1. Cellular Respiration: Krebs cycle and ETC
IB HL Biology 1

2. Cell Respiration- controlled release of energy from organic compounds in cells to form ATP
Aerobic Cellular Respiration a metabolic pathway
with over 20 reactions, using 20 enzymes

3. Why cell respiration?
Cells require a constant source of energy to perform various tasks
Movement
Transport
Division

4. Summary Equation C6H12O6 + O2  CO2 + H2O + ATP
The summary equation for cellular respiration is:
C6H12O6 + O2  CO2 + H2O +
Glucose + oxygen  carbon dioxide + water + ATP
ATP

5. Key players in this process
Glucose: source of fuel
NAD+: electron carrier
Enzymes: mediate entire process
Mitochondria: site of aerobic respiration
ATP: principal end product
Protons/Electrons: sources of potential energy
Oxygen: final electron acceptor

6. Redox Reactions
Reduction: reducing overall positive charge by gaining electrons
Oxidation: loss of electrons
OIL RIG
Oxidation Is Loss of e- and addition of oxygen
Reduction Is Gain of e- and loss of oxygen
Redox reactions produce energy change
Reduction absorbs energy (endergonic)
Oxidation releases energy (exergonic)

7. Respiration is a controlled release of energy
It’s a highly exergonic, but well-controlled process
Mediated by enzymes, electron carriers
Otherwise, it would be like an explosion
Not compatible with life!

8. Phosphorylation
Addition of a phosphate group to a molecule; ex. Adding PO4 to ADP to form ATP
Occurs in two ways
Substrate level phosphorylation
Oxidative phosphorylation

9. Substrate-level phosphorylation
An enzyme transfers a phosphate group from a substrate to ADP
Making ATP with an enzyme
Ineffective in generating large amounts of ATP
Occurs during glycolysis; addition of PO4 to glucose and ATP is made when pyruvate loses PO4 to ADP

10. Oxidative phosphorylation
Refers to phosphorylation that occurs due to redox reactions transferring electrons from food to oxygen
Occurs in electron transport chains in mitochondrion
Makes ATP using energy derived redox reactions

11. Three stages of cell respiration
Stage 1: Glycolysis (energy investment)
Some ATP is made, some is used
Stage 2: Krebs Cycle (oxidation of pyruvate)
Generation of CO2
Stage 3: Oxidative Phosphorylation -ETC
Generation of most ATP

12. Three stages of respiration

13. Glycolysis General Outline No Oxygen Anaerobic Oxygen Aerobic
Glucose
Glycolysis
No Oxygen
Anaerobic
Oxygen
Aerobic
Pyruvic Acid
Transition Reaction
Fermentation
Krebs Cycle
ETS
2 ATP
36-38 ATP

14. Mitochondrion--Structure and Function 2 phospholipids bilayers with embedded protein
Inner Membrane
Contains folds called cristae
Cristae contain carriers of ETC
Inter-membrane Space
Space between outer and inner membrane
ETC pumps H+ here for oxidative phosphorylation
Matrix
Space inside inner membrane
Contains enzymes, ribosomes, small loops of DNA and metabolites
acetylCoA formed here
Krebs cycle occurs here

15. Mitochondrion Mitochondria are membrane-bound organelles
The outer membrane is fairly smooth. But the inner membrane is highly convoluted, forming folds called cristae.
The cristae greatly increase the inner membrane's surface area.
On these cristae organic molecules are combined with oxygen to produce ATP - the primary energy source for the cell. 15

16. 8.1.3 Structure of the mitochondria.
Location of aerobic respiration
Pyruvate, the product of glycolysis can be further oxidized here to release more energy.
Mitochondria are only found in eukaryotic cells.
Cells that need a lot of energy will have many mitochondria ( liver cell) or can develop them under training (muscles cells).
There is a double membrane.
The inner membrane is folded to form 'cristae'.
There is a space between the two membranes which is important for creating a place to concentrate H+ (see 8.1.6 )
The inner space is called the matrix.
Mitochondria contain some of their own DNA (mDNA).
8.1.6 Relationship between the structure and function of the mitochondria.
1. Cristae folds increase the surface area for electron transfer system.
2. The double membrane creates a small space into which the H+ can be concentrated.
3. Matrix creates an isolated space in which the Krebs cycle can occur. 16

