Photosynthesis and Cellular Respiration
-
living things depend on energy from sun
-
photoautotrophs transform light energy into chemical potential energy (e.g. glucose) through
photosynthesis
Overall equation for photosynthesis:
chlorophyll
6CO2(g) + 6H2O(l) + light energy
C6H12O6(aq) + 6O2(g)
This reaction is endergonic and anabolic.
-
heterotrophs rely on autotrophs for energy (e.g. glucose)
heterotrophs use cellular respiration to break the covalent bonds in glucose causing the release
of free energy
Overall equation for aerobic cellular respiration:
C6H12O6(aq) + 6O2(g)
6CO2(g) + 6H2O(l) + energy
Aerobic - oxygen used in process
Anaerobic - oxygen not used in process
This reaction is exergonic and catabolic.
NOTE: Photosynthesis and cellular respiration are complimentary processes.
Redox Reactions in Cellular Respiration
Redox Reaction – a chemical reaction involving a transfer of electrons from one atom to another
OXIDATION
 Loss of electrons
 Loss of hydrogen
 Gain of oxygen
Can be any of these
REDUCTION
 Gain of electrons
 Gain of hydrogen
 Loss of Oxygen
Can be any of these
Oxidation
Xe-
+
Y
→
X
+
Ye-
Reduction



Whenever there is oxidation, there must also be reduction (redox)
During cellular respiration, glucose is oxidized
oxidation
oxidation
C6H12O6
+
6O2
→
6CO2
+
6H2O
Reduction



Electrons are removed from glucose and their energy is used to make ATP
Most organic molecules (carbohydrates, lipids, proteins) contain a lot of C and H and are
excellent fuels because high energy ELECTRONS and HYDROGEN can be removed from them
The electron carriers in cellular respiration are NAD+ and FAD
OXIDIZED FORM
REDUCED FORM
NAD+
NADH + H+
Pick up 2 electrons
and 2 H+
FAD
FADH2
Overview of Aerobic Cellular Respiration as a Redox Reaction
Oxygen oxidizes C-H covalent bonds in glucose in 2 ways:
1)
-
12 H atoms break away from C atoms in glucose and attach to 6 O atoms (3 O 2) to become 6
H2O
O is more electronegative than C therefore the shared electrons are closer to O
as electrons (in H atoms) move from less electronegative C atoms (in glucose) to highly
electronegative O atoms, they lose potential energy
decrease in free energy means exergonic process
2)
6 C atoms and 6 O atoms from glucose attach to 6 remaining O atoms (3 O2) to become 6 CO2
once bound to C atoms, the highly electronegative O atoms draw electrons to themselves,
therefore loss of potential energy
- decrease in free energy means exergonic process
Overall, valence electrons move from a higher free energy state in glucose to a lower free energy
state in CO2 and H2O.
-
The overall energy change yields 2870kJ of free energy per mole of glucose.
Of this energy:
- 34% is trapped
- 66% is lost
Cellular Respiration and Activation Energy
-
the reactants, oxygen gas and glucose are stable covalent compounds
-
therefore, no reaction occurs when collisions occur at 37C
-
the activation energy is relatively high for combustion of glucose which prevents spontaneous
combustion
-
specific enzymes catalyze every step in aerobic respiration allowing the reaction rate to meet the
needs of the cell