Chapter 9 Cellular Respiration
Cellular respiration-stepwise series of catabolic reactions that break down complex molecules to CO2
and H2O and release energy.
 These reactions occur in a controlled manner
 They are catalyzed by enzymes at every step
 Some of the energy from these reactions is stored in ATP
Formula for respiration:
C6H12O6 + 6O2-------------- 6CO2 + 6H2O + energy
(enzymes)
Single reactions in a cell do not use glucose as a direct source of energy because glucose contains too
much energy for one reaction.
THREE METABOLIC STAGES OF CELLULAR RESPIRATION
 Glycolysis-occurs in cytosol
 Krebs (Citric Acid) Cycle (TAC cycle) –occurs in mitochondrial matrix
 Electron Transport Chain and Oxidative phosphorylation- occurs in inner mitochondrial
membrane.
Redox reactions: a chemical reaction in which there is a transfer of one or more electrons from one
reactant to another (Oxidation-reduction reactions). Be sure to know terms: oxidation and reduction,
reducing agent and oxidizing agent)
In respiration Glucose is oxidized to carbon dioxide and oxygen is reduced to water.
For each molecule of glucose broken down into CO2 and H2O the cell makes about 36 to 38 ATP
molecules.
1. Glycolysis- In several steps, glucose (a 6-carbon sugar) splits into two 3-carbon molecules of
pyruvic acid and a small amount of ATP forms.
2. Krebs Cycle (Citric Acid Cycle)-The two molecules of pyruvic acid are broken down to CO2 and
additional ATP forms.
Glycolysis and Krebs cycle reactions release hydrogen atoms. Special molecules carry these hydrogen
atoms to the third stage of respiration.
3. Electron Transport Chain-Most ATP is synthesized in this step. Each hydrogen atom is separated
into an electron and a proton (H+) and transferred through many small steps to oxygen, finally
forming water.
ONLY THE VERY LAST STEP IN CELLULAR RESPIRATION INVOLVES OXYGEN.
Aerobic respiration- with oxygen
Anaerobic respiration-without oxygen
The hydrogen carriers are NADH AND FADH2.
Hydrogen carried by NADH AND FADH2 is used to synthesize ATP in the ETC.
The Reactions of Respiration
A. Glycolysis
1. Molecules of glucose enter a cell.
2. An enzyme converts glucose to glucose-6-phosphate (ATP provides the phosphate and the free
energy to power the reaction).
3. Another enzyme rearranges the glucose-6-phosphate and a second molecule of ATP donates a
phosphate group.
4. The molecule splits into two 3-carbon phosphates.
5. The two 3-carbon phosphates go through a series of rearrangements (each catalyzed by specific
enzyme).
6. Four molecules of ATP, two molecules of NADH, and 2 molecules of 3-carbon pyruvic acid are
formed.
THE RESULT IS A NET GAIN OF 2 ATP MOLECULES SINCE 2 ATP MOLECULES USED TO BEGIN
GLYCOLYSIS.
SUMMARY-The role of glycolysis is to produce:
 ATP
 NADH
 Pyruvic Acid
 and form carbon skeletons.
AT THE END OF GLYCOLYSIS THE PRESENCE OF ABSENCE OF OXYGEN DICTATES THE FATE OF
PYRUVIC ACID.
 LITTLE OR NO OXYGEN-Lactic acid fermentation occurs. In fermentation animal cells can
convert NADH and Pyruvic Acid into NAD+ and Lactate (NAD+ cycles back to glycolysis and
the pathway continues to provide a small amount of ATP until oxygen becomes available
again.
In plants and bacterial cells fermentation often results in the production of alcohol instead of
lactate. FERMENTATION IS ANAEROBIC RESPIRATION.
IN PLANTS
IN ANIMALS

OXYGEN PRESENT-Pyruvic acid is transported to the mitochondria.
A complex enzyme system changes pyruvic acid into acetate (2-carbon acid) and CO2. This
step also produces a molecule of NADH from NAD+.
MITOCHONDRIA-Special compartment that house the Krebs Cycle and the ETC.
 The inner mitochondrial membrane contains so many enzymes that it is more protein than
lipid.
 The outer mitochondria membrane regulates what molecules enter and leave.
 Cristae-has enzymes of the ETC, the enzymes for ATP formation, and some of the enzymes of
the Krebs Cycle (most of the Kreb Cycle enzymes re in the matrix.
LINKING STEP: CONVERSION OF PYRUVATE TO ACETYL CoA: -one Acetyl CoA produced plus one
carbon dioxide molecule released , one NADH produced.
KREBS CYCLE
1. An enzyme combines the acetyl group of acetyl CoA with a 4-carbon acid (Oxaloacetate)
making a 6-carbon acid (Citric Acid) and releasing CoA.
2. In several steps other enzymes rearrange the 6-carbon acid and convert it into a 5-carbon acid
(Ketoglutarate). These reactions release a molecule of CO2 and form a molecule of NADH from
NAD+.
3. An enzyme converts the 5-carbon acid into a 4-carbon acid and releases a molecule of CO2 and
forms a molecule of NADH.
4. Enzymes rearrange the 4-carbon acid two times forming one molecule of ATP and FADH2.
5. Enzymes convert the rearranged 4-carbon acid into Oxaloacetate and form a molecule of
NADH.
6. The cycle continues as Oxaloacetate enters step A.
PRODUCTS OF THE KREBS CYCLE
Reaction
CO2
NADH
Pyruvic acid to acetyl CoA
1
1
Krebs Cycle
2
3
TOTAL for linking step & one
3
4
turn of Krebs cycle
X 2 (per glucose to pyruvate)
6
8
FADH2
0
1
1
ATP
0
1
1
2
2