1. How Cells Release Stored Energy
Chapter 7

2. “Killer” Bees
Descendents of African honeybees that were imported to Brazil in the 1950s
More aggressive, wider-ranging than other honeybees
Africanized bee’s muscle cells have enlarged mitochondria

3. Producing the Universal Currency of Life
All energy-releasing pathways
require characteristic starting materials
yield predictable products and by-products
produce ATP

4. ATP Is Universal Energy Source
Photosynthesizers get energy from the sun
Animals get energy second- or third-hand from plants or other organisms
Regardless, the energy is converted to the chemical bond energy of ATP

5. Making ATP Plants make ATP during photosynthesis
Cells of all organisms make ATP by breaking down carbohydrates, fats, and protein

6. Main Types of Energy-Releasing Pathways
Anaerobic pathways
Evolved first
Don’t require oxygen
Start with glycolysis in cytoplasm
Completed in cytoplasm
Aerobic pathways
Evolved later
Require oxygen
Start with glycolysis in cytoplasm
Completed in mitochondria

7. Main Pathways Start with Glycolysis
Glycolysis occurs in cytoplasm
Reactions are catalyzed by enzymes
Glucose 2 Pyruvate
(six carbons) (three carbons)

8. Overview of Aerobic Respiration
C6H O CO2 + 6H20
glucose oxygen carbon water
dioxide

9. Overview of Aerobic Respiration
CYTOPLASM
glucose
ATP
GLYCOLYSIS
energy input to
start reactions
e- + H+
(2 ATP net)
2 NADH
2 pyruvate
MITOCHONDRION
e- + H+
2 CO2
2 NADH
e- + H+
8 NADH
4 CO2
KREBS CYCLE
e- + H+ 2
2 FADH2
ATP e-
ELECTRON
TRANSPORT PHOSPHORYLATION 32
ATP H+
water
e- + oxygen
TYPICAL ENERGY YIELD: 36 ATP

10. The Role of Coenzymes
NAD+ and FAD accept electrons and hydrogen from intermediates during the first two stages
When reduced, they are NADH and FADH2
In the third stage, these coenzymes deliver the electrons and hydrogen to the transport system

11. Glucose
A simple sugar
(C6H12O6)
Atoms held together by covalent bonds

12. Glycolysis Occurs in Two Stages
Energy-requiring steps
ATP energy activates glucose and its six-carbon derivatives
Energy-releasing steps
The products of the first part are split into three-carbon pyruvate molecules
ATP and NADH form

13. Energy-Requiring Steps
glucose
ATP
ADP
2 ATP invested P
glucose-6-phosphate P
fructose-6-phosphate
ATP
ADP P
fructose-1,6-bisphosphate

14. Energy-Releasing Steps
PGAL
PGAL
Energy-Releasing Steps
NAD+
NAD+
NADH
NADH Pi Pi P P P P
1,3-bisphosphoglycerate
1,3-bisphosphoglycerate
substrate-level phosphorylation
ADP
ADP
ATP
ATP
2 ATP invested P P
3-phosphoglycerate
3-phosphoglycerate P P
2-phosphoglycerate
2-phosphoglycerate
H2O
H2O P P
PEP
PEP
substrate-level phosphorylation
ADP
ADP
ATP
ATP
2 ATP invested
pyruvate
pyruvate

15. Net Energy Yield from Glycolysis
Energy requiring steps:
2 ATP invested
Energy releasing steps:
2 NADH formed
4 ATP formed
Net yield is 2 ATP and 2 NADH

16. Second-Stage Reactions
PREPARATORY STEPS
pyruvate
Second-Stage Reactions
coenzyme A (CoA)
NAD+
NADH
(CO2)
CoA
Acetyl–CoA
KREBS CYCLE
CoA
Occur in the mitochondria
Pyruvate is broken down to carbon dioxide
More ATP is formed
More coenzymes are reduced
oxaloacetate
citrate
H2O
NADH
H2O
NAD+
malate
isocitrate
NAD+
H2O
NADH
fumarate
a-ketogluterate
FADH2
CoA
FAD
NAD+
NADH
succinate
CoA
succinyl–CoA
ATP
ADP + phosphate group (from GTP)

17. Two Parts of Second Stage
Preparatory reactions
Pyruvate is oxidized into two-carbon acetyl units and carbon dioxide
NAD+ is reduced
Krebs cycle
The acetyl units are oxidized to carbon dioxide
NAD+ and FAD are reduced

18. Preparatory Reactions
pyruvate + coenzyme A + NAD+
acetyl-CoA + NADH + CO2
One of the carbons from pyruvate is released in CO2
Two carbons are attached to coenzyme A and continue on to the Krebs cycle

19. What is Acetyl-CoA? A two-carbon acetyl group linked to coenzyme A
CH3
C=O
Coenzyme A
Acetyl group

20. The Krebs Cycle Overall Reactants Overall Products Acetyl-CoA 3 NAD+
FAD
ADP and Pi
Overall Products
Coenzyme A
2 CO2
3 NADH
FADH2
ATP

21. Results of the Second Stage
All of the carbon molecules in pyruvate end up in carbon dioxide
Coenzymes are reduced (they pick up electrons and hydrogen)
One molecule of ATP is formed
Four-carbon oxaloacetate is regenerated

22. Coenzyme Reductions During First Two Stages
Glycolysis 2 NADH
Preparatory
reactions NADH
Krebs cycle FADH NADH
Total FADH NADH

23. Electron Transport Phosphorylation
Occurs in the mitochondria
Coenzymes deliver electrons to electron transport systems
Electron transport sets up H+ ion gradients
Flow of H+ down gradients powers ATP formation

