Photosynthesis

Frage	Antworten
Overall equation for photosynthesis LOOK AT DIAGRAM P18	6CO2+6H2O---> C6H12O6+ 6CO2 2 photosynthesis stages- light dependent/ light independent stage Light dependent- converts light energy to chemical- photolysis of water- releases electrons and protons- energy carried by electrons creates proton gradient across thylakoid membrane- energy used for photophosphorylation of ADP-endergonic reaction bonding phosphate ion to a molecule of ADP, using light energy, making ATP- protons and electrons reduce NADP- light independent- ATP and reduced NADP reduce CO2- produce energy containing glucose
Chloroplasts and light trapping	Photosynthesis happens in chloroplasts- surrounded by double membrane- folded inward to form thylakoid lamellae- combine in stacks- granum- groups grana- in grana is most of photosynthetic pigments for light dependent stage Stroma= fluid filled interior- bathes thylakoids/ grana- where light independent reactions occur If starch grains were present- would appear white as stain for electron micrographs binds to lipids not carbs
Where is chlorophyll found	only in parts that are exposed to light- leaves, petioles and stem- leaf= main organ of photosynthesis- chloroplasts found in palisade mesophyll but also in spongy mesophyll- only epidermal cells containing chloroplasts= guard cells
Structural features and significance for photosynthesis- leaf	Large SA- Capture as much light as possible Thin- light penetrates right through leaf Stomatal pores- allow CO2 to diffuse into leaf Air spaces in spongy mesophyll- Allow CO2 to diffuse into photosynthesising cells Spaces between palisade cells- Allow CO2 to diffuse to photosynthesising cells
Structural features and significance for photosynthesis- cells	Cuticle and epidermis are transparent/ cellulose cell walls are thin= light penetrates straight through the mesophyll palisade cells have a large vacuole- chloroplasts form a single layer at periphery (edge) of each cell- so do not shade each other Palisade cells are cylindrical, elongated at right angles to the surface of the leaf-leaves can accomodate large no. palisade cells- light only passes through 2 epidermal cells and 1 palisade cell to reach chloroplast- if cells were stacked horizontally light would have to pass through many cell walls- won't reach chloroplast
Structural significance- chloroplasts	Large SA- max light absorption Chloroplasts move within palisade cells- move toward top on dull days- for max light absorption- if light intensity = very high moves to bottom-prevent pigment bleaching Chloroplasts rtate in palisade cells/ Pigments in thylakoids= in single layer at surface of thylakoid membrane-max absoption 5x as many chloroplasts in palisade than spongy mesophyll cells- palisade= top of leaf- exposed to more light- so max light absorption
Diagram	p20
Chloroplasts as transducers	bio transducers- more efficient than artificial- waste less energy- chloroplast= transducer- turns energy from photons of light to chemical energy- made available through ATP- incorporated into molecules eg glucose
Englemann's exp	Used filamentous green algae- determined which wavelengths of light were most effective- put algae in suspension of evenly distributed, motile aerobic bacteria - exposed to range of wavelengths- blue and red light- bacteria clustered near chloroplasts- deduced wavelengths resulted in high photosynthesis- where there was most O2- bacteria flocked to
Photosynthetic pigments	Pigment= molecule that absorbs specific wavelengths of light- different photosynthetic pigments trapping different wavelengths- large range absorbed- more useful- 2 main classes of pigments= transducers- chlorophylls/ carotenoids
Chlorophylls	chlorophyll a- peak wavelengths- 435,670-680= blue-green- in all (mosses, ferns, conifers, flowering plants) chlorophyll b- peak wavelength- 480, 650- yellow- green - higher plants (conifers, flowering plants)
Carotenoids	B (beta)- carotene- 425-489- orange- all xanthophylls- 400- 500- yellow- most
Absorption and action spectra	Absorption spectrum- graph showing how much light is absorbed at different wavelengths Action spectrum- graph showing rate of photosynthesis at different wavelengths different pigments absorb different wavelengths- seen by making seperate solutions of pigment- shining light through- chlorophylls a/b - absorb mainly at red/ blue/violet- reflect green- gives leaves colour carotenoids- absorb blue/ green, reflect longer wavelengths- leaves appear orange
action spectrum more	shows rate of photosynthesis at different wavelengths measured by mass of carbs synthesised by plants exposed at different wavelengths If action spectrum is superimposed on absorption spectrum- close correlation can be seen- suggests pigments responsible for absorbing light= used in photosynthesis
ton of diagrams	p22
What's in photosystems P1	Antenna complex- contains photosynthetic pigments- chlorophylls/ carotenoids anchored in phospholipids in thylakoid membrane- held together by clusters of protein molecules= antenna complex- combo of pigments- range of wavelengths absorbed
What's in photosystems P2	Reaction centre- within antenna complex- contains 2 molecules of primary pigment chlorophyll a - when molecules absorb light- excitation causes both to release 1 electron- 2 types of reaction centre- Photosystem 1- arranged around 2 chlorophyll molecules - absorption peak of 700nm Photosystem 2- named last but comes first- arranged around 2 chlorophyll molecules absorption peak of 680nm
