Cellular respiration involves three main stages - glycolysis, the Krebs cycle, and the electron transport chain. Glycolysis breaks down glucose into pyruvate and produces a small amount of ATP. In the presence of oxygen, pyruvate enters the mitochondria and is further oxidized in the Krebs cycle, producing more ATP, NADH, and FADH2. These reduced coenzymes donate their electrons to the electron transport chain, where oxygen is the final electron acceptor. This powers ATP synthase to produce the majority of the cell's ATP through chemiosmosis. In total, the complete oxidation of one glucose molecule yields approximately 38 ATP.

1. Cellular Respiration
Harvesting Chemical Energy

2. Decomposition pathways
 Release stored energy by breaking down
complex molecules
 Fermentation – partial breakdown of sugars
in absence of oxygen
 - anaerobic
 Cellular respiration – breakdown of organic
compounds requiring oxygen
 - aerobic

3. Cellular Respiration
 Overall process
Organic compounds + Oxygen
 Carbon dioxide + Water + Energy

4. Cellular Respiration
 Breakdown of glucose
C6H12O6 + 6 O2  6 CO2 + 6 H2O +
energy
 Energy released stored in ATP
 ATP provides energy to power most life
processes

5.  How do cells make ATP from the energy in
food?

6. Cellular Respiration
 Three stages
 Glycolysis
 Krebs cycle
 Electron transport chain (ETC)

7. Cellular Respiration
 Glycolysis does not need O2
 Krebs Cycle and electron transport chain do

8. Glycolysis
 Decomposition pathway
 Occurs in the cytoplasm
 1st stage of both anaerobic and aerobic
respiration

9. Glycolysis
 Glucose + ATP → Glucose-6-phosphate +
ADP
 G–6–P rearranges and combines with
another ATP → Fructose diphosphate

10. Glycolysis
 Fructose diphosphate → 2 3-carbon molecules
(PGAL)
 PGAL molecules rearranged to form pyruvic acid
(pyruvate)
 Releases energy, stored in 2 ATP molecules for each
reaction
 Electrons released from PGAL convert NAD+ →
NADH

12. Summary of Glycolysis
 For each molecule of glucose:
 2 molecules pyruvate
 2 NADH
 2 ATP (net): (4 ATP – 2ATP)

13. What happens next?
 Depends on whether or not O2 is present
 If O2 present, pyruvate enters mitochondria
for aerobic respiration
 If no O2 present, fermentation

14. Fermentation (Anaerobic respiration)
 NADH + pyruvate → NAD+ and lactate (also
a 3-carbon acid)
 NAD+ returns to glycolysis
 = Lactic Acid fermentation

15. Lactic acid fermentation
 Human muscle cells (when O2 scarce)
 Cheese and yogurt

16. Fermentation (Anaerobic respiration)
 Yeast:
 pyruvate + NADH → ethanol and CO2 + NAD+.
= Alcoholic Fermentation
 In both lactic acid and alcoholic fermentation,
no more ATP produced.

19. Aerobic vs. Anaerobic Respiration

20. Mitochondria
 Further steps of aerobic
respiration involve an
electron transport system
 In eukaryotic cells, the membrane for this is
in the mitochondria

22. Mitochondria
 “Powerhouse of the cell”
 Where most of the ATP is synthesized
 10-1000’s of mitochondria/cell

23. As the pyruvate is transported to the
mitochondria …..
 Pyruvate → Acetate
 Acetate + coenzyme A → Acetyl CoA
 Molecule of CO2 released
 NAD+ → NADH

24. Krebs cycle
 Decomposition pathway
 Occurs in mitochondrial matrix
 Acetyl CoA into CO2

25. Krebs Cycle
 Acetyl CoA + oxaloacetate (4-C)→ 6 carbon acid citrate (6-C)
 Coenzyme A released and recycled
 Citrate rearranged and oxidized (removal of electrons)
 Two of citrate’s carbons removed to form carbon dioxide
 Hydrogen atoms removed convert NAD+ → NADH

