1. Harvesting Energy: Glycolysis and Cellular Respiration
Chapter 8
Harvesting Energy: Glycolysis and Cellular Respiration

2. Chapter 8 At a Glance
8.1 How Do Cells Obtain Energy?
8.2 What Happens During Glycolysis?
8.3 What Happens During Cellular Respiration?
8.4 What Happens During Fermentation?

3. 8.1 How Do Cells Obtain Energy?
Most cellular energy is stored in the chemical bonds of energy-carrier molecules like adenosine triphosphate (ATP)
Cells break down glucose in two stages: glycolysis, which liberates a small quantity of ATP, followed by cellular respiration, which produces far more ATP

4. 8.1 How Do Cells Obtain Energy?
Photosynthesis is the ultimate source of cellular energy
Photosynthetic organisms capture the energy of sunlight and store it in the form of glucose
The overall equation for photosynthesis is
6 CO2 + 6 H2O + light energy  C6H12O6 + 6 O2

5. Author Animation: Overview: Photosynthesis and Respiration

6. Photosynthesis Provides the Energy Released by Glycolysis and Cellular Respiration
energy from sunlight
photosynthesis 6
CO2 6
H2O 6 O2
C6H12O6
cellular
respiration
glycolysis
ATP
Fig. 8-1

7. 8.1 How Do Cells Obtain Energy?
Glucose is a key energy-storage molecule
All cells metabolize glucose for energy
In humans, energy is stored as long chains of glucose, called glycogen, or as fat
These storage molecules are converted to glucose to produce ATP for energy harvesting

8. 8.1 How Do Cells Obtain Energy?
An overview of glucose breakdown
The first stage of glucose breakdown is glycolysis
Glycolysis begins by splitting glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon sugar)
Two ATP molecules are produced in glycolysis
Glycolysis proceeds in the same way under aerobic (with oxygen) or anaerobic (without oxygen) conditions
Glycolysis occurs in the cytoplasm

9. 8.1 How Do Cells Obtain Energy?
An overview of glucose breakdown (continued)
The second stage of glucose breakdown is cellular respiration and occurs when oxygen is available
In this stage, two pyruvate molecules produced by glycolysis are broken down into six carbon dioxide molecules and six water molecules
For every two pyruvate molecules, an additional 34 or 36 ATP molecules are generated
Cellular respiration occurs in mitochondria, organelles specialized for the aerobic breakdown of pyruvate

10. 8.1 How Do Cells Obtain Energy?
An overview of glucose breakdown (continued)
If oxygen is not available, the second stage of glucose breakdown is fermentation
Fermentation does not produce any ATP
In fermentation, pyruvate remains in the cytoplasm and is converted into lactate or ethanol + CO2

11. Author Animation: Fate of Pyruvate

12. 8.1 How Do Cells Obtain Energy?
An overview of glucose breakdown (continued)
The overall equation for the complete breakdown of glucose is
C6H12O O2  6 CO H2O + ATP + heat

13. A Summary of Glucose Breakdown
(cytoplasmic
fluid)
glucose
glycolysis 2
ATP
lactate
2 pyruvate
fermentation
ethanol +
CO2
If O2 is available
If no O2 is available 6 O2
cellular
respiration 34 or 36
ATP 6
CO2 6
H2O
mitochondrion
Fig. 8-2

14. 8.2 What Happens During Glycolysis?
Glycolysis has two parts, each with several steps
Glucose activation
Energy harvesting

15. 8.2 What Happens During Glycolysis?
Glucose activation
A glucose molecule is activated when it receives two phosphates from two ATPs, becoming fructose bisphosphate
Two ATPs are converted into two low-energy adenosine diphosphate (ADP) molecules

16. 8.2 What Happens During Glycolysis?
Glucose activation (continued)
Although the formation of fructose bisphosphate costs the cell two ATP molecules, this initial investment of energy is necessary to produce greater energy returns later
The glucose molecule is relatively stable; the added phosphates make fructose bisphosphate highly reactive

