Cellular Respiration Continued:

The Krebs Cycle and Electron Transport Chain

Krebs Cycle aka Citric Acid Cycle

• Uses OXYGEN!! Occurs in mitochondria

• At the end of glycolysis, 90% of chemical energy in glucose is still unused. Need oxygen to remove this energy = aerobic respiration

• 2nd stage of cellular respiration – Krebs cycle (also called citric acid cycle) – pyruvic acid from glycolysis is broken down into carbon dioxide in a series of energy extracting reactions

What comes out:

•2 ATPs (one for each pyruvic acid)

•NADH, FADH2

•CO2 (lots)

What goes in:

•Pyruvic acid (from glycolysis

•Oxygen

Steps of Krebs cycle (Citric Acid Cycle)

• Occurs in mitochondria• A) Pyruvic acid from glycolysis

enters mitochondria. One carbon atom from pyruvic acid becomes part of carbon dioxide which is released. Two of the 3 carbon atoms are then joined to coenzyme A to form acetyl-CoA. Acetyl-CoA then adds the 2-carbon acetyl group to a 4-carbon molecule, producing a 6-carbon molecule called citric acid

Summary: Pyruvic acid turned into citric acid and CO2 released

Steps of Krebs cycle (Citric Acid Cycle)

(continued)

• B) Citric acid then is broken down into a 4-carbon molecule, more CO2 is released and electrons are transferred to energy carriers (NADH)

Summary: Citric acid turned into CO2 and high-energy electrons gathered

Steps of Krebs cycle (Citric Acid Cycle)

(continued)

• 6 carbon atoms in citric acid – 2 carbons removed as CO2 leaving a 4-carbon molecule which is ready to accept another 2-carbon acetyl group to start the cycle over again (leftover carbons go back into the cycle)

• For each turn of the cycle, an ATP is produced as well as 5 pairs of high-energy electrons captured by 5 carrier molecules – 4 NADH and 1 FADH2 (similar to NADH)

Citric Acid Production

 The Krebs Cycle

Mitochondrion

Citric Acid Production

 The Krebs Cycle

Mitochondrion

Krebs Cycle

• ATP produced in Krebs cycle is then used for cellular activities. In the presence of oxygen, high energy electrons are used to generate HUGE amounts of energy in one last step…

Electron Transport Chain

• Uses OXYGEN!! Occurs in mitochondria

• Electrons passed from carriers in the Krebs cycle to the electron transport chain, where the electrons are used to convert ADP to ATP

What goes in:

•NADH and FADH2 (from Krebs Cycle and Glycolysis)

•Oxygen

What comes out:

•32 ATPs

•H2O

Steps in the Electron Transport Chain

• A) Electrons from NADH and FADH2 passed along to chain. In eukaryotes, the chain is located in the inner membrane of the mitochondria. High energy electrons are passed from one carrier protein to the next. At the end of the chain, electrons bind with H+ and oxygen to form water. Oxygen is final electron acceptor. (Oxygen removes “used” electrons)

Summary: Electrons passed down chain to oxygen; water released

Steps in the Electron Transport Chain

(continued)

• B) High-energy electrons transport H+ ions across the membrane. During electron transport, H+ ions build up on one side of the membrane = positively charged. The other side becomes negatively charged = difference in charges

Summary: During electron transport down chain, H+ brought into membrane = + charge on one side and – charge on the other…

Steps in the Electron Transport Chain

(continued)

• C) Charge difference fuels ATP synthase to change ADP into ATP

Summary: Use charge difference to fuel ATP production

The Electron Transport Chain

Electron TransportHydrogen Ion Movement

ATP Production

ATP synthase

Channel

Matrix

The Totals of ATP Production through Entire Cellular Respiration

• Glycolysis produces = 2 ATPs

• Krebs Cycle and electron transport chain produce = 34 ATP (Kreb’s Cycle produces 2 ATPs and electron transport chain produces 32 ATPs)

• Total = 36 ATP molecules produced

Energy and Exercise

• Quick energy – uses ATP quickly (sprinting) and then lactic acid kicks in

• Long-term energy – ATP continues to be built constantly, supply the body with energy (distance running)