What's in photosystems P3	Some photons absorbed by chlorophyll directly- but many absorbed first by accessory (antenna) pigments- photons excite these pigments- energy passed on to reaction centre- electrons of chlorophyll get excited/ raised to higher energy level-chlorophyll a= most significant molecule of reaction centre- passes energy to subsequent reactions f photosynthesis
2 stages of photosynthesis	Photosynthesis= sequence of reactions- takes place on thylakoid membranes - ATP provides chemical energy transduced from light energy-synthesize energy rich molecules eg glucose- reduced NADP- uses reducing power to synthesize molecules eg glucose from CO2 O2= by- product- derived from water- diffuses out of chloroplast, out of photosynthetic cells and out of leaf via stomata Reactions using ATP and reduced NADP occurs in stroma solution- doesn't require light
Light dependent stage- photophosphorylation	Photosynthesis has 2 pathways of photophosphorylation- non cyclic- includes both PSI and PSII- in linear direction- cyclic= includes only PSI- in cycle
Cyclic photophosphorylation	Light is absorbed by photosystem I -passed to the photosystem I primary pigment (P700) An electron in the chlorophyll a is excited to a higher energy level and is emitted in a process known as photoactivation This excited electron is captured by an electron acceptor, transported via a chain of electron carriers known as an electron transport chain before being passed back to the chlorophyll molecule in photosystem I (hence: cyclic) As electrons pass through the electron transport chain they provide energy to transport protons (H+) from the stroma to the thylakoid lumen via a proton pump A build-up of protons in the thylakoid lumen can then be used to drive the synthesis of ATP from ADP and an inorganic phosphate group (Pi) by the process of chemiosmosis The ATP then passes to the light-independent reactions
Non cyclic	Light is absorbed by photosystem II (located in the thylakoid membrane) and passed to the photosystem II primary pigment (P680) An electron in the primary pigment molecule (ie. the chlorophyll molecule) is excited to a higher energy level and is emitted from the chlorophyll molecule in a process known as photoactivation This excited electron is passed down a chain of electron carriers known as an electron transport chain, before being passed on to photosystem I During this process to ATP is synthesised from ADP and an inorganic phosphate group (Pi) by the process of chemiosmosis The ATP then passes to the light-independent reactions Photosystem II contains a water-splitting enzyme called the oxygen-evolving complex which catalyses the breakdown (photolysis) of water by light: H2O → 2H+ + 2e- + ½O2
Non cyclic p2- look at diagram p24	H2O → 2H+ + 2e- + ½O2 As the excited electrons leave the primary pigment of photosystem II and are passed on to photosystem I, they are replaced by electrons from the photolysis of water Photosystem I At the same time as photoactivation of electrons in photosystem II, electrons in photosystem I also undergo photoactivation The excited electrons from photosystem I also pass along an electron transport chain These electrons combine with hydrogen ions (produced by the photolysis of water) and the carrier molecule NADP to give reduced NADP: 2H+ + 2e- + NADP → reduced NADP The reduced NADP (NADPH) then passes to the light-independent reactions to be used in the synthesis of carbohydrate
Photolysis of water	In thylakoid spaces- water molecules absorb light- indirectly causes them to dissociate to hydrogen, O2 and electrons- splitting of water by light= photolysis- enhanced by protein complex in PSII- only known enzyme to oxidise water- electrons produced replace those lost by PSII, protons from water and electrons from PSI reduce NADP- O2 diffuses out of chloroplast into intercellular airspaces then out via stomata as waste product
Passage of protons and photophosphorylation	electrons pass through proton pump of thylakoid membrane- H+ ions join protons from water accumulate- builds electrochemical gradient between thylakoid membrane and stroma-chemiosmosis happens- down ATP synthase back to stroma- releases energy- phosphorylate ADP Once in stroma H+ ions passed to oxidising NADP- reducing it NADP+2H+ + e-----> reduced NADP
Light dependent- equation and comparison	Electron flow- cyclic, non cyclic chlorophyll at reaction centre- P700, P680 ATP production- yes, yes Photolysis of water- no, yes O2 production- no, yes NADP production- no, yes Occurrence- all photosynthetic organisms, plants/ algae/cyanobacteria
Light independent stage of photosynthesis	Occurs in solution in stroma of chloroplast- involves many reactions - each catalysed by a different enzyme- used products of light dependent- ATP= source of energy- reduced NADP= source of reducing power- reduces CO2
Light independent- calvins cycle	5 carbon acceptor molecule- ribulose biphosphate (RUBP)- combines w/ CO2- catalysed by ribose biphosphate carboxylase (rubisco) - unstable 6 carbon compound formed- immediately splits into 2 glycerate 3 phosphate molecules- -
Light independent p2	glycerate 3 phosphate reduced to triose phosphate by reduced NADP- requires energy provided by ATP from light dependent- triose phosphate= first carb made in photosynthesis and NADP= reformed- some triose phosphate converted to glucose phosphate then to starch by condensation- most of triose phosphate- goes through series of reactions- regenerates RuBP - cycle continues- ATP from light dependent stage used for this
Product synthesis	Photosynthetic molecules can make all the molecules they need- like animals ned fats, carbs and proteins- can be made by 3C carbons produced by carbon cycle
