Glycolysis, the Krebs cycle, and oxidative phosphorylation work together to break down glucose and harvest energy in the form of ATP. Glycolysis produces 2 ATP and feeds pyruvate into the Krebs cycle in the mitochondria. The Krebs cycle further processes pyruvate and generates more ATP, as well as NADH and FADH2. These molecules are used in oxidative phosphorylation to pump protons across the mitochondrial membrane, driving ATP synthase to produce approximately 32 additional ATP. Aerobic respiration therefore yields about 36 ATP per glucose molecule.

1. Glycolysis, Krebs Cycle, and other Energy-Releasing Pathways Goal: take pyruvate and put it into the Krebs cycle, producing NADH
and FADH2
All organisms produce ATP by releasing energy stored in glucose and other Where: the mitochondria
sugars. There are two steps
o The Conversion of Pyruvate to Acetyl CoA
Plants make ATP during photosynthesis. o The Krebs Cycle proper
All other organisms, including plants, must produce ATP by breaking The Krebs cycle and the conversion of pyruvate to Acetyl CoA
down molecules such as glucose produce 2 ATP's, 8 NADH's, and 2FADH2's per glucose molecule
Aerobic respiration - the process by which a cell uses O2 to "burn" molecules The Oxidation of Pyruvate to form Acetyl CoA for Entry Into the Krebs
and release energy Cycle
The reaction: C6H12O6 + 6O2 >> 6CO2 + 6H2O 2 NADH's are generated (1 per pyruvate)
2 CO2 are released (1 per pyruvate)
Note: this reaction is the opposite of photosynthesis
This reaction takes place over the course of three major reaction pathways
Glycolysis
The Krebs Cycle
Electron Transport Phosphorylation (chemiosmosis)
Glycolysis (glyco = sugar; lysis = breaking)
Goal: break glucose down to form two pyruvates
Who: all life on earth performs glyclolysis
Where: the cytoplasm The Krebs Cycle
Glycolysis produces 4 ATP's and 2 NADH, but uses 2 ATP's in the
process for a net of 2 ATP and 2 NADH Krebs Cycle Animation
NOTE: this process does not require O2 and does not yield much energy 6 NADH's are generated (3 per Acetyl CoA that enters)
2 FADH2 is generated (1 per Acetyl CoA that enters)
The First Stage of Glycolysis 2 ATP are generated (1 per Acetyl CoA that enters)
4 CO2's are released (2 per Acetyl CoA that enters)
Glucose (6C) is broken down into 2 PGAL's (Phosphoglyceraldehyde
- 3Carbon molecules)
This requires two ATP's
The Second Stage of Glycolysis
2 PGAL's (3C) are converted to 2 pyruvates
This creates 4 ATP's and 2 NADH's
The net ATP production of Glycolysis is 2 ATP's
Therefore, the total numbers of molecules generated in the oxidation
of pyruvate and the Krebs Cycle is:
o 8 NADH
o 2 FADH2
o 2 ATP
o 6 CO2
Electron Transport Phosphorylation (Chemiosmosis)
Goal: to break down NADH and FADH2, pumping H+ into the outer
Oxidation of Pyruvate and the Krebs Cycle (citric acid cycle, TCA cycle) compartment of the mitochondria

