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Cellular Respiration
Introduction – all
forms of life depend
directly or indirectly on
light energy captured
during photosynthesis
– glucose molecules
are broken down back
into carbon dioxide
and water (molecules
the plant started with)
ATP – Adenosine triphosphate
most common energy carrier in cells
 nucleotide composed of adenine, the
sugar ribose, and three phosphate groups
 synthesized from adenosine diphosphate
(ADP) and inorganic phosphate – process
is called phosphorylation
 during glucose breakdown, energy is
release and stored in bonds of ATP
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Summary of complete glucose metabolism:
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Photosynthesis:
6CO2 + 6H2O + sunlight energy  C6H12O6 + 6O2
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Complete glucose metabolism:
C6H12O6 + 6O2  6CO2 + 6H2O + chemical and heat energies
Glycolysis
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first stage of aerobic respiration
does not require O2 (anaerobic) and proceeds in
exactly the same way under both aerobic (with
oxygen) and anaerobic (without oxygen) conditions
Splits apart a single glucose molecule (6 carbon) into
two molecules of pyruvate (3 carbon)
under anaerobic conditions, pyruvate converted by
fermentation to lactic acid or ethanol
occurs in cytoplasm
pyruvate may enter mitochondria if oxygen available –
breaks pyruvate down completely to CO2 and water
generating an additional 34 to 36 ATP – aerobic
respiration
each step (reaction) is catalyzed by an enzyme
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products are 2 molecules of ATP and 2
molecules of NADH
– nicotinamide adenine dinucleotide – an
electron carrier that transports energy in
form of energetic electrons - Coenzyme
– electrons are held in high-energy outer
electron shells – NAD+ NADH
– donates the electrons and their energy to
other molecules
– hydrogen ions are often picked up
simultaneously
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formed through oxidation/reduction
reactions – involves two complementary
reactions
– oxidation – liberates energy from the
oxidation substance; results from the
removal of one more electrons, alone or
with H+
– reduction – stores energy in a reduced
compound; reduction results from addition
of one or more electrons, alone or with H+
Glycolysis consists of two major sets of
reactions:
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Step 1 - glucose activation
– 2 ATP are used to convert stable glucose
into highly unstable fructose bisphosphate
(6 C)
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Step 2 – Energy harvest
fructose bisphosphate splits into two 3 C molecules of
glyceraldehyde 3-phosphate (G3P or PGAL)
each G3P molecule goes through series of reactions
that convert it into pyruvate (pyruvic acid)
2 ATPs are made per G3P for a total of 4 – however,
net gain is only 2 ATPs
During these reactions, 2 high energy electrons and a
H+ are added to NAD+ to form “energized” carrier
NADH – 2 NADH are made (one from each PGAL)
Fermentation
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In the absence of oxygen, pyruvate acts as
electron acceptor from NADH producing ethanol
or lactic acid – this process is called
fermentation
NADH production is not used as method to
capture energy – used to get rid of hydrogen
ions and electrons made when glucose is broken
down
 NAD+ is regenerated by pyruvate acting as final
electron acceptor - pyruvate may be converted
to lactic acid (lactic acid fermentation – occurs in
human muscles during strenuous exercise) or
ethanol and CO2 (alcoholic fermentation)
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Aerobic Respiration
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In presence of oxygen, oxygen is the electron
acceptor (in the electron transport system)
allowing pyruvate to be fully broken down
(back into CO2 and water) to make even more
ATP
Aerobic Cellular Respiration – series of
reactions, occurring under aerobic conditions,
in which large amounts of ATP are produced –
pyruvate is broken down into carbon dioxide
and water – oxygen serves as final electron
acceptor – each step catalyzed by enzymes
Aerobic Respiration
occurs in the mitochondria
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double membrane – inner folds are called cristae
inner compartment contains fluid matrix
intermembrane compartment separates the two membranes
Mitochondria have their own DNA (circular chromosome) and
ribosomes (70s – smaller than eukaryotic ribosomes)
Aerobic Respiration
Step 1 – Glycolysis
