Section 9.3
Getting Energy to make ATP
Overview:
*Remember, humans do not have chlorophyll in their
skin, therefore they cannot use light energy to
make ATP like a plant does. Instead, we have
mitochondria in our cells that can convert
carbohydrates, like those made by plants, into
ATP.
I.
Cellular Respiration
1. Definition - cellular respiration is the process by
which mitochondria break down food molecules to
produce ATP.
2. 3 stages of cellular respiration:
a) glycolysis
b) citric acid cycle (Krebs Cycle)
c) electron transport chain
3 Parts of Cellular Respiration:
*Glycolysis is anaerobic and thus does not require
oxygen; the citric acid cycle and ETC are aerobic
therefore oxygen is needed for them to occur.
A. Glycolysis
1. Definition - glycolysis is a series of reactions in
the cytoplasm of a cell in which glucose (a 6
carbon molecule) is broken down into two
molecules of pyruvic acid (3 carbon molecules).
2. ATP - it takes 2 molecules of ATP to start the
process of glycolysis, and only 4 ATPs are made,
therefore this process is not very energy efficient.
*only 2 molecules of ATP are produced from the
breakdown of one glucose molecule.
3. NAD+ (nicotinamide dinucleotide) - just as
photosynthesis has the energy carrier NADP+;
glycolysis has an energy carrier called NAD+.
*NAD+ forms NADH when carrying an electron.
4. At the end of glycolysis the pyruvic acid
molecules produced move to the mitochondria,
the powerhouses or ATP producers of the cell.
Glycolysis
Molecular details of glycolysis
5. Post-glycolysis reactions - before the pyruvic acid
molecules can enter the citric acid cycle (the next
stage of cellular respiration) some modifications need
to be done.
a) pyruvic acid loses a molecule of CO2 and
combines with Coenzyme A to form a molecule
of Acetyl-CoA.
b) the rxn w/ Coenzyme A makes a molecule of
NADH + H+
Post-glycolysis Reactions: Formation of Acetyl-CoA
B. The Citric Acid Cycle: “The breakdown of Glucose”
1. Definition - the citric acid cycle is a series of
chemical reactions similar to the Calvin Cycle, but
opposite in purpose.
Calvin Cycle - forms glucose in photosynthesis
Citric Acid Cycle - breaks down glucose in
cellular respiration
2. Materials needed :
to break down glucose, two electron carriers are
needed:
a) NAD+
b) FAD (flavin adenine dinucleotide)
3. What’s formed:
The Citric Acid Cycle (CAC) produces a number
of molecules:
a) 1 ATP is produced
b) 3 NADH + H+ are produced
c) 1 FADH2 molecule is produced
4. Steps of the Citric Acid Cycle (CAC):
a) formation of citric acid - a 2 carbon acetyl CoA
combines w / 4 carbon compound called
oxaloacetic acid, forming a 6 carbon molecule
called citric acid.
b) formation of CO2 - one molecule of CO2 is
formed from the citric acid cycle which reduces
the citric acid molecule to a 5 carbon molecule
called ketoglutaric acid.
*from this rxn, one molecule of NADH +
H+ is made from one NAD+
c) formation of second CO2 - another molecule of
CO2 is formed and released from the ketoglutaric
acid; this results in a 4 carbon compound called
succinic acid.
*from this rxn, one molecule of ATP and one
molecule of NADH + H+ are formed.
d) recycling of oxaloacetic acid - succinic acid
undergoes a series of rxns which form FADH and
NADH + H+ and oxaloacetic acid; this is then
available for the next cycle to occur.
Succinic -> fumaric -> malic -> oxaloacetic
The Citric Acid Cycle (CAC):
Hans Krebs
C. The Electron Transport Chain:
1. Function - move energized molecules; NADH &
FADH2 pass energized molecules from protein to
protein releasing small amounts of energy with each
pass.
