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CONCEPTUAL UFE SCIENCE
Cellular Respiration
Introduction
Cellular respiration is the oxidative, chemical attack on energy-rich molecules to
provide useful energy for the cell. Enzymes catalyze the oxidation reactions.
These reactions are known as catabolic reactions because they break molecules
down to release energy.
Anaerobic respiration
The first part ofrespiratory pathways in the cell is anaerobic. This term means
thatoxygenisnotinvolved. Evencellsoforganismsthatutilizeoxygen,suchashumans,
have an anaerobic component. The anaerobic component is known as glycolysis
and the Embden-Meyerhof pathway carries it out. This pathway produces ATP
and NADli as energy-storage molecules. NADH is similar to the NADPH
produced in photosynthesis. In anaerobic respiration, this NADH needs to be
recycled. The"total anaerobic pathway iric1uding the recycling process is
calledfermentation. We will consider two kinds of fennentation reactions. The
first will be the laclic acidfermentation as it occurs in muscles. The second will
be the alcoholfermentation as it'occurs in yeast.
Glycolysis
Glycolysisisapathwaythatthecellusestooxidizeglucose. Someofthe steps in this
pathway are the reverseofstepsfound in thedarkreactionsofphotosynthesis. .
Glycolysis is part of the fennentation process found in muscle and yeast cells.
It is important to avoid confusion between photosynthesis and cellular respiration.
Photosynthesis is reductive and anabolic. It produces glucose and releases oxygen
as a waste product. In contrast, cellular respiration is .oxidative and catabolic. It
uses glucose and oxidizes it to produce C<h as the waste product.
Fermentation
Fermentation is used by muscle cells to provide quick energy. Yeast cells use it
to produce alcohoJ and C<h. This is why yeast is used for brewing and baking.
Lactic acid bacteria produce lactic acid, which is used to make fermented foods
like pickles, sauerkraut and yogurt.
Fermentation is required under anaerobic conditions because ofthe need to
recycle NADH. The Embden-Meyerhofpathway produces two molecules
ofNADH. These cannot be used to make ATP under anaerobic conditions so
they are recycled and oxidized using the fermentation process. This recycling
pennits the Embden-Meyerhof pathway to continue functioning.
" -I
Glycolysis is the conversion ofglucose to pyruvate.
Glucose 7" ~ 2 'GAL 7" ~ 2'GA 7"~ 21\fnm11a
ZAT' ZAD, ZHAD. ZHADH • AD' • AT'
•
• •1
Figure 7-1. Glycolysis. Energy is made available all the time. Two ATP are used
to startandthepathwaymakes4ATP,sothereisanetyieldoftwoATP. ThePOALto
PGAstepisthereverseofa similarstepintheCalvin-Bensoncycle. Theresultingtwo
NADH molecules must be reoxidized (recycled) to permit the pathway to
continue
working.
Fermentationisthereductionofpyruvate(usingtheNADH)toproduce 8reduced
product such as lactic acid or alcohol.
Net reactions ofanaerobic respiration.
A. Lactic acid fermentation in muscle
Glucose + 2 Pi + 2 ADP -+ 2 lactate + 2 ATP
B. Alcohol fermentation in yeast
Glucose + 2 Pi + 2 ADP -+ 2 ethanol + 2 C<h + 2 ATP
, Each ofthe rea~ons beginswithglucose and producesanetyieldoftwoATP.
.ThislownumberofATPresultsfromthelackofaerobic(oxidative)conditioDS. Muscles
use fennentation under conditions when oxygen is low or when energy is needed
quickly.
Yeast cells are used in the processes ofbrewins and baking. In brewing, the
, product, such as wine, contains alcohol. Ii1 the case of'beer, the product contains
both" ,'.alcohol and e<h. In baking, the product ofinterest is the e<h. The carbon
dioxide helps the baked products rise. That is why yeast is used in making bJad,
cakes and t)ther baked products.
Recycling ofNADH
The NADH produced in the glycolysisreactioDS is in its reduced fonn. It must
become oxidized again in order for the pathway to function. The final
fermentation
reactions are used to regenerate or recycle NAD+ for use in the
Embden-Meyerhof
pathway. '
a. Reoyelng of NADH In IDUsoIe
2 pyruvate
r , 2 ..c....
~
2NADH
2NAif
b. RecycUng of NADH in yeIISt
2 pyruvate 7'""""\) 2 ethanol + 2 CO 2
2
NADH 2 NAD·
Figure 7-2. Production ofNAD+ by recycling.
The centralreactionofthe Embden-Meyerhofpathwayrequirestheoxidizedfonn
ofNAD+. The pathway converts it to the reduced fonn ofNADH. In order for the
pathway to continue, the NADH must be recycled.
