Cellular Respiration:
Harvesting Chemical Energy
The Metabolic Pathway of Cellular Respiration
• Cellular respiration is an example of a metabolic
pathway
– A series of chemical reactions in cells
• All of the reactions involved in cellular respiration
can be grouped into three main stages
– Glycolysis
– The Krebs cycle
– Electron transport
A Road Map for Cellular Respiration
Cytosol
Mitochondrion
High-energy
electrons
carried
mainly by
NADH
High-energy
electrons
carried
by NADH
Glycolysis
Glucose
2
Pyruvic
acid
Krebs
Cycle
Electron
Transport
Figure 6.7
Stage 1: Glycolysis
• A molecule of glucose is split into two molecules of
pyruvic acid
– These molecules then donate high energy electrons to
NAD+, forming NADH
– Glycolysis makes some ATP directly when enzymes
transfer phosphate groups from fuel molecules to
ADP
– Glycolysis occurs in the cytosol of the cell.
2 Pyruvic acid
Glucose
PGAL
Figure 6.8
If oxygen is available, pyruvic acid enters the
pathways of aerobic respiration (respiration with
oxygen)
Stage 2: The Krebs Cycle
• The Krebs cycle completes the breakdown of sugar
• Occurs inside of the mitochondria (mitochondrial
matrix)
• In the Krebs cycle, pyruvic acid from glycolysis is first
“prepped” into a usable form, Acetyl-CoA
2
Acetic
acid
Pyruvic
acid
CO2
Coenzyme A
Acetyl-CoA
(acetyl-coenzyme A)
Mitochondria
• The Krebs cycle extracts the energy of sugar by
breaking the acetic acid molecules all the way down
to CO2
– The cycle uses some of this energy to make ATP
– The cycle also forms NADH and FADH2
Hans Krebs
2
3
4
6
Things to Remember
One glucose molecule causes two turns of the
Krebs cycle
The two turns produce 6 NADH, 2 FADH2,
2 ATP, and 4 CO2.
So now there have been 4 molecules of ATP
created up to this point (remember the 2 created
during glycolysis)
Stage 3: Electron Transport
• Electron transport releases the energy your cells
need to make the most of their ATP
• The molecules of electron transport chains are built
into the inner membranes of mitochondria
– The chain functions as a chemical machine that uses
energy released by the “fall” of electrons to pump
hydrogen ions across the inner mitochondrial
membrane
– These ions store potential energy
• When the hydrogen ions flow back through the membrane,
they release energy
– The ions flow through ATP synthase
– ATP synthase takes the energy from this flow and
synthesizes ATP (Chemiosmosis)
Protein
complex
Electron
carrier
Inner
mitochondrial
membrane
Electron
flow
Electron transport chain
ATP synthase
Figure 6.12
The Versatility of Cellular Respiration
• Cellular respiration can
“burn” other kinds of
molecules besides glucose
– Diverse types of
carbohydrates
– Fats
– Proteins
Food
Polysaccharides
Sugars
Glycerol
Fats
Fatty acids
Proteins
Amino acids
Amino groups
Glycolysis
AcetylCoA
Krebs
Cycle
Electron
Transport
Figure 6.13
Adding Up the ATP from Cellular Respiration
Cytosol
Mitochondrion
Glycolysis
Glucose
2
Pyruvic
acid
2
AcetylCoA
Krebs
Cycle
Electron
Transport
Maximum
per
glucose:
by direct
synthesis
by
direct
synthesis
by
ATP
synthase
Figure 6.14
FERMENTATION: ANAEROBIC HARVEST OF
FOOD ENERGY
• Some of your cells can actually work for short
periods without oxygen
– For example, muscle cells can produce ATP under
anaerobic conditions
• Fermentation
– The anaerobic harvest of food energy
Fermentation in Human Muscle Cells
• Human muscle cells can make ATP with and
without oxygen
– They have enough ATP to support activities such as
quick sprinting for about 5 seconds
– A secondary supply of energy (creatine phosphate)
can keep muscle cells going for another 10 seconds
– To keep running, your muscles must generate ATP
by the anaerobic process of fermentation
• Glycolysis is the metabolic
pathway that provides ATP
during fermentation
– Pyruvic acid is reduced by
NADH, producing NAD+,
which keeps glycolysis
going
– In human muscle cells,
lactic acid is a by-product
2 ADP+ 2
Glycolysis
2 NAD
2 NAD
Glucose
2 Pyruvic
acid
+ 2 H
2 Lactic
acid
(a) Lactic acid fermentation
Figure 6.15a
Fermentation in Microorganisms
• Various types of
microorganisms perform
fermentation
– Yeast cells carry out a
slightly different type of
fermentation pathway
– This pathway produces
CO2 and ethyl alcohol
• The food industry uses yeast to produce various
food products
Figure 6.16
Two Types of Fermentation