1. CHAPTER 6 How Cells Harvest Chemical Energy

2. How is a Marathoner Different from a Sprinter?
Long-distance runners have many slow fibers in their muscles
Slow fibers break down glucose for ATP production aerobically (using oxygen)
These muscle cells can sustain repeated, long contractions

3. Sprinters have more fast muscle fibers
Fast fibers make ATP without oxygen— anaerobically
They can contract quickly and supply energy for short bursts of intense activity

4. INTRODUCTION TO CELLULAR RESPIRATION
Nearly all the cells in our body break down sugars for ATP production
Most cells of most organisms harvest energy aerobically, like slow muscle fibers
The aerobic harvesting of energy from sugar is called cellular respiration
Cellular respiration yields CO2, H2O, and a large amount of ATP

5. 6.2 Cellular respiration banks energy in ATP molecules
Cellular respiration breaks down glucose molecules and banks their energy in ATP
The process uses O2 and releases CO2 and H2O
Glucose
Oxygen gas
Carbon dioxide
Water
Energy
Figure 6.2A

6. The efficiency of cellular respiration is quite high (and comparison with an auto engine)
Energy released from glucose banked in ATP
Energy released from glucose (as heat and light)
Gasoline energy converted to movement
100%
About 40%
25%
Burning glucose in an experiment
“Burning” glucose in cellular respiration
Burning gasoline in an auto engine
Figure 6.2B

7. ATP powers almost all cell and body activities
6.3 Connection: The human body uses energy from ATP for all its activities
ATP powers almost all cell and body activities
Table 6.3

8. BASIC MECHANISMS OF ENERGY RELEASE AND STORAGE
6.4 Cells tap energy from electrons transferred from organic fuels to oxygen
Glucose gives up energy as it is oxidized, producing CO2, H2O and ATP.
Loss of hydrogen atoms
Energy
Glucose
Gain of hydrogen atoms
Figure 6.4

9. Dehydrogenase and NAD+
6.5 Hydrogen carriers such as NAD+ shuttle electrons in redox reactions
However, ATP will not be produced directly most of the time during cellular respiration.
Instead, enzymes remove electrons from glucose molecules and transfer them to a coenzyme (for example, NAD+)
OXIDATION
Dehydrogenase and NAD+
REDUCTION
Figure 6.5

10. NAD

11. 6.6 Redox reactions release energy when electrons “fall” from a hydrogen carrier to oxygen
NADH delivers electrons to a series of electron carriers in an electron transport chain
As electrons move from carrier to carrier, their energy is released in small quantities
Energy released and now available for making ATP
ELECTRON CARRIERS of the electron transport chain
Electron flow
Figure 6.6

12. 6.7 Two mechanisms generate ATP
Cells use the energy released by “falling” electrons to pump H+ ions across a membrane
The energy of the gradient is harnessed to make ATP by the process of chemiosmosis
High H+ concentration
ATP synthase uses gradient energy to make ATP
Membrane
Electron transport chain
ATP synthase
Energy from
Low H+ concentration
Figure 6.7A

13. Organic molecule (substrate) New organic molecule (product)
ATP can also be made by transferring phosphate groups from organic molecules to ADP
Enzyme
Adenosine
Organic molecule (substrate)
This process is called substrate-level phosphorylation
Adenosine
New organic molecule (product)
Figure 6.7B

14. 6.8 Overview: Respiration occurs in three main stages
STAGES OF CELLULAR RESPIRATION AND FERMENTATION
6.8 Overview: Respiration occurs in three main stages
Cellular respiration oxidizes sugar and produces ATP in three main stages
Glycolysis occurs in the cytoplasm
The Citric acid cycle (Krebs cycle ) and the electron transport chain occur in the mitochondria

15. An overview of cellular respiration
High-energy electrons carried by NADH
GLYCOLYSIS
ELECTRON TRANSPORT CHAIN AND CHEMIOSMOSIS
Citric acid cycle
Glucose
Pyruvate
Cytoplasmic fluid
Mitochondrion
Figure 6.8

16. 6.9 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
Figure 6.9A

17. PREPARATORY PHASE (energy investment)
Steps – A fuel molecule is energized, using ATP.
Glucose 1 3
Details of glycolysis
Step 1
Glucose-6-phosphate 2
Fructose-6-phosphate 3
Fructose-1,6-diphosphate
Step A six-carbon intermediate splits into two three-carbon intermediates. 4 4
Glyceraldehyde-3-phosphate (G3P)
ENERGY PAYOFF PHASE 5
Step A redox reaction generates NADH. 5
1,3-Diphosphoglyceric acid (2 molecules) 6
Steps – ATP and pyruvate are produced.
3-Phosphoglyceric acid (2 molecules) 6 9 7
2-Phosphoglyceric acid (2 molecules) 8
2-Phosphoglyceric acid (2 molecules) 9
Pyruvate
Figure 6.9B
(2 molecules per glucose molecule)

18. 6.10 Pyruvate is chemically groomed for the Krebs cycle
Each pyruvate molecule is broken down to form CO2 and a two-carbon acetyl group (acetyl-CoA), which enters the citric acid cycle
Pyruvate
Acetyl CoA (acetyl coenzyme A)
CO2
Figure 6.10

