6
Pathways that Harvest and
Store Chemical Energy
Chapter 6 Pathways that Harvest and Store Chemical Energy
Key Concepts
• 6.1 ATP, Reduced Coenzymes, and
Chemiosmosis Play Important Roles in
Biological Energy Metabolism
• 6.2 Carbohydrate Catabolism in the Presence
of Oxygen Releases a Large Amount of Energy
• 6.3 Carbohydrate Catabolism in the Absence of
Oxygen Releases a Small Amount of Energy
Chapter 6 Pathways that Harvest and Store Chemical Energy
• 6.4 Catabolic and Anabolic Pathways Are
Integrated
• 6.5 During Photosynthesis, Light Energy Is
Converted to Chemical Energy
• 6.6 Photosynthetic Organisms Use Chemical
Energy to Convert CO2 to Carbohydrates
Chapter 6 Opening Question
Why does fresh air inhibit the formation of
alcohol by yeast cells?
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play
Important Roles in Biological Energy Metabolism
Cellular respiration is a major catabolic pathway.
Glucose is oxidized:
Photosynthesis is a major anabolic pathway.
Light energy is converted to chemical energy
(CO2 is reduced):
Figure 6.7 ATP, Reduced Coenzymes, and Metabolism
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological
Energy Metabolism
Five principles governing metabolic pathways:
1. Chemical transformations occur in a series of
intermediate reactions that form a metabolic
pathway.
2. Each reaction is catalyzed by a specific
enzyme.
3. Most metabolic pathways are similar in all
organisms.
Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological
Energy Metabolism
4. In eukaryotes, many metabolic pathways
occur inside specific organelles.
5. Each metabolic pathway is controlled by
enzymes that can be inhibited or activated.
1. How do heterotrophs obtain free energy?
From food!
A lot of energy is released when reduced
molecules (ex: glucose) are fully oxidized to
CO2.
Figure 6.8 Energy Metabolism Occurs in Small Steps
This is the equation for aerobic cellular respiration:
Endergonic or exergonic?
Catabolic or Anabolic?
Require or Release energy?
Positive or Negative change in Free
Energy?
This is the equation for aerobic cellular respiration:
Which substance is being oxidized?
To what is this substance oxidized to?
Which substance is being reduced?
To what is this substance being reduced to?
Figure 6.9 Energy-Releasing Metabolic Pathways
What is an “electron shuttle” or “electron carrier”? What are the
two major electron carriers in respiration?
Transfer electrons and protons to other areas of
cells to continue metabolism (transfer of energy!)
NAD+ and FAD (oxidized forms)
NADH and FADH2 (reduced form)
Glycolysis:
Glycolysis:
a. Cytoplasm
b. 10 reactions
c. Glucose, 2NAD+, 2ATP
d. 2 ATP
e. 4 ATP
f. Substrate Level Phosphorylation
g. NAD+ is reduced to NADH; 2
h. 2 ATP, 2 NADH, 2 Pyruvate
Figure 6.10 Glycolysis Converts Glucose into Pyruvate (Part 1)
Figure 6.10 Glycolysis Converts Glucose into Pyruvate (Part 2)
Figure 6.10 Glycolysis Converts Glucose into Pyruvate (Part 3)
Why is glycolysis considered to be anaerobic?
Glycolysis does not require oxygen!
Why is glycolysis considered to be the most ancient metabolic
pathway?
It occurs in the cytoplasm of ALL living
things!
8. Before each Pyruvate molecule can enter the citric acid cycle, it
is converted into...
Acetyl CoA
a. Mitochondria
b. CO2
c. Reduced
Comparing inputs/outputs per pyruvate and per glucose molecule
Inputs
Outputs
Acetyl CoA
preparation reaction
per Pyruvate
1 Pyruvate
1 NAD+
1 Coenzyme A
1 Acetyl CoA
1 NADH
1 CO2 (waste
product)
Acetyl CoA
preparation reaction
per Glucose
2 Pyruvate
2 NAD+
2 Coenzyme A
2 Acetyl CoA
2 NADH
2 CO2 (waste
product)
Figure 6.11 The Citric Acid Cycle
9. Citric Acid Cycle (The Krebs Cycle): 2 Turns per Glucose
Molecule
a. Mitochondrial Matrix
b. CO2; 2
c. 3
d. No; FAD is also used in this cycle
e. 1
f. 1; Substrate Level Phosporylation
g. To enable the cycle to continue
g. Citric Acid Cycle Summary
Inputs
Per Acetyl CoA
Molecule
Outputs
1 Acetyl CoA
3 NAD+
1 FAD
1 ADP
2 CO2
3 NADH
1 FADH2
1 ATP
Per Glucose Molecule 2 Acetyl CoA
6 NAD+
2 FAD
2 ADP
4 CO2
6 NADH
2 FADH2
2 ATP
Although the CAC doesn’t produce a lot of ATP…
**Most of the
energy from
glucose is stored
in electron
shuttles (NADH,
FADH2) during
this
process…This
energy will be
released in the
next step!**
Figure 6.12 Electron Transport and ATP Synthesis in Mitochondria
Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen
Releases a Large Amount of Energy
a. Inner mitochondrial membrane (cristae)
b. NADH and FADH2 are oxidized to NAD+ and
FAD
 Move back to the Cytoplasm (glycolysis) or the
Matrix (Citric Acid Cycle) to be reused
c. Chain of membrane-associated electron carriers
that electrons from NADH and FADH2 pass
through
d. Actively Pump protons from the matrix into the
inter-membrane space creating an
electrochemical gradient
e. Oxygen, Water is formed
How is ATP generated?
