Cellular Respiration
LA Charter School Science Partnership
28 Apr 2012
Nick Klein
Today’s Talk
• Part 1: Big picture: review of photosynthesis,
redox
• Part 2: Macromolecules, enzymes, and catalysis
• Part 3: Respiration & Fermentation
Part 1: The big picture
• Let’s think back to photosynthesis.
– Photosynthesis is the process by which
organisms use the energy in sunlight to
chemically transform carbon dioxide (CO2)
into organic carbon compounds such as
sugars
12H2O + 6CO2  C6H12O6 + 6O2 + 6H2O
Part 1: The big picture
• Photosynthesis and respiration both
involve reduction/oxidation (redox)
reactions— chemical reactions that
involve the movement of electrons from
one molecule to another
• In photosynthesis, when carbon dioxide is
fixed, it is reduced (electrons are added to
it) which produces organic carbon
compounds
Part 1: The big picture
Loss of
Electrons is
Oxidation
goes
Gain of
Electrons is
Reduction
Part 1: The big picture
• Respiration is in many ways
photosynthesis BACKWARDS.
Photosynthesis uses sun energy to turn
CO2 into glucose. Respiration releases
that stored energy from glucose.
C6H12O6 + 6O2  6CO2 + 6H2O
Part 1: The big picture
• So if photosynthesis involves the chemical
reduction of CO2 into glucose, and
respiration is very similar to
photosynthesis backwards…
• Respiration is the oxidation of glucose
back into CO2, which releases the stored
chemical energy!
Part 1: The big picture
• Organisms that make their own food are
called autotrophs. Organisms that make
food using photosynthesis are
photoautotrophs
• All animals, including humans, are
heterotrophs—we have to consume other
organisms as food
Part 1: The big picture
• Photosynthesis respiration work together
in what is called the carbon cycle
Part 1: The big picture
Image courtesy NASA Earth Observatory
Break!
Part 2: Macromolecules & Catalysis
• Before we get to the details of cellular
respiration, let’s cover a few more basics of
biochemistry that will help us understand
both photosynthesis and respiration better!
• Specifically, we’re going to briefly discuss
the basic building blocks and machinery of
biology
Part 2: Macromolecules & Catalysis
• What are the basic building blocks of life?
– Amino acids (proteins)
– Sugars (carbohydrates)
– Lipids (fats)
– Nucleic acids (DNA & RNA)
• All of these “building blocks” string together
to form chains called macromolecules or
biopolymers
Part 2: Macromolecules & Catalysis
• Remember glucose? Glucose is the basic
unit of a large number of different sugars
(carbohydrates)
Part 2: Macromolecules & Catalysis
• Glucose can bond with other glucose
molecules in several different ways
Sucrose (table sugar)
Part 2: Macromolecules & Catalysis
• Glucose can bond with other glucose
molecules in several different ways
Lactose (milk sugar)
Part 2: Macromolecules & Catalysis
• Glucose can also form long chains
Cellulose (woody part of plants)
Part 2: Macromolecules & Catalysis
• Glucose can also form long chains
Starch
Part 2: Macromolecules & Catalysis
• Amino acids chain together to form proteins
Catalase
Part 2: Macromolecules & Catalysis
• Nucleic acids chain together to form DNA &
RNA
DNA
Part 2: Macromolecules & Catalysis
• Our body has to break down sugar
polymers into the individual sugar
monomers (glucose) before we can use it
in cellular respiration
• Can our bodies use cellulose? Why or why
not?
• We don’t have the right biochemical
machinery to digest cellulose! We would
need a cellulase enzyme…
Part 2: Macromolecules & Catalysis
• Enzymes are proteins (chains of amino
acids) that act as biological catalysts: they
speed the rate of a chemical reaction, but
are left unchanged by the reaction
• Example demo: catalase
Part 2: Macromolecules & Catalysis
• Catalase speeds the
reaction:
2H2O2  2H2O + O2
• What do you think will
happen when I pour
H2O2 on the potato?