2. Location - the inner membrane of the mitochondria
The Mitochondria:
ETC:
3. The Process:
a) NADH & FADH2 pass energized molecules
from protein to protein; small amounts of
energy are released with each pass.
b) some energy is used to form ATP, while some
is used to pump H+ ions into the center of the
mitochondria.
c) as H+ ions are pumped into the center of the
mitochondria, the center becomes more (+),
while the outside becomes more (-). Since
the outside is more (-) it will attract more (+)’s
or more H+ ions,creating an electrochemical
gradient.
d) The electrochemical gradient drives the inner
membrane of the mitochondria to form ATP.
e) The final electron acceptor in the ETC is Oxygen.
The oxygen reacts with H+ ions to form water
molecules.
* this is why our bodies need Oxygen
The Electron Transport Chain in the Mitochondria
Another view of the ETC
4. The importance of Oxygen (O2)
If oxygen is not available for the ETC, then the chain
cannot pass along energized electrons; if electrons
cannot be passed, then there is no room to accept
more electrons and a blockage results. Therefore,
cellular respiration cannot occur.
5. Overall production
The ETC results in the production of 32 ATP
molecules
This is the most efficient means for production of ATP
Think: Aerobic (jogging) vs, Anaerobic (sprinting) which can be done longer?
II. Fermentation
*sometimes your cells may be deprived of oxygen for
a short time
*fermentation can occur during extremely strenuous
activities
1. Fermentation - anaerobic process that occurs
when your cells are w/o O2 for a short time. It occurs
after glycolysis and provides a way to continue
producing ATP until oxygen is available again.
2. 2 main types of fermentation:
a) lactic acid fermentation
b) alcoholic fermentation
A.
Lactic Acid Fermentation
*occurs during anaerobic conditions when oxygen
is not available as the final electron acceptor in the
ETC, therefore a “back-up” occurs.
1. What happens:
a) as NADH and FADH2 try to pass their
energized electrons onto the next protein in the
ETC, they are rejected.
b) if NADH and FADH2 cannot pass on their
energized electrons, then NADH and FADH2
cannot be converted back to NAD+ & FAD, which
are needed to keep the CAC and glycolysis
going.
c) the cell has no way of replacing FAD during
anaerobic conditions, but NAD+ can be replaced via
Lactic acid fermentation.
d) in lactic acid fermentation, 2 molecules of pyruvic
acid (from glycolysis) use NADH to form 2 molecules
of lactic acid.
e) when the NADH is used to make lactic acid, which
is becomes stored in the muscles, NAD+ is released
and can be reused in glycolysis to make 2 more ATP
molecules.
f) the lactic acid is then transferred from the muscle
cells to the liver where it is changed back into pyruvic
acid.
Lactic Acid Fermentation:
*when lactic acid builds up in muscle cells it causes
fatigue; you feel as though you “hit a wall” or have
“jello legs”
B. Alcoholic Fermentation
*often used by yeast cells to produce CO2 and ethyl
alcohol.
*anaerobic process - used to make bread dough
“rise” and brew alcohols.
III. Comparing Photosynthesis and Cellular Respiration
*both use an ETC to form ATP
*do opposite jobs:
photosynthesis - produces high energy
carbohydrates and O2 from the
sun’s energy
cellular respiration - uses O2 to break down
carbohydrates with much lower
energy level
Comparisons:
Photosynthesis
Food is made or
accumulated
Cellular Respiration
Food is Broken down
Energy from sun is stored as Energy from glucose is
glucose
released to be used by body
Carbon dioxide (CO2) is
taken in
CO2 is given off as a waste
product
Oxygen (O2) is given off as
waste
Oxygen is needed and is
taken in
Produces glucose from
PGAL
Produces CO2 and H2O as
waste
Can happen only when
some light is available
Can occur all day and all
night
Requires Chlorophyll, can
only happen in plants
Occurs in all living cells plants and animals