 Step 2 – Oxidative Decarboxylation
– two molecules of pyruvate produced by
glycolysis are transported across both
mitochondrial membranes into matrix
– each pyruvate is split into CO2 and a 2 C
acetyl group which immediately attaches to
coenzyme A to form acetyl CoA – during this
reaction NADH is produced
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Step 3 – Krebs Cycle (Citric Acid Cycle)
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the acetyl CoA enter Krebs cycle by briefly combining
with oxaloacetate to form citrate – coenzyme A is
released to be reused
Kreb’s cycle rearranges citrate to regenerate
oxaloacetate giving off 2 CO2, 1 ATP and four electron
carriers (1 FADH2 and 3 NADH) per pyruvate molecule
(x2 per glucose molecule)
Electron Transport System (Oxidative Phosphorylation)
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energetic electrons from NADH and FADH2 are
used to generate more ATP (3 ATP are
generated per NADH and 2 ATP per FADH2)
located in inner mitochondrial membrane
electrons move from molecule to molecule along
transport system – energy released by electrons
is used to pump hydrogen ions from the matrix
across the inner membrane into the
intermembrane compartment (used for
chemiosmosis)
at the end of the ETS, oxygen and hydrogen ions
accept the electrons to form water – clears out
transport system for more electrons to run
through
Chemiosmosis
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hydrogen ions pumped across the inner
membrane generate a large H+ concentration
gradient (high concentration in intermembrane
compartment and low concentration in matrix)
inner membrane is impermeable to hydrogen
ions except at protein channels that are part of
ATP-synthesizing enzymes (ATP synthase) whole thing is called the F1 complex
during chemiosmosis, hydrogen ions move
down the concentration gradient from
intermembrane compartment to matrix by
means of the F1 complex
the flow of hydrogen ions provides energy to
synthesize 32 – 34 ATPs from ADP
Coupled Reactions
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Many steps involved in respiration are
coupled - reactions in which exergonic
reactions drive endergonic reactions
Some reactions occur together with two
reactions sharing a common
intermediate molecule
Metabolism of Fats and Proteins
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cells can extract energy from fats and proteins
breakdown of fat and proteins creates products that
can be fed into the enzyme pathways of respiration
Fats
starts with hydrolysis into glycerol and fatty acids
glycerol is converted into G3P and enters pathway
fatty acids are converted into acetyl-CoA and enter
pathway
Proteins
amino acids are broken down in a number of ways
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the amino group is first removed (deamination)
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some amino acids are converted into pyruvic acid,
some into acetyl-CoA, and some into other
compounds in the Krebs cycle
Body Temperature and Metabolism
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cellular respiration captures energy in the bonds of ATP
however, much of the energy is lost as heat (approx
60% of energy available is lost)
the majority of animals and plants quickly lose this
thermal energy to the environment – referred to as
poikilothermic (“of variable heat”) or ectothermic
(“externally heated”)
body heat comes from external sources, body temp
fluctuates with environmental temp
metabolic rate (organism’s rate of oxygen consumption
or release of CO2) increases with temp (enzymes more
active at higher temps)
ectotherms are more active with higher temps and
sluggish with lower temps
many have behavioral adaptations to assist with temp
control (basking)
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Mammals and birds (and some other
organisms) make use of heat produced
during metabolism – have evolved
mechanisms to conserve heat (insulation
with fat, hair, feathers)
endothermic (“internally heated”) – have
fairly high body temps (higher than
environment)
tend to maintain fairly constant body temp
– homeothermic even when environmental
temp fluctuates
metabolic rate stays fairly constant
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metabolic rate is inversely related to body
size in both endo and ectotherms
endothermic, smaller animals have a higher
surface area-to- volume ratio and therefore
a larger relative heat loss to the
environment
must have faster metabolism to replace
heat – have to consume relatively large
amounts of food
metabolic rate is higher in smaller
ectotherms too but has never been fully
explained as to why