Sources of energy
Cells derive energy from:
1. Sunlight
2. The oxidation of organic
molecules
3. The oxidation of
inorganic molecules
Photosynthesis
The trapping and conversion of light energy into chemical energy
Light energy  ATP + NADH /NADPH
Photosynthesis
The trapping and conversion of light energy into chemical energy
The reduction and incorporation of CO2 into organic material
Roles of photosynthesis
Provides organisms with the energy to synthesize organic
material
organisms serve as the base of most food chains
Replenishes the planet’s supply of O2
Photosynthesis
Almost all energy on Earth is ultimately derived from solar
energy
More than half of all photosynthesis is carried out by
microorganisms
Photosynthesis
Light reaction
Light energy converted to chemical energy
Dark reaction
Chemical energy used to reduce and incorporate CO2 into
organic molecules
Light absorbing pigments
Absorb energy of light and transfers energy via excited electrons
Chlorophylls
Most important photosynthetic
pigments
Planar rings composed of 4 pyrole
rings with a central magnesium
atom
Hydrophobic tail aids in
attachment to membranes
Chlorophylls a and b
Common in eukaryotes
Absorb light at 650 + 430 nm
Transmit green light
 green color
Chlorophylls a and b
Common in eukaryotes
Absorb light at 650 + 430 nm
Transmit green light
 green color
Bacteriochlorophylls
Found in green and purple
photosynthetic bacteria
Absorb light at longer
wavelengths
Bacteriochlorophylls
Accessory pigments
Carotenoids - long molecules (usually yellow)
Phycobiliproteins - protein attached to a tetrapyrole ring
Algae
Diatoms + dinoflagellates
Red algae and cyanobacteria
Accessory pigments
Carotenoids
Phycobiliproteins
Absorb energy from wavelengths that chlorophylls cannot
Transfer energy to chlorophylls
Antennas
Arrays composed of chlorophylls and accessory pigments
Large surface area to trap photons
Light energy transferred to reaction-center chlorophyll
Photosystems I and II
Eukaryotes and many prokaryotes
possess 2 photosystems
I - absorbs light greater than
680 nm
II absorbs light lower than 680 nm
Photosystem I
P700 -reaction-center chlorophyll
Absorbed energy transferred to
P700
Donates excited electron to an
acceptor (Chlorophyll a or FeS)
Electron transferred to ferredoxin
then travels 1 of 2 pathways
Cyclic pathway
Electron moves through a series
of carriers
Travels back to P700
PMF formed and used to generate
ATP
 cyclic photophosphorylation
Noncyclic pathway
Ferredoxin reduces NADP+ to
NADPH + H+
Photosystem II donates electrons
to P700
Photosystem II
P680 absorbs light energy
Electron moves to pheophytin a
Electron moves to Q then to P700
Photosystem II
P680 obtains electron from H2O
 O2
Synthesis of ATP is by noncyclic
photophosphorylation
Photosynthetic electron transport chain
Takes place in a membrane
Structure referred to as a
thylakoid
Photosynthetic electron transport chain
In plants takes place in the
thylakoid membrane of
chloroplasts
Green and purple photosynthetic bacteria
Do not use water as electron donor
Do not produce O2
Photosynthesis that produces O2 = oxygenic
Photosynthesis that does not produce O2 = anoxygenic
Green and purple photosynthetic bacteria
Purple system - like Photosystem II
Green system - like Photosystem I
Green and purple photosynthetic bacteria
Use H2, H2S, S0 or organic molecules as electron donors
Possess bacteriochlorophylls (absorb at longer wavelengths)
Green and purple photosynthetic bacteria
Possess only 1 photosystem  restricted to cyclic
photophosphorylation
Use ATP or proton motive force to reduce NAD+
The dark reaction
Involves the fixing of CO2 (conversion into organic material)
Known as the Calvin or the Calvin-Benson cycle
Alternate CO2 fixing pathways exist
The dark reaction
Divided into three phases
1. Carboxylation phase
2. Reduction phase
3. Regeneration phase
Carboxylation phase
Ribulose-1,5-bisphosphate
carboxylase adds CO2 to ribulose1,5-bisphosphate (RuBP)
Forms 2 molecules of
3-phosphoglycerate (PGA)
Reduction phase
PGA is reduced to
glyceraldehyde-3-phosphate using
NADPH as an electron donor
Regeneration phase
Regenerates RuBP and produces
carbohydrates
Involves transketolase and
transaldolase reactions
6RuBP + 6CO2  12 PGA
12 PGA  6RuBP + fructose-6-P
The dark reaction
6CO2 + 18 ATP + 12 NADPH + 12H2O
 glucose + 18 ADP + 18 Pi + 12 NADP+
Biosynthesis
Also known as anabolism
Construction of complex
molecules from simple
precursors
Energy derived from catabolism
used in biosynthesis
Principles governing biosynthesis
Large macromolecules (proteins, nucleic acids, polysaccharides)
are made up of a few small structural units (monomers)
Cells often use the same enzymes for anabolism and catabolism
In amphibolic pathways, some steps may be catalyzed by 2
different enzymes (one for anabolism, one for catabolism)
Principles governing biosynthesis
Principles governing biosynthesis
Some biosynthetic reactions are coupled to the breakdown of ATP
so as to drive them in the forward direction
Anabolic and catabolic pathways often use different cofactors
(e.g. NADH as electron acceptor and NADPH as electron donor)
Gluconeogenesis
Microorganisms that cannot
perform photosynthesis must
synthesize sugars from reduced
organic compounds
Synthesis of glucose from
noncarbohydrate precursors
known as gluconeogenesis
Reversal of glycolysis
Gluconeogenesis
Three steps are irreversible and
must be catalyzed by other
enzymes
Synthesizes both glucose and
fructose  other sugars
synthesized from glucose and
fructose
Nucleoside diphosphate sugars
Some sugars are synthesized
while attached to a nucleoside
diphosphate
Most common nucleotide
diphosphate sugar is uridine
diphosphate glucose (UDPG)
Nucleoside diphosphate sugars
UDPG can be used to synthesize
galactose or glucuronic acid
Nucleoside diphosphate sugars
play a central role in the
synthesis of starch and glycogen
Phosphorus assimilation
Phosphate required for synthesis of nucleic acids, proteins,
phospholipids, ATP and coenzymes
Inorganic phosphate can be assimilated by incorporating it into
ATP in one of three ways:
Photophosphorylation
Oxidative phosphorylation
Substrate level phosphorylation
Phosphorus assimilation
Microorganisms may also hydrolyze phosphate from organic
phosphates