Aerobic Respiration

Aerobic RespirationSBI4U1

Aerobic Respiration: catabolic pathway that requires oxygen

C6H12O6 (s) + 6O2 (g) 6CO2 (g) + 6H2O (l) + energy

• Plants and animals rely on it to form ATP– Obligate anaerobes (organisms that must have oxygen)

• Energy from food molecules is transferred to ATP

• Cells use ATP to power endergonic rxns

Goals of Cellular Respiration

1. Break bonds b/t C-atoms in glucose to form 6CO2

2. Move H atom electrons from glucose to oxygen, to form 6H2O

3. Trap free energy released in the process in form of ATP

C6H12O6 (s) + 6O2 (g) 6CO2 (g) + 6H2O (l) + energy

• Cellular respiration decreases potential energy and increases entropy

• Yields 2870kJ of free energy per mol of glucose

• ΔG = -2870 kJ per mol of glucose

4 Steps in Cellular Respiration

1. Glycolysis (cytoplasm, 10 steps)

2. Pyruvate Oxidation (mitochondrial matrix, 1 step)

3. Krebs Cycle (mitochondrial matrix, 8 steps)

4. Oxidative Phosphorylation/Electron Transport Chain (cristae, most ATP generation)

#1

#2#3

#4

Glycolysis• glykos = sweet, lysis = splitting• Breaks down glucose into 2 molecules of

pyruvate • There are two stages (each w/ 5 steps, 10 total)• Does not require oxygen (essential anaerobic)

Animation: http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter25/animation__how_glycolysis_works.html

1st Stage: Glucose Split into Two1. Phosphorylation of

glucose by ATP2-3. Molecule is rearranged

and 2nd ATP phosphorylation

4-5. 6-C molecule split into two 3-C molecules• One glyceraldehyde 3-

phosphate (G3P) and one that will be converted later

2nd Stage: Forms Pyruvate6. Oxidation then

phosphorylation to produce NADH to 2 BPG (NAD+ reduced)

7. 2 ADP removes high energy phosphates, leaving 2 3PG molecules

8-9. H2O removed leaving 2 PEP molecules

10. 2 ADP removes high energy phosphates, leaving 2 pyruvate molecules

ReactantsC6H12O6

2 NAD+

2 ADP2 Pi

Products2 C3H4O3 (pyruvate)

2 NADH + H+

2 ATP (net)2 H2O

Net Reaction for Glycolysis:

Glucose + 2 NAD+ + 2 ADP + 2Pi 2 pyruvate + 2H2O + 2NADH + 2ATP

Animation (one more time!): http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter25/animation__how_glycolysis_works.html

Pyruvate Oxidation• Carboxyl group removed from pyruvate as CO2

• C2 fragment is oxidized into acetic acid (as NAD+ is reduced to NADH)

• Coenzyme A (a sulfur containing vitamin B derivative) forms an unstable bond with acetic acid

• Two molecules of Acetly-CoA and NADH produced

Acetyl CoA• Acetyl CoA is a

pivotal molecule in cellular metabolism

• Most molecules used to provide an organism with energy are converted to acetyl CoA(reversible)

Krebs Cycle

• A.k.a. Citric Acid Cycle• Cyclic metabolic pathway• Acetyl CoA is oxidized to CO2

• Regenerates compound that picks up more acetyl CoA

• Converts released energy to ATP, NADH, and FADH2

For each turn of the cycle:– 2 C atoms enter as

acetyl group and 2 C leave as CO2

– much of acetyl group’s energy is transferred as high energy electrons to reduce 3NAD+ → 3NADH and 1 FAD → 1 FADH2

– some of acetyl group’s energy is used in the substrate level phosphorylation of 1 ADP → 1ATP

– for each glucose molecule oxidized 2 acetyl CoA are produced

– there are 2 turns of the Kreb’s cycle for each 1 glucose

Reactants2 acetyl CoA2 oxaloacetate6 NAD+

2 ADP2 FAD2 Pi

Products2 CoA4 CO2

2 oxaloacetate6 NADH 6 H+

2 FADH2

2 ATP (net)

Reactants and Products of the Krebs Cycle:

Animation:http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter25/animation__how_the_krebs_cycle_works__quiz_1_.html

Oxidative Phosphorylation• During glycolysis and 2 rounds of the Krebs

cycle carbon from glucose CO2

• Very few ATP molecules have been produced at this point

• Energy is in NADH and FADH2

• Oxygen is used here to produce majority of ATP

Animation: http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::525::530::/sites/dl/free/0072464631/291136/electron_transport.swf::electron_transport.swf

Electron Transport Chain (ETC)• System of enzymes

(with cofactors) embedded in the inner mitochondrial membrane

• Enzymes pass electrons from NADH and FADH2 to O2 in a series of redox reactions

• Each component of the ETC is more electronegative than the previous

• Final electron acceptor, O2, is one of the most electronegative substances on earth

•Electron transfer from NADH to O2 is highly exergonic•Intermediate steps help release the energy in manageable amounts to accommodate the change in energy form

•FADH2 enters the ETC after the first part of the

multi-enzyme complex•Therefore yields 1/3 less energy

• NADH from cytosol cannot move into the mitochondrial matrix

• Electrons must be shuttled, usually via an FADH2 • Therefore usually yields 1/3 less energy then NADH

produced in the mitochondrial matrix

ETC

Animation (one more time!): http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::525::530::/sites/dl/free/0072464631/291136/electron_transport.swf::electron_transport.swf

Chemiosmosis

• H+ ions pumped using energy released in the electron cascade through the ETC

• Creates a high [H+] in the inner membrane space• This gradient drives phosphorylation of ADP ATP

ATP Synthase

• ATP is formed (oxidative phosphorylation)

• As the H+ re-enter the matrix through special protein channels that are coupled with ATP synthase

Yield of ATP from Aerobic Respiration

• It is possible to generate 36 or 38 molecules of ATP from one glucose molecule– 38 for prokaryotes b/c they do not need to use 2 ATP

transport NADH from glycolysis across mitochondrial membrane

• These #’s are theoretical• Experimental yields are lower. Why?

(read text for reasons to account for lower values)

Interconnections of Metabolic Pathways

Humans eat more than just glucose. So what happens to the other molecules?

• Compounds from all nutrients can be broken down

• They can enter glycolysis and the Krebs cycle– E.g. Glycerol from fatty acids can be converted to G3P– E.g. Amino acid alanine is directly converted to

pyruvate

Things You Should Know...

• Aerobic respiration• 4 stages in respiration– Know where they occur– Know the basic steps (not all intermediates, but

major molecules)– Know electron carriers (NAD+ and FAD)– Reactants/products for each (esp. ATP)

• Examples of common intermediates between fat, carbohydrates, protein and stages in respiration.