Cellular Respiration Continued:

The Citric Acid Cycle and Electron Transport

Chain

Citric Acid Cycle aka Krebs 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 – Citric Acid – pyruvic acid from glycolysis is broken down into carbon dioxide in a series of energy extracting reactions

Where: mitochondria

What comes out:

•2 ATPs (one for each pyruvic acid)

•NADH, FADH2

•CO2 (lots)

What goes in:

•Pyruvic acid (from glycolysis

•Oxygen

Steps of Citric Acid Cycle

• Occurs in mitochondria• 1) Pyruvic acid from glycolysis

enters mitochondria. One carbon atom from pyruvic acid becomes part of carbon dioxide which is released (acetic acid now). 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 Citric Acid Cycle(continued)

• 2) Citric acid then is broken down into a 4-carbon molecule, more CO2 is released and electrons are transferred to energy carriers (NADH, FADH2)

Summary: Citric acid turned into CO2 and high-energy electrons gathered, NADH, FADH2

Steps of Citric Acid Cycle(continued)

• 3) 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 Citric Acid Cycle

Mitochondrion

Citric Acid Production

 The Citric Acid Cycle

Mitochondrion

Citric Acid Cycle

• ATP produced in Citric Acid 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 Citric Acid cycle to the electron transport chain, where the electrons are used to convert ADP to ATP

Where: mitochondria

What goes in:

•NADH and FADH2 (from Citric Acid Cycle and Glycolysis)

•Oxygen

What comes out:

•28 ATPs (up to 32)

•H2O

Steps in the Electron Transport Chain

• 1) 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)

• 2) High-energy electrons transport H+ ions across the membrane. During electron transport, H+ ions build up on one side of the membrane = concentration gradient – more H+ on inside = want to go outside membrane

Summary: During electron transport down chain, H+ brought into membrane = concentration gradient – more H+ on inside = want to go outside membrane

Steps in the Electron Transport Chain

(continued)

• 3) Concentration gradient fuels ATP synthase to change ADP into ATP

Summary: Use concentration gradient to fuel ATP synthase to make ATP

The Electron Transport Chain

12

Spacebetweenmembranes

Innermitochondrialmembrane

Electroncarrier

Proteincomplex

Electronflow

Matrix Electron transport chain ATP synthase

NADH NAD

FADH2 FAD

ATPADP

H2OO2 2

P

– –

– –

H

H

H

H

H

HH

H

H

H

H

H

H

H

H H

H

H

H1

2

4

6

53

H

Cellular Respiration Review

The Totals of ATP Production through Entire Cellular Respiration

• Glycolysis produces = 2 ATPs

• Citric Acid Cycle and electron transport chain produce = 30 ATP (Citric Acid Cycle produces 2 ATPs and electron transport chain produces 28 ATPs (can produce up to 32))

• Total = 32 ATP molecules produced (can be as high as 36)