17. 8.2 What Happens During Glycolysis?
Energy harvesting
The six-carbon fructose bisphosphate is split into two, three-carbon molecules of glyceraldehyde-3-phosphate (G3P)
In a series of reactions, each of the two G3P molecules is converted into a pyruvate, generating two ATPs per conversion, for a total of four ATPs
Because two ATPs were used to activate the glucose molecule, there is a net gain of two ATPs per glucose molecule

18. 8.2 What Happens During Glycolysis?
Energy harvesting (continued)
As each G3P is converted to pyruvate, two high-energy electrons and a hydrogen ion are added to an “empty” electron-carrier nicotinamide adenine dinucleotide (NAD+) to make the high-energy electron-carrier molecule NADH
Because two G3P molecules are produced per glucose molecule, two NADH carrier molecules are formed as well

19. 8.2 What Happens During Glycolysis?
Summary of glycolysis
Each molecule of glucose is broken down to two molecules of pyruvate
A net of two ATP molecules and two NADH (high-energy electron carriers) are formed

20. Author Animation: Glycolysis

21. The Essentials of Glycolysis2
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
Fig. 8-3

22. 8.3 What Happens During Cellular Respiration?
Cellular respiration in eukaryotic cells occurs in mitochondria in three stages
First, pyruvate is broken down in the mitochondrial matrix, releasing energy and CO2
Keep in mind that each glucose molecule produces two pyruvate molecules
Second, high-energy electrons travel through the electron transport chain
Third, ATP is generated by chemiosmosis

23. 8.3 What Happens During Cellular Respiration?
First, pyruvate is broken down in the mitochondrial matrix, releasing energy and CO2 (continued)
In eukaryotic cells, cellular respiration occurs within mitochondria, organelles with two membranes that produce two compartments
The inner membrane encloses a central compartment containing the fluid matrix
The outer membrane forms the outer surface of the organelle, and an intermembrane space lies between the two membranes

24. A Mitochondrion matrix inner membrane intermembrane space
Fig. 8-4
outer membrane

25. 8.3 What Happens During Cellular Respiration?
First, pyruvate is broken down in the mitochondrial matrix, releasing energy and CO2 (continued)
Glucose is first broken down into pyruvate, through glycolysis, in the cell cytoplasm
Pyruvate is next transported into the mitochondrion matrix (in eukaryotes), where further breakdown occurs in two stages:
The formation of acetyl coenzyme A (acetyl CoA)
The Krebs cycle

26. 8.3 What Happens During Cellular Respiration?
First, pyruvate is broken down in the mitochondrial matrix, releasing energy and CO2 (continued)
The formation of acetyl CoA
To generate acetyl CoA, pyruvate is split, forming an acetyl group and releasing CO2
The acetyl group reacts with CoA, forming acetyl CoA
During this reaction, two high-energy electrons and a hydrogen ion are transferred to NAD+, forming NADH

27. 8.3 What Happens During Cellular Respiration?
First, pyruvate is broken down in the mitochondrial matrix, releasing energy and CO2 (continued)
The Krebs cycle
The Krebs cycle begins by combining acetyl CoA with a four-carbon molecule to form six-carbon citrate, and coenzyme A is released
As the Krebs cycle proceeds, enzymes in the matrix break down the acetyl group, releasing two CO2 molecules and regenerating the four-carbon molecule for use in future cycles

28. 8.3 What Happens During Cellular Respiration?
First, pyruvate is broken down in the mitochondrial matrix, releasing energy and CO2 (continued)
The Krebs cycle (continued)
Chemical energy released by breaking down each acetyl group is captured in energy-carrier molecules
Each acetyl group produces one ATP, three NADH, and one FADH2
Flavin adenine dinucleotide (FAD), a high-energy electron carrier similar to NAD, picks up two electrons and two H+, forming FADH2

29. Author Animation: Acetyl CoA and the Krebs Cycle

30. Reactions in the Mitochondrial Matrix1
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
Fig. 8-5
ATP

31. 8.3 What Happens During Cellular Respiration?
First, pyruvate is broken down in the mitochondrial matrix, releasing energy and CO2 (continued)
During the mitochondrial reactions, CO2 is generated as a waste product
CO2 diffuses out of cells and into the blood, which carries it to the lungs, where it is exhaled

