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PHOTOSYNTHESIS - Coggle Diagram
PHOTOSYNTHESIS
REQUIREMENT OF PHOTOSYNTHESIS
- chlorophyll pigments
- CO2
- water
- optimum temperature
- light
- CHLOROPHYLL
- thylakoid in chloroplast have several pigment that different pigment absorb light of different wavelength
- chlorophyll is the main pigment of photosynthesis which absorb light primarily in blue and red region of visible spectrum
- chlorophyll a is the most abundant pigment in plant and it absorb ligh mainly in the blue-violet region
- chlorophyll b absorb light about 453nm and 642nm an helps to increase the range of light a plant can utilize for photosynthsis
- carotenoid absorb light maximally in the vlue-violet region
- its acts as accessory pigment which pass the light energy to chlorophyll a in the reaction centre.
- it also protect the chlorophyll from oxidation
MECHANISM OF PHOTOSYNTHESIS
a. the light dependent reaction
- require light energy & its biochemical in nature
b. light independent reaction
- consist of series enzymatic biochemical reaction that involve the bonding CO2 to complex organic compound
A. LIGHT DEPENDENT REACTION
- occur in the thylakoid of chloroplast which contain complex photosynthesis pigments
- involve process
i. photoactivation
ii. photolysis of water
iii. photophosphorylation (produce ATP & NADPH)
i. PHOTOACTIVATION
- when molecule of cholorphyll absorb a photon, one of the molecule’s electron elevated to an orbital where it has more potential energy and the pigment molecule is said to be in excited state )photoactivation process)
- the energy of an absorbed photon is converted into potential energy of electron raised from the ground state to the excited state
- chlorophyll and accessory pigment are grouped into 2 photosystem :
a. Photosystem I (PSI)
- have a reaction centre chlorophyll, P700 centre that has absorption peak at 700nm
b. Photosystem II (PSII)
- have reaction centre at peak 680nm
- the 2 photosystem work together to use the light energy to generate ATP and NADPH
- the photosystem embedded in the thylakoid membranes of chloroplast and contain a complex of 200-300 pigment molecules
ii. PHOTOLYSIS OF WATER
- a process which water is split during the light reaction of photosynthesis
- occur in the space of the thylakoid & catalysed by a water splitting enzyme
- enzyme passes the electron from water to the reaction centre of PS II & forming O2 which is liberated
H2O >>. O2 + H+. + 2e-
- H+ combine with NADP+ >> NADPH + H+
iii. PHOTOPHOSPHORYLATION
- process of generating ATP from ADP + Pi during the light reactions of photosynthesis
- light reaction consist of :
a. non-cyclic photophosphorylation
b. cyclic photophosphorylation
a. NON-CYCLIC PHOTOPHOSPHORYLATION
- involve 2 PS ( PSI & PSII )
- light energy absorbed by antenna pigments of PSII
- energy transferred from one antenna molecule to another molecule then transferred to the reaction centre in P680 in PSII
- P680 become photoexcited & high energy is released and capture by the primary e- accepto
- this create electron deficiency in PSII and each photoexcited e- passes from the primary e- acceptor of PSII to PSI via e- transport chain. this chain is very similar to the one that functions in cellular respiration
- electrons from the primary electron acceptor then transferred through plastoquinone (Pq) & cytochrome complex
- the e- then passed to plastocyanin (Pc) & then to PSI
- the energy released in the passage from Pq to cytochrome is used by he cell to make ATP from ADP + Pi
- when e- reach the bottom of the e- transport chain it fills an e- hole in P700 (the chlorophyll a molecule in the reaction centre of PSI)
- this replace the e- that light energy drives from chlorophyll to the primary e- acceptor of PSI
- primary e- acceptor of PSI passes the photoexited e- to ferredoxin (Fd)
- an enzyme called NADP+ reductase then transfer the e- from Fd to NADP+.This is the redox reaction that store high energy e- in NADPH, the molecule that will provide reducing power for the synthesis of sugar in the Calvin cycle
H+. + e- + NADP+ >> NAPH +. H+
- the H+ ions formed in water splitting is used in formation of NADPH
b. CYCLIC PHOSPHORYLATION
- under certain condition, photoexcited e- take alternative path called cyclic phosphorylation which use PSI not PSII
- light energy absorbed by antenna molecules is transferred to P700 in PSI
- electron from P700 is photoexcited & passed to primary e- acceptor
- primary e- acceptor passes e- to ferredoxin (Fd)
- in cyclic phosphorylation, alernative route is used which e- passed from Fd to cytochrome complex
- energy is released during e- flow & this used in chemiosmosis to generate more ATP
- the e then transferred to plastocyanin (Pc) & passed back to PSI
b. LIGHT INDEPENDENT REACTION (CALVIN CYCLE)
