1. CHAPTER 7
Cellular Respiration

2. Section 1: Glycolysis and Fermentation

3. Food serves as a source of raw materials for the cells in the body and as a source of energy.
Cells break down food into simpler molecules in a process that releases energy to power cellular activities.
Animal Cells
Animal
Mitochondrion
Plant
Photo Credits: left: ©Bob Gurr/DRK Photo; middle bottom: ©John Durham/Science Photo Library/Photo Researchers, Inc. ; middle top: ©Ron Boardman/Stone; right: ©Keith Porter/Photo Researchers, Inc.
Plant Cells

4. Harvesting Chemical Energy
Cellular respiration is the process by which cells break down organic compounds (food) to produce ATP.
Both autotrophs and heterotrophs perform cellular respiration.
The primary fuel for cellular respiration is glucose, which is formed when carbohydrates such as starch and sucrose are broken down.
If too few carbohydrates are available, other molecules, such as fats and proteins can be broken down to make ATP.

5. Harvesting Chemical Energy
When cells break down food molecules, some of the energy in the molecules is released as heat.
The remaining energy is stored in molecules of ATP.
ATP is the energy “currency” of cells.

6. Adenosine Triphosphate
ATP supplies energy for 3 main types of biological work:
Mechanical functions of cells
Active transport of ions and molecules across cell membranes
Synthesis and breakdown of large molecules
The phosphate groups of ATP store energy.
This energy is released when the bonds that hold the phosphate groups together are broken.
The cell uses this energy to do work.

7. The ATP-ADP Cycle

8. Overview of Cellular Respiration
Electrons carried in NADH
Electrons carried
in NADH and
FADH2
Pyruvic acid
Glucose
Glycolysis
Cytoplasm
Cellular respiration is the process that releases energy by breaking down food molecules in the presence of oxygen. Glycolysis takes place in the cytoplasm. The Krebs cycle and electron transport take place inside the mitochondria.
Mitochondrion

9. Overview of Cellular Respiration
Cellular respiration is the process that releases energy by breaking down glucose and other food molecules in the presence of oxygen.
The equation for cellular respiration is:
6O2 + C6H12O6 → 6CO2 + 6H2O + Energy
oxygen + glucose → carbon dioxide + water + Energy

10. Overview of Cellular Respiration
Glycolysis takes place in the cytoplasm. The Krebs cycle and electron transport take place in the mitochondria.
Glycolysis
Cellular respiration is the process that releases energy by breaking down food molecules in the presence of oxygen. Glycolysis takes place in the cytoplasm. The Krebs cycle and electron transport take place inside the mitochondria.
Cytoplasm
Mitochondrion

11. Glycolysis ATP Production
At the beginning of glycolysis, the cell uses up 2 molecules of ATP to start the reaction.
2 ATP
2 ADP
4 ADP
4 ATP
Glycolysis is the first stage in cellular respiration. During glycolysis, glucose is broken down into 2 molecules of pyruvic acid.
Glucose
2 Pyruvic
acid

12. Glycolysis
When glycolysis is complete, 4 ATP molecules have been produced.
2 ATP
2 ADP
4 ADP
4 ATP
Glucose
2 Pyruvic
acid

13. Glycolysis This gives the cell a net gain of 2 ATP molecules. 2 ATP
2 ADP
4 ADP
4 ATP
Glucose
2 Pyruvic
acid

14. Glycolysis NADH Production
One reaction of glycolysis removes 4 high-energy electrons, passing them to an electron carrier called NAD+.
2 ATP
2 ADP
4 ADP
4 ATP
Glucose
2NAD+
2 Pyruvic
acid

15. Glycolysis
Each NAD+ accepts a pair of high-energy electrons and becomes an NADH molecule.
2 ATP
2 ADP
4 ADP
4 ATP
Glucose
2NAD+
2 Pyruvic
acid 2

16. Glycolysis
The NADH molecule holds the electrons until they can be transferred to other molecules.
2 ATP
2 ADP
4 ADP
4 ATP
2NAD+
2 Pyruvic
acid 2
To the electron transport chain

17. Glycolysis The Advantages of Glycolysis
The process of glycolysis is so fast that cells can produce thousands of ATP molecules in a few milliseconds.
Glycolysis does not require oxygen.