32. 8.3 What Happens During Cellular Respiration?
In the second stage of cellular respiration, high-energy electrons travel through the electron transport chain
During glycolysis and the mitochondrial matrix reactions, the cell captures many high-energy electrons in carrier molecules: 10 NADH and 2 FADH2 for every glucose molecule that was broken down
These carriers each release two electrons into an electron transport chain (ETC), many copies of which are embedded in the inner mitochondrial membrane

33. 8.3 What Happens During Cellular Respiration?
In the second stage of cellular respiration, high-energy electrons travel through the electron transport chain (continued)
These high-energy electrons jump from molecule to molecule in the ETC, losing small amounts of energy at each step
This resembles the process that occurs in the thylakoid membrane of chloroplasts during photosynthesis
Much of this energy is harnessed to pump H+ from the matrix across the inner membrane and into the intermembrane space, producing a concentration gradient of H+

34. 8.3 What Happens During Cellular Respiration?
In the second stage of cellular respiration, high-energy electrons travel through the electron transport chain (continued)
The buildup of H+ in the intermembrane space is used to generate ATP during chemiosmosis
At the end of the ETC, the energy-depleted electrons are transferred to oxygen, which acts as an electron acceptor
Energy-depleted electrons, oxygen, and hydrogen ions combine to form water
One water molecule is produced for every two electrons that traverse the ETC

35. 8.3 What Happens During Cellular Respiration?
In the second stage of cellular respiration, high-energy electrons travel through the electron transport chain (continued)
Without oxygen, electrons would be unable to move through the ETC, and H+ would not be pumped across the inner membrane
The H+ gradient would dissipate, and ATP synthesis by chemiosmosis would stop
ATP generation continues only when there is a steady supply of oxygen

36. The Electron Transport Chain
(matrix)
ADP
ATP  P H+ 3 4 1 1
NADH 2 e– 2 e–
FADH2 O2  2 H+  2 e–
H2O 2
NAD+
FAD
ATP
synthase
(inner
membrane)
ETC H+ 2 H+ H+ H+ H+ H+
(intermembrane space)
Fig. 8-6

37. 8.3 What Happens During Cellular Respiration?
The third stage of cellular respiration generates ATP by chemiosmosis
Chemiosmosis is the process by which energy is first used to generate a gradient of H+, and then captured in the bonds of ATP as H+ flows down its gradient
As the ETC pumps H+ across the inner membrane, it produces a high concentration of H+ in the intermembrane space and a low concentration in the matrix

38. 8.3 What Happens During Cellular Respiration?
The third stage of cellular respiration generates ATP by chemiosmosis (continued)
The energy present in this non-uniform H+ distribution across the inner membrane is released when hydrogen ions flow down their concentration gradient
The H+ ions flow across the membrane through the ATP synthase channels, and their movement generates ATP from ADP and phosphate

39. 8.3 What Happens During Cellular Respiration?
The third stage of cellular respiration generates ATP by chemiosmosis (continued)
The flow of H+ through the synthase channel provides the energy to synthesize 32 or 34 molecules of ATP for each molecule of glucose
The newly formed ATP leaves the mitochondrion and enters the cytoplasm, where it provides the energy needed by the cell

40. Author Animation: The Electron Transport Chain

41. 8.3 What Happens During Cellular Respiration?
A summary of glucose breakdown in eukaryotic cells
Glycolysis occurs in the cytoplasm, with one glucose molecule producing two three-carbon pyruvate molecules and releasing a small fraction of the energy stored in glucose
Some of the energy is used to generate two ATP molecules, and some is captured in two NADH molecules, which will feed their electrons into the ETC during cellular respiration, generating more ATP

42. 8.3 What Happens During Cellular Respiration?
A summary of glucose breakdown in eukaryotic cells (continued)
During cellular respiration, the two pyruvate molecules enter the mitochondrion
First, each reacts with coenzyme A (CoA), a process that captures high-energy electrons in two NADH, produces two molecules of acetyl CoA, and liberates two molecules of CO2
The acetyl CoA enters the Krebs cycle

43. 8.3 What Happens During Cellular Respiration?
A summary of glucose breakdown in eukaryotic cells (continued)
The Krebs cycle releases four molecules of CO2, produces two ATP, and captures high-energy electrons in six NADH and two FADH2
These electrons are passed to the ETC, where their energy is used during chemiosmosis to generate a gradient of H+, yielding a net of 32 or 34 ATP

