1. How Cells Release Chemical Energy
Chapter 7
How Cells Release Chemical Energy

2. Electron Transfers The bonds hold a lot of energy that can be released
Burning reaction with oxygen; release energy from organic molecules
Cells break organic molecules apart in small, manageable steps
Most of these steps are oxidation– reduction reactions (redox reactions)

3. Electron Transfers (cont’d.)
In a typical redox reaction, one molecule accepts electrons (reduced) from another molecule (oxidized)

4. Electron Transfers (cont’d.)
Figure 5.17 Visible evidence of oxidation–reduction: a glowing protist, Noctiluca scintillans (below). The pathway that produces the glow involves an enzyme, luciferase, and its substrate, luciferin. It occurs when the cells are mechanically stimulated, as by waves (shown in the chapter opening photo) or an attack by a protist-eating predator.

5. Electron Transfers (cont’d.)
Electron transfer chain: enzymes and other molecules that transfer electrons in sequence
The energy of the electrons is released with each step of the sequence
Important for photosynthesis and aerobic respiration

6. Electron Transfers (cont’d.)
glucose 1 + H+ e-
oxygen 2
carbon
dioxide +
water
Figure 5.16 {Animated} Comparing uncontrolled and controlled energy release.
A Left, glucose in a metal spoon reacts (burns) with oxygen inside a glass jar. Energy in the form of light and heat is released all at once as CO2 and water form.
B In cells, the same overall reaction occurs in a stepwise fashion
that involves an electron transfer chain, represented here by a staircase. Energy is released in amounts
that cells are able to use.
1 An input of activation energy splits glucose into carbon dioxide, electrons, and hydrogen ions (H+).
2 Electrons lose energy as they move through an electron transfer chain. Energy released by electrons
3 3Electrons, hydrogen ions, and oxygen combine to form water 3 e-

7. 7.1 How Do Cells Access the Chemical Energy in Sugars?
Cells use the energy stored in molecules,
There are two mechanisms by which organisms break down sugars to make ATP

8. 7.3 What Is Glycolysis? Glycolysis found in ALL cells
Net yield of 2 ATP per glucose
Electrons from glucose are transferred to 2 electron carrier NADH
Produces 2 three-carbon pyruvate molecules

9. What Is Glycolysis? (cont’d.)

10. Aerobic Respiration and Fermentation Compared (cont’d.)
Aerobic respiration follows glycolysis when oxygen is present
Produces more ATP
Main energy-releasing pathway in nearly all eukaryotes and some bacteria

11. 7.4 What Happens During the Second Stage of Aerobic Respiration?
Aerobic part occurs inside mitochondria
Breaks down the pyruvate produced during glycolysis

12. What Happens During the Second Stage of Aerobic Respiration? (cont’d.)
mitochondrion
cytoplasm
2 pyruvate
outer membrane
inner membrane
The breakdown of 2 pyruvate (from glycolysis) to 6 CO2 yields 2 ATP and 10 reduced coenzymes (8 NADH and 2 FADH2).
2 acetyl–CoA
6 CO2
Figure 7.3 The second stage of aerobic respiration, acetyl–CoA formation and the Krebs cycle, occurs inside mitochondria. Left, an inner membrane divides a mitochondrion’s interior into two fluid-filled compartments. Right, the second stage of aerobic respiration takes place in the mitochondrion’s innermost compartment, or matrix.
matrix 2 8
Electrons carried by the coenzymes will power ATP formation in the third stage of aerobic respiration. 2

13. Acetyl–CoA Formation
Each pyruvate is split into CO2 and a two- carbon acetyl group (Acetyl CoA)
Electrons are removed combine with NADH

14. The Krebs Cycle (a.k.a. Citric acid cycle)
Each acetyl groups are transferred forming citric acid
ATP formed
Three NADH form and 1 FADH2
Two CO2 released

15. The Krebs Cycle (cont’d.)
2nd stage of
aerobic respiration
pyruvate (2)
carbon dioxide (6)

16. 7.5 What Happens During the Third Stage of Aerobic Respiration?
Last stage occurs on the inner mitochondrial membrane
NADH and FADH2 deliver electrons and H+ to electron transfer chains