ATP synthase in the membrane uses the H+
gradient to synthesize ATP by chemiosmosis.
Summary of Oxidative Phosphorylation
Inputs
Oxidative
10 NADH
Phosphorylation: ETC 2 FADH2
and Chemiosmosis
Oxygen
Outputs
~32-34 ATP
Water
10 NAD+
2 FAD
How much ATP is produced per glucose molecule?
Between 32 and 36 molecules of ATP are
produced from 1 glucose molecule
(Hypothetical yield is 38 ATP)
Why isn’t it an exact number?
ATP synthesis and oxidation of electron carriers
are NOT directly coupled, and therefore we
cannot give an exact number of ATP produced
• ~ 3ATP per NADH, ~2 ATP per FADH2
Other macromolecules can enter various points of cell respiration:
Polysaccharides are hydrolyzed to glucose,
which enter glycolysis.
Lipids break down to fatty acids and glycerol.
Fatty acids can be converted to acetyl CoA.
Proteins are hydrolyzed to amino acids that can
feed into glycolysis or the citric acid cycle.
Cellular Respiration is a highly regulated process:
This is accomplished by regulation of enzymes—
allosteric regulation, feedback inhibition.
Example: Glycolytic enzymes (a glycolysis
enzyme) are a major site of control; It is
stimulated by AMP, and inhibited by ATP
and citrate (produced in the Krebs cycle)
Overview of Glycolysis: Location, Inputs, Net Outputs
• Cytoplasm
• Glucose, 2NAD+, 2 ATP
• 2 Pyruvate, 2 NADH, 2 ATP
Two types of anaerobic respiration/fermentation pathways:
What happens after glycolysis if oxygen is not available?
To prevent a cell from depleting its
NAD+ molecules, fermentation
occurs
Fermentation allows the NADH
produced in glycolysis to be
oxidized back to NAD+ to enable
anaerobic respiration to occur
NAD+ is needed for glycolysis
Fermentation takes place in the
cytoplasm
In both types of anaerobic respiration, what is the overall yield of
ATP? Where are these ATPs produced?
2 ATPs
Glycolysis
Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen
Releases a Small Amount of Energy
Lactic acid fermentation:
End product is lactic acid.
NADH is used to reduce pyruvate to lactic acid,
thus regenerating NAD+.
Which type of animal cells can carry out lactic
acid fermentation? Why?
Skeletal Muscle Cells when experiencing an
Oxygen debt
Figure 6.13 A Fermentation
Alcohol Fermentation
Alcoholic fermentation:
End product is ethanol.
Pyruvate is reduced to ethanol, CO2 is
produced, and NADH is regenerated.
Figure 6.13 B Fermentation
If the ATP is generated during glycolysis in both types of fermentation, what is
the major purpose of fermentation?
To regenerate NAD+ so glycolysis can continue
to occur!
Which process is more efficient?
Aerobic is 19x more efficient than anaerobic
respiration in terms of usable energy
production (~38 ATP compared to 2 ATP)
Cell Fraction
CO2
Lactic Acid
Mitochondria incubated with
glucose
Absent
Absent
Mitochondria incubated with
pyruvate
Present
Absent
Cytoplasm incubated with
glucose
Absent
Present
Cytoplasm incubated with
pyruvate
Absent
Present
Figure 6.14 Relationships among the Major Metabolic Pathways of the Cell
Answer to Opening Question
Pasteur’s findings:
Catabolism of the beet sugar is a cellular
process, so living yeast cells must be present.
With air (O2) yeasts used aerobic metabolism to
fully oxidize glucose to CO2.
Without air, yeasts used alcoholic fermentation,
producing ethanol, less CO2, and less energy
(slower growth).
Figure 6.23 Products of Glucose Metabolism