44. 8.3 What Happens During Cellular Respiration?
A summary of glucose breakdown in eukaryotic cells (continued)
Energy-depleted electrons exiting the ETC are picked up by H+ released from NADH and FADH2, and combine with oxygen to form water

45. 8.3 What Happens During Cellular Respiration?
A summary of glucose breakdown in eukaryotic cells (continued)
The total energy captured from the breakdown of a single glucose molecule from glycolysis and cellular respiration is 36 or 38 ATP
The reason two different numbers exist for ATP synthesis is that some cell types have to expend two ATPs to transport the two NADH molecules created during glycolysis into the mitochondrion; others are more efficient and don’t require two
The ATP enters the cytoplasm for cellular metabolic activities

46. Author Animation: Overview of Glucose Metabolism

47. Author Animation: Summary of Cellular Respiration

48. The Energy Sources and ATP Harvest from Glycolysis and Cellular Respiration
(cytoplasmic
fluid)
glucose 2
NADH
glycolysis 2
ATP
2 pyruvate
mitochondrion
CoA 2
NADH 2
CO2
2 acetyl CoA 6
NADH
Krebs
cycle 2
ATP 2
FADH2 4
CO2 O2
H2O 32 or 34
electron transport chain
ATP
Fig. 8-7
total: 36 or 38 ATP

49. 8.4 What Happens During Fermentation?
Why is fermentation necessary?
For glycolysis to continue, the NAD+ used to generate NADH must constantly be regenerated
Under aerobic conditions, NADH donates its high-energy electrons and hydrogen produced in glycolysis to ATP-generating reactions in the mitochondria, ultimately being donated to oxygen during the creation of water and making NAD+ available to recycle during glycolysis
Under anaerobic conditions, with no oxygen to allow the ETC to function, the cell must regenerate the NAD+ for glycolysis using fermentation

50. 8.4 What Happens During Fermentation?
Why is fermentation necessary? (continued)
Fermentation does not produce more ATP, but is necessary to regenerate NAD+, which must be available for glycolysis to continue
If the supply of NAD+ were to be exhausted, glycolysis would stop, energy production would cease, and the organism would rapidly die

51. 8.4 What Happens During Fermentation?
Why is fermentation necessary? (continued)
Organisms use one of two types of fermentation to regenerate NAD+
Lactic acid fermentation produces lactic acid from pyruvate
Alcohol fermentation generates alcohol and CO2 from pyruvate
Because fermentation does not break down glucose completely and does not use the energy of NADH to produce more ATP, organisms that rely on fermentation must consume far more sugar to generate the same amount of ATP than do those relying on cellular respiration

52. 8.4 What Happens During Fermentation?
Some cells ferment pyruvate to form lactate
Muscles that are working hard enough to use up all the available oxygen ferment pyruvate to lactate
To regenerate NAD+, muscle cells ferment pyruvate to lactate, using electrons from NADH and hydrogen ions
A variety of microorganisms use lactic acid fermentation, including the bacteria that convert milk into yogurt, sour cream, and cheese

53. Glycolysis Followed by Lactic Acid Fermentation2
NAD+ 2
NADH 2
NADH 2
NAD+ C C C C C C 2 C C C 2 C C C
(glycolysis)
(fermentation)
glucose
pyruvate
lactate 2
ADP 2
ATP
Fig. 8-8

54. 8.4 What Happens During Fermentation?
Some cells ferment pyruvate to form alcohol and carbon dioxide
Many microorganisms, such as yeast, engage in alcohol fermentation under anaerobic conditions
As in lactic acid fermentation, the NAD+ must be regenerated to allow glycolysis to continue
During alcohol fermentation, H+ and electrons from NADH are used to convert pyruvate into ethanol and CO2; this releases NAD+, which can accept more high-energy electrons during glycolysis

55. Author Animation: Fermentation

56. Glycolysis Followed by Alcohol Fermentation2
NAD+ 2
NADH 2
NADH 2
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. 8-9

57. Fermentation in Action
Fig. 8-10