1. Krebs Cycle (Citric Acid Cycle)
&
Quick and Long Energy

2. Glycolysis
Glucose
2 molecules of
pyruvate
To the electron transport chain

3. Energy accounting of glycolysis
2 ATP 2 ADP
glucose pyruvate
6C 2x 3C
4 ADP 4 ATP All that work!
And that’s all
I get?
2 NAD+ 2 But
glucose has
so much more
• Net gain = 2 ATP + 2 NADH to give!
– some energy investment (-2 ATP)
– small energy return (4 ATP + 2 NADH)
• 1 6C sugar 2 3C sugars

4. Krebs Cycle
• British biochemist Hans Krebs, discovered
1937
• During Krebs cycle, PYRUVATE is broken
down into CARBON DIOXIDE in a series
of energy extracting reactions
• AKA the CITRIC ACID CYCLE because
citrate (also called citric acid) is the first
product of the cycle

5. From Glycolysis comes the
Pyruvates….

6. Krebs Cycle

7. • Citric Acid Production
Step 1
– Pyruvate ( 3-carbons) enter
mitochondria Matrix
– One carbon is removed as CO2
(WASTE product) and electrons
are removed by NAD+ (making
NADH which goes to the ETC)
– Co-enzyme A joins the 2-carbon
molecule (that used to be
pyruvate) making Acetyl-CoA
– Now Acetyl-CoA can enter the
Krebs cycle
– Acetyl Co-A combines with 4-
carbon molecule called
OXALOACETATE , making
citrate (citric acid), a 6-carbon
molecule

8. Step 2
• Citric acid (6 carbon molecule) is
broken down into a few different
5-carbon compounds, then into a
few different 4-carbon
compounds.
• Each step releases CO2, NADH
and FADH2, and ATP
– CO2 is a waste product
(breath out!)
– NADH and FADH2 (taxi cabs)
goes onto the ETC (where
the party is at)
– ATP is used for cell to do
work (mechanical, chemical,
or transport)

9. Count the carbons!
pyruvate acetyl CoA
3C 2C
citrate
4C 6C
4C oxidation 6C
This happens
twice for each of sugars
CO2
glucose
molecule x2
4C 5C
4C 4C CO2

10. Count the electron carriers! CO2
pyruvate acetyl CoA
3C 2C
NADH
citrate
NADH 4C 6C
4C reduction 6C
This happens of electron
twice for each carriers CO2
glucose
NADH
molecule x2 5C
4C
FADH2 CO2
4C 4C
NADH
ATP

11. So we fully
oxidized
(broke down)
glucose
C6H12O6
CO2
& ended up
with 4 ATP!
What’s the
point?

12. Electron Carriers = Hydrogen Carriers
H+
H+
 Krebs cycle
H+ H+
+
H+ H+ H H+
produces large
quantities of
electron carriers ADP
+ Pi
 NADH ATP
H+
 FADH2
 go to Electron
Transport Chain!
What’s so
important about
electron carriers?

13. Energy accounting of Krebs
cycle
4 NAD + 1 FAD 4 NADH + 1 FADH2
2x pyruvate CO2
3C 3x 1C
1 ADP 1 ATP
ATP
Net gain = 2 ATP
= 8 NADH + 2 FADH2

14. Value of Krebs cycle?
• If the yield is only 2 ATP then how was the Krebs
cycle an adaptation?
– value of NADH & FADH2
• electron carriers & H carriers
• to be used in the Electron Transport Chain
like $$
in the
bank

16. Summary
• In one turn of the Krebs • For one Glucose
Cycle: molecule how many
– 3 CO2 (1 from right times does the Krebs
before Krebs) Cycle turn?
• Released when we exhale
• What are the totals
– 1 ATP (E for cell work)
from the Krebs Cycle
– 1 NADH from right before
Krebs for one Glucose
– 3 NADH from Krebs (to molecule?
ETC) – 6 CO2s
– 1 FADH2 (to ETC) – 2 ATPs
– Water leaves and then – 8 NADHS
reenters so we don’t – 2 FADH2
count it in the products

17. So we use Krebs if we have
oxygen….what if there is NO
oxygen???
• Then we can’t even enter the mitochondria
and go to the Krebs cycle…
• We are stuck using Glycolysis…
• Anaerobic respiration (NO oxygen)
– 2 types
• Lactic acid fermentation
• Alcohol fermentation

18. Pyruvate is a branching point
Pyruvate
O2 O2
fermentation
anaerobic
respiration
mitochondria
Krebs cycle
aerobic respiration

