Photosynthesis 1 Flashcards
The green algae and plants and prokaryotic cyanobacteria use a ?
Water-cleaving P/S reaction and evolve O2 (oxygenic P/S ), The reaction is driven by light energy (hv), water serves as the reductant (e−donor)
Many prokaryotes perform?
Anoxygenic P/S using e− donors other than H2O, such as H2S in the purple sulphur bacteria
Photosynthetic organisms - divided into ?
Oxygenic and anoxygenic types:
- Oxygenic evolve O2 use H2O as their hydrogen & e- donor.
- Anoxygenic (usually anaerobic bacteria) do not evolve O2 and use a variety of hydrogen donors ( not H2O but e.g. H2S )
Explain in depth Prochlorococcus ?
- It’s the smallest and most abundant photosynthetic cell on Earth!
- Prochlorococcus(Pro) cyanobacteria are the most abundant phytoplankton in the ocean
- An estimated 3 x 1027of them in the global oceans
- Constitute half of the chlorophyll over vast ocean ecosystems
contribute almost half of the yearly global net primary production
Difference between mitochondria and chloroplast ?
- Compared to a mitochondrion, the chloroplast has an extra compartment - the thylakoid space (lumen).
- All the thylakoid lumens are connected thus giving a continuous third compartment separated from the stroma.
- The thylakoid lumen is bounded by the thylakoid membrane, which houses the two photosystems with their traps, the electron transport chain with its components and the ATP synthase.
- Stroma - gel-like matrix with various inclusions e.g. ions, ribosomes, starch, lipid droplets, ATP, NADP, (C3) cycle (Calvin cycle) with the enzymes for CO2 fixation
Chloroplasts have three distinct membranes:
- the outer membrane
- the inner membrane
- the thylakoid membrane
- Outer double membrane is the envelope (with various carriers (transporters) present and enclosing the intermembrane space
What is the purpose of Thylakoid membrane ?
Light absorption by chlorophylls a & b, electron transport, O2 and the synthesis of ATP and NADPH
All cytochromes contain ?
Fe bound to four N atoms in a haem ring and to N or S ligands from the protein. There are four types of haem group: a, b, c and d - differing in the side chains attached to the haem ring.
Chloroplasts have only ?
b- and c- type cytochromes
The haem gives a ?
Bright red colour with clear absorption bands. The position of the highest wavelength band (α - band) distinguishes the cytochromes
Iron (Fe) in a haem group can serve as ?
An electron acceptor
What are three major groups of photosynthetic pigments?
- Chlorophylls - chl a (primary) and chl b (accessory)
- Carotenoids - carotenes and xanthophylls
- Phycobilins - water soluble pigments
Explain the pigments of the photosystems in more detail ?
- Chlorophylls (chl) absorb ~ 400 - 700 nm, present in antennae systems, feed their absorbed energy to the dimer traps
(reaction centres) designated as P680 or as P700 - Carotenoids - absorb ~ 400 - 500 nm, also known as accessory pigments, can feed (30 - 40 % efficiency) their absorbed energy to the reaction centres but also have a protective function in dissipating the energy from excited chlorophyll molecules (preventing photoxidation)
- Phycobilins - absorb ~ 500 - 700 nm (not present in green plants but common in aquatic systems). [water absorbs the blue wavelengths of light ]. Consist of an open chain of four pyrrole rings (tetrapyrrole). Note chlorophylls are also composed of four pyrroles, but arranged as a large heterocyclic porphyrin ring with a metal atom (Mg) at their centre rather than as an open chain like the phycobilins
Explain light absorption in a chlorophyll molecule?
- The ability to harvest light energy stems from the e- in a chlorophyll molecule being distributed in a decentralised cloud around the light-absorbing porphyrin ring (blue) + Mg
- When light of an appropriate wavelength hits a chlorophyll molecule it excites e- in this diffuse network disturbing the way they are distributed
- This state is unstable and the excited chlorophyll molecule seeks to get rid of this excess energy and so it returns to a more stable ground state
Absorption of Oxygenic and Anoxygenic ?
Oxygenic organisms use visible light of ~400 -700 nm but anoxygenic can use less energetic (near infrared) i.e. > 700 nm
Not all colours of light have equal energy. With blue light absorption, the chlorophyll molecule ?
Gains a higher excited state than that with red light
The catalytic centre for water-splitting is ?
A Mn4CaO5-cluster organized as an asymmetric, distorted chair form embedded within the protein matrix of PSII
The water-splitting reaction requires ?
Manganese and the associated cofactors
Manganese deficiency in chloroplasts specifically affects ?
O2 evolution.
Manganese atoms appear to undergo successive oxidations to yield a strongly oxidising complex in the S4 state that is capable of water oxidation.
Extraction of four electrons (H+) and one molecule of molecular oxygen (O2) requires ?
2 H2O and 4 molecules of photooxidised chlorophyll a+
The uptake of one electron by plastoquinone results in?
The radical semiplastoquinone, an additional electron and two protons result in PLQol
Explain how Electron and proton flow through the cytochromeb6f complex?
- Plastoquinol (PQH2), formed in PSII, is oxidized by the cytochromeb6fcomplex.
- One electron from PQH2 passes to the Fe-S center of the Rieske protein, the other to hemebLof cytochromeb6.
- The net effect is passage of electrons from PQH2 to the soluble protein plastocyanin, which carries them to PSI.
- Electrons are funneled to plastocyanin, a single electron carrier, which carries the electron to PSI.
Explain the reaction centre of PSI receiving e- from plastocyanin ?
- The photon energy is used to excite the reaction centre P700 of photosystem I (PSI).
- P700 loses electron to the iron-sulfur protein ferredoxin.
- The photo-oxidized P700+ is a powerful oxidizing agent.
- P700+ receives electrons e- from plastocyanin.
