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Concept 9.1 .
Q: What is cellular respiration?
A: How a cell releases the energy stored in organic compounds.
Q: Where does this energy originally come from?
A: The Sun.
Fig. 9.2.
a) The energy flows into an ecosystem as sunlight and leaves as
heat.
b) Chemical recycling of elements essential for Life (1st Law of
Thermodynamics).
Q: Where does cellular respiration occur in Eukaryotes?
A: Mitochondria.
Q: What are the organic compounds used in cellular respiration?
A: Carbohydrates, fats and proteins. Glucose is the one most
often used.
General equation:
Organic compound + O2
CO2 + H2O + Energy
Specific equation:
C6H12O6 + 6O2
6CO2 + 6H2O + Energy (for ATP and Heat)
ΔG = -686Kcal
Q: What does this tell you about the reaction?
A: Exergonic and spontaneous (no energy input required).
Q: What kind of metabolic pathway is this?
A: Catabolic (breaking larger molecules into smaller ones). NO
ATP MADE in this reaction, just energy to make ATP.
Q: How is the energy to make ATP released?
A: The transfer of electrons during chemical reactions. This
moving of electrons releases the energy stored in the organic
compounds. This is known as a REDOX REACTION.
Q: What is a redox reaction?
A: Transfer of electron(s) from one reactant to another. When
one loses the other gains electron(s).
Loss of electron(s) = oxidation
Gain of electron(s) = reduction (reduction of positive charge of
atom).
Fig. 9.3 – Not always a complete transfer of electrons – can be a
change in the amount of electron sharing in a covalent bond.
- Covalent electrons in methane equally shared as H and C
have almost equal affinity.
- The two oxygens share electrons equally too, BUT react
with the hydrogen from methane to form water. In the H2O
formed the electrons are attracted to the O therefore it
has partially gained electrons and is reduced. As O is so
electronegative it is a very strong oxidizing agent
(oxidation).
- In the CO2 formed, the electrons are further away from
the C atoms and closer to the O (very electronegative).
Therefore C has partially “lost” the shared electrons and so
the CH4 has been oxidized.
- As electrons move from a less electronegative atom to a
more electronegative atom they LOSE PE as it is harder to
remove the electron.
Q: Why?
A: Stronger pull on the electron.
- This energy loss is released as chemical energy available for
work.
- Organic compounds have lots of Hs which have “hilltop”
electrons that lose PE as they fall to oxygen. Stored energy
of glucose is released (-ΔG) and is available for ATP
synthesis.
- Enzymes lower activation energy to allow oxidation of
carbohydrates and fats in a series of steps.
Q: Why is the release of energy step wise?
A: If all the energy was released at once hard to harness for
useful work. Ex. gas tank explodes hard to drive the car very
far!
Glucose is broken down in a series of steps catalyzed by
enzymes.
Fig. 9.4.
a) Dehydrogenase removes a pair of H atoms (2e and 1p) from
sugar (substrate) = oxidation.
b) Enzyme in a) delivers the 2e and 1p to coenzyme NAD+. The
other p is released as H+ into the solution. NAD+ accepts e’s
and is an oxidizing agent. NAD+ is the most useful e
acceptor in cellular respiration and works in several redox
steps when sugar is broken down.
c) E’s efficiently transferred from food to NAD+ to form
NADH.
d) E’s “fall down” energy gradient from NADH to O2 and the
stored energy is tapped to make ATP.
Fig. 9.5.
- Electron transport chain breaks “fall” into a series of
smaller steps so the released energy can be stored. Rest is
given off as heat.
- Start at the top with NADH and end with oxygen capturing
e’s with H+ to form water.
- ΔG = -53Kcal so lose a little energy in each step.
- ETC made up of a series of proteins in the inner membrane
of mitochondria.
Cellular respiration occurs in (Fig. 9.6) and is made up of:
a) Gylcolysis – Start of breakdown of glucose into pyruvate.
Occurs in cytosol of cell.
b) Citric Acid Cycle – Completes breakdown of glucose. Occurs
in matrix of mitochondria.
c) ETC and chemiosmosis – Make ATP. Occurs in inner
membrane of mitochondria.