1. Harvesting Energy: Glycolysis and Cellular Respiration
Chapter 7
Harvesting Energy: Glycolysis and Cellular Respiration

2. 7.1 What Is The Source Of A Cell’s Energy?
The energy for cellular activities is stored until use in bonds of molecules such as carbohydrates and fats.
In order to be used, stored energy is transferred to the bonds of energy-carrier molecules such as adenosine triphosphate (ATP).
Glucose is a key energy-storage molecule, and energy stored in this molecule is harvested to power many biological processes in cells.

3. 7.1 What Is The Source Of A Cell’s Energy?
Photosynthesis is the ultimate source of cellular energy.
Photosynthetic cells capture and store sunlight energy, and this energy is later used by cells.
These cells can be the photosynthetic organisms themselves, or can be other organisms that directly or indirectly consume photosynthesizers.

4. 7.1 What Is The Source Of A Cell’s Energy?
Glucose metabolism and photosynthesis are complementary processes.
The products of each reaction provide reactants for the other.
The symmetry is apparent by comparing the equations that describe each process.
Photosynthesis:
6 CO2 + 6H2O + sunlight energy  C6H12O6 + 6 O2
Complete glucose metabolism:
C6H12O6 + 6O2  6 CO2 + 6 H20 + ATP + heat energy

5. 7.2 How Do Cells Harvest Energy From Glucose?
Glucose metabolism occurs in stages; the first stage is glycolysis.
Glycolysis occurs in the cytoplasm of cells.
Glucose (a six-carbon sugar) is split into two three-carbon pyruvate molecules.
The split releases a small amount of energy that is used to make two ATP molecules.

6. Electron transport chain
(cytoplasmic fluid)
glucose
Glycolysis 2
ATP 2 C C C 2 C C C
lactate
pyruvate or 2 C C + 2 C
Fermentation
ethanol
CO2
Cellular respiration C C
2 acetyl CoA 4 C
CO2 C
2 CO2
Krebs cycle 2
ATP
electron carriers
intermembrane compartment
H2O
Electron transport chain
32 or 34
ATP
(mitochondrion) O2
Fig. 7-1

7. 7.2 How Do Cells Harvest Energy From Glucose?
Pyruvate produced by glycolysis is further processed in the second stage, called cellular respiration.
Cellular respiration occurs in the mitochondria.
Cellular respiration breaks pyruvate down to CO2 and H20, yielding 34 or 36 ATP molecules.
Oxygen is needed for cellular respiration to proceed.

8. 7.2 How Do Cells Harvest Energy From Glucose?
When oxygen is not available, the second stage of glucose metabolism is fermentation.
In the absence of oxygen, pyruvate remains in the cytoplasm.
Cytoplasmic pyruvate may be converted to lactate or ethanol by fermentation.
Fermentation does not produce additional ATP energy.

9. 7.3 What Happens During Glycolysis?
Glycolysis splits one molecule of glucose into two molecules of pyruvate.
During glycolysis, one molecule of glucose yields two ATP and two molecules of the electron carrier nicotinamide adenine dinucleotide (NADH).
Glycolysis involves two major parts:
Glucose activation
Energy harvest

10. 7.3 What Happens During Glycolysis?
Glucose activation
The activation of glucose uses up some ATP energy.
Two ATP molecules are used to phosphorylate glucose to form fructose bisphosphate. 2
ATP 2
ADP C C C C C C C C C C C C
glucose P
fructose bisphosphate P 1
Glucose activation
Fig

11. 7.3 What Happens During Glycolysis?
Energy harvest from glycolysis
Two molecules of fructose bisphosphate split into two glyceraldehyde-3-phosphate (G3P) molecules.
Each G3P molecule is converted to pyruvate. 4
ADP 4
ATP 2 C C C 2 C C C
G3P
pyruvate P 2
NAD+ 2
NADH 2
Energy harvest
Fig