18. Fermentation
When oxygen is not present, glycolysis is followed by a different pathway. The combined process of this pathway and glycolysis is called fermentation.
Fermentation does not require oxygen—it is an anaerobic process.
Fermentation does not produce ATP, but it does regenerate NAD+, which allows for the continued production of ATP through glycolysis.

19. Fermentation
There are many types of fermentation. Two of the most common types are:
Alcoholic fermentation
Lactic acid fermentation

20. Alcoholic Fermentation
When oxygen is not present, yeasts and certain bacteria use alcoholic fermentation, forming ethyl alcohol and carbon dioxide as wastes.
The equation for alcoholic fermentation after glycolysis is:
pyruvic acid + NADH → alcohol + CO2 + NAD+
Yeasts, added to crushed grapes, eat the grapes’ sugars and produce wine when there is no oxygen present.
Yeasts, added to the grain barley, eat the grain’s sugars and produce beer when there is no oxygen present.

22. Alcoholic Fermentation
The CO2 released by the yeast causes the carbonation of some alcoholic beverages, such as champagne and beer.
Yeasts, added to dough, digest sugars (derived from starches in dough) and produce carbon dioxide, causing the dough to rise.

23. Lactic Acid Fermentation
In some cells, pyruvic acid that accumulates as a result of glycolysis can be converted to lactic acid when oxygen is not present.
This type of fermentation is called lactic acid fermentation. Like alcoholic fermentation, it regenerates NAD+ so that glycolysis can continue.
The equation for lactic acid fermentation after glycolysis is:
pyruvic acid + NADH → lactic acid + NAD+

24. Lactic Acid Fermentation
During strenuous exercise, oxygen is scarce; therefore, human muscle cells switch from aerobic respiration to lactic acid fermentation.
Lactic acid that accumulates as a waste product may cause muscle soreness, but it is gradually carried away by the blood to the liver.
Lactic acid is converted back to pyruvic acid by liver cells.

25. Lactic Acid Fermentation
Lactic acid fermentation by certain fungi and bacteria is used in the dairy industry to make cheese and yogurt.
Milk bacteria digest the milk sugar lactose and produce lactic acid, which acts with the added enzyme rennet to curdle the milk. The cheesemaker drains off the whey and compacts the curds, which various microbes then ripen into a mature cheese.
Lactic acid fermentation is also used to make pickles and sauerkraut.
The cucumbers and cabbage are soaked in a salt brine and sealed, allowing the growth of bacteria that eat the vegetable’s sugars and produce tart-tasting lactic acid.

28. The first part of the equation is glycolysis.
Lactic acid fermentation converts glucose into lactic acid. The first part of the equation is glycolysis. The second part shows the conversion of pyruvic acid to lactic acid.

29. The second part shows the conversion of pyruvic acid to lactic acid.
Lactic acid fermentation converts glucose into lactic acid. The first part of the equation is glycolysis. The second part shows the conversion of pyruvic acid to lactic acid.

30. Efficiency of Glycolysis
After glycolysis, only about 2% of the energy contained in glucose has been transferred to ATP.
Most of the energy is still stored in pyruvic acid.
Large organisms that require more energy will meet their needs by undergoing aerobic respiration.

31. Section 2: Aerobic Respiration

32. Overview of Aerobic Respiration
After glycolysis, if oxygen is present, fermentation will not occur. Instead, 2 pathways called the Krebs cycle and the Electron Transport Chain will occur.
Because these pathways of cellular respiration require oxygen, they are aerobic.
Aerobic respiration produces nearly 20 times as much ATP as is produced by glycolysis alone.

33. Both plant and animal cells carry out the Krebs cycle and the Electron Transport Chain in the mitochondria.
Animal Cells
Outer membrane
Intermembrane space
Mitochondrion
Inner membrane
Photo Credits: left: ©Bob Gurr/DRK Photo; middle bottom: ©John Durham/Science Photo Library/Photo Researchers, Inc. ; middle top: ©Ron Boardman/Stone; right: ©Keith Porter/Photo Researchers, Inc.
Matrix
Plant Cells

34. Overview of Aerobic Respiration

35. The Krebs Cycle Discovered by Hans Krebs in 1937
He received the Nobel Prize in physiology or medicine in 1953 for his discovery.
Forced to leave Germany prior to WWII because he was Jewish
The Krebs cycle occurs in the mitochondrial matrix.
Mitochondrial
Matrix

