ketoglutarate Isocitrate Citric acid cycle SCoA SH NADH FADH 2 FAD GTP GDP NAD + ADP P i NAD + CO 2 CoA SH CoA SH CoA S H2OH2O + H + H2OH2O C CH 3 O OCCOO – CH 2 COO – CH 2 HO C COO – CH 2 COO – CH 2 HCCOO – HOCH COO – CH CH 2 COO – HO COO – CH HC COO – CH 2 COO – CH 2 CO COO – CH 2 CO COO – Glycolysis Oxidative phosphorylation NAD + + H + ATP Citric acid cycle Figure 9.12

Kreb’s Cycle Summary pyruvate  Acetyl-CoA + 1 NADH Each turn of cycle uses 1 pyruvate – So… 1 glucose molecule produces 2 turns of Kreb’s cycle 1 turn of cycle yields 4 NADH, 1 ATP, and 1 FADH 2 and 3 CO2 (as waste product) Remember to multiply by 2…why?

Stage #3: Oxidative Phosphorylation ( Electron Transport Chain (ETC) + Chemiosmosis) Chemiosmosis couples electron transport to ATP synthesis NADH and FADH 2 – Donate e-s to ETC, which powers ATP synthesis using oxidative phosphorylation **OCUURS IN CRISTAE (folds of inner membrane)

The Pathway of Electron Transport In the ETC… – e-s from NADH and FADH 2 lose energy in several steps **NEEDS O 2 TO PROCEED (unlike glycolysis)

Electron transport chain (ETC) Occurs on the inner membrane of the mitochondria; Energy from NADH and FADH 2 power ATP synthesis The ETC is a series of proteins throughout the membrane; the electrons lose energy every time they get passed down the chain… OXYGEN IS THE FINAL ELECTRON ACCEPTOR!!! → oxygen combines with the electrons and H+ to make WATER

Main Goal of the ETC → It creates a proton gradient that powers chemiosmosis which creates ATP through ATP Synthase NADH and FADH2 drop off electrons to the ETC As the electrons get passed down the chain, they lose energy The ETC uses that energy (from the electrons) and pumps the protons OUT of the matrix into the intermembrane space This creates a concentration gradient The protons then diffuse back INTO the matrix through the ATP synthase (chemiosmosis) -This creates a TON of ATP  either 32 or 34 ATPs Final e- acceptor after the electrons go down the ETC is OXYGEN (from the atmosphere) It combines with e- and H+ to make the final product = WATER Electron transport chain (ETC)

ETC Characteristics Lots of proteins (cytochromes) in cristae  increases surface area – 1000’s (many) copies of ETC ETC carries e-’s from NADH/FADH 2  O 2 O 2 = pulls e-s “down” ETC due to electronegativity (high affinity for e-s)

Chemiosmosis and the electron transport chain Oxidative phosphorylation. electron transport and chemiosmosis Glycolysis ATP Inner Mitochondrial membrane H+H+ H+H+ H+H+ H+H+ H+H+ ATP P i Protein complex of electron carriers Cyt c I II III IV (Carrying electrons from, food) NADH + FADH 2 NAD + FAD + 2 H / 2 O 2 H2OH2O ADP + Electron transport chain Electron transport and pumping of protons (H + ), which create an H + gradient across the membrane Chemiosmosis ATP synthesis powered by the flow Of H + back across the membrane ATP synthase Q Oxidative phosphorylation Intermembrane space Inner mitochondrial membrane Mitochondrial matrix Figure hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535:: 535::/sites/dl/free/ /120071/bio11. swf::Electron%20Transport%20System%20and %20ATP%20Synthesis Protin motive force is used!

At the end of the chain – Electrons are passed to oxygen, forming water – O 2 = final e- acceptor NAD delivers e- higher than FAD  NAD provides 50% more ATP What happens at the end of the ETC chain?

ETC is a Proton (H+) Pump Uses the energy from “falling” e-s (exergonic flow) to pump H+’s from matrix  to outer compartment A H+ (proton) gradient forms inside mitochondria ETC does NOT make ATP directly but provides stage for CHEMIOSOMOSIS to occur

RECALL…Chemiosmosis Is an energy-coupling mechanism that uses energy in the form of a H + gradient across a membrane to drive cellular work Uses ATP synthase Makes ~90% of ATP in Cell Resp. Proposed by Peter Mitchell (1961)

Chemiosmosis: The Energy- Coupling Mechanism ATP synthase – Is the enzyme that actually makes ATP INTERMEMBRANE SPACE H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ P i + ADP ATP A rotor within the membrane spins clockwise when H + flows past it down the H + gradient. A protein anchored in the membrane holds the knob stationary. A rod (or “stalk”) extending into the rotor/ knob also spins, activating catalytic sites in the knob. Three catalytic sites in the stationary knob join inorganic Phosphate to ADP to make ATP. MITOCHONDRIAL MATRIX Figure m/life- science/metabolomics/learni ng-center/metabolic- pathways/atp-synthase.html

There are three main processes in this metabolic enterprise Electron shuttles span membrane CYTOSOL 2 NADH 2 FADH 2 2 NADH 6 NADH 2 FADH 2 2 NADH Glycolysis Glucose 2 Pyruvate 2 Acetyl CoA Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis MITOCHONDRION by substrate-level phosphorylation by substrate-level phosphorylation by oxidative phosphorylation Maximum per glucose: About 36 or 38 ATP + 2 ATP+ about 32 or 34 ATP or Figure ATP

About 40% of the energy in a glucose molecule – Is transferred to ATP during cellular respiration, making ~ ATP

Overall (Aerobic Respiration) FADH2NADHCO2ATP Total ATP Gained Glycolysis 242 (net) Pyruvate  Acetyl- CoA 2 Kreb’s Cycle ETC ~32-34 ATP GRAND TOTAL = ~36-38 ATP per 1 GLUCOSE

Cellular Respiration overview ATP Summary → Glycolysis – 2 ATP Krebs – 2 ATP ETC – 32/34 ATP