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Chapter 8
How Cells Release Stored Energy
AKA: Cellular Respiration
How do cells make ATP?
• ATP is the prime energy carrier for all cells
• Aerobic Respiration (with oxygen) is the
main pathway for energy release from
carbohydrates to ATP
• All energy-releasing pathways start with
glycolysis
– Glucose is split into two pyruvate molecules
– Glycolysis reactions occur in the cytoplasm
Overview of Aerobic Respiration
• Aerobic Respiration yields 36 ATP
• Anaerobic Respiration (without oxygen)
yields 2 ATP
Aerobic respiration route:
C6H12O6 + 6O2  6CO2 + 6H2O
(Reverse equation to photosynthesis)
Three steps to aerobic respiration
• 1- Glycolysis: is the breakdown of glucose
to pyruvate
– Small amount of ATP are generate (2 ATP)
– Takes place in the cytoplasm
• 2- Kreb Cycle: degrades pyruvate to
carbon dioxide, water, ATP, H+ ions and
electrons (accepted by NAD+ and FAD)
– Takes place in the mitochondrian
– Makes 2 ATP
Continue…
• 3- Electron Transfer Phosphorylation:
processes the H+ ions and electrons to
generate high yields of ATP; oxygen is the
final electron acceptor
– Takes place in the mitochondrion
– Yields 32 ATP (this is the real takes place)
Glycolysis: First stage of energyreleasing pathways
• 2 ATP is required to start glycosis
• Enzymes in the cytoplasm catalyze several
steps in glucose breakdown
– Glucose is first phosphorylated in energyrequiring steps, then the six-carbon
intermediate is split to form two molecules of
PGAL (which gives a phosphate to make ATP)
– Enzymes remove H+ and electrons from PGAL
and transfer them to NAD+ which becomes
NADH (used later in the electron transfer)
– By substrate-level phosphorylation, four ATP
are produced
• The end product to glycolysis is:
– 2 ATP (net gain)
– 2 pyruvates
– 2 NADH
For each glucose moleucule degraded
Continue…
• The pyruvic acid diffuses into the inner
compartment of the mitochondrion where
a transition reaction occurs that serves
to prepare pyruvic acid for entry into the
next stage of respiration:
– (a) pyruvic acid acetic acid + CO2 (a waste
product of cell metabolism) + NADH +
– (b) acetic acid + co-enzyme A -> acetyl CoA
Second Stage of the Aerobic
Pathway: Kreb Cycle
• Takes place in the inner mitochondria
matrix
• Pyruvate enters the mitochondria and is
converted to acetyl-CoA, which then joins
oxaloacetate already present from a
previous “turn” of the cycle.
• During each turn of the cycle, three carbon
atoms enter (as pyruvate) and three leave
as three carbon dioxide molecules
Functions of the second stage
• H+ and e- are transferred to NAD+ and
FAD (coenzymes)
– Ten coenzymes are loaded with electrons and
hydrogen
• Two molecules of ATP are produced by
substrate-level of phosphorlyation
Continue…
• Most of the molecules are recycled to
conserve oxaloacetate for continuous
processing of acetyl-CoA
• Carbon dioxide is produced as a byproduct
Third Stage of the Aerobic
Pathway
• This is where the real work is done
• NADH and FADH2 give up their electrons
to transfer (enzyme) system embedded in
the mitochondrial inner membrane
Electron Transfer
Phosphorylation
• According to the chemiosmotic model,
energy is released in the passage of
electrons through components of the
transfer series
• Oxygen joins with the “spent” electrons
and H+ to yield water
Summary of the Energy Harvest
Net Gain
• Electron transfer
• Glycolysis
• Kreb Cycle
32 ATP
2 ATP
2 ATP
Total 36 ATP
(per glucose molecule)
Continue…
• Normally, for every NADH produced within
the mitochondria and processed by
electrons transfer chain, three ATP are
produced
• FADH2 produced 2 ATP
Continue…
• NADH from the cytoplasm cannot enter
mitochondrian and must transfer its
electrons!!
– In most cells (skeletal and brain) the electrons
are transferred to FAD and thus yield two ATP
(for a total yield of 36)
– But in the liver, heart, and kidney cells, NAD+
accepts the electrons to yield three ATP
because two NADH are produced per
glucose, this total yield of 38 ATP
Anaerobic Respiration
• Cellular respiration without using oxygen
(or very limited)
– Pyruvate from glycolysis is metabolized to
produce molecules other than acetyl-CoA
• Example: Single Yeast Cells
Fermentation Pathways
• With an energy yield of only 2 ATPs
• Glycolysis serves the first stage (just like
aerobic respiration)
Lactate Fermentation
• Certain bacteria (as in bacteria) and
muscles cells have the enzymes capable
of converting pyruvate to lactate
– Example: Muscle Cramps
• No additional ATP beyond the net two
from glycolysis is produced but NAD+ is
regenerated
Alcoholic Fermentation
• Fermentation begins with glucose
degradation to pyruvate
• Cellular enzymes convert pyruvate to
acetaldehyde, which then accepts
electrons from NADH to become alcohol.
• Yeast are valuable in the baking industry
(Carbon dioxide byproduct makes dough
“rise”) and in alcoholic beverage
production
Anaerobic Electron Transfer
• Some kinds of bacteria are able to strip
electrons from organic compounds and
send them through a special electron
transfer in their membranes to produce
ATP
• Example: Such bacteria include those that
reduce sulfate to hydrogen sulfide (foul
smelling gas) and those that convert
nitrate to nitrite
Alternative Energy Sources in
the Human Body
• Excess carbohydrate intake is stored as
glycogen in the liver and muscle for future
use.
• Free glucose is used until it runs low, then
glycogen reserves are tapped
Energy from Fats
• Excess fats (including those made from
carbohydrates) are stored away in cells of
adipose tissue
• Fats are digested into glycerol, which
enters glycolysis, and fatty acids, which
enter the Kreb Cycle
• Fatty acids have more carbon and
hydrogen atoms, they degraded more
slowly and yield greater amounts of ATP
Energy from Proteins
• Amino acids are released by digestion and
travel in the blood
• After the amino group is removed, the
amino acid is removed, the amino acid
remanant is fed into in the Kreb Cycle
Perspective on the Molecular
Unity of LIfe
• Photosynthesis and cellular respiration are
intimately connected
• Life is not some mysterious force, but a
series of chemical reactions under highly
integrated control.