17. DRAW AND LABEL A DIAGRAM SHOWING THE STRUCTURE OF A MITOCHONDRION AS SEEN IN ELECTRON MICROGRAPHS.17

18. General Outline of Aerobic Respiration
Glycolysis
Transition or
Link Reaction
Krebs Cycle
Electron Transport System

19. Aerobic Cellular Respiration
electrons
electrons
Glycolysis
Krebs Cycle
ETC &
chemiosmosis
glucose
pyruvate
ATP
ATP
ATP

20. Krebs Cycle
When O2 is present, pyruvate enters mitochondrion, enzymes of the Krebs cycle complete the oxidation of the organic fuel
Upon entering, pyruvate is converted to acetyl coenzyme A (acetyl CoA)
This step is the transition between glycolysis and Krebs cycle; is called the link reaction
It is accomplished by a multi-enzyme complex that catalyzes three reactions
Glycolysis releases less than a quarter of the chemical energy stored in glucose
Most of the energy remains stocked in the two molecules of pyruvate

21. Transition to the Krebs cycle
Link Reaction
Step 1: carboxyl group removed, CO2 diffuses out
Step 2: remaining 2-carbon fragment is oxidized, forms acetate
enzyme transfers extracted electrons to NAD+, storing energy in form of NADH
Step 3: coenzyme A attaches to acetate by unstable bond, makes acetyl group very reactive
The final product is acetyl Co-A, which enters Krebs cycle for further oxidizing

22. Conversion of pyruvate to acetyl CoA
Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the Krebs cycle, involves three reactions:
Step 1:
carboxyl group removed, CO2 diffuses out
Step 2:
remaining 2-carbon fragment is oxidized, forms acetate (ionized form of acetic acid)
enzyme transfers extracted electrons to NAD+, storing energy in form of NADH
Step 3:
coenzyme A attaches to acetate by unstable bond, makes acetyl group very reactive
The final product is acetyl Co-A, which enters Krebs cycle for further oxidizing

23. Krebs cycle has eight steps, each catalyzed by a specific enzyme
For each turn, two carbons enter as acetate, and two different carbons leave in oxidized form of CO2
Acetate joins the cycle by its enzymatic addition to oxaloacetate, forming citrate
Subsequent steps decompose oxaloacetate, giving off CO2
It is the regeneration of oxaloacetate that accounts for the “cycle” in the Krebs cycle

24. Most of energy harvested by oxidative steps is conserved in NADH.
for each acetate that enters the cycle, three NAD+ are reduced to NADH.
In one oxidative step, electrons are transferred to FAD (flavin adenine dinucleotide).
reduced form FADH2 donates its electrons to the electron transport chain
Another step forms an ATP by substrate-level phosphorylation
NADH and FADH2 relay extracted electrons to electron transport chain to make ATP by oxidative-phosphorylation
Krebs cycle turns twice, once for each of the two pyruvate generated by glycolysis

25. Krebs Cycle Animation

26. Summary of Krebs Cycle Results in:
3 CO2 (including the one from pre-Krebs cycle conversion of pyruvate to acetyl-CoA)
1 ATP per turn by substrate phosphorylation
Most of chemical is transferred during redox rxns to NAD+ and FAD. The reduced coenzymes, NADH and FADH2, shuttle the energy-electrons to electron transport chain, which uses the energy to synthesize ATP by oxidative-phosphorylation
TOTAL x 2:
3 CO2 x 2 = 6 CO2
1 ATP x 2 = 2 ATP
4 NADH x 2 = 8 NADH
1 FADH2 x 2 = 2 FADH2

27. Transition or Link Reaction (refer to p273 Clegg)
Links glycolysis with reactions occuring in mitochodria
Pyruvate diffuses into mitochondrial matrix
It is decarboxylated by removal of CO2 and oxidized by removal of H+
NAD is reduced by addition of e- and H+
A two-carbon acetyl group is formed and combines with Coenzyme A to make acetyl CoA, which enters the Krebs cycle
Pyruvic Acid
Acetyl CoA
CO2

28. Stage 2: Krebs cycle (refer to p273 Clegg)
Where:
Matrix of mitochondria, but only if O2 present
Why:
To oxidize pyruvate to CO2
To build up a H+ ion gradient used in electron transport
FAD and NAD are reduced
FAD is chief H-carrying coenzyme in ETC

