Photosynthesis in Higher pLants 2 Flashcards
describe cyclic electron transport
(i) In photosystem II the reaction centre chlorophyll a absorbs 680 nm
wavelength of red light causing electrons to become excited and jump into an orbit farther from the atomic nucleus. These electrons are picked up by an electron acceptor which passes them to an electrons transport system consisting of cytochromes (Figure
(ii) This movement of electrons is downhill, in terms of an oxidation-reduction or redox potential scale. The electrons are not used up
as they pass through the electron transport
chain, but are passed on to the pigments of
photosystem PS I.
(iii) Simultaneously, electrons in the reaction centre of PS I are also excited when they receive red light of wavelength 700 nm and are transferred to another accepter molecule that has a greater redox potential. These electrons then are moved downhill again,
this time to a molecule of energy-rich NADP+
The addition of these electrons reduces NADP+
to NADPH + H+
.
what is the z scheme
This whole scheme of transfer
of electrons, starting from the PS II, uphill to
the acceptor, down the electron transport chain
to PS I, excitation of electrons, transfer to
another acceptor, and finally down hill to NADP+
reducing it to NADPH +
H+
is called the Z scheme, due to its characterstic shape (Figure 11.5).
This shape is formed when all the carriers are placed in a sequence on a
redox potential scale.
how does psII ensure continuous supply of water
The
electrons that were moved from photosystem II must be replaced. This is
achieved by electrons available due to splitting of water. The splitting of
water is associated with the PS II; water is split into 2H+
, [O] and electrons.
This creates oxygen, one of the net products of photosynthesis. The
electrons needed to re We need to emphasise here that the water splitting complex is associated
with the PS II, which itself is physically located on the inner side of the
membrane of the thylakoid.
define phosphorylation
The process through which ATP is synthesised by cells (in mitochondria and
chloroplasts) is named phosphorylation. Photophosphorylation is the synthesis of ATP from
ADP and inorganic phosphate in the presence of
light.
explain non cylic photophosphorylation
-, when light falls on PSI, the electrons get excited into a higher orbit which is then received by an electron accepter from where it is transferred to ETS
- The electrons travels through ETS and comes back to PSI and hence it returns to ground state.
- During transfer of electron, ATP is made by joining ADP+iP. NADPH is not made because nadp reductase absent.
differentiate cyclic and non cyclis phosphotrylation
Cyclic
-PSI only
- Done in stromal lamellae onlae
- No photolysis of water/ no evolution of oxygen
- ATP
- requires wavelength of 680nm and more
Non-CYclic
- PSI and PSII only
- photolysis of water and evolution of oxygen present
- Thylakoids only
- ATP and NADPH+H2
- requires wavelengths of 680-700nm
what is chemiosmotic theory
ATP synthesis is
linked to development of a proton gradient across a membrane. This time
these are the membranes of thylakoid. There is one difference though, here
the proton accumulation is towards the inside of the membrane, i.e., in the
lumen.
explain how the H+ gradient is maintained
- when light falls on PSII, it emits e-s which pass through a series of components in ETS, until it reaches PSI. PSII enters an excited state. It requires e-s to return to gorund state. Photolysis of water in water splitting complex causes e-s and protons to be released. These e-c are used to PSII but protons are accumulated in lumen
-As electrons move through the photosystems, protons are transported
across the membrane. This happens because the primary accepter of
electron which is located towards the outer side of the membrane
transfers its electron not to an electron carrier but to an H carrier.Hence, this molecule removes a proton from the stroma while
transporting an electron. When this molecule passes on its electron
to the electron carrier on the inner side of the membrane, the proton
is released into the inner side or the lumen side of the membrane.
-when PSI gets excited it emits electrons which eventually reaches the NADP Reductase enzyme n the stroma side of the
membrane. Along with electrons that come from the acceptor of
electrons of PS I, protons are necessary for the reduction of NADP+
to
NADPH+ H+
. These protons are also removed from the stroma.
within the chloroplast, protons in the stroma decrease in
number, while in the lumen there is accumulation of protons. This creates
a proton gradient across the thylakoid membrane as well as a measurable
decrease in pH in the lumen.
why is the proto gradient important? what isit used for?
This gradient is
important because it is the breakdown of this gradient that leads to the
synthesis of ATP. The gradient is broken down due to the movement of
protons across the membrane to the stroma through the transmembrane channel of the CF0
of the ATP synthase.
explain the working of ATP Synthase molecule
The ATP synthase enzyme consists of two parts: one called the CF0 is embedded in the thylakoid membrane and forms a transmembrane channel that carries out facilitated diffusion
of protons across the membrane.
The other portion is called CF1 and protrudes on the outer surface of the thylakoid membrane on the side that faces the stroma.
The break down of the gradient provides enough
energy to cause a conformational change in the CF1 particle of the ATP
synthase, which makes the enzyme synthesise several molecules of energypacked ATP.
what are the conditions required for maintainence of h+ gradient
Chemiosmosis requires a membrane, a proton pump, a proton
gradient and ATP synthase. Energy is used to pump protons across a
membrane, to create a gradient or a high concentration of protons within
the thylakoid lumen. ATP synthase has a channel that allows diffusion of
protons back across the membrane; this releases enough energy to activate
ATP synthase enzyme that catalyses the formation of ATP.
