1. Living systems require energy from outside sources
Overview: Life Is Work
Living systems require energy from outside sources
Different organisms have different strategies
For the Discovery Video Space Plants, go to Animation and Video Files.

2. An ecosystem is an open system Energy enters as light and
leaves as heat
or entropy
Energy enters as light
Light
energy
ECOSYSTEM
Photosynthesis in chloroplasts and
cyanobacteria
Organic
molecules
CO2 + H2O
+ O2
Cellular respiration
in mitochondria
Figure 9.2 Energy flow and chemical recycling in ecosystems
ATP
ATP powers most cellular work
Heat
energy

3. We usually use energy harvesting pathways that are catabolic pathways as our examples
Chemotrophic pathways

4. Concept 9.1: Energy-harvesting catabolic pathways
The breakdown of organic molecules (sugars) is exergonic and involves several multistep pathways
Fermentation is a partial degradation of organic molecules that occurs without O2
Aerobic respiration complete degradation of organic molecules and requires O2
Anaerobic respiration is similar to aerobic respiration but requires compounds other than O2 -used by many types of microbes but has lower energy yield
Cellular respiration includes both aerobic and anaerobic respiration but is usually used to refer to aerobic respiration

5. Aerobic respiration is most relevant to humans
Discussion Outline
Aerobic Respiration
Anaerobic Respiration
Fermentation

6. C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy (ATP + heat)
We trace aerobic respiration by following the path of the atoms of glucose (although many other substances are also consumed as fuel) as it is used as a source of chemical energy.
C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy (ATP + heat)

7. The movement of electrons during chemical reactions helps to release and move energy stored in organic molecules
The electrons carry energy (key point) as they are transferred from one substance to another

8. In electron transfer reactions, one substance loses electrons and another gains them.
In an oxidation, a substance loses electrons, or is oxidized
In a reduction, a substance gains electrons, or is reduced
They must occur together
Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions

9. becomes oxidized becomes reduced
Example
becomes oxidized
becomes reduced
In an aqueous environment, protons from water follow the electrons.
Therefore the reduced form of Y might be written as YH
And the reduced form of X as XH

10. Electrons carry the energy in these transfer reactions but something is needed to carry the electrons from reaction to reaction
Electrons from organic compounds are usually first transferred to NAD+, a coenzyme (other coenzymes can be used as well)
As Each NADH (the reduced form of NAD+) represents stored energy that can be tapped to synthesize ATP or do something else

11. Oxidation/Reduction of NAD
2 e– + 2 H+
2 e– + H+
NADH H+
Dehydrogenase
Reduction of NAD+
NAD+ +
2[H] + H+
Oxidation of NADH
Nicotinamide
(reduced form)
Nicotinamide
(oxidized form)
Hydrogen (H) follows electrons in an aqueous environment
An enzyme that catalyzes an oxidation is a
“dehydrogenase”
Figure 9.4 NAD+ as an electron shuttle

12. NADH passes high energy electrons to the electron transport chain
This chain hands off electrons in a series of exergonic steps
Finally they reach O2 which becomes reduced
This is the last stop for the electrons so O2 is called the terminal electron acceptor
The energy given off is used to regenerate ATP

13. There are three important stages:
Glycolysis (breaks down glucose into two molecules of pyruvate) and yields a little ATP directly by non-oxidative reaction-substrate level phosphorylation
The Citric acid cycle (completes the breakdown of glucose) and also yields a little ATP directly reaction-substrate level phosphorylation
Oxidative phosphorylation (accounts for most of the ATP synthesis) includes electron transport and reduction of oxygen

14. Electrons carried via NADH Electrons carried via NADH and FADH2
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
Glycolysis
Citric
acid
cycle
Glucose
Pyruvate
Mitochondrion
Cytosol
Figure 9.6 An overview of cellular respiration
ATP
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
Oxidative
phosphorylation

15. Oxidative phosphorylation involves transfer of electrons from reduced coenzymes to oxygen, the terminal electron acceptor.
A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation

