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Cellular Respiration
Chapter 4: Cells and Energy
Sections 4 and 5
Cellular Respiration
Overview:
• Cellular respiration is the process of releasing
chemical energy from sugars and other Carbon
based molecules to make ATP when oxygen is
present.
CRITICAL THINKING:
Discuss the relationship between photosynthesis
and cellular respiration with a partner.
Cellular Respiration
• The equation for cellular respiration is:
C6H12O6 + 6O2  6CO2 + 6H2O + Energy
Does this equation look familiar?
The equation for photosynthesis is:
6CO2 + 6H2O  C6H12O6 + 6O2
Cellular Respiration:
C6H12O6 + 6O2  6CO2 + 6H2O + Energy
Cellular Respiration
• 3 Phases of Cellular Respiration:
1. Glycolysis
2. Krebs Cycle
3. Electron Transport Chain
• Recall: In the definition of cellular respiration,
we stated that it takes place in the presence of
Oxygen.
• However, Glycolysis does NOT need oxygen in
order to occur
Aerobic vs. Anaerobic
• Aerobic – requires the use of
oxygen
– Krebs Cycle
– Electron Transport Chair
• Anaerobic – in the absence
of oxygen
– Glycolysis
Glycolysis
• Glycolysis is the process of breaking down
glucose
• Occurs in the cytoplasm
• Step 1:
– 2 ATP Molecules transfer their terminal Phosphate
groups to a glucose molecule.
Glycolysis
• NOTE: Some energy is needed in order to start
the process of glycolysis.
CRITICAL THINKING:
When the 2 Phosphate groups are added to
glucose, the 6-Carbon molecule splits into TWO 3Carbon molecules.
Think back to what you know about how glucose is
formed.
What are the two 3-Carbon molecules?
Glycolysis
• Step 2:
– 2 Phosphate groups are removed from G3P.
• The phosphate group is added to ADP
• What molecule will be created?
– Electrons and H+ (protons) are removed from G3P
• NAD+ is an electron carrier that receives the electrons
and protons.
• NAD+ is converted to NADH
• (This molecule acts similarly to NADPH in
photosynthesis).
Glycolysis
• Step 3:
– The G3P molecules are converted into 2
molecules of Pyruvate.
Glycolysis
CRITICAL THINKING:
There is a net gain of 2 ATP during glycolysis.
Why is it not a net gain of 4 ATP molecules?
2 ATP molecules were USED to start glycolysis.
4 ATP molecules are created.
4-2 = 2
Therefore, the NET GAIN is 2 ATP molecules.
Phase II: Krebs Cycle
• After glycolysis, there are two pyruvate
molecules produced.
– Despite a net gain of 2 ATP, most of the energy is
still contained in the pyruvate.
• Keep in mind: Krebs Cycle is an AEROBIC
process
What does that mean?
Krebs Cycle
• Krebs Cycle is AEROBIC (requires O2)
– In the presence of O2, pyruvate is transported into
the mitochondrial matrix
– Kreb Cycle is the series of reactions in which
pyruvate is broken down into CO2, ATP and
energy-carrying molecules.
• Also known as:
– Tricarboxylic acid (TCA) Cycle
– Citric Acid Cycle
Krebs Cycle
• Prior to beginning, pyruvate reacts with
Coenzyme A (CoA).
– Pyruvate is a 3-Carbon compound
– Coenzyme A combines with pyruvate to form a 2carbon intermediate
• 2-Carbon compound is called Acetyl CoA
• CO2 is released at the same time
• NAD+ is converted to NADH
Pyruvate  Acetyl CoA
Krebs Cycle
• Now the Krebs Cycle can begin!
• Step One:
– Acetyl CoA combines with 4-Carbon compound
– Forms a 6-Carbon compound called citric acid
• Hence the alternative “Citric Acid Cycle” name
• Step Two:
– Citric Acid released CO2
• Citric Acid (6 Carbons) – CO2 (1 Carbon) = 5 carbon
compound
– Generates: NADH
Krebs Cycle
• Step 3:
– Now there is a 5-Carbon compound
• Carbon dioxide is released leaving a 4-Carbon
compound
– Generates NADH and ATP
• Step 4:
– Enzymes rearrange the 4-Carbon compound into a
new 4-Carbon compound
– Generates a new energy carrying molecule called
FADH2, as well as NADH
Krebs Cycle
RECALL:
Each cycle of the Krebs Cycle involves one
pyruvate molecule.
However, 2 molecules of pyruvate were
generated from one glucose.
Therefore, TWO cycles of the Krebs Cycle are
needed for one molecule of glucose.
Krebs Cycle
• The net yield of the Krebs Cycle:
– 6 CO2
– 8 NADH
– 2 FADH2
• NADH and FADH2 are important for the next
and final step of cellular respiration
Phase III: Electron Transport
• This is the phase where the most ATP is produced
• High energy electrons and H+ from NADH and
FADH2 are used to convert ADP to ATP
• Still occurs within the mitochondrial matrix
Critical Thinking:
Where have we seen an electron transport chain
before?
Electron Transport Chain
Step 1:
• Electrons move along mitochondrial
membrane from one protein to another
Step 2:
• NADH and FADH2 transfer electrons to
proteins in the mitochondrial matrix
– Replenishing electrons in membrane proteins
Electron Transport Chain
Step 3:
• H+ is pumped in to the mitochondrial matrix
• Building a high concentration of H+ inside the
mitochondria
Step 4:
• Chemiosmosis!!!
Turn to a neighbor and discuss what you know
about chemiosmosis
Electron Transport Chain
Step 4:
• In chemiosmosis, H+ diffuse along the
concentration gradient through the protein
channel ATP SYNTHASE
• ATP Synthase harvests the energy in order to
add a Phosphate group to ADP, creating ATP
Electron Transport Chain
Step 5:
• Oxygen serves as the final electron acceptor
• ½ Oxygen + e- + 2H+ 
OVERALL, the electron transport produces 32
ATP
– Each NADH  3 ATP
• 10 NADH x 3 = 30 ATP
– 3 molecules of FADH2  2 ATP
• 1 group of 3 x 2 = 2 ATP
Cellular Respiration
• Glycolysis  2 ATP
• Krebs Cycle  2 ATP
• Electron Transport  32 ATP
During cellular respiration (under ideal
conditions), a eukaryotic cell will produce 36 ATP
molecules.