12. 7.3 What Happens During Glycolysis?
Energy harvest from glycolysis (continued)
Two ATPs are used to activate glucose.
Two ATPs are made for each pyruvate (four total).
Each conversion to pyruvate forms one molecule of NADH (two total).
Net gain from glycolysis: 2ATP + 2 NADH

13. 7.3 What Happens During Glycolysis?
The essentials of glycolysis 2
ATP 2
ADP 4
ADP 4
ATP C C C C C C C C C C C C 2 C C C 2 C C C
G3P
glucose
pyruvate P
fructose bisphosphate P P 2
NAD+ 2
NADH 1
Glucose activation 2
Energy harvest

14. 7.4 What Happens During Cellular Respiration?
Cellular respiration is the second stage of glucose metabolism, and it occurs only in the presence of oxygen.
Cellular respiration occurs in the mitochondria.
Large amounts of ATP are produced by this process, which converts pyruvate to CO2 and H2O.

15. Fig. 7-3 intermembrane compartment mitochondrion inner membrane
outer membrane
matrix
glucose
cristae
Glycolysis 1
coenzyme A
2 pyruvate 2
ATP H+
ATP
acetyl CoA 8
ATP-synthesizing enzyme 7
CO2 H+ 6 H+
(cytoplasmic fluid) H+
ADP
ATP
Krebs cycle
H2O
electron transport chain 5 H+
1/2 O2 H+
2e– H+ 4
2 H+
(matrix)
outer membrane H+ 3
energized electron carriers: NADH, FADH2
CO2
intermembrane compartment
2e–
inner membrane
depleted carriers: NAD+, FAD
Fig. 7-3

16. 7.4 What Happens During Cellular Respiration?
Steps of cellular respiration
Step 1: Two molecules of pyruvate produced by glycolysis are transported into the matrix of a mitochondrion.
Step 2: Each pyruvate is split into CO2 and acetyl CoA, which enters the Krebs cycle.
The Krebs cycle produces one ATP from each pyruvate, and donates electrons to NADH and flavin adenine dinucleotide (FADH2).

17. 7.4 What Happens During Cellular Respiration?
Steps of cellular respiration (continued)
Step 3: NADH and FADH2 donate energized electrons to the electron transport chain of the inner membrane.
Step 4: In the electron transport chain, electron energy is used to transport hydrogen ions (H+) from the matrix to the intermembrane compartment.
Step 5: Electrons combine with O2 and H+ to form H2O.

18. 7.4 What Happens During Cellular Respiration?
Steps of cellular respiration (continued)
Step 6: Hydrogen ions in the intermembrane compartment diffuse across the inner membrane, down their concentration gradient.
Step 7: The flow of ions into the matrix provides the energy to produce ATP from ADP.
Step 8: ATP moves out of mitochondrion into the cytoplasm.

19. 7.4 What Happens During Cellular Respiration?
The Krebs cycle breaks down pyruvate in the mitochondrial matrix.
Pyruvate produced by glycolysis reaches the matrix and reacts with coenzyme A, forming acetyl CoA.
During this reaction, two electrons and a H+ are transferred to NAD+ to form NADH.
Acetyl CoA enters the Krebs cycle and produces one ATP, one FADH2, and three NADH.

20. 7.4 What Happens During Cellular Respiration?
The reactions in the mitochondrial matrix 1
Formation of acetyl CoA 3
NADH 3
NAD+
FAD
FADH2 C
CO2
coenzyme A
coenzyme A C C
CoA 2
Krebs cycle C C C 2 C
CO2
acetyl CoA
pyruvate
NAD+
NADH
ADP
ATP
Fig. 7-4

21. 7.4 What Happens During Cellular Respiration?
Energetic electrons are carried to the electron transport chain.
Step 1: Energized carriers deposit their electrons in the electron transport chains (ETC) in the inner mitochondrial membrane.
Step 2: Electrons in the ETC move from one molecule to the next, transferring energy that is used to pump H+ out of the matrix and into the intermembrane compartment.
Step 3: At the end of the ETC, oxygen atoms combine with two H+ and two depleted electrons to form H2O.