19. or
2

20. • Cells cannot get enough oxygen
• Build up of pyruvic acid and NADH and no oxygen to break it down
• Cells begin fermentation
– Lactic Acid fermentation
• Pyruvic acid + NADH lactic acid + NAD+
• Get about 90 seconds of energy without having to use oxygen
• HOWEVER, oxygen will be paid back double when you are done
(think heavy breathing)
• Occurs in muscle cells, and microorganisms, such as the ones that
turn milk into cheese and yogurt
• Lactic acid causes muscle cramping and burning sensation
• Oxygen is required to break down lactic acid and get it out of body
– Alcohol fermentation
• Pyruvic acid + NADH ethyl alcohol + NAD+ + CO2
• Occurs in yeast cells and other microorganisms, such as the ones
involved in the production of bread and wine

21. How is NADH recycled to NAD+?
with oxygen without oxygen
Another molecule aerobic respiration anaerobic respiration
must accept H from pyruvate “fermentation”
NADH
H2O NAD+
CO2
O2 NADH NADH acetaldehyde
recycle acetyl-CoA NADH
NAD+
NADH
lactate NAD+
lactic acid
fermentation
which path you Krebs
cycle ethanol
use depends on alcohol
who you are… fermentation

22. Fermentation (anaerobic)
• Bacteria, yeast
pyruvate ethanol + CO2
3C 2C 1C
NADH NAD+
back to glycolysis
 beer, wine, bread
 Animals, some fungi
pyruvate lactic acid
3C 3C
NADH NAD+
back to glycolysis
 cheese, anaerobic exercise (no O2)

23. Alcohol Fermentation bacteria yeast
recycle
pyruvate ethanol + CO2 NADH
3C 2C 1C
NADH NAD+ back to glycolysis
 Dead end process
 at ~12%
ethanol, kills
yeast
 can’t reverse the
reaction
Count the
carbons!

24. animals
some fungi
Lactic Acid Fermentation
recycle
O2
pyruvate lactic acid NADH
3C 3C
NADH NAD+ back to glycolysis
 Reversible process
 once O2 is available,
lactate is converted
back to pyruvate by
the liver
Count the
carbons!

25. Quick Energy
• 3 ways to obtain energy
– ATP stored in muscles (glycogen) (short)
– ATP from lactic acid (short)
– ATP from cellular respiration (long)
• Cells initially have small amount of ATP from cell resp. and glycolysis
• Think of running a 200 m sprint
– Gun goes off
• Muscles of runner contract, turning glycogen in muscle cells into
glucose, but this only provides for a few seconds of intense activity
– You pass the 50m mark
• most ATP the was initially stored is now gone
• Muscle cells are producing ATP from lactic acid fermentation
• This lasts about 90 seconds
– End of Race
• Lots of lactic acid build up
• Only way to get rid of lactic acid is a chemical pathway that requires oxygen
• Thus, at the end of the race, you are breathing heavily and you should follow
an intense work out with a slow jog

28. Long Term
• Energy for running long races or other endurance sports
• Cellular respiration is the only way to get enough ATP to last the length of the race
• Cellular respiration makes ATP more slowly than lactic acid fermentation
• Athletes must pace themselves
• Glycogen an important molecule
– Carbohydrate
– Polysaccharide (monosaccharide is glucose)
– Muscle and liver cells store E as glycogen
– Glycogen is broken down by the hormone Glucagon
– Glycogen break down is also stimulated by muscle contraction
• When you work out, muscles contract and they can use energy stored in glycogen
– Increase glycogen storage, increase the duration of exhaustive work your muscles can do
– Stores of glycogen last about 15-20 minutes
• After glycogen is used up, body starts to break down other molecules to get energy
– Fats and proteins
– Fatty acids are broken down and carried to mitochondrial matrix and enter the membrane in fragments
as acetyl-CoA
– Proteins are broken down into aa and the these modified aa’s are fed back into the Krebs cycle (NAD+
and FAD)
• Aerobic exercise is good for weight control because it leads to break down of fats

29. Training to Improve Function of
ATP production
• Anaerobic training
– Increase levels of glycogen in muscle cells
and increase tolerance of lactic acid build up
• Aerobic training
– Increases size and number of mitochondria in
muscle cells and increase delivery of oxygen
to muscle cells by improving heart and lung
efficiency

30. What’s the
point?
The point
is to make
ATP!
ATP
2007-2008