19. 6.11 The citric acid cycle completes the oxidation of organic fuel, generating many NADH and FADH2 molecules
Acetyl CoA
The citric acid cycle is a series of reactions in which enzymes strip away electrons and H+ from each acetyl group 2
KREBS CYCLE
CO2
Figure 6.11A

20. 1 5 2 4 3 2 carbons enter cycle Oxaloacetate Citric acid (citrate)
CO2 leaves cycle 5
Citric acid cycle 2
malate 4
-ketoglutarate 3
CO2 leaves cycle
Succinate
Step
Acetyl CoA stokes the furnace
Steps and
NADH, ATP, and CO2 are generated during redox reactions.
Steps and
Redox reactions generate FADH2 and NADH. 1 2 3 4 5
Figure 6.11B

21. 6.12 Chemiosmosis powers most ATP production
The electrons from NADH and FADH2 travel down the electron transport chain to oxygen
Energy released by the electrons is used to pump H+ into the space between the mitochondrial membranes
In chemiosmosis, the H+ ions diffuse back through the inner membrane through ATP synthase complexes, which capture the energy to make ATP

22. ELECTRON TRANSPORT CHAIN
Chemiosmosis in the mitochondrion
Protein complex
Intermembrane space
Electron carrier
Inner mitochondrial membrane
Electron flow
Mitochondrial matrix
ELECTRON TRANSPORT CHAIN
ATP SYNTHASE
Figure 6.12

23. Cyanide, carbon monoxide ELECTRON TRANSPORT CHAIN
6.13 Connection: Certain poisons interrupt critical events in cellular respiration
Rotenone
Cyanide, carbon monoxide
Oligomycin
ELECTRON TRANSPORT CHAIN
ATP SYNTHASE
Figure 6.13

24. 6.14 Review: Each molecule of glucose yields many molecules of ATP
For each glucose molecule that enters cellular respiration, chemiosmosis produces up to 38 ATP molecules
Cytoplasm
Mitochondrion
Electron shuttle across membranes
KREBS CYCLE
GLYCOLYSIS
2 Acetyl CoA 2
Pyruvate
Citric acid cycle
ELECTRON TRANSPORT CHAIN AND CHEMIOSMOSIS
Glucose
by substrate-level phosphorylation
used for shuttling electrons from NADH made in glycolysis
by substrate-level phosphorylation
by chemiosmotic phosphorylation
Maximum per glucose:
Figure 6.14

25. 6.15 Fermentation is an anaerobic alternative to aerobic respiration
Under anaerobic conditions, many kinds of cells can use glycolysis alone to produce small amounts of ATP
But a cell must have a way of replenishing NAD+
Therefore, cells developed fermentation to spend NADH so NAD+ can be regenerated

26. In alcoholic fermentation, pyruvate is converted to CO2 and ethanol
This recycles NAD+ to keep glycolysis working
Alcoholic fermentation only happens in bacteria and yeast.
released
GLYCOLYSIS
2 Pyruvate
2 Ethanol
Glucose
Figure 6.15A
Figure 6.15C

27. In lactate fermentation, pyruvate is converted to lactate
As in alcoholic fermentation, NAD+ is recycled
Lactate fermentation happens in animals and Lactobacillus
Lactate fermentation is used to make cheese and yogurt
GLYCOLYSIS
2 Pyruvate
2 Lactate
Glucose
Figure 6.15B

28. INTERCONNECTIONS BETWEEN MOLECULAR BREAKDOWN AND SYNTHESIS
6.16 Cells use many kinds of organic molecules as fuel for cellular respiration
Polysaccharides can be hydrolyzed to monosaccharides and then converted to glucose for glycolysis
Proteins can be digested to amino acids, which are chemically altered and then used in the citric acid cycle
Fats are broken up and fed into glycolysis and the citric acid cycle

29. ELECTRON TRANSPORT CHAIN AND CHEMIOSMOSIS
Pathways of molecular breakdown
Food, such as peanuts
Polysaccharides
Fats
Proteins
Sugars
Glycerol
Fatty acids
Amino acids
Amino groups
ELECTRON TRANSPORT CHAIN AND CHEMIOSMOSIS
Glucose
G3P
pyruvate
Acetyl CoA
Citric acid cycle
GLYCOLYSIS
Figure 6.16

30. 6.17 Food molecules provide raw materials for biosynthesis
In addition to energy, cells need raw materials for growth and repair
Some are obtained directly from food
Others are made from intermediates in glycolysis and the citric acid cycle
Biosynthesis consumes ATP

31. ATP needed to drive biosynthesis Cells, tissues, organisms
Biosynthesis of macromolecules from intermediates in cellular respiration
ATP needed to drive biosynthesis
GLUCOSE SYNTHESIS
Citric acid cycle
Acetyl CoA
pyruvate
G3P
Glucose
Amino groups
Amino acids
Fatty acids
Glycerol
Sugars
Proteins
Fats
Polyscaccharides
Cells, tissues, organisms
Figure 6.17

32. 6.18 The fuel for respiration ultimately comes from photosynthesis
All organisms have the ability to harvest energy from organic molecules
Plants, but not animals, can also make these molecules from inorganic sources by the process of photosynthesis
Figure 6.18