CHAPTER 5
Harvesting Chemical Energy
Chemical Energy
and Food
• All organisms require energy to carry out their
life functions.
• Evolution has produced a number of biochemical
processes that organisms use to obtain energy
stored in food.
• Food provides:
– Energy for living things and their cells to function
• How much energy is in food?
– Calorie: amount of energy needed to raise the
temperature of one gram of water one degree
Celsius
– Food labels are kilocalories (1,000 calories)
Metabolism
• Metabolism – all chemical reactions in an
organism
• Composed of 2 parts:
1) Synthesis
a) Reactions that require energy
b) Use carbon skeletons and energy for cell
growth and maintenance
c) Example:Photosynthesis!
Metabolism, continued
• Composed of 2 parts:
2) Decomposition
a. Release energy from food
b. Produce carbon skeletons
c. Example:
6O2 + C6H12O6  6CO2 + 6H2O + Energy
 Which of the above makes ATP/ energy?
 Which of the above uses ATP/ energy?
Overview of Cellular Respiration
• Main Points:
– Decomposition reaction
– Provides cells with energy they need to
function
– Each step catalyzed by an enzyme
– Releases energy by oxidizing sugars and
other organic substances
Two Types of Respiration
1) Aerobic Respiration (which is typically referred to
as Cellular Respiration)
a.
b.
c.
d.
e.
Requires oxygen
Electrons flow to oxygen
Raw materials are fats, proteins, and carbohydrates
Energy is released
Overall Equation:
6O2 + C6H12O6
Enzymes
6CO2 + 6H2O + ATP
2) Anaerobic Respiration
a. Occurs without oxygen
b. Electrons flow to another acceptor, such as nitrogen or
sulfur in bacteria and yeast
The Stages of Aerobic
Respiration
Glycolysis
Location
What goes in?
What comes out?
ATP formed
Krebs Cycle
Electron Transport
Cytoplasm
Mitochondrial
Matrix
Mitochondrial
Membranes
Glucose
Pyruvate/
Acetyl CoA
O2, NADH and
FADH2
Pyruvate &
NADH
CO2, NADH, &
FADH2
H2O
2 Net
2
34
Glycolysis
• The first stage for both aerobic and
anaerobic respiration.
• Definition:
– The anaerobic (does not require O2) process
of breaking down glucose into 2 molecules of
pyruvic acid
– Each step is catalyzed by different enzymes
• Location in the Cell:
– Cytoplasm of the cell
Steps of Glycolysis
1. Glucose is converted into glucose-6phosphate
–
Requires the use of 2 ATP, which releases its
phosphates
2. Glucose-6-phosphate is rearranged and
eventually splits into two 3-C sugarphosphates
3. Partial oxidation of these 3-C molecules
results in …
– the formation of pyruvic acid (pyruvate)
– the production of ATP
– NAD+ is reduced to form NADH, which will be
sent to the ETS
Glycolysis Totals
• Total Energy Made:
– To start glycolysis 
– During glycolysis 
Net gain:
• Electron carrier:
– 2 NAD+
accepts e-
2 ATP needed
4 ATP made
2 ATP
2 NADH
– NAD+ accepts 4 high energy electrons and forms
NADH. NADH holds them until they are passed to
other molecules.
The FATE of PYRUVATE
• The PRESENCE or ABSENCE of
OXYGEN in the cell determines the
FATE of PYRUVATE.
PYRUVATE
Anaerobic
(no oxygen)
LACTATE
Aerobic
(oxygen)
Enters
MITOCHONDRIA
Lactic Acid Fermentation
• Convert NADH and pyruvate into NAD+ and lactate
• NAD+ cycles back to glycolysis
• Small amounts of ATP are made.
Other types of fermentation:
• Alcoholic fermentation…
– Produces ethyl alcohol and CO2.
– EX: yeast used to make bread
• Acetic Acid fermentation…
– Produces vinegar
– EX: bacteria used to make yogurt, cheese, and sour
cream
Fermentation cont.
• Total Energy Output of Fermentation=
No new ATP made!
(only 2 ATP from glycolysis)
This is why we can’t exercise rapidly for
long periods of time!
Cellular Respiration
(Aerobic!)
The FATE of PYRUVATE
• The PRESENCE or ABSENCE of
OXYGEN in the cell determines the
FATE of PYRUVATE.
PYRUVATE
Anaerobic
(no oxygen)
LACTATE
Aerobic
(oxygen)
Enters
MITOCHONDRIA
Glycolysis Review
• At the end of glycolysis, there is still a
lot of unused energy stored in the 2
molecules of pyruvic acid
• To access this energy, cells need O2.
• Therefore, the final steps in cellular
respiration are aerobic because they
require O2
Pyruvate enters the
Mitochondria…
• Produces acetate (acetic acid)
• Produces NADH from NAD+
• CoEnzyme A picks up acetate and forms
Acetyl CoA
• CoA delivers acetate to the Krebs
Cycle
• Also known as the Citric Acid Cycle
The Mitochondria
– Cite of ATP synthesis
– Compartments in which the Krebs cycle and
electron transport chain occurs
– Number in cells vary from 10 to 1000s based
on activity
– Made of 2 membranes:
1. Inner – contains many enzymes, made of more
protein than lipids, forms cristae (folds) which
increase surface area.
2. Outer – regulates movement of molecules in and out
of the mitochondria
Steps of the Kreb’s Cycle
•
•
•
•
An enzyme combines the 2C acetyl group of
acetyl CoA with a 4C acid (oxaloacetate),
forming a 6 C acid (citric acid) and releasing
CoA.
Enzymes rearrange the 6C acid and convert it
into a 5 C acid (Ketogluterate). These
reactions release CO2 into the atmosphere and
harvest electrons to form NADH from NAD+.
Enzymes rearrange the 4 C acid two times,
forming one molecule of ATP (2) and FADH2
Finally, enzymes convert the rearranged 4 C
acid into oxaloacetate and form a third
molecule of NADH. The cycle continues as
oxaloacetate enters the beginning.
Pyruvic
acid or
Pyruvate
Acetyl- CoA
CO2
Citric acid
or Citrate
atmosphere
Other
molecules
CO2
2 ATP
H+
electrons
Captured by NAD+ and FAD
To E.T.C
Remember…
• The CO2 released is the source
of the CO2 in your breath when
you exhale
• ATP produced directly in the
Kreb’s cycle can be used in
cellular activities
• When O2 is present, the high
energy electrons can be used to
generate huge amounts of ATP
Steps of the Electron Transport
System
1. H atoms are carried by NAD and FAD to the ETS
2. Each system consists of a series of electron
carriers, enzymes, and other proteins known as
cytochromes which are embedded in the cristae
of the mitochondria.
3. H atoms are accepted by the system and
separated into electrons and protons.
4. The electron carriers transfer the electrons step
by step through the system to a cytochrome.
5. The cytochrome combines the electrons with
protons and O2 forming H2O (this step requires
oxygen!)
Steps of the Electron
Transport System
6. At each step the electrons release
free energy, some is used by
proton pumps to actively transport
protons from the matrix across
the cristae to the intermembrane
space.
7. A high concentration of protons
accumulate causing it to be
unstable
8. Protons diffuse back to the
matrix of the mitochondria, they
pass through the ATP synthase
(where ATP is made.)
9. ATP can be transferred out of the
mitochondria and used by the cell.
NADH NAD+ + e- + H+
FADH2 FAD + e- + H+
ATP
ATP
ATP
e-
ATP
e-
e-
ee-
O2
H+
H2O
Exhaled as
waste
2
2
32 - 34