Download Glycolysis, Krebs Cycle, and other Energy

Lect., No. :
Dr. Rihab Edan
Glycolysis, Krebs Cycle, and other Energy-Releasing Pathways
All organisms produce ATP by releasing energy stored in glucose and other sugars.
1- Plants make ATP during photosynthesis.
2- All other organisms, including plants, must produce ATP by breaking down
molecules such as glucose.
Aerobic respiration : the process by which a cell uses O2 to "burn" molecules and
release energy.
C6H12O6 + 6O2  6CO2 + 6H2O
Note: this reaction is the opposite of photosynthesis
This reaction takes place over the course of three major reaction pathways
1- Glycolysis
2- The Krebs Cycle
3- 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
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.
NOTE: This process does not require O2 and does not yield much energy.
The First Stage of Glycolysis
1- Glucose (6C) is broken down into 2 PGAL's (3C)
2- This requires two ATP's
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Lect., No. :
Dr. Rihab Edan
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
Krebs Cycle (citric acid cycle, TCA cycle)
Goal: take pyruvate and put it into the Krebs cycle, producing NADH and FADH2
Where: the mitochondria
There are two steps:The Conversion of Pyruvate to Acetyl CoA
The Krebs Cycle proper
 In the Krebs cycle, all of Carbons, Hydrogens, and Oxygeng in pyruvate end up as
CO2 and H2O.
 The Krebs cycle plus the Conversion of Pyruvate produces 2 ATP's, 8 NADH's, and
2FADH2's per glucose molecule.
The Conversion of Pyruvate to Acetyl CoA for Entry Into the
Krebs Cycle
 2 NADH's are generated.
 2 CO2 are released.
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Lect., No. :
Dr. Rihab Edan
The Krebs Cycle
Krebs Cycle Animation
 6 NADH's are generated
 2 FADH2 is generated
 2 ATP are generated
 4 CO2's are released
Therefore, for each glucose molecule that enters into the Krebs cycle
(including the preparatory conversion to Acetyl CoA), the net production of products
are:
 8 NADH
 2 FADH2
 2 ATP
 6 CO2
 Remember, glycolysis produced 2 ATP and 2 NADH, so there is a net production
of 4 ATP and 10 NADH up to now.
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Lect., No. :
Dr. Rihab Edan
Electron Transport Phosphorylation (Chemiosmosis)
Goal: to break down NADH and FADH2, pumping H+ into the outer compartment of
the mitochondria.
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.
Net Energy Production from Aerobic Respiration
 Glycolysis: 2 ATP
 Krebs Cycle: 2 ATP
 Electron Transport Phosphorylation: 32 ATP
 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 transport the NADH into the mitochondria, so
there is a net gain of 2 ATP for each NADH produced in gylcolysis.
 Each NADH produced in the conversion of pyruvate to acetyl COA and Krebs
Cycle is worth 3 ATP (8 x 3 = 24).
 Each FADH2 is worth 2 ATP (2 x 2 = 4).
 4 + 24 + 4 = 32
 Net Energy Production: 36 ATP.
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Lect., No. :
Dr. Rihab Edan
Anaerobic Respiration
Goal: to reduce pyruvate, thus generating NAD+
Where: the cytoplasm
Why: in the absence of oxygen, it is the only way to generate NAD+ and ADP.
 Alcohol Fermentation: Occurs in yeasts and in many bacteria.
 The product of fermentation, alcohol, is toxic to the organism.
 Lactic Acid Fermentation  Occurs in humans and other mammals.
 The product of Lactic Acid fermentation, lactic acid, is toxic to mammals.
 This is the "burn" felt when undergoing strenuous activity.
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Lect., No. :
Dr. Rihab Edan
 The only goal of fermentation reactions is to convert NADH to NAD+ (to use in
glycolysis).
 No energy is gained.
 Note differences - fermentation - 2 ATP's produced, aerobic respiration - 36 ATP's
produced.
 Thus, the evolution of an oxygen-rich atmosphere, which facilitated the evolution
of aerobic respiration, was crucial in the diversification of life.
Photosynthesis: 6 CO2 + 6 H2O  C6H12O6 + 6 O2
Respiration: C6H12O6 + 6 O2  6 CO2 + 6 H2O
Notice that these reactions are opposites - this is important since the earth is a closed
system. 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.
 Energy Yields:
 Glucose: 686 kcal/mol
 ATP: 7.5 kcal/mol
 7.5 x 36 = 270 kcal/mol for all ATP's produced
 270 / 686 = 39% energy recovered from aerobic respiration
 Related Catabolic Processes Beta Oxidation
 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.
 A comparison between Plants and Animals
 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.
 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
Ref. : Basic Notes and pictures obtained from the following link:
http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect12.
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