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Unit 7: Cellular Energetics
Chapters 6-8
By Brook Wigginton
Chapter 6
An Introduction to Metabolism
Metabolic Pathways
• In a metabolic pathway, a specific molecule
is altered in a series of defined steps,
resulting in a product.
• Some metabolic pathways release energy by
breaking down complex molecules to simpler
ones. These degradative processes are
called catabolic pathways.
-Glycolysis
• Anabolic pathways, in contrast, consume
energy to build complicated molecules from
simpler ones.
Types of Energy
• Energy is the capacity to cause change.
• Kinetic energy can be associated with the relative
motion of objects
• Thermal Energy is the kinetic energy associated with
the random movement of atoms or molecules
• Thermal energy in transfer from one object to another
is called heat.
• Potential energy is energy that is not kinetic
-Example
• Chemical energy is a term used by biologists to refer
to the potential energy available for release in a
chemical reaction.
Laws of Energy Transformation
• According to the first law of
thermodynamics, the total amount of
energy in the universe is constant. It
cannot be created nor destroyed.
• The second law of thermodynamics
states that the spontaneous directional
energy flow is from high to low quality
forms.
Enzymes
• Enzymes are catalysts that speed up certain
reactions by lowering the activation energy.
• Enzymes can be controlled by:
-Temperature. High temperatures decrease
reaction rates.
-pH balance. They function best at a level
of 7.
-Salinity. If enzymes are dissolved in fluids
that exceed the tolerance level, the hydrogen
bonds will break, thus inactivating the
enzyme.
Endergonic vs. Exergonic
• Endergonic reactions require energy input
resulting in products with more energy than
the reactants had.
-Photosynthesis
• Exergonic reactions release energy such that
the products have less energy than the
reactant.
-Cellular Respiration
Redox Reactions
• In electron transfer chains, molecules accept
and give up electrons in an orderly, stepwise
manner to control the release of energy.
• Oxidation-reduction reactions are simply
electron transfers between molecules. The
donor molecule loses and electron and is
oxidized, then the donor molecule gains an
electron and is reduced.
• Bioluminescence comes from an enzyme
called ‘luciferase’.
• Reactions begin when ATP is in the presence
of oxygen, which transfers a phosphate group
to luciferin.
• As the enzymes start to react, they release the
extra energy in the form of fluorescent light at
each step in the reaction.
Chapter 7
Cellular Respiration and
Fermentation
Carbohydrate Breakdown Pathways
• Anaerobic respiration and fermentation can
release small quantities of energy without the
use of oxygen
• Aerobic respiration is the main energyreleasing pathway leading to ATP formation in
eukaryotes.
Aerobic Respiration
• The formula for aerobic respiration is as
following: C6H12O6 + 6O2  6CO2 + 6H2O
• Three reactions are required for aerobic
respiration:
1.
2.
3.
Glycolysis- the breakdown of glucose to pyruvate; small
amounts of ATP are generated.
The Krebs Cycle- degrades the pyruvate to CO2, ATP is
produced, and NAD+ and FAD accept H+ and electrons to
be carried to the electron transfer chain.
Electron Transfer Phosphorylation- processes the H+ and
electrons to generate high yields of ATP; oxygen is the final
electron acceptor.
Fermentation vs. Aerobic Respiration
• Anaerobic respiration and fermentation take
place when there is no oxygen present.
• Aerobic respiration takes place when in the
presence of oxygen.
Important Formulas
• Glycolysis- glucose + oxygen  oxygen and
water
• Fermentation- pyruvate  acetyl aldehyde +
NADH + alcohol
• Krebs Cycle- acetyl Co A + hydrogen  ATP +
NADH + FADH + CO2
• Electron transfer phosphorylation- FADH2 +
NADH  H +ATP + H2O
Glycolysis
• Enzymes in the cytoplasm catalyze several steps in the
breakdown of a 6 C sugar glucose into two molecules of
pyruvate.
• Glycolysis begins when glucose is first phosphorylated, then
split to form two molecules of PGAL using two ATP
molecules.
• Then, enzymes remove H+ and electrons from PGAL to
change NAD+ to NADH.
-4 ATPs are produced
• Overall, glycolysis produces 2 pyruvates, 2 ATP, and 2 NADH
• Additionally, glycolysis occurs in the cytoplasm of the cell.
After Glycolysis
• Acetyl-Co A formation occurs as each pyruvate enters
the mitochondria. 1 C is removed and attaches to the
oxygen, forming carbon dioxide; the 2 C fragment
remaining joins coenzyme A to form Acetyl-Co A
• The Krebs Cycle/The Citric Acid Cycle
- Begins when the two pyruvates are converted
into two acetyl-coenzyme A (acetyl-CoA), two carbon
dioxide molecules, and two NADH. Then, during the
series of eight reactions that make up the citric acid cycle,
the two acetyl-coA molecules are oxidized, yielding two
more molecules of carbon dioxide and 2 ATP.
After the Krebs Cycle
• Electron Transfer Phosphorylation is when
NADH and FADH2 give up their electrons to
transfer systems embedded in the
mitochondrial inner membrane. The energy is
used to pump H+ out of the inner
compartment. When H+ flow back through
the ATP synthase in the channels, the cuopling
of P to ADP yields ATP
(cont.)
• Oxygen joins with the used electrons from the
glucose molecules and the H+ to yield water.
• Without oxygen, the transfer chain becomes
clogged, and no H+ gradient forms. No
gradient means no ATP, which means the cell
will die.
Total Energy
• Overall, electron transfer yields 32 ATP,
glycolysis yields 2 ATP, and the Krebs cycle
yields 2 ATP for a total of 36 ATP per glucose
molecule! That is a lot of energy.
