1. Biology, 9th ed,Sylvia Mader
Chapter 08
Chapter 08
Cellular Respiration
Cellular Respiration

2. Outline Glycolysis Preparatory Reaction Citric Acid (Krebs) Cycle
Electron Transport System
Fermentation
Metabolic Pool
Catabolism
Anabolism

3. Cellular Respiration Exergonic reaction releases energy.
Glucose is high-energy molecule; CO2 and H2O are low-energy molecules
Endergonic reactions requires energy.
Buildup of ATP

4. Cellular Respiration
A cellular process that requires oxygen and gives off carbon dioxide
Usually involves breakdown of glucose to carbon dioxide and water
Energy extracted from glucose molecule:
Released step-wise
Allows ATP to be produced efficiently (39%)
Oxidation-reduction enzymes include NAD+ and FAD as coenzymes

5. Glucose Breakdown: Summary Reaction

6. NAD+ and FAD NAD+ (nicotinamide adenine dinucleotide)
Called a coenzyme of oxidation-reduction it can
Oxidize a metabolite by accepting electrons
Reduce a metabolite by giving up electrons
Each NAD+ molecule used over and over again
FAD (flavin adenine dinucleotide)
Also a coenzyme of oxidation-reduction
Sometimes used instead of NAD+
Accepts two electrons and two hydrogen ions (H+) to become FADH2

7. NAD+ Cycle

8. Cellular Respiration: Overview of 4 Phases
Glycolysis:
Occurs in cytoplasm
Glucose broken down to two molecules of pyruvate
ATP is formed
Preparatory reaction:
Both pyruvates are oxidized
Electron energy is stored in NADH
Two carbons are released as CO2
Citric Acid (Krebs) Cycle:
Electron energy is stored in NADH and FADH2
Four carbons are released as CO2
Electron transport chain:
Extracts energy from NADH & FADH2
Produces 32 or 34 molecules of ATP

9. Glucose Breakdown: Overview of 4 Phases

10. Glucose Breakdown: Glycolysis
Occurs in cytoplasm outside mitochondria
Energy Investment Steps:
Two ATP are used to activate glucose
Glucose splits into two G3P molecules
Energy Harvesting Steps:
Two electrons (as hydrogen atoms) are picked up by two NAD+
Four ATP produced by substrate-level phosphorylation
Net gain of two ATP
Both G3Ps converted to pyruvates

11. Energy Investment stage ends with G3P
Steps of Glycolysis
Energy Investment stage ends with G3P
Energy Harvesting stage ends with Pyruvate

12. Energy Investment stage ends with G3P
Steps of Glycolysis
Energy Investment stage ends with G3P

13. Energy Harvesting stage ends with Pyruvate
Steps of Glycolysis
Energy Harvesting stage ends with Pyruvate

14. Glycolysis: The Balance Sheet

15. Substrate-level Phosphorylation

16. Glycolysis

17. Glucose Breakdown: The Preparatory (Prep) Reaction
End product of glycolysis, pyruvate, enters the mitochondrial matrix
Pyruvate converted to 2-carbon acetyl group
Attached to Coenzyme A to form acetyl-CoA
Electron picked up (as hydrogen atom) by NAD+
CO2 released, and transported out of mitochondria into the cytoplasm

18. Mitochondrion: Structure & Function

19. Preparatory Reaction

20. Glucose Breakdown: The Citric Acid Cycle
A.K.A. Krebs cycle
Occurs in matrix of mitochondria
Both acetyl (C2) groups received from the preparatory reaction:
Join with an enzyme CoA molecule to make acetyl-CoA
Acetyl (C2) group transferred to oxaloacetate (C2) to make citrate (C6)
Each acetyl oxidized to two CO2 molecules
Remaining 4 carbons from oxaloacetate converted back to oxaloacetate (thus “cyclic”)
NADH, FADH2 capture energy rich electrons
ATP formed by substrate-level phosphorylation

21. The Citric Acid Cycle

22. The Citric Acid Cycle

23. Citric Acid Cycle: Balance Sheet

24. Electron Transport Chain
Location:
Eukaryotes: cristae of the mitochondria
Aerobic Prokaryotes: plasma membrane
Series of carrier molecules:
Pass energy rich electrons along
Complex arrays of protein and cytochromes
Cytochromes are respiratory molecules
Complex carbon rings with metal atoms in center
Receives electrons from NADH & FADH2
Produce ATP by oxidative phosphorylation

25. Electron Transport Chain
The fate of the hydrogens:
Hydrogens from NADH deliver enough energy to make 3 ATPs
Those from FADH2 have only enough for 2 ATPs
“Spent” hydrogens combine with oxygen
Recycling of coenzymes increases efficiency
Once NADH delivers hydrogens, it returns (as NAD+) to pick up more hydrogens
However, hydrogens must be combined with oxygen to make water
If O2 not present, NADH cannot release H
No longer recycled back to NAD+

26. Electron Transport System
Electron transport system is located in cristae of mitochondria; consists of carriers that pass electrons
Electrons that enter the electron transport system are carried by NADH and FADH2
Some protein carriers are cytochrome molecules
Chemiosmosis

27. NADH delivers electrons to system; by the time electrons are received by O2, three ATP are formed
If FADH2 delivers electrons to system, by the time electrons are received by O2, two ATP are formed.

28. Electron Transport Chain & Chemiosmosis

29. Organization of Cristae

30. Glucose Catabolism: Overall Energy Yield
Net yield per glucose:
From glycolysis – 2 ATP
From citric acid cycle – 2 ATP
From electron transport chain – 32 ATP
Energy content:
Reactant (glucose) 686 kcal
Energy yield (36 ATP) 263 kcal
Efficiency 39%; balance is waste heat

31. Overall Energy Yielded per Glucose Molecule

32. Fermentation (1) When oxygen limited: Fermentation:
Spent hydrogens have no acceptor
NADH can’t recycle back to NAD+
Glycolysis stops because NAD+ required
Fermentation:
“Anaerobic” pathway
Can provide rapid burst of ATP
Provides NAD+ for glycolysis
NADH combines with pyruvate to yield NAD+

33. Fermentation

34. Fermentation (2) Pyruvate reduced by NADH to:
Lactate
Animals & some bacteria
Cheese & yogurt; sauerkraut
Ethanol & carbon dioxide
Yeasts
Bread and alcoholic beverages
Allows glycolysis to proceed faster than O2 can be obtained
Anaerobic exercise
Lactic acid accumulates
Causes cramping and oxygen debt
When O2 restored, lactate broken down to acetyl-CoA and metabolized

35. Products of Fermentation

36. Efficiency of Fermentation
InLine Figure 143