16. Substrate Level Phosphorylation
Enzyme
Enzyme
ADP P
Substrate +
ATP
Figure 9.7 Substrate-level phosphorylation
Product
Direct transfer of high energy phosphate
from substrate to substrate

17. Oxidative phosphorylation involves a membrane and the movement of electrons (and protons)

18. Glycolysis occurs in the cytoplasm and has two major phases:
Concept 9.2: Glycolysis harvests chemical energy by converting glucose to pyruvate
Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate
Glycolysis occurs in the cytoplasm and has two major phases:
Energy investment phase
Energy payoff phase

19. Overview of Glycolysis
Glucose
Energy investment phase
2 ADP + 2 P
2 ATP
used
4 ADP + 4 P
4 ATP
formed
Energy payoff phase
2 NAD e– + 4 H+
2 NADH
+ 2 H+
Figure 9.8 The energy input and output of glycolysis
2 Pyruvate + 2 H2O
Net
Glucose
2 Pyruvate + 2 H2O
4 ATP formed – 2 ATP used
2 ATP
2 NAD e– + 4 H+
2 NADH + 2 H+

20. Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules (except for coenzymes)
In the presence of O2 , pyruvate is transported into the mitochondrion
First, pyruvate must be converted to acetyl CoA, which links the citric acid cycle to glycolysis

21. Aerobic Metabolism of Pyruvate
CYTOSOL
MITOCHONDRION
NAD+
NADH
+ H+ 2 1 3
Acetyl CoA
Figure 9.10 Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the citric acid cycle
Pyruvate
Coenzyme A
CO2
Transport protein
CoA is derived from a vitamin- Vitamin B5 or pantothenic acid
Acetyl CoA links carbohydrate and fatty acid catabolism (beta oxidation)

22. The citric acid cycle, also called the Krebs cycle, takes place within the mitochondrial matrix
The cycle oxidizes organic fuel via Acetyl-CoA, generating 1 ATP, 3 NADH, and 1 FADH2 per turn

23. Pyruvate CO2 NAD+ CoA NADH + H+ Acetyl CoA CoA CoA Citric acid cycle 2
Figure 9.11 An overview of the citric acid cycle
FADH2
3 NAD+
FAD 3
NADH
+ 3 H+
ADP + P i
ATP

24. Can be made into many substances
Acetyl CoA
CoA—SH
NADH
+H+ 1
H2O
NAD+ 8
Oxaloacetate 2
Can be made into many substances
Malate
Citrate
Carbon
Skeletons
Isocitrate
NAD+
Citric
acid
cycle
NADH 3 7
+ H+
H2O
CO2
Fumarate
CoA—SH
-Keto-
glutarate
Figure 9.12 A closer look at the citric acid cycle 4 6
CoA—SH
FADH2 5
CO2
NAD+
FAD
Succinate P
NADH i
GTP
GDP
Succinyl
CoA
+ H+
ADP
ATP
You do not need to memorize this

25. Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis
Following glycolysis and the citric acid cycle, NADH and FADH2 carry most of the energy extracted from food in the form of high energy electrons
These two electron carriers hand off the electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation
For the Cell Biology Video ATP Synthase 3D Structure — Side View, go to Animation and Video Files.
For the Cell Biology Video ATP Synthase 3D Structure — Top View, go to Animation and Video Files.

26. The electron transport chain is in the cristae of the mitochondrion
Most of the chain’s components are proteins, which exist in the form of huge multiprotein “complexes”
The carriers alternate reduced and oxidized states as they accept and donate electrons
Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O

27. You do not need to memorize this
NADH 50 2 e–
NAD+
FADH2 2 e–
FAD
Multiprotein
complexes  40
FMN
FAD
Fe•S 
Fe•S Q

Cyt b
Fe•S 30
Cyt c1 IV
Free energy (G) relative to O2 (kcal/mol)
Cyt c
Cyt a
Cyt a3 20
Figure 9.13 Free-energy change during electron transport 2 e– 10
(from NADH
or FADH2)
2 H+ + 1/2 O2
H2O
You do not need to memorize this