Anaerobic Pathways
• Anaerobic pathways operate when there is no
oxygen present. Pyruvate from glycolysis is
metabolized to produce molecules other than
acetyl CoA.
• Fermentation pathways
-Fermentation yields enough energy for many
single-celled anaerobic organisms and is sufficient
for some aerobic organisms when oxygen levels are
diminished, but as a whole, is ineffective for large,
multi-celled organisms.
Types of Fermentation
• Alcoholic fermentation begins with glucose
degradation into pyruvate. Then, cellular
enzymes convert pyruvate to acetaldehyde,
which then accepts electrons from NADH to
become alcohol.
• Lactate fermentation occurs when certain
bacteria, such as milk, and muscle cells have the
enzymes capable of converting pyruvate into
lactate. No additional ATP beyond the net 2 from
glycolysis is produced, but NAD+ is regenerated.
Twitches
• Slow twitch muscle fibers support steady,
prolonged activity because they have an
abundance of mitochondria making lots of
ATP.
• Fast twitch muscle fibers have few
mitochondria and produce small amounts of
ATP by lactate fermentation, so they support
quick, non-sustainable demands for energy.
Alternate Energy Sources
• Excess fats are stored in adipose tissue, which is
digested into glycerin, which enters glycolysis.
• Fatty acids enter the Krebs cycle, and since they
have so many carbon and hydrogen atoms, they
are broken down more slowly and yield more ATP.
• Amino acids travel in the blood, and after the
amino group is removed, the amino acid is fed
into the Krebs cycle.
Chapter 8
Photosynthesis
Light Properties
• Light energy is packaged as photons, which
vary in energy as a function of wavelength.
The light reflected in each pigment gives the
pigment it’s color.
• Only a small range (380-750) of wavelengths
are used in photosynthetic reactions
Taste the Rainbow
• Chlorophyll is the most common pigment
used in photosynthesis by plants,
photosynthetic protists, and cyanobacteria.
• A pigment absorbs light of specific
wavelengths by acting as an antenna for
receiving photon energy.
• Photosynthesis begins when photosynthetic
pigments absorb a photon of light.
Photo Flat
• Photosynthesis takes place in the chloroplast.
• The semifluid interior, the stroma, is the site
for the second stage of reactions
• Thylakoids, which are like flattened sacs, are
interconnected by channels weaved through
the stroma.
Two Types of Reactions
• Light dependent reactions convert light
energy to chemical bond energy of ATP
• Light independent reactions assemble sugars
and other organic molecules using ATP,
NADPH, and CO2
Light Dependent Reactions
• Begins when light is ‘harvested’ by pigments in
the chloroplasts.
-One pathways begins when chlorophyll P680
in photosystem II absorbs energy. A boosted
electron moves through a transport system, which
releases energy for ATP.
-The other pathway begins when chlorophyll
P700 in photosystem I absorbs energy. The boosted
electron from P700 passes to the acceptor, then the
electron transport chain, and finally joins NADP+ to
form NADPH.
Noncyclic Photophosphorylation
• Step 1:
• Electrons trapped by P680 in photosystem II
are energized by light.
• Step 2:
• Two energized electrons are passed to a
molecule called the primary electron acceptor.
This electron acceptor is called “primary”
because it is the first in a chain of electron
acceptors.
Electron Transport Chain
• Step 3:
• Electrons pass through an electron transport
chain. This chain consists of proteins in the
thylakoid membrane of the chloroplast that pass
electrons from one carrier protein to the next.
Some carrier proteins, such as the cytochromes,
include nonprotein parts containing an ion. The
electron transport chains in photosynthesis are
analogous to those in the inner mitochondrial
membrane.
Phosphorylation
• Step 4:
• As the two electrons move “down” the
electron transport chain, they lose enrgy. The
energy lost by the electrons as they pass along
the ETC is used to phosphorylate, on average,
about 1.5 ATP molecules
Photosystem I & NADPH
• Step 5:
• The ETC terminates with P700. Here the
electrons are again energized by sunlight and
passed to a primary electron acceptor.
• Step 6:
• The two electrons pass through a short ETC. At
the end of the chain, the two electrons combine
with NADP+ and H+ to form NADPH. NADPH is a
coenzyme, electron acceptor, and forms an
energy rich molecule.
Parting the Red Sea
• Step 7:
• The two electrons that originated in PS II are
now incorporated into NADPH. The loss of
these two electrons from PS II is replaced
when H2O is split into two electrons, 2 H+,
and ½ O2. The two electrons from the water
replace the lost electrons from PS II, one of
the H+ provides the H in NADPH, and the ½ O2
contributes to the oxygen gas that is released.
Calvin-Benson Cycle
• Step 1: Carbon fixation
-6 carbon dioxide combine with 6 RuBP to
produce 12 PGA. The enzyme rubisco catalyzes
the merging of carbon dioxide and RuBP. The
Calvin Cycle is referred to as C3 Photosynthesis
because the first product formed, PGA, contains
three carbon atoms.
Step 2
• Reduction: 12 ATP and 12 NADPH are used to
convert 12 PGA to 12 G3P. The energy in the
ATP and NADPH molecules is incorporated
into G3P, thus making it a very energized
molecule.
Step 3
• Regeneration: 6 ATP are used to convert 10
G3P to 6 RuBP. Regenerating the 6 RuBP
originally used to combine with 6 CO2 allows
the cycle to repeat.
Step 4
• Carbohydrate synthesis: Note that 12 G3P
were made in step 2, but only 10 were used in
step 3. These two remaining G3P are used to
build glucose. Other monosaccharides, like
fructose and maltose, can also be formed. In
addition, glucose molecules can be combined
to form disaccharides, like sucrose, and
polysaccharides, like starch and cellulose.