22. 7.4 What Happens During Cellular Respiration?
Energetic electrons are carried to the electron transport chain (continued).
Oxygen accepts electrons after they have passed through the ETC and given up most of their energy.
If O2 is not present, electrons accumulate in the ETC, H+ pumping out of the matrix stops, and cellular respiration ceases.

23. 7.4 What Happens During Cellular Respiration?
The electron transport chain in the mitochondrial matrix
(matrix)
FADH2 3
1/2 O2 + 2H+
NADH
NAD+
FAD
2e–
2e–
H2O 1
electron carriers
(inner membrane) H+ H+ H+ 2
energy to drive
ATP
synthesis
(intermembrane compartment)
Fig. 7-5

24. 7.4 What Happens During Cellular Respiration?
Energy from a hydrogen-ion gradient is used to produce ATP.
Hydrogen ions accumulate in the intermembrane compartment and diffuse back into the matrix.
The energy released when hydrogen ions move down their concentration gradient is used to make ATP in a process called chemiosmosis.
During chemiosmosis, 32 to 34 molecules of ATP are produced from each molecule of glucose.
This ATP is transported from the matrix to the cytoplasm, where it is used to power metabolic reactions.

25. Suggested Media Enhancement: Cellular Respiration
To access this animation go to folder C_Animations_and_Video_Files and open the BioFlix folder.

26. 7.5 What Happens During Fermentation?
When oxygen is not present (anaerobic conditions), glucose cannot be metabolized by cellular respiration; instead, fermentation takes place.
Unlike cellular respiration, fermentation generates no ATP, but instead, regenerates NAD+ that is used to get ATP from glycolysis.

27. 7.5 What Happens During Fermentation?
In fermentation, pyruvate acts as an electron acceptor from the NADH produced during glycolysis.
When pyruvate accepts electrons from NADH, it recycles the NAD+ so that more glucose can be converted to pyruvate, generating a small amount of ATP in the process.
When no O2 is present, glycolysis becomes the main source of ATP and NADH production.

28. 7.5 What Happens During Fermentation?
There are two types of fermentation: one converts pyruvate to ethanol and CO2, and the other converts pyruvate to lactate.
Alcoholic fermentation is the primary mode of metabolism in many microorganisms.
The reactions use hydrogen ions and electrons from NADH, thereby regenerating NAD+.
Alcoholic fermentation is responsible for the production of many economic products, such as wine, beer, and bread.

29. 7.5 What Happens During Fermentation?
Glycolysis followed by alcoholic fermentation
regeneration
NAD+
NADH
NADH
NAD+ C C C C C C 2 C C C 2 C C
+ 2 C
(glycolysis)
(fermentation)
glucose
pyruvate
ethanol
CO2 2
ADP 2
ATP
Fig. 7-6

30. 7.5 What Happens During Fermentation?
Other cells ferment pyruvate to lactate, and include microorganisms that produce yogurt, sour cream, and cheese.
Lactate fermentation also occurs in aerobic organisms when cells are temporarily deprived of oxygen, such as muscle cells during vigorous exercise.
These muscle cells ferment pyruvate to lactate, which uses H+ and electrons from NADH to regenerate NAD+.

31. 7.5 What Happens During Fermentation?
Glycolysis followed by lactate fermentation
regeneration
NAD+
NAD+
NADH
NADH C C C C C C 2 C C C 2 C C C
(glycolysis)
(fermentation)
glucose
pyruvate
lactate 2
ADP 2
ATP
Fig. 7-8

32. 7.5 What Happens During Fermentation?
Fermentation limits human muscle performance.
The average speed of a long distance run is slower than a 100-meter sprint.
During a sprint, muscles use more ATP than can be delivered by cellular respiration because O2 cannot be delivered to muscles fast enough.
Glycolysis can deliver a small amount of ATP to rapidly contracting muscles, but toxic buildup of lactate will occur.
Long distance runners must therefore pace themselves so that cellular respiration can power their muscles for most of the race.

33. 7.5 What Happens During Fermentation?
PLAY
Animation—Glucose Metabolism