Overview of Cellular Respiration,
Photosynthesis and Redox
Reactions
Lecture 6
Fall 2008
Working Cells
• All organisms need ATP to do cellular work
• Cells get ATP from converting carbon
compounds (chemical energy) through cellular
respiration
• Cellular Respiration:
– The conversion of chemical energy of carbon
compounds into another form of chemical energy,
ATP
Where does an organism get the chemical energy
(carbon compounds) used in cellular respiration?
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Food Chains
Producers
• Autotrophs (self-feeders)
• Use photosynthesis or
chemosynthesis to make organic
food molecules
• Plants, algae (phytoplankton),
cyanobacteria
• Basis for all other ecosystem growth
Primary productivity
• Productivity: amount of biomass
produced in a given area during a
given period of time
– Biomass: dry weight of all organic
matter in an organism
Fig. 54.11
Cellular Respiration & Photosynthesis
Photosynthesis
• The process by which light energy from the
sun is converted into chemical energy
– Chemical energy in the form of organic
compounds
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Food Chains
Consumers
• Heterotrophs (other-feeders)
• Obtains organic food molecules by
eating other organisms or
substances derived from other
organisms
• Primary = herbivores
– eat producers
• Secondary = carnivores
– eat herbivores
• Tertiary = top carnivores
– eat other carnivores
Fig. 54.11
Metabolic Diversity
Phototrophs:
Chemotrophs:
Autotrophs:
Heterotrophs:
Organisms that obtain energy from light
Organisms that obtain energy from
inorganic or organic compounds in their
environment
Organisms that need only inorganic
molecules (CO2 or CH4 ) as a carbon source
Organisms that require at least one organic
nutrient (e.g., glucose) as a carbon source
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Cellular Respiration &
Photosynthesis
•Cellular respiration requires carbon
compounds & oxygen
•Carbon compounds & oxygen
produced by photosynthesis
See Fig. 9.2
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Cellular Respiration
• Catabolic pathway
• Aerobic respiration
– Consumes oxygen
• Anaerobic respiration
– Use other molecules (not oxygen)
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Cellular Respiration
• Cellular respiration is not the same as gas
exchange (respiration)
– Gas exchange is the exchange of O2 and CO2
between an organism and its environment
• Cellular respiration is not the same as breathing
– “Breathing” refers specifically to process in
lungs
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Cellular Respiration
• Glucose molecule held
together by covalent bonds
• Covalent bonds share
electrons
• Cellular respiration changes
which atoms are bonded
together
• ΔG = - 686 kcal/mol
Redox Reactions
Redox reactions =oxidation-reduction reactions
• Chemical reactions that transfer electrons from
one substance to another
– Oxidation
• The loss of electrons
– Reduction
• The addition of electrons
• Reduction reduces the positive charge of an atom
– Reducing agent = electron donor
– Oxidizing agent = electron acceptor
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Redox Reactions
• Energy needed to pull electrons away from an
atom
• Requires more energy the more electronegative
the atom
– Electronegativity : The tendency of an atom to attract
electrons towards itself
• Electrons lose potential energy as they move
from less electronegative to more
electronegative atom
– Releases energy
Redox Reactions in Cellular Respiration
• When glucose is broken down, hydrogen and its
electrons change partners
• Electrons lose potential energy along the way and
energy is released
Oxidation
• Glucose oxidized
• Reducing agent - Loses electrons
Reduction
• Oxygen reduced
• Oxidizing agent - Gains electrons
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Electron Acceptors
• High electronegativity
• Electrons move from molecule to molecule by being
“passed” to a stronger electron acceptor
Electron Acceptors
• NAD+ (nicotinamide adenine dinucleotide)
– Positively charged electron acceptor
– NAD+ is reduced to NADH
• NADP+ (nicotinamide adenine dinucleotide phosphate)
– Reduced to NADPH
• FAD (flavin adenine dinucleotide)
– FAD is reduced to FADH2
• Oxygen
– Can accept (“pull”) electrons from both NADH & FADH2
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Electron Transport Chain
Why can’t the electrons be
transferred from glucose to
oxygen in one step?
• Too explosive
– Energy released as heat and
light energy
– Not readily usable to do work
• Breaking it down into steps
allows the energy to be used
to perform work
= Electron Transport Chain
Fig. 9.5
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Electron Transport Chain
Electron Transport Chain
• Used to slow the fall of
electrons from glucose to
oxygen
• “chain” of protein
complexes (and other
molecules)
• Electrons pass from
molecule to molecule in a
series of redox reactions
• At each step, energy is
released
– Used in the synthesis of ATP
Fig. 9.5
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Electron Transport Chain
• At bottom of chain, the
electron “drops” to
oxygen
• Oxygen also picks up
hydrogen and forms
H20
• Oxygen is driving the
“fall” by attracting the
electrons
Fig. 9.5
6.6
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