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1. Microbio: PHOTOSYNTHESIS
 Phototrophy – the use of light energy
 Photosynthesis
o Is the conversion of light energy to
chemical energy (carbohydrate).
 Phototrophs (autotrophs)
- Organisms that carry out photosynthesis
- Organisms capable of making their own
food
 Photoautotrophy
- Capable of growing with CO2 as the sole
carbon source
- Requires ATP production and CO2
reduction.
 Photoheterotrophy
- organic carbon as their carbon source
Photoautotrophy (photosynthesis)
o that two distinct sets of reactions operate in parallel:
o ATP Production
o CO2 reduction
o for autotrophic growth,
o energy is supplied from ATP
o electrons for reduction of CO2 come from
NADH (or NAPH).
o In purple and green bacteria the donor could be a
reduced sulfur compound:
o hydrogen sulfide (H2S)
o molecular hydrogen (H2)
o green plants, algae, and cyanobacteria derive
electrons from
o water (H2O).
 Chlorophyll and Bacteriochlorophyll
- related to tetrapyrroles that are the parent
structure of the cytochromes.
- chlorophylls contain magnesium instead of iron
at the center of the ring.
 Chlorophyll – light-sensitive pigments required in
photosynthesis.
 Organisms must produce some form of chlorophyll to be
photosynthetic.
CHLOROPHYLL (prokaryotic phototrophs)
o Exist within the membranes in the cell.
o In eukaryotic phototrophs, photosynthesis takes
place in intracellular organelles, the chloroplasts,
where the chlorophylls are attached to sheetlike
membranes.
o contain specific substituents on the tetrapyrrole
ring and a hydrophobic alcohol that helps anchor
the chlorophyll into photosynthetic membranes.
o Related to porphyrins ----- tetrapyrroles (parent
material of cytochromes).
o Chlorophyll a, the principal chlorophyll of
oxygenic phototrophs.
 Chlorophyll a, the principal chlorophyll of Cyanobacteria
o Absorbs red and blue light and transmits green
light.
o Absorption spectrum shows strong absorbance
near 680nm and 430nm.
o Different chlorophylls have different absorption
spectra.
 Prochlorophytes produce chlorophyll a
and b.
BACTERIOCHLOROPHYLL (prokaryotic phototrophs)
o Photosynthetic pigments used by purple and green
bacteria (anoxygenic phototrophs).
o Bacteriochlorophyll a
 Present in most purple bacteria
 Absorbs maximally between 800
and 925nm
The existence of different forms of chlorophyll or bacteriochlorophyll
that absorb light of DIFFERENT WAVELENGTHS allows phototrophs to
make better use of the available energy in the electromagnetic
spectrum and COEXIST in the same habitat.
 Photocomplexes – consisting of anywhere from 50
to 300 chlorophyll/bacteriochlorophyll molecules.
o Reaction centers – participate directly in the
conversion of light energy to ATP.
o Antenna pigments – funnel/concentrate
light energy to reaction centers.
“low light intensities that are often found in nature, this arrangement
for concentrating energy allows reaction centers to receive light
energy that would otherwise be missed”
CHLOROPLASTS
o Intracellular organelles
o Chlorophylls are attached to sheet-like membranes
(thylakoids)
o Grana stacks of thylakoids
o Stroma – matrix space that surround the thylakoids.
- Chloroplasts are absent from prokaryotic
phototrophs.
Chromatophores
- The photosynthetic pigments in purple
bacteria are integrated into internal
membrane systems that arise from
invagination of the cytoplasmic membrane
forming membrane vesicles.
- Lamellae – stacks of chromatophores.
CHLOROSOMES
- Found in green sulfur bacteria.
- Contain bacteriochlorophyll c, d, or e.
 Antenna pigments absorbed light energy.

