Cellular Respiration
How do living things obtain energy to carry out life function?
Energy=ability to do work
Burning stored fuel
Define burning.
Burning=combustion with oxygen releasing heat and light
Cellular respiration-
releasing energy in food in a biologically useful form (not heat or light)
Respiration
conversion of stored chemical energy into biologically useful work
P + ADP + E <---> ATP + H2O
Cellular respiration involves the breakdown of glucose to
release energy which converts ADP->ATP
The ATP is used for cell activities
The breakdown of glucose can produce many ATPs
(If all the energy is 1 glucose were to be released at once a cell couldn't use it)
There are other compounds that are used for this, but ATP is the most common
[Oxidation-Reduction
Reactions
-chemistry preview....don't copy]
Oxidation=chemical change where atom or molecule loses electrons
(Obviously if one loses something else MUST gain)
Reduction=chemical change where atom or molecule gains electrons
Oxygen is not required in oxidation-reduction reactions
-the name comes from the fact that historically reactions with oxygen were first studied
Sometimes, hydrogen is transferred instead of an electron-this is still oxidation (to lose a hydrogen) and reduction (to gain one)
The molecule losing the hydrogen (or electron) generally loses energy, with the molecule gaining hydrogen (or electron) gaining energy
----back to biology notes----
Biochemical pathway-
sequence of reactions with enzymes and coenzymes
Hydrogen acceptors are needed to facilitate reactions involved in harvesting energy from glucose
Two such co-factors:
NAD+ <-> NADH
FADH2 <->FAD
What is the final acceptor of the electrons and hydrogens in cellular respiration?
(What do you think?)
(Think about what was the original source of protons and electrons in photosynthesis...and that respiration is the opposite of photosynthesis)
CELLULAR RESPIRATION is a 2 step process
1- glycolysis
followed by either
2-anaerobic
or
2-aerobic respiration
Glycolysis
Glyco lysis
sugar break apart
Glycolysis takes place in the cytosol
glucose + 2 ATP -->
P-C-C-C-C-C-C-P
breaks apart into
2(P-C-C-C )
PGAL
as PGAL loses its P's it reduces 2 NAD+ -->
2 NADH and
4 ATPs
(Note 2 ATP are needed to start the process and 4 are produced.
Net: 2 ATP for glycolysis
Carbons from glucose are in 2 molecules of pyruvate
(C-C-C)
Pyruvic acid can serve as the end hydrogen acceptor
re-oxidizing NAD+ producing either lactic acid or ethanol and CO2
Anaerobic Fermentation
(2 possible paths)
- No oxygen
lactic acid fermentation
alcoholic fermentation
In chordates (higher animals)
pyruvate is converted to lactic acid (another 3C compound)
and NADH is restored to NAD+ so the cycle can continue
in mammals-
pyruvic acid becomes the terminal hydrogen acceptor and is
converted to lactic acid
Rest and oxygen allow lactic acid to be oxidized back to pyruvic acid-
the amount of oxygen needed is called the oxygen debt
It hurts
Muscle fatigue demonstration
pain
In alcoholic fermentation
pyruvic acid is broken down to a 2 Carbon compound releasing CO2
The 2 carbon compound is then converted to ethyl alcohol restoring the NADH to NAD+
Fermentation - alcoholic
In yeast and some bacteria
Glucose -->2Alcohol + 2CO2 + 2ATP
In either case most of glucose's energy is wasted (as ethyl alcohol or lactic acid) --- you can burn alcohol, so you know it has energy!
Efficiency < 4%
Aerobic respiration
starts with glycolysis
then 2 stages
Krebs cycle
Electron transport chain
Goals:
Krebs cycle
oxidizes pyruvic acid
produces NADH
ETC uses NADH to produce ATP
both Krebs and ETC require oxygen
Pyruvic acid (produced by glycolysis) diffuses from cytosol into mitochondrion matrix (space inside double membrane)
highly folded so that there is a lot of surface area for membrane bound enzymes
In mitochondria (enzymes)
Requires molecular oxygen
pyruvic acid + coenzyme A -->
acetyl coenzyme A
this process also reduces NAD+ -> NADH and releases a CO2
Krebs Cycle
In a series of steps (biochemical pathway) Acetyl CoA
releases CO2
reducing
3 NAD+ -> NADH
FAD--> FADH2
and producing 1 ATP
Since 1 glucose produced 2 pyruvic acid which produced 2 Acetyl Co A
for 1 glucose you get
6 NADH
2 FADH2
2 ATP
from the Krebs cycle
(and 4 CO2)
Energy totals so far (per glucose)
glycolysis
2 ATP
2 NADH
conversion of pyruvic acid -> acetyl CoA
2 NADH
Krebs Cycle
6 NADH
2 FADH2
2 ATP
-------------------------total
4 ATP
10 NADH
2 FADH2
Electron Transport Chain
NADH and FADH2 release their extra hydrogens in the
mitochondrial matrix
Those hydrogens are basically separated into protons and
electrons. The electrons are passed through a chain of molecules
located ON the inner membrane.