2. Where: the mitochondria
In this reaction, the ETS creates a gradient which is used to produce
ATP, quite like in the chloroplast
Electron Transport Phosphorylation typically produces 32 ATP's
ATP is generated as H+ moves down its concentration gradient
through a special enzyme called ATP synthase
The only goal of fermentation reactions is to convert NADH to
NAD+ (to use in glycolysis).
No energy is gained
Note differences - anaerobic respiration - 2 ATP's produced (from
glycolysis), aerobic respiration - 36 ATP's produced (from glycolysis,
Krebs cycle, and Oxidative Phosphorylation)
Net Engergy Production from Aerobic Respiration Thus, the evolution of an oxygen-rich atmosphere, which facilitated
the evolution of aerobic respiration, was crucial in the diversification
of life
Glycolysis: 2 ATP
Krebs Cycle: 2 ATP
Photosynthesis: 6 CO2 + 6 H2O >> C6H12O6 + 6 O2
Electron Transport Phosphorylation: 32 ATP
o Each NADH produced in Glycolysis is worth 2 ATP (2 x
2 = 4) - the NADH is worth 3 ATP, but it costs an ATP to Respiration: C6H12O6 + 6 O2 >> 6 CO2 + 6 H2O
transport the NADH into the mitochondria, so there is a
net gain of 2 ATP for each NADH produced in gylcolysis
Notice that these reactions are opposites - this is important since the earth is
o Each NADH produced in the conversion of pyruvate to a closed system
acetyl COA and Krebs Cycle is worth 3 ATP (8 x 3 = 24)
o Each FADH2 is worth 2 ATP (2 x 2 = 4)
o 4 + 24 + 4 = 32 All life has a set amount of natural materials to work with, so it is important that
they all be cycled through effectively and evenly
Net Energy Production: 36 ATP!
Energy Yields:
Anaerobic Respiration
Glucose: 686 kcal/mol
Goal: to reduce pyruvate, thus generating NAD+
ATP: 7.5 kcal/mol
Where: the cytoplasm
7.5 x 36 = 270 kcal/mol for all ATP's produced
Why: in the absence of oxygen, it is the only way to generate NAD+
270 / 686 = 39% energy recovered from aerobic respiration
Alcohol Fermentation - occurs in yeasts in many bacteria Related Catabolic Processes - Beta Oxidation
o The product of fermentation, alcohol, is toxic to the
organism
Fats consist of a glycerol backbone with two or three fatty acids
connected to it
The body absorbs fats and then breaks off the fatty acids from the
glycerol
Glycerol is converted to glyceraldehyde phosphate, an intermediate of
glycolysis
The fatty acids are broken down into two-carbon units which are then
converted to acetyl CoA.
o An eight-carbon fatty acid can produce 4 acetyl CoA's
o Each acetyl CoA is worth 12 ATP's (3 NADP, 1 FADH2, 1
ATP)
o Therefore, this short fatty acid is worth 48 ATP's, a fat
with three chains of this length would be worth 144
ATP's!
o This is why fats are such a good source of energy, and are
hard to lose if you want to lose weight
Lactic Acid Fermentation - occurs in humans and other mammals
o The product of Lactic Acid fermentation, lactic acid, is
toxic to mammals A comparison between Plants and Animals
o This is the "burn" felt when undergoing strenuous activity
Animal cells and Plant cells contain mitochondria!
o However, animal cells contain many more mitochondria
than plant cells
Animal cells get most of their ATP from mitochondria

3. Plant cells get most of their ATP from the chloroplast
o The ATP generated from the mitochondria is only used
when the plant cannot generate ATP directly from the
light-dependent reactions
Other Uses for Molecules used in Glycolysis and the Krebs Cycle
Not all of the molecules that enter Glycolysis and the Krebs Cycle are
used for energy
Some are used to synthesize fats, nucleotides, amino acids, and other
biologically important molecules.
Regulation of Glycolysis and the Krebs Cycle
Step 3 of Glycolysis - The conversion of Fructose 6-phosphate to
Fructose 1,6-bisphosphate
o Enzyme catalyzing this reaction = Phosphofructokinase
o "Committing Step" - Fructose 6-phosphate can be used by
the cell for lots of things, but fructose 1,6-bisphosphate
has limited use except in glycolysis
o Phosphofructokinase inhibited by high levels of ATP
 ATP is also a substrate - odd, eh?
 Enzyme has two ATP binding sites, one in the
active site and one in an allosteric site
 Low to mid levels of ATP cause ATP to bind
to the active site
 High levels of ATP also enable ATP to bind to
allosteric site, causing a conformation change
and shutting down the enzyme
Conversion of Pyruvate to Acetyl CoA
o Enzyme involved in catalyzing this reaction = pyruvate
dehydrogenase
o High levels of ATP slow down this reaction by
phosporylating the enzyme, changing its shape and
shutting it down
 High levels of NADH and Acetyl CoA also
inhibit this enzyme
o NAD+, CoA, or AMP (an indicator of low ATP) can
speed up the reaction