In oxidative conditions, the NADH is not recycled but is sent to the Electron
TransPOrtlChemiosmosis pathway instead. Under oxidative conditions,
fennentation does not take place. The pyruvate resulting from the
Embden-Meyerhofpathway is scrit to the Krebs Citric Acid Cycle, instead.
Aerobic respiration
Aerobic respiration consists ofthree parts.
•
Glycolysis
•
Krebs Ci1ric Acid Cycle
•
Electron
TransportIChemiosmosis
Overall reaction ofaerobic respiration
Aerobicrespirationisthereverseofphotosynthesis. Theglucosemadebytheplantis
oxidized in the cell to produce the waste products, Cen and H20, and energy.
Cellular respiration is the opposite ofphotosynthesis. The energy stored by the
plant is released in our cells for our use.
Glycolysis
Glycolysis produces two pyruvates, two ATP and two NADH. Unlike the
situation in fennentation, the two NADH molecules are not recycled. The
process occurs in the cytoplasm ofthe cell.
II
Krebs Citric Acid Cycle
TheKrebs Citric Acid Cycle isaseriesofeightreactionsthatgo ar01md in a
circle. It is found in mitochondria. Each pyruvate molecule resulting from
glycolysis enters the cycle and is oxidized to Produce three C02, and the
following energy molecules:
4 NADH, I FADH2 and I ATP.
Electron TransportlChemiosmosis
This is the final pathway of aerobic respiration. It occurS in the mitochondria.
The pathway removes electrons and protons from the NADH and FADH2
molecules. FAD is Flavin Adenine Dinucleotide. It is an electron carrier like
NADH. FADH2 is the reduced fonn that carries the electrons and protons.
. Electron Transport and ChemioSJDosis occur in the mitochondria The electrons
are passed through the electron transport chain, aseries ofenzymesthat transport
electroDS. The electrons eventually end up on oxygen, which is the terminal
electron acceptor of the chain. The protODS are pumped out of the
mitQChondria through the mitochondrial membrane. The pumping mechanism
is called chemiosmosis. Energy. from chemioSJDOsis Produces 3 ATP for each
NADH and 2 ATP for each FADH2. So, the result ofall ofthese pathways is to
make useful energy for the cell in the fonn of ATP.
Final elimination ofelectrons is done by adding electrons and protons to a
molecule ofoxygen. The result is the production ofhydrogen peroxide (H2(h).
.Hydrogen peroxide is a toxic substance. The cell has an enzyme called catalase
that destroys hydrogen peroxide producing oxygen and water, thereby rendering
in harmless by removing it
Figure 7-3. The catalase reaction.
Virtually all enzymes are proteins. Their job is to catalyze biological
reactions andallowthesereactionstooccuratroomtemperature.
Catalaseisthenameofa SPeCific enzyme that perfonns the reaction responsible
for the destruction ofhydrogen peroxide. Each enzyme is a catalyst, which is
specific for its own substrate. The substrate is the specific chemical substance
reacted on by the enzyme. The enzyme changes the substrate in~o products. In
the case ofthe destruction ofhydrogen peroxide
by catalase, the substrate was hydrogen peroxide (the starting material) and the
products were water and oxygen (the resulting materials).
Total energy yield for one glucose by aerobic respiration
•
•
Yield from glycolysis: 2ATP direct 6 ATP from 2 NADH
Krebs Cycle (per pyruvate): 1ATP direct
2ATP from FADH2
12 ATP from
NADH
c.
Total Yield: 8ATP from glycolysis 30 ATP from 2
pyruvates in Krebs Cycle 38 ATP total yield
Note: Normal totals in eukaryotic ce]]s are 36 molecules ofATP. The figme
of38 ATP comes from prokaryotic cens, which do not have,mitochondria.
Study guide for cellular respiration
General considerations
1
Respiration is used to obtain energy.
2
TheenergyisinthefonnofATP..
3
The by-products ofrespiratiQn are H20 and CCh.
4
The process ofrespiration is conducted by aJI living cells, all ofthe
time.
,B. Anaerobic respiratiOI.1 (fermentation) .
1
Anaerobic'respiration does not requiJe air or Ch.
2
The process is used to obtain energy.
3
Few ATP molecules (2 ATP) are produced.
A.
4
S.
C.
are:
The
end-products
a.
muscle:
lactic acid
b.yeast: alcohol and CO2
The process occurs in the cytoplasm ofthe cell.
Aerobic respiration
1
Aerobic respiration uses anaerobic respiration (glycolysis) as the tint set
ofreactions.
2
ConversionofpyruvatetoCChandH20givesmuchmoreenergythan
anaerobic respiration.
3
Aerobic respiration occurs in the mitochondria.
4
ATP is produced from NAOH and FADH2.