26. Krebs Cycle
 4-carbon molecule produced by citrate losing
2 carbons is rearranged and further oxidized
to become oxaloacetate
 Oxidation converts NAD+ to NADH and FAD
to FADH2→
 Energy released in these reactions used to
convert ADP + P → ATP

27. Krebs Cycle

28. Krebs cycle summary

29. For every turn of Krebs cycle
 Two C’s enter as acetyl CoA
 Two C’s are oxidized and leave as CO2
 Coenzymes NAD+ and FAD are reduced
 3 NADH and 1 FADH2 are produced
 One ATP produced
 Oxaloacetate is regenerated
 Note: Two turns are required for complete
oxidation of glucose

30.  Animation: How the Krebs Cycle Works (Quiz
1)

31. Cellular Respiration
 Three stages
 Glycolysis
 Krebs cycle
 Electron transport chain (ETC) and

32. Electron transport chain
 Uses inner membrane of mitochondrion
 Accepts energized electrons from reduced
coenzymes (NADH and FADH2)
 Uses the electrons for ATP synthesis
 produces most (90%) of ATP of aerobic
respiration

34. Electron transport chain (ETC)
 Electron carrier molecules called
cytochromes embedded in inner membrane
 H atoms of NADH and FADH2 are separated
into electrons and protons.
 Cytochromes transfer the electrons from one
to the next
 Last cytochrome is an enzyme that combines
electrons and protons with oxygen, forming
water

35. ETC
 With each transfer along the ETC, electrons
release free energy
 Used by proteins in the inner membrane to
actively transport protons from matrix to the
intermembrane space

36. Chemiosmosis
 Protons diffuse back across the membrane,
passing through an ATP-synthase complex
 ADP + P → ATP

37. ATP synthase
 Protons diffuse through ATP synthase
complex which converts ADP to ATP
 Cristae increase the surface area available
for chemiosmosis

39. ETC
 Electrons from each NADH can drive
synthesis of up to 3 ATP
 Electrons from each FADH2 can drive
synthesis of up to 2 ATP
 The 6 NADH and 2 FADH2 made from each
molecule of glucose can drive production of a
total of 22 ATP through the ETC

40. Overview of Cellular Respiration

41. Cellular Respiration
36 – 38 ATPs per glucose
Process Reduced
Coenzyme
ETC
Total
ATP
glycolysis
Net
2 ATP 2 NADH 4 - 6 ATP 6 - 8
Oxidation
of pyruvate
--
2 NADH 6 ATP 6
Krebs
cycle 2 ATP
6 NADH
2 FADH2
18 ATP
4 ATP 24

42. ETC
 For every two NADH, one O2 is reduced to
two H2O molecules

43. Cellular Respiration
 Estimated efficiency = 38%
 Energy lost in the process is lost as heat.

44. Related Metabolic Processes

45. Bacteria
 No mitochondria
 ETC occur in cell membrane
 Some bacteria use oxidizers other than
oxygen
 E.g. SO4
-2 , NO3
-

46. Facultative anaerobes
 Organisms capable of growth in either
aerobic or anaerobic environments
 Yeast, many bacteria, mammalian muscle cells

47. Obligate anaerobes
 Bacteria that are poisoned by O2.
 Can only do fermentation or anaerobic
respiration.

48. Obligate aerobes
 Us! (And all other animals and plants)
 Cannot survive without O2

49. Evolutionary significance of glycolysis
 First prokaryotes produced ATP by glycolysis
 Why do we think this?

50.  Complex molecules can be hydrolyzed to
intermediates of glycolysis or Krebs cycle

51.  Starch, glycogen, disaccharides  glucose

52.  Proteins  amino acids  deaminated  . . .

53.  Fats 
 Glycerol  glyceraldehyde phosphate (PGAL)
 Fatty acids  acetyl CoA

56. Yeast that are facultative anaerobes use
more glucose when there is less oxygen
 Why?
 ATP acts as an inhibitor of
phosphofructokinase
 With fermentation, less ATP is produced
 Less inhibition of glycolysis

57. Control of Respiration
 What happens to glucose taken in as food?
 Depends on energy needs
 If low energy demand, stored as starch
(plants) or glycogen (animals). Fats also
stored.
 If high energy demand, broken down in cell
respiration