17. What Happens During the Third Stage of Aerobic Respiration? (cont’d.)
E.T.C. move H+ actively across the inner membrane
Ion gradient causes the ions to flow through the ATP synthase
Oxygen accepts electrons at the end of mitochondrial electron transfer chains

18. Summary of aerobic respiration:
For each glucose molecule, 4 ATP form in the first- and second-stage reactions
The twelve electron carriers produce 32 additional ATP during the third stage
36 net ATP are produced in total

19. What Happens During the Third Stage of Aerobic Respiration? (cont’d.)
glucose
Stage 1
Glycolysis in cytoplasm splits a glucose molecule into 2 pyruvate; 2 NADH and 4 ATP also form. An investment of 2 ATP began the reactions, so the net yield is 2 ATP. 2 4
2 (net)
2 NADH
2 pyruvate
Stage 2
Acetyl–CoA formation and the Krebs cycle
in the mitochondrial matrix break down
the pyruvate to CO2, which leaves the cell. Ten additional coenzymes are reduced. Two ATP form.
2 NADH
2 NADH
2 CO2
2 acetyl–CoA
Stage 3
In electron transfer phosphorylation, the reduced
coenzymes give up electrons and hydrogen ions to electron transfer chains in the inner mitochon-drial membrane. Energy lost by the electrons as they move through the chains is used to move H+ across the membrane. The resulting gradient causes H+ to flow through ATP synthases,
which drives ATP synthesis.
4 CO2
6 NADH
2 FADH2 2
Figure 7.6 Summary of the three stages of aerobic respiration in a mitochondrion. 32
oxygen
H2O

20. 7.6 What Is Fermentation? Pyruvate is not fully broken down to CO2
No additional ATP forms
The net yield is two ATP

21. What Is Fermentation? (cont’d.)
Alcoholic fermentation: pathway that produces ATP, CO2, and ethanol
Lactate fermentation: pathway that produces ATP and lactate

22. Alcoholic Fermentation (cont’d.)
NADH
NAD+ +
carbon
dioxide
pyruvate
acetaldehyde
ethanol

23. Lactate Fermentation (cont’d.)
NADH
NAD+
pyruvate
lactate

24. Lactate Fermentation (cont’d.)B
Figure 7.8 Lactate fermentation. C

25. 7.7 Can the Body Use Any Organic Molecule for Energy?
36 ATP by fully oxidizing glucose
Other carbohydrates, fats, and proteins can be converted to molecules that enter glycolysis or the Krebs cycle

26. 7.7 Can the Body Use Any Organic Molecule for Energy? (cont’d.)
Food
a triglyceride (fat)
glycerol head
Fats
Complex Carbohydrates
Proteins
fatty acids
glycerol
glucose, other simple sugars
amino acids 2 3 1 4
acetyl–CoA
PGAL
acetyl–CoA
fatty acid tails
NADH
pyruvate
intermediate
of Krebs cycle
Figure 7.9 {Animated} A variety of organic compounds from food can enter the reactions of aerobic respiration.
NADH, FADH2

30. 7.8 Application: Mitochondrial Malfunction
Sometimes when oxygen enters an electron transfer chain, it escapes as a free radical
Free radicals cause damage by oxidizing biological molecules and breaking carbon backbones
Antioxidants in the cytoplasm detoxify free radicals

31. Application: Mitochondrial Malfunction (cont’d.)
A genetic disorder or encounter with a toxin can result in a missing antioxidant or defective electron transfer chain
Free radicals accumulate and destroy first the function of mitochondria, then the cell
This tissue damage is called oxidative stress
Hundreds of incurable disorders are associated with such defects
Cancer, hypertension, Alzheimer’s, and Parkinson’s diseases

32. Application: Mitochondrial Malfunction (cont’d.)
Figure 7.10 This cross-section of a nerve shows how these cells are packed with mitochondria. Mitochondria are powerhouses of all eukaryotic cells. When they malfunction, the lights go off in cellular businesses.