photosynthesis Flashcards
structure of the chloroplast
- lens shaped structure surrounded by a double membrane
- has internal membrane system which consists of flattened sacs called thylakoids and intergranal lamella
- fluid within chloroplast: stroma
- stroma contains circular DNA, 70S ribosomes, enzymes and starch grains
- chlorophyll molecules are located on the thylakoid membrane
- ATP synthase complex on thylakoid membrane project into stroma
function of chloroplast
Chloroplasts contain chlorophyll which convert solar energy to chemical energy
through photosynthesis via
1) Site of light-dependent reactions (i.e. cyclic & non-cyclic
photphosphorylation) which occurs in the thylakoid membrane
2) Site of light-independent reactions (i.e. Calvin cycle) which occurs in the stroma
types of photosynthetic pigments and their functions
Chlorophyll a: most abundant photosynthetic pigment – special chl a (chlorophyll P680 and P700) found in reaction centers
Chl b: accesory pigment that absorbs light and channels the light energy to chl a in reaction centers
Carotenoids: accessory pigments, have photoprotective role (absorb and dissipate excess light energy that could damage chlorophyll/bind to reactive oxygen species that can cause oxidative damage to the cell)
the 2 types of photosystems
PS l: contains P680: special chl a in reaction center most effectively absorbs light of wavelength 680nm
PS ll: contains P700: special chl a in reaction center most effectively absorbs light of wavelength 700nm
describe photoactivation
Process
When a photon of light* strikes and is absorbed by a chlorophyll molecule, one of the pigment molecule’s electrons is excited to a higher energy state;
Energy is relayed from pigment to pigment, via resonance transfer of energy, until it reaches one of the two specialized chlorophyll a* (P680/700) in the reaction center*
of photosystem II/I.
Excited electron emitted from chlorophyll a (P680/P700) is captured by the primary electron acceptor*, in the reaction centre
what happens during noncyclic photophosphorylation
(photoactivation at PSll)
photoactivation:
When a photon of light is absorbed by an accessory pigment molecule in the light harvesting complex (LHC) of PS II, one of its electrons is
excited to a higher energy level. As the excited electron drops to its ground state, the energy released is passed on to the next pigment molecule. This
resonance transfer of energy continues until P680, the special chlorophyll a molecule in the reaction centre (RC) is reached.
* When P680 absorbs the energy from the accessory pigments of the light harvesting apparatus, it loses an electron, leaving an electron hole in PSII.
The displaced electron is accepted by a primary electron acceptor (X) in the reaction centre.
*
*
what happens in noncyclic photophosphorylation
(photolysis of water)
The electron hole in PSII is filled by an electron released from the splitting of water in an enzyme-catalysed reaction in the thylakoid space.
During the splitting of water, the H+ released contributes to a high concentration of H+ in the thylakoid space while the O atom combines with another O atom,
forming molecular oxygen (O2) as a by-product
what happens in noncyclic photophosphorylation
(1st ETC from PS ll to PS l)
The electron from the primary e- acceptor (X) is then passed down a series of increasingly electronegative electron carriers (of the 1st ETC)
losing energy during the transfer. The energy lost during this electron flow is used to actively pump H+ from the stroma to the thylakoid space,
generating a proton gradient across the membrane. Chemiosmosis occurs when H+ diffuse down the proton gradient back into the stroma via ATP
synthase, & ADP is phosphorylated to ATP.
what happens in noncyclic photophosphorylation
(light harvesting at PSl)
Meanwhile, PSI loses an electron in a manner similar to PSII. When P700 absorbs the energy from the accessory pigments in the light harvesting
apparatus, it loses an electron, leaving an electron hole in PSI. The displaced electron is accepted by a primary electron acceptor (Y) in the reaction
centre. The electron hole in PSI is filled by the displaced electron from PSII when it reaches the end of the first electron transport chain.
what happens in noncyclic photophosphorylation
(2nd ETC from PS l to NADP+)
The electron from the primary electron acceptor (Y) is then is passed down a series of electron carriers of a 2nd ETC. (Energy is not released during
electron transfer down this 2nd ETC.).
The electron is finally accepted by NADP (the final electron acceptor) which is reduced to NADPH by NADP reductase which is found on the thylakoid membrane.
The ATP & NADPH produced during non-cyclic photophosphorylation will be used in the Calvin cycle.
reduction of NADP+ equation
(NADP+e-+H+→NADPH)
what happens in cyclic photophosphorylation
Photoactivation occurs = displaces excited electron from P700 of PSl = electron hole created
Electron flow is cyclical passing from PS I to middle of 1st ETC, further down the ETC and transported back to primary electron acceptor Y of PSl
Excited electron travels down ETC consisting of electron carriers of increasing electronegativity = energy lost is coupled to pumoing of H+ from stroma to thylakoid space = generate proton gradient = used in formation of ATP via chemiosmosis
NADPH is not produced and the ATP produced is used in the Calvin cycle
how does thylakoid membrane help in photophosphorylation
Provides large surface area to embed many photosynthetic pigments for light absorption
Maintains sequential arrangement of electron carriers of ETC for flow of electrons
Maints proton gradient for ATP synthesis: hydrophobic core of (phospholipid bilayer) thylakoid membrane repels charged H+ ions = impermeable and allows them to accumulate
Allows many ATP synthase to be embedded in the correct orientation = ATP can be produeced as protons flows down the gradient via chemiosmosis
how is proton gradient maintained
by accumulation of H+ from photolysis of water in thylakoid space
Usage of H+ in stroma when NADP is reduced to NADPH by NADP reductase = low [H+] in stroma contributes to steepness of gradient
Lack of permeability of thylakoid membrane
Proton pump that pumps H+ into thylakoid space
describe calvin cycle (carbon fixation)
1 CO2 combines with RuBP (Co2 acceptor) in the presence of RuBisCO to form unstable 6C intermediate
Intermediate breaks down into 2 molecules of glycerate phosphate (3C)
describe calvin cycle (reduction)
GP is reduced to glyceraldehyde-3-phosphate (G3P), requiring ATP and NADPH (1 ATP and 3 NADPH for every 1 GP)
NADPH provides the reducing power for the reaction as it stores high energy electrons
produces ADP and Pi
describe calvin cycle (regeneration of RuBP)
5 molecules of G3P (each 3C) is used to regenerate 3 RuBP (each 5C), driven by 3 ATP from light dependent reaction
enable calvin cycle can continue
use of G3P in synthesis of glucose
2 G3P (3C) is needed to form 1 glucose (6C)
For every 3 CO₂ molecules fixed, the Calvin cycle produces 6 molecules of G3P = 1 G3P molecule exits the cycle to contribute to carbohydrate synthesis.
Since 2 G3P molecules are required to form 1 glucose molecule, the Calvin cycle must run twice (fixing 6 CO₂ molecules in total) to produce the necessary 2 G3P molecules for glucose synthesis.
how much of each reactnat is needed to generate one glucose molecule which is 6C
2 G3P
6 CO2
18 ATP
12 NADPH
how are the products of photophosphorylation used in the calvin cycle
ATP is source of energy and
NADPH is the reducing power for the reduction* of GP to G3P;
ATP is also used in the regeneration of RuBP*;
role of NADP in linking the light dependent reactions to Calvin cycle
NADP is the final electron acceptor* at the end of the non-cyclic light dependent
reaction, which is then reduced to NADPH.
2. Reduced NADP/NADPH is used to drive the reduction of glycerate phosphate (GP) to
glyceraldehyde-3-phosphate (G3P), with the use of ATP in the Calvin cycle
limiting factors of p/s
light intensity
co2 concentration
temperature
pH