29. Krebs cycle summary Per acetyl CoA that enters: 1 ATP made
2 CO2 given off
3 NADH produced
1 FADH2 produced
Think: how many acetyl CoA entered the cycle?
How many times must this cycle happen to break down ONE glucose?
Refer to p274 Clegg text Table 9.1

30. Aerobic Cellular Respiration
electrons
electrons
Glycolysis
ETC &
chemiosmosis
Krebs Cycle
glucose
pyruvate
ATP
ATP
ATP

31. Stage 3: Oxidative phosphorylation
Where:
Inner membrane of mitochondria (on cristae)
Why:
To produce ATP from H+ ion gradient generated during Krebs cycle
Requires oxygen!
ETC Movie

32. Oxidative Phosphorylation and ETC (refer to p275 Clegg)
Chemiosmosis is process by which synthesis of ATP is coupled with ETC by H+ movement
Occurs in the cristae folds of inner mitochondrial membrane where e- carrier proteins are arranged
4 protein-based complexes that work in sequence moving H+
Carrier proteins oxidize the reduced coenzymes
Energy from this process pumps H+ from matrix to the inter-membrane space (proton pumps)

33. Oxidative Phosphorylation and ETC (refer to p275 Clegg)
A concentration of H+ ions accumulate between the inner and outer membranes; the H+ contain energy like a dam and pH decreases
H+ concentration gradient creates a potential difference across the membrane
This causes the synthesis of ATP by chemiosmosis as H+ flow back into membrane via channels in ATPase enzymes and APTase uses energy from gradient to make ATP
Energized e- & H+ from the 10 NADH2 and 2 FADH2 (produced during glycolysis & Krebs cycle) are transferred to O2 to produce H2O (redox reaction) O2  +  4e-  +  4H+  2H2O

34. Summary: Oxidative Phosphorylation
34 ATP made
H2O generated
NADH oxidized back to NAD
Very efficient process! Produces a lot of energy.

35. Electron Transport Chain:
Found in the inner mitochondrial membrane or cristae
Contains 4 protein-based complexes that work in sequence moving H+ from the matrix across the inner membrane (proton pumps)
A concentration gradient of H+ between the inner & outer mitochondrial membrane occurs
H+ concentration gradient causes the synthesis of ATP by chemiosmosis
Energized e- & H+ from the 10 NADH2 and 2 FADH2 (produced during glycolysis & Krebs cycle) are transferred to O2 to produce H2O (redox reaction)
O e H+ 2H2O

36. Electron Transport Chain & Chemiosmosis
intermembrane space
 Chemiosmosis
 ATP synthetase: proton pump
inner mito. membrane
The importance of e-?!?
Force the displacement of H+ from the matrix to intermembrane space
ADP + Pi
ATP
matrix

37. Electron Transport Chain & Chemiosmosis
NAD+
NADH H+
+ H+ H+ H+

38. Electron Transport Chain & Chemiosmosis
NAD+
NADH H+ H+
+ H+

39. Electron Transport Chain & Chemiosmosis
NAD+
NADH H+
+ H+

40. Electron Transport Chain & Chemiosmosis
2 H+ + ½ O2
H20
NAD+
NADH
+ H+
electon transport chain
chemiosmosis

41. Electron Transport Chain & Chemiosmosis
2 H+ + ½ O2
H20
NAD+
NADH
ADP + P
ATP
+ H+

42. C6H12O6 + 6O2 --> 6CO2 + 6H2O+ energy to make 38 ATP
Stage
Location
Reactants
Products
ATP molecules
Glycolysis
cytosol
1 Glucose,
2 ATP, 4ADP,
2 NAD +
2 Pyruvate
4 ATP
2 NADH 2
Link Reaction
Upon entering mitochondrion
2 pyruvate
2 CoenzymeA
2 Acetyl CoA
2 CO2
Krebs Cycle
Mitochondria
matrix
6 NAD +
2 FAD, 2 ADP
4 CO2
6 NADH
2 FADH ATP
Electron Transport Chain
Cristae (inner membrane)
10 NADH
2 FADH2
6O2
34 ATP
6H2O 34

43. Other molecules can be used in respiration
Proteins: must be deaminated, then converted to pyruvate
Fats: undergo beta-oxidation
Cells prefer carbs