24. Electron Transport
Electron transport systems are embedded in inner mitochondrial compartment
NADH and FADH2 give up electrons that they picked up in earlier stages to electron transport system
Electrons are transported through the system
The final electron acceptor is oxygen

25. Creating an H+ Gradient
OUTER COMPARTMENT
NADH
INNER COMPARTMENT

26. Making ATP: Chemiosmotic Model
INNER
COMPARTMENT
ADP + Pi

27. Importance of Oxygen
Electron transport phosphorylation requires the presence of oxygen
Oxygen withdraws spent electrons from the electron transport system, then combines with H+ to form water

28. Summary of Energy Harvest (per molecule of glucose)
Glycolysis
2 ATP formed by substrate-level phosphorylation
Krebs cycle and preparatory reactions
Electron transport phosphorylation
32 ATP formed

29. Energy Harvest from Coenzyme Reductions
What are the sources of electrons used to generate the 32 ATP in the final stage?
4 ATP - generated using electrons released during glycolysis and carried by NADH
28 ATP - generated using electrons formed during second-stage reactions and carried by NADH and FADH2

30. Energy Harvest Varies
NADH formed in cytoplasm cannot enter mitochondrion
It delivers electrons to mitochondrial membrane
Membrane proteins shuttle electrons to NAD+ or FAD inside mitochondrion
Electrons given to FAD yield less ATP than those given to NAD+

31. Energy Harvest Varies Skeletal muscle and brain cells
Liver, kidney, heart cells
Electrons from first-stage reactions are delivered to NAD+ in mitochondria
Total energy harvest is 38 ATP
Skeletal muscle and brain cells
Electrons from first-stage reactions are delivered to FAD in mitochondria
Total energy harvest is 36 ATP

32. Efficiency of Aerobic Respiration
686 kcal of energy are released
7.5 kcal are conserved in each ATP
When 36 ATP form, 270 kcal (36 X 7.5) are captured in ATP
Efficiency is 270 / 686 X 100 = 39 percent
Most energy is lost as heat

33. Anaerobic Pathways Do not use oxygen
Produce less ATP than aerobic pathways
Two types
Fermentation pathways
Anaerobic electron transport

34. Fermentation Pathways
Begin with glycolysis
Do not break glucose down completely to carbon dioxide and water
Yield only the 2 ATP from glycolysis
Steps that follow glycolysis serve only to regenerate NAD+

35. Lactate Fermentation GLYCOLYSIS C6H12O6 2 ATP energy input 2 ADP
2 NAD+ 2
NADH 4
ATP
2 pyruvate
energy output
2 ATP net
LACTATE FORMATION
electrons, hydrogen from NADH
2 lactate

36. Alcoholic Fermentation
GLYCOLYSIS
Alcoholic Fermentation
C6H12O6 2
ATP
energy input
2 ADP
2 NAD+ 2
NADH 4
ATP
energy output
2 pyruvate
2 ATP net
ETHANOL FORMATION
2 H2O
2 CO2
2 acetaldehyde
electrons, hydrogen from NADH
2 ethanol

37. Yeasts Single-celled fungi Carry out alcoholic fermentation
Saccharomyces cerevisiae
Baker’s yeast
Carbon dioxide makes bread dough rise
Saccharomyces ellipsoideus
Used to make beer and wine

38. Anaerobic Electron Transport
Carried out by certain bacteria
Electron transport system is in bacterial plasma membrane
Final electron acceptor is compound from environment (such as nitrate), NOT oxygen
ATP yield is low

39. Carbohydrate Breakdown and Storage
Glucose is absorbed into blood
Pancreas releases insulin
Insulin stimulates glucose uptake by cells
Cells convert glucose to glucose-6-phosphate
This traps glucose in cytoplasm where it can be used for glycolysis

40. Making Glycogen
If glucose intake is high, ATP-making machinery goes into high gear
When ATP levels rise high enough, glucose-6-phosphate is diverted into glycogen synthesis (mainly in liver and muscle)
Glycogen is the main storage polysaccharide in animals

41. Using Glycogen
When blood levels of glucose decline, pancreas releases glucagon
Glucagon stimulates liver cells to convert glycogen back to glucose and to release it to the blood
(Muscle cells do not release their stored glycogen)

42. Energy Reserves
Glycogen makes up only about 1 percent of the body’s energy reserves
Proteins make up 21 percent of energy reserves
Fat makes up the bulk of reserves (78 percent)

43. Energy from Proteins Proteins are broken down to amino acids
Amino acids are broken apart
Amino group is removed, ammonia forms, is converted to urea and excreted
Carbon backbones can enter the Krebs cycle or its preparatory reactions

44. Energy from Fats Most stored fats are triglycerides
Triglycerides are broken down to glycerol and fatty acids
Glycerol is converted to PGAL, an intermediate of glycolysis
Fatty acids are broken down and converted to acetyl-CoA, which enters Krebs cycle

45. Evolution of Metabolic Pathways
When life originated, atmosphere had little oxygen
Earliest organisms used anaerobic pathways
Later, noncyclic pathway of photosynthesis increased atmospheric oxygen
Cells arose that used oxygen as final acceptor in electron transport

46. Processes Are Linked Aerobic Respiration Photosynthesis Reactants
Sugar
Oxygen
Products
Carbon dioxide
Water
Photosynthesis
Reactants
Carbon dioxide
Water
Products
Sugar
Oxygen

47. Life Is System of Prolonging Order
Powered by energy inputs from sun, life continues onward through reproduction
Following instructions in DNA, energy and materials can be organized, generation after generation
With death, molecules are released and may be cycled as raw material for next generation