37. The Krebs Cycle
During the Krebs cycle, pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions.

38. The Krebs cycle begins when pyruvic acid produced by glycolysis enters the mitochondrion.

39. One carbon atom is removed, forming CO2, and electrons are removed, changing NAD+ to NADH.

40. Coenzyme A joins the 2-carbon molecule, forming acetyl-CoA.

41. Acetyl-CoA then adds the 2-carbon acetyl group to a 4-carbon compound, forming citric acid, a 6-carbon compound.
Citric acid

42. Citric acid is broken down into a 5-carbon compound, then into a 4-carbon compound.

43. Two more molecules of CO2 are released and electrons join NAD+ and FAD, forming NADH and FADH2. NADH and FADH2 carry high-energy electrons and are referred to as “Electron Taxis”.

44. In addition, one molecule of ATP is generated.
However, the Krebs cycle turns twice (once for each pyruvic acid molecule produced in glycolysis)!
The complete breakdown of glucose through cellular respiration, including glycolysis, results in the production of 36 molecules of ATP.

45. The Krebs Cycle Totals after the Krebs Cycle: 2 ATP
8 NADH (plus 2 NADH from glycolysis = 10 total NADH)
2 FADH2
6 CO2
Most of the energy released in the breakdown of glucose still has not been transferred to ATP.
The 10 NADH molecules and the 2 FADH2 molecules drive the next stage of aerobic respiration where most of the energy transfer occurs.

46. Electron Transport Chain
The electron transport chain (or ETC) uses the high-energy electrons from NADH and FADH2 to convert ADP into ATP by moving protons down their concentration gradient (chemiosmosis).
The ETC is located in the inner mitochondrial membrane in folds called cristae.
Inner
Mitochondrial
Membrane

47. High-energy electrons from NADH and FADH2 are passed along the electron transport chain from one carrier protein to the next.

48. At the end of the chain, an enzyme combines these electrons with hydrogen ions and oxygen to form water.

49. As the final electron acceptor of the electron transport chain, oxygen gets rid of the low-energy electrons and hydrogen ions.
The Importance of Oxygen
ATP can only be produced if electrons keep moving down the electron transport chain
Without oxygen to accept electrons, the electron transport chain stops and no more ATP can be produced

50. When 2 high-energy electrons move down the electron transport chain, their energy is used to move hydrogen ions (H+) across the membrane.

51. During electron transport, H+ ions build up in the intermembrane space, so it is positively charged.

52. The other side of the membrane, from which those H+ ions are taken, is now negatively charged.

53. The inner membranes of the mitochondria contain proteins called ATP synthases.

54. H+ ions move down their concentration gradient through channels into the ATP synthase. This causes the ATP synthase to spin.
Channel
ATP synthase
The electron transport chain uses high-energy electrons from the Krebs cycle to convert ADP to ATP.

55. As it rotates, the enzyme grabs a low-energy ADP, attaching a phosphate, forming high-energy ATP.
Channel
ATP synthase
The electron transport chain uses high-energy electrons from the Krebs cycle to convert ADP to ATP.
ADP

56. Electron Transport Chain Animation

57. Electron Transport Chain Song

58. The Totals
Glycolysis produces just 2 ATP molecules per molecule of glucose.
The complete breakdown of glucose through aerobic cellular respiration, including glycolysis, results in the production of 36 molecules of ATP.
Efficiency of Cellular Respiration
This represents about 37% of the energy stored in glucose.
The remaining energy is released as heat.

59. The Totals
The complete breakdown of glucose through cellular respiration, including glycolysis, results in the production of 36 molecules of ATP.

60. Comparing Aerobic & Anaerobic Cellular Respiration Pathways
Aerobic (needs oxygen)
Anaerobic
(no oxygen)
Occurs in:
Most organisms
Mostly yeast and bacteria
1 glucose makes:
6 CO2 + 6 H2O
Ethanol + CO2 or lactic acid
Net ATP production: 36 2

61. Comparing Photosynthesis and Cellular Respiration
The energy flows in photosynthesis and cellular respiration take place in opposite directions.

62. Photosynthesis-Cellular Respiration Cycle

63. Comparing Photosynthesis and Cellular Respiration
On a global level, photosynthesis and cellular respiration are also opposites.
Photosynthesis removes carbon dioxide from the atmosphere and cellular respiration puts it back.
Photosynthesis releases oxygen into the atmosphere and cellular respiration uses that oxygen to release energy from food.

64. The
end