Catalase
Part 2: Macromolecules & Catalysis
• Enzymes work by lowering the activation
energy. The activation energy is a
measure of how much chemical energy a
molecule must have before it will undergo a
reaction.
• Enzymes lower this “hill” and cause
reactions to happen that would otherwise
only go very slowly
Part 2: Macromolecules & Catalysis
Part 2: Macromolecules & Catalysis
• Other examples of enzymes: lactase,
cellulase, amylase
• If you’re lactose intolerant, your body does
not produce enough lactase to digest
lactose sugar very well
• Similarly, we cannot digest the woody part
of plants since our bodies don’t produce
cellulase—cows and other herbivores have
bacteria in their guts that make cellulase
Part 2: Macromolecules & Catalysis
• We explored the action of amylase in one
of our morning activities
Break!
Part 3: Respiration & Fermentation
12H2O + 6CO2  C6H12O6 + 6O2 + 6H2O
C6H12O6 + 6O2  6H2O + 6CO2
Respiration is photosynthesis backwards!
Part 3: Respiration & Fermentation
Pigments
2H2O
Photosystem
4e- + 4H+ + O2
Part 3: Respiration & Fermentation
Electron Transport
Chain
Pigments
Photosystem
4e-
NADPH
Part 3: Respiration & Fermentation
ATP + NADPH
CO2
C6H12O6
The Calvin Cycle (light independent reactions)
Part 3: Respiration & Fermentation
• In photosynthesis, we used light energy to
split electrons out of a water molecule,
then used the electron transport chain to
take energy from those electrons and
convert it into ATP
• Then we used ATP and the leftover
electrons (in the form of NADPH) to fix
(reduce) CO2 into glucose using the
Calvin Cycle
Part 3: Respiration & Fermentation
• In respiration, we oxidize glucose (add
oxygen to transform it into 6CO2) to “pull”
electrons out of it
• These electrons are then put through an
electron transport chain to generate ATP
• What do we need for respiration?
– Glucose
– Oxygen
Part 3: Respiration & Fermentation
• First step in respiration is glycolysis
• In glycolysis, glucose (6 carbons) is split
into two molecules of pyruvate (3 carbons
each)
• This yields 2 ATP and 2 NADH (electron
carriers)
• If no O2 is available, glycolysis is the only
way to get energy from glucose and
fermentation occurs
Part 3: Respiration & Fermentation
• In fermentation, we get 2 ATP from
glycolysis but can’t continue to the Krebs
cycle, which requires O2 to function
• Have to recycle the NADH, so the
electrons the NADH carries are
transferred to pyruvate and glycolysis can
continue
• Different organisms transform pyruvate to
different waste molecules in
fermentation—in humans, lactic acid.
Part 3: Respiration & Fermentation
• But, if we have O2 we can put the pyruvate into
the Krebs cycle and yield 38 ATP total instead
of 2 for each glucose!
• Krebs cycle is complex, but in basic terms
pyruvate is added to a 4-carbon molecule to
make citrate, which is then oxidized one CO2 at
a time
• Each time a carbon is removed from citrate,
CO2 is produced and we pull electrons out and
transfer them to NADH or FADH2
Part 3: Respiration & Fermentation
• In the Krebs Cycle, we’ve oxidized pyruvate into
CO2 and produced NADH and FADH2 (electron
carriers)
• These electron carriers now move the electrons
to the electron transport chain (remember from
photosynthesis?)
• As the electrons flow through the transport
chain, their energy is used to create a proton
gradient
• Then, when the protons flow back in, they drive
ATP synthase (an enzyme!) which makes ATP
Part 3: Respiration & Fermentation
Part 3: Respiration & Fermentation
• Recap: in glycolysis, we split 6-carbon glucose
into two 3-carbon pyruvate and yield 2 ATP
• Stop at glycolysis if no oxygen available, then
fermentation
• If oxygen is available, Krebs Cycle oxidizes
pyruvate and strips the electrons from it
• NADH and FADH2 carry electrons stripped from
glucose to electron transport chain where they
are used to make ATP (energy)
Part 3: Broader context