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