2. - Ultimate structure for capturing low light
intensities
- Function as massive antenna complexes
- Bacteriochlorophyll molecules in the
chlorosomes are not attached to proteins
Phototrophic organisms have accessory pigments in addition to
chlorophyll, including:
CAROTENOIDS
- The most widespread accessory pigments in
phototrophs
 Common carotenoid is B-carotene.
- typically yellow, red, brown, or green and
absorb blue
- energy absorbed by carotenoids can be
transferred to a reaction center.
- Prevent photooxidative damage to cells
 Photooxidation produce toxic form of
oxygen the singlet oxygen.
- function primarily as photoprotective agents.
PHYCOBILIPROTEINS
- Main light-harvesting systems of these
phototrophs.
- Form intro aggregates within the cell called
phycobilisomes attach to cyanobacterial
thylakoids.
- Consist of red or blue-green linear
tetrapyrroles called bilins.
- Allow cell to capture more light energy than
chlorophyll alone.
Types of phycobiliproteins:
 Phycoerythrin – red, 550nm
 Phycocyanin – blue, 620nm
 Allophycocyanin – 650nm
ANOXYGENIC PHOTOSYNTHESIS
 Electrons traverse an ETC arranged in photosynthetic
membrane in order of their increasingly more
electropositive reduction potential.
 Generates proton motive force that drives ATP
synthesis.
 Found in four phyla of Bacteria
 Photosynthesis apparatus is embedded in
membranes.
 ETC occur in the reaction center of anoxygenic
phototrophs.
 Reducing power for CO2 fixation comes from
reductants present in the environment (H2S, Fe, or
NO)
o Requires reverse electron transport for NADH
production in purple phototrophs.
o Electrons are transported in the membrane
through a series of proteins and cytochromes.
Electron flow in anoxygenic photosynthesis in a purple
bacterium.
 For purple bacterium to grow autotrophically, the
formation of ATP is not enough – CO2 fixation.
o Reducing power (NADH) is necessary
o Reduced substances such as H2S are
oxidized, and the electrons eventually end up
in the quinone pool of the photosynthetic
membrane.
OXYGENIC PHOTOSYNTHESIS
 Oxygenic phototrophs use light to generate ATP and
NADPH.
 The two light reactions are called photosystem l and
photosystem ll.
 Z scheme of photosynthesis.
 ATP can also be produced by cyclic
photophosphorylation.

3. Anoxygenic photosynthesis in Oxygenic phototrophs
Condition: when Photosystem ll is blocked.
- Cyclic but not noncyclic
photophosphorylation occurs.
- The reducing power for CO2 will come from
other sources than water.
Cyanovacteria: can use H2S as an electron donor.
Green Algae: can use H2
“from an evolutionary standpoint, the process of cyclic
photophosphorylation in both oxygenic and anoxygenic phototrophs
is on of many indication of their close relationship”
AUTOTROPHIC PATHWAYS
 Autotrophy
- A process by which an energy-poor and highly
oxidized form of carbon (CO2) is reduced and
assimilated into cell material.
- Includes all phototrophs and
chemolithotrophs.
 THE CALVIN CYCLE (Carbon fixation, reduction of
CO2)
- Named from its discoverer, Melvin Calvin
- Fixes CO2 intro cellular material for
autotrophic growth
- Requires NADPH, ATP, Ribulose bisphosphate
carboxylase (RubisCO), and
phosphoribulokinase.
- 6 molecules of CO2 are required to make 1
molecule of glucose.

4. “calvin cycle reactions based on the incorporation of 6 molecules of CO2, as this is what is required to make one hexose (C6H12O6)”
CARBOXYSOMES
- A mechanism for concentrating CO2 in the cell and making it readily available to RubisCO.
- Functions to restrict access of RubisCO to O2 (alternative substrate for this enzyme).
- RubisCO carboxylates rather than oxygenates ribulose bisphosphate.
o If ribulose 1,5-bisphosphate is oxygenated, more energy and reducing power are required to incorporate t into central
metabolic pathways than if it is carboxylated.
Other autotrophic pathways, aside from calvin cycle.
 Green sulfur bacteria use the reverse citric acid cycle to fix CO2, succinate CoA as the substrate to carboxylized.
 Green nonsulfur bacteria use the hydroxypropionate pathway to fix CO2, acetyl CoA as the substrate to be carboxylize.