- consist of 4 main stages :
a. carbon dioxide fixation
b. reduction phase
c. regeneration of CO2 acceptor
d. product synthesis phase
a. CARBON DIOXIDE FIXATION
- CO2 from the atmosphere diffuse through the stomata into the intercellular spaces of the leaf. then it diffuse into the stroma in the chloroplast of the palisade & spongy mesophyll cells
- a five carbon acceptor , ribulose biphosphate (RuBP) combines with molecule of CO2 to form an unstable siz-carbon sugar
- the process is catalysed by the enzyme RuBP carboxylase
- the six-carbon compound immediately splits into 2 molecules of glycerate-3-phosphate (PGA) (3- carbon compound)
b. REDUCTION PHASE
- glycerate 3-phosphate receive an additional phosphate group from ATP to become glycerate 1,3-diphosphate
- glycerate 1,3-diphosphate combines with hydrogen atoms from reduces dinucleotide phosphate (NADPH + H+) & is converted into 3-phosphoglycerate (PGAL triose phosphate)
c. REGENERATION OF CO2 ACCEPTOR
- some of PGAL molecules are rearranged in a series of complex reactions to regenerate ribulose biphosphte (this process requires ATP)
d. PRODUCT SYNTHESIS PHASE
- the rest of glyceraldehyde-3-phosphate is used to assimilate organic molecules such as glucose, amino acids , proteins & lipids
PHOTORESPIRATION
- RuBP carboxylase (Rubisco) the enzyme that catalysed the carboxylation of ribulose biphosphate can also catalyse the oxidation of RuBP by molecular O2
- CO2 & O2 are alternative substrate that compete with each other for the same active sites on the enzyme
-when concentration of CO2 is high, O2 low, the CO2 occupies the active sites, carboxylation is favoured & carbohydrates synthesis by the Calvin cycle proceed
- but when concentration of CO2 is low & O2 high, the site is occupied by O2, oxidation is favoured or photorespiration occur
- this occur during hot dry days which cause water stress in plant
- as result of water stress, the plant close their stomata ( to conserve water)
- once stomata close, O2 produced during photosynthesis is accumulate in the chloroplast
- so Rubisco bind RuBP to O2 instead of CO2
O2 + RuBP >> Phosphoglycolate + glycerate-3-phosphate
- the 2 phosphoglycolate molecules then undergo a series of reaction requiring O2 & ATP to produce one molecule of glycerate-3-phosphate & then release
= photorespiration is wasteful because organic carbon (phosphoglycolate) is converted into CO2 with no net production of ATP or other energy rich metabolites
= photorespiraion can reduce the potential photosynthetic yield from between 30-40%
= this degradation process is called photorespiration because : i. occur during daylight
ii. require O2 (like aerobic respiration)
iii. produce CO2 & H2O (like aerobic respiration)
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C3, C4 AND CAM PLANTS
C4 PLANTSeg : maize, sugar cane, sorghum, sunflower
- they have 2 distinct type of photosynthetic cells :
a. bundle sheath cells
- arrange into tightly packed sheets around the veins of the leaf
b. mesophyll cells
- arrange more loosely between the bundle sheets & the leaf surface
= know that the arrangement of the vascular bundle called as Krantz’ anatomy
- C4 plant are not found in hot climates (tropical)
- under hot conditions, C4 plant attains higher photosynthetic rate compared to C3 plants
C4 PATHWAY
- CO2 in atmosphere diffuse into the mesophyl cell of C4 plant where they will combine with PEP (phosphoenolpyruvate) (3C) to produce oxaloacetate (4C)
- this process catalysed by the enzyme Pepco (phosphoenol-pyruvate carboxylase) which has higher affinity for CO2 at low CO2 concentration
- oxaloacetate is reduce to malate (4C). Malate is shunted through plasmodesmata into the bundle sheath cells
- Malate then oxidised to pyruvate (3C) by the removal of hydrogen & CO2
- increase the concentration of CO2 in bundle cells so the CO2 undergoes fixation as in C3 pathway
- high CO2 concentration in bundle sheath cells inhibit the photorespiration & as result production of carbohydrate increase
- pyruvate diffuse back into mesohphyll cells & phophorylated to regenerate PEP (phosphoenolpyruvate)
CAM PLANTS
= Crassulcean Acid Metabolismeg : succulent plant like cacti & pineapples
- this succulent plant avoid water loss in their hot environment by closing their stomata during the day & opening them at night
- CAM only contain mesophyll cells & no Krantz’ anatomy
CAM PATHWAY
- it is the same as C4 but carbon fixation take place at night when stomata are open
- PEP (phosphoenolpyruvate) (3C) combines with CO2 to form oxaloacetate (4C)
- malate stored in cell vacuole at night to prevent pH changes in cytoplasm
- during daytime, malate oxidised producing pyruvate & CO2
- concentration of CO2 increase in mesophyll cells * photorespiration is prevented
- CO2 is used in Calvin Cycle producing organic molecules (glucose)