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Summary Metabolic diversity and photosynthesis
- Instelling
- Rijksuniversiteit Groningen (RuG)
Summary about the lecture metabolic diversity and photosynthesis inducing corresponding literature.
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HC7 Metabolic diversity &Photosynthesis (BOOK)
Chapter 14.1 – 14.5 (no other pathways of CO2 fi xati on than Calvin cycle)
CH14 Metabolic Diversity of Microorganisms
14.1 Photosynthesis and Chlorophylls
Organisms that carry out photosynthesis are called phototrophs, which are autotrophs.
Photoautotrophy is comprised of two distinct sets fo reactions that operate in parallel:
1. Light reactions that produce ATP
2. Light-independent dark reactions that produce CO2 to cell material
When O2 is produced as a waste product, the photosynthetic process is called oxygenise
photosynthesis. When there is no O2 produced, it is called anoxygenic photosynthesis.
Chlorophyll and bacteriochlorophyll
Chlorophyll and bacteriochlorophyll are tetrapyrroles that are related to the parent structure of the
cytochromes.
Chlorophyll a is green because it absorbs red and blue light, and transmits green light. Several
structurally distinct chlorophylls are known, each distinguished by its unique absorption spectrum.
Cyanobacteria contain chlorophyll a while their relatives (prochlorophytes) contain chlorophylls a
and b.
Anoxygenic phototrophs produce one or more bacteriochlorophylls.
The existence of different forms of chlorophyll or bacteriochlorophyll that absorb light of different
wavelengths allow phototrophs to make better use of the available energy in the electromagnetic
spectrum.
In oxygenic phototrophs and in purple anoxygenic phototrophs, (bacterio)chlorophyll molecules are
attached to proteins and housed within membranes, forming photocomplexes.
Reaction centers are structures that participate directly in het reactions that lead to energy
conservation.
Antenna pigments are light harvesting (bacterio)chlorophylls that function to absorb light and funnel
some of the energy to the reaction center.
Eukaryotes and prokaryotes
In eukaryotic phototrophs, photosynthesis takes plates in intracellular organelles, the chloroplasts
which contain sheetlike photosynthetic membrane systems called thylakoids (stacked up from
grana). The inner space of the thylakoids is called the lumen, and the outer space is called the
stroma. Chloroplasts are absent from prokaryotic phototrophs.
In purple bacteria, the photosynthetic pigments are integrated in membrane systems (membrane
vesicles called chromatophores and membrane stacks called lamellae).
The ultimate structure for capturing energy is the chlorosome which are present in filamentous
anoxygenic phototrophs, anoxygenic green sulfur bacteria and photosynthetic Acidobacteria.
, 14.2 Carotenoids and Phycobilins
Although (bacterio)chlorophyll is required for photosynthesis, phototrophic organisms contain other
pigments as well, carotenoids and phycobilins.
Carotenoids
Carotenoids are hydrophobic pigments that are firmly embedded in the photosynthetic membrane.
They are typically yellow, red, brown or green and absorb blue light.
Carotenoids function primarily as photoprotective agents. They quench toxic oxygen species by
absorbing much of the harmful light and prevents in this way dangerous photooxidations.
Phycobilins
Phycobiliproteins are the main light-harvesting systems of cyantobacteria and red algae. They
contain bilins bound to proteins.
Phycobiliproteins assemble into aggregates called phycobilisomes that attach to cyanobacterial
thylakoids.
14.3 Anoxygenic Photosynthesis
Photosynthetic reaction centres are complex macromolecular structures which interact with both
antenna pigments and components of the electron transport chain. Photosynthetic pigments funnel
light energy to the reaction center to excite a special pair of (bacterio)chlorophylls, thereby
generating high-potential electrons that can be donated to subsequent electron transport reactions.
There are different types of reaction centers, quinone type (Q-type) and iron-sulfur type (FeS-type).
Quinone type
Purple bacteria use a quinone type (Q-type) reaction center which contains three types of
polypeptides (L, M and H). These polypeptides bind two molecules of bacteriochlorophyll a —>
together called the special pair.
Photosynthetic light reactions begin when light energy absorbed by antenna systems is transferred to
the special pair, which excites the special pair converting it from weak, to a strong electron donor.
ATP is synthesized during photosynthetic electron flow from the activity of ATPase that couples the
proton motive force to ATOP synthesis —> photophosphorylation. In cyclic phosphorylation, there is
no net input or consumption of electrons, they simply travel a circuitous route, returning from
whence they came.
However, for a purple bacteria to grow as a photoautotroph, the formation of ATP is not enough.
Reducing power (NADH) is also necessary to reduce CO2 to cell material.
This requires an electron donor. When electrons are formed, they end up in the ‘quinone pool’’.
However, the E0’ of quinone is insufficiently electronegative to reduce NAD+ to NADH and therefore
electrons from the quinone pool must travel backwards against the electrochemical gradient —>
reverse electron transport. This is driven by the energy of proton motive force.
14.4 Oxygenic Photosynthesis
In contrast to anoxygenic phototrophs wo have either Fes-type or Q-type, oxygenic phototrophs can
have both. Two distinct photosystems; photosystem I or PSI or P700 (FeS-type reaction center) and
photosystem II or PSII or P680 (Q-type reaction center). PSI and PSII interact in the Z-scheme of
photosynthesis.
PSII performs the first step which is the splitting of water into oxygen and electrons.
Oxygenic photosynthesis is a form of noncylic photophosphorylation because electrons do not cycle
back to reduce the oxidised P680 but are used in the reduction of NADP+ instead.
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