28. Electrons are transferred from NADH or FADH2 to the electron transport chain
Electrons are passed through a number of proteins including cytochromes (each with an iron atom) and iron-sulfur proteins
The electron transport chain generates no ATP directly
The chain’s function is to break the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts

29. Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space
H+ then moves back across the membrane, passing through channels in ATP synthase
ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP
This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work
(Note-this is an important concept)

30. The energy stored in the electrochemical H+ gradient across a membrane connects or couples the redox reactions of the electron transport chain to ATP synthesis
The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work

31. Electron transport chain 2 Chemiosmosis
Protein complex
of electron
carriers
Cyt c V Q
 
ATP synthase 
2 H+ + 1/2O2
H2O
FADH2
FAD
NADH
NAD+
Figure 9.16 Chemiosmosis couples the electron transport chain to ATP synthesis
ADP + P
ATP i
(carrying electrons
from food) H+ 1
Electron transport chain 2
Chemiosmosis
Oxidative phosphorylation

32. How much ATP is produced?
During cellular respiration, most energy flows in this sequence:
glucose  NADH  electron transport chain  proton-motive force  ATP
Roughly 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP

33. CYTOSOL
Electron shuttles
span membrane
MITOCHONDRION
2 NADH or
2 FADH2
2 NADH
2 NADH
6 NADH
2 FADH2
Glycolysis
Oxidative
phosphorylation:
electron transport
and
chemiosmosis 2
Pyruvate 2
Acetyl
CoA
Citric
acid
cycle
Glucose
+ 2 ATP
+ 2 ATP
+ about 32 or 34 ATP
Figure 9.17 ATP yield per molecule of glucose at each stage of cellular respiration
About
36 or 38 ATP
Maximum per glucose:

34. Concept 9.5: Without oxygen-cells use fermentation or anaerobic respiration to produce ATP
Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions)
In the presence of O2 glycolysis couples with aerobic respiration and uses oxygen as terminal electron to produce ATP
In the absence of O2, glycolysis couples with fermentation or anaerobic respiration to produce ATP

35. Anaerobic respiration uses an electron transport chain with an electron acceptor other than O2, for example sulfate, ending with H2S not H2O
Fermentation uses phosphorylation instead of an electron transport chain to generate ATP
Note: Fermentation is defined to consist of reactions that regenerate NAD+-these vary from organism to organism

36. Types of Fermentation
alcohol fermentation
lactic acid fermentation
butyric acid fermentation
biohydrogen fermentation

37. Alcohol fermentation 2 ADP + 2 P 2 ATP Glucose Glycolysis 2 Pyruvate
2 NADH 2
CO2
+ 2 H+
Figure 9.18a Fermentation
2 Acetaldehyde
2 Ethanol
Alcohol fermentation

38. Lactic acid fermentation
2 ADP + 2 P
2 ATP i
Glucose
Glycolysis
2 NAD+
2 NADH
+ 2 H+
2 Pyruvate
Figure 9.18b Fermentation
Lactic acid fermentation
2 Lactate

39. Fermentation vs. aerobic respiration
Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate
The processes have different terminal electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration
Cellular respiration produces 38 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule

40. Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2
Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration

41. Citric acid cycle Oxidative phosphorylation
Proteins
Carbohydrates
Fats
Amino
acids
Sugars
Glycerol
Fatty
acids
Glycolysis
Glucose
Glyceraldehyde-3- P
NH3
Pyruvate
Acetyl CoA
Figure 9.20 The catabolism of various molecules from food
Citric
acid
cycle
Oxidative
phosphorylation

42. Figure 9.21 The control of cellular respiration
Glucose
AMP
Glycolysis
Fructose-6-phosphate
Stimulates +
Phosphofructokinase – –
Fructose-1,6-bisphosphate
Inhibits
Inhibits
Pyruvate
ATP
Citrate
Acetyl CoA
Figure 9.21 The control of cellular respiration
Citric
acid
cycle
Oxidative
phosphorylation