The hydrogens are pumped to the other side of the membrane.
The high concentration of protons in the space between the
inner membrane and the outer mitochondrial membrane drives an ATP
synthase "pump" which produces ATP.
The electrons combine with oxygen and the protons (H+) after
they come through the syntase producing water.
Oxygen is the final electron and hydrogen acceptor
water the product
Each NADH can produce 3 ATP
Each FADH2 can produce 2 ATP
Thus...
ETC produces 34 ATP
compared with 2 for glycolysis and 2 for the first step
creating Acetyl CoA
Aerobic Cellular Respiration Total:
38 ATP
(2 may be used to get the NADH from the cytosol to the ETC, so most books list the efficiency of aerobic respiration based on 36) ATP/glucose
Using 38 ATP
efficiency = 66%
Summary:
Glycolysis
Glucose + 2ATP
2PGAL + 4ATP
2 NADH
Alcoholic fermentation
2 Pyruvate
2 Ethanol
2 Carbon dioxide
Glycolysis
Glucose + 2ATP
2 PGAL + 4 ATP
2 NADH
Lactic Acid Fermentation
2 Pyruvate
2 lactic acid
Glycolysis
Glucose + 2 ATP
2 PGAL + 4 ATP
2 NADH
2 PGAL + 2 CoA ->
2 Acetyl CoA + 2 carbon dioxide
Krebs Cycle
2 Acetyl CoA
4 CO2
6 NADH
2 FADH2
2 ATP
ETC
34 ATP
Aerobic (phase) - efficient
Water is both a product and a reactant so the actual equation is
C6H12O6 + 6H2O + 6O2 -->
6CO2 +12H2O +energy
(36 - 38 ATP)
The C for the CO2
comes from the glucose-obviously
The O in the CO2
comes from the sugar and the initial water
The O from the initial O2
all ends up in the water at the end
Efficiency:
Aerobic respiration about 20X better than fermentation
About 45-66% of energy in food converted to ATP
In a conventional combustion engine 25% is converted to useful
work
Respiration of Fats and Proteins
Breakdown of Fats produces 2X as much ATP as glucose (per
gram)
Glycerol is converted to pyruvic acid and enters the cycle there
Fatty acids are directly converted to Acetyl CoA and enter the cycle there
Breakdown of Protein about the same number of ATP as glucose (per gram)
Amino Acids are converted into Pyruvic Acid and enter the
cycle there
Pens down-
story time:
Once upon a time the earth was very different from today.
There was no free oxygen.
There was lots of energy from UV radiation (no ozone), lightning etc.
Molecules formed freely in the "organic" soup of the shallow seas of this primitive earth.
The first "organisms" got their energy by using energy in pre- formed compounds
They were heterotrophs
Heterotroph hypothesis
The heterotrophs used up the early chemicals and released CO2
When they ran out of pre-formed compounds- mass extinction-
few with primitive photosynthetic abilities
survived
This primitive photosynthetic microbes didn't produce oxygen, but rather broke apart other organic compounds in photosynthesis
Eventually since producing oxygen from water is more energy efficient those organisms that did that outgrew their ancestors
Producing oxygen was also a problem.
Oxygen is toxic to most living reactions.
The oxygen first "rusted" all the iron on the earth's surface
Then as the percentage of oxygen in the air increased the ozone layer developed, and the high energy environment that first led to life disappeared.
As more oxygen became available organisms that could not adapt to it became extinct
This led to the success of heterotrophs that respired oxygen instead of other chemicals.
(The reason you didn't need to copy this, is it is part of the notes on evolution.)
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