Download lecture 3 -photosynthesis

LECTURE PHOTOSYNTHESIS
DR AKM SHAFIQUL ISLAM
ELECTRON TRANSPORT PATHWAY
• Occurs within the inner mitochondrial membrane
• Electrons are removed from NADH and shuttled through
a series of electron acceptors
– Energy is removed from the electrons
with each transfer
• This energy is used to make ATP
– NADH  3 ATP
– FADH2  2 ATP
– O2 is the terminal electron acceptor
• ½O2 + 2H+ + 2e-  H2O
– Anaerobic respiration utilizes a
molecule other than O2 as the
terminal electron acceptor
• e.g., NO3-, SO42-, CO2, etc.
• Respiratory chain reaction
NADH + H+ +1/2 O2 + 3 ADP + 3Pi
NAD+ + 4 H2O + 3 ATP
and
FADH2 + ½ O2 + 2 ADP + 2Pi
FAD + 3 H2O + 2 ATP
Overall reaction
Glucose + 6 O2
6 CO2 + 6 H2O
DGo’= -696 kcal/mol
Review of Cellular Respiration
– one 6-C glucose oxidized to 6 CO2 molecules
– 2 ATP from substrate level phosphorylation in
glycolysis
– 2 ATP from substrate level phosphorylation in
citric acid cycle
– Each NADH generates a maximum of 3 ATP
• 10 NADH = maximum of 30 ATP
– Each FADH2 generates a 2 ATP
• 2 FADH2 = 4 ATP
– TOTAL possible ATP = 38
Photophosphorylation
• Only occurs in photosynthetic cells which
contain light trapping pigments such as
chlorophyll
• Light causes chlorophyll to give up electrons
• Energy released from
the transfer of
electrons (oxidation)
of chlorophyll through
a system of carrier
molecules is used to
generate ATP
Photosynthesis
• Photosynthesis is largely reverse of respiration
• Energy in the form of light is captured and used
for conversion of carbon dioxide to glucose and
its polymers
• Photosynthesis is the prime supplier of energy
for biosphere
• 6 CO2 + 6 H2O + light  C6H12O6 + O2
PHOTOSYNTHESIS
Who does photosynthesis?
• Though plants can photosynthesize,
microorganisms are responsible for the
majority of the photosynthesis occurring
on the planet
CHLOROPLASTS
• Chloroplasts are the site of photosynthesis
– photosynthetic bacteria don’t have chloroplasts, they
essentially are chloroplasts
• Chloroplasts contain the pigment chlorophyll
– Pigments absorb light
– Multiple similar forms of chlorophyll exist
– The light absorbed is ultimately used to reduce CO2 to
glucose
• In procaryctes, (cyanobacteria, green sulfur
bacteria, purple sulfur bacteria) photosynthesis
occurs in stacked membranes
• While organelle called the chloroplast conducts
photosynthesis for eucaryotes (algae, plants)
• Both systems contain chlorophylls which
strongly absorb visible light
•
Two different light-harvesting and
reaction system.
1. Photosystem I – activated about 430 nm.
2. While phosphosystem II activated by
light with wavelength below 680 nm.
In the case of phosphosystem I, the electron used to
reduce NADP+.
Excited electron from phosphsystem II are passed to
phosphosystem I with ADP phosphorylation
accomplished once for every excited electron pair
transported.
Thus the overall stoichiometry for the photosynthetic
eucaryotes is
H2 O + 4hv + NADP+ + ADP + Pi 
NADPH + H+ +1/2 O2 + (ATP + H2O)
Additional ATPs may be generated from absorbed light
energy via cyclic photophosphorylation
• Additional ATPs may be generated from
absorbed light energy via cyclic
photophosphorylation. Electron excited from
P700 to P430 flow to cyt b563 and back to P700,
causing ADP phosphorylation in the process.
3hv + ADP + Pi  ATP + H2O
Biosynthesis
• The phenomenon that characterize the
microbial process are
– Substrate or nutrient utilization
– Cell growth
– Product release
Biosynthesis influences all the three
• Nutrient requirements are directed by cells
need for precursor molecule, stored
chemical energy and reducing power
• The rate of cell growth is determined by
the rate of biosysnthesis, the rate at which
new cell materials are formed
• Cell growth rate – varies widely. E. coli
bacteria can double in 20 min.
Synthesis of Small Molecules
• Monomeric building blocks are constructed.
• Approximately 70 different compounds are
required in this purpose
–
–
–
–
–
4 ribonucleotides
4 hydroxyribonucleotides
20 amino acid
about 15 monosaccharides
about 20 fatty acids and lipids
• ATP, NAD, other carrier and coenzymes involved
in the synthesis
• Living cell assimilate nitrogen by incorporating it
into the amino acids glutamic acid and glutamine
First, glutamic acid is formed by reaction between
ammonia and a-ketoglutamic acid, one of the TCA cycle
intermediate
HOOC(CH2)2COCOOH + NH4+ + NADH glutamate
dehydrogenase
HOOC(CH2)2CHNH2COOH + NAD+ + H2O
Additional ammonia can be accepted by adding it to
glutamic acid to give glutamine
+
glutamine
synthase
HOOC(CH2)2CHNH2COOH + NH4 + ATP
HOOCNH2CH(CH2)2CONH2 + ADP + Pi + H+
The second reaction require metabolic energy and
ammonia. In some bacteria direct amination of pyruvate
to alanine occurs with consumption of NADH
• Other amino acids are formed by conversion of
glutamate or by transfer of its amino group to
other carbon skeletons
– Example, glutamate is converted to poline in a
sequence of two enzyme-catalyzed reaction plus a
spontaneous hydrolysis step
C5NH8O4- + ATP + 2(NADP + H+)
C5NH8O2- + ADP + Pi + 2NADP+ + H2O
• Transpotation from glutamate to alanine
and aspartate
Glutamate + oxaloacetate  a-ketoglutarate + aspartate
Glutamate + pyruvate  a-ketoglutarate + alanine