Product= carbs	Carbs- first hexose made from fructose bisphosphate- can be converted to glucose and combined w/ fructose to make sucrose for transport around plant- a glucose molecules may be converted to starch for storage or to B glucose which is polymerised into cellulose for cell walls
Product= fats-	acetyl co-enzyme A can be synthesized from glycerate 3 phosphate- made in calvin cycle - converted to fatty acids- triose phosphate can be converted directly to glycerol- fatty acids and glycerol undergo condensation reactions to form triglycerides
Product- proteins	glycerate 3 phosphate can also be converted into amino acids for protein synthesis - amino group derived from NH4+ ions made from nitrate ions- (NO3-)- taken in at roots- transported through plant
Limiting factors in photosynthesis	Plants need suitable environment to be efficient at photosynthesis: Reactants= CO2, water Light at high enough intensity and suitable wavelengths Suitable temp If any is lacking= no photosynthesis- each factor has optimum- rate of photosynthesis is highest- if any is sub-optimal- rate= reduced- limiting factor- if factor increases- rate increases
CO2 concentration	CO2 conc increases from 0- rate of light independent reactions and therefore photosynthesis increases= limiting factor- yet above 0.5% rate of photosynthesis remains constant- CO2 not affecting rate- no longer limiting- rate decreases above 1%- stomata close- prevent CO2 entering- rate of slowest reaction in a sequence determines overall rate of process (rate limiting step)- in light independent- reaction catalysed by rubisco= step
CO2 conc p2	CO2= usually a limiting factor for terrestrial plants- 0.04% CO2 in air- crop plants most efficient at around 0.1%, Aquatic plants and algae get CO2 from hydrogen carbonate ion HCO3- , some algae can increase their concs with carbonic anhydrase- so it's not a limiting factor- optimum conc for algae is approx 0.1 mol dm-3
Light intensity	Light intensity= controls rate of photosynthesis - if plant= in darkness- light independent can still happen but light dependent can't- no oxygen produced LIght intensity increases- light dependent= more efficient- overall rate of photosynthesis increases- light intensity controlling rate= limiting factor 10,000 lux light dependent at max rate- increasing intensity doesn't produce faster reactions- not limiting- even higher intensity- photosynthetic pigments= damaged- light dependent fails
Sun and shade plants	Plants have different adaptations for different light intensities- sun plants most efficient at photosynthesis for photosynthesis- shade plants most efficient at low light intensities
Light compensation point	The light intensity at which plant has no net gas exchange- volume of gases used and produced in respiration and photosynthesis are equal-photosynthesis can be assessed on CO2 uptake- rate of light dependent and consequently independent decreases- rate of CO2 uptake decreases- reaches point respiration provides all CO2 for photosynthesis and photosynthesis provides all O2 for respiration- no net gas exchange- LCP- occurs at lower light intensity for shade plants
Temp/ water	Increased temp- increased photosynthesis rate- kinetic energy of molecules increases in reactions- Above certain temp (optimum) enzymes progressively denature- rate decreases- temp= limiting Scarce water- plant cells plasmolyse- stomata close- wilting happens - experiments w/ water deficient plants- slight water deprivation can reduce carbs made= limiting but as so many systems affected hard to see its affect on photosynthesis alone
Limiting factors can combine	Sometimes plant has optimal values for all environmental factors- then photosynthesis will occur efficiently and grow well- but if plant has several of these factors limiting eg temp/ light intensity= low- photosynthesis= reduced grow slowly - rate is controlled by factor closest to its minimum value
Minerals nutrition	Inorganic nutrients needed may be limiting factors for metabolism- have various roles: Structural eg calcium in middle lamella walls Synthesis of compounds needed for plant growth- eg enzyme activators such as magnesium required by ATPase and DNA polymerase May form an integral part of magnesium in chlorophyll iron in carriers of electron transport chain and manganese in photosystem II Macronutrients include nitrogen, potassium, sodium, magnesium, calcium, nitrate and phosphate- required in greater quantities than micronutrients eg maganese and copper.
Nitrogen	Most of nitrogen in soil is in humus, in organic molecules of decaying organisms- inorganic nitrogen occurs as ammonium ions NH4+ or nitrate ions NO3- nitrogen is largely taken up by roots as nitrates but rhizobium in root nodules delivers ammonium ions to the plant- ions transported to the xylem- delivered to cells- ions are transported in the xylem and delivered to cells - nitrogen converted to ammonium ions which becomes amino( -NH2) groups of amino acids- amino acids= transported into phloem used for synthesis of proteins, chlorophylls and nucleotides
Because of nitrogens role in protein and nucleic acid synthesis...	symptoms of nitrogen deficiency causes reduced growth in whole of plant- nitrogen is also a component of chlorophyll and so deficiency causes chlorosis- yellowing of leaves due to inadequate chlorophyll production- chlorosis first appears in older leaves
Magnesium	absorbed as Mg 2+ - transported in xylem - required by all tissues but especially leaves- required by all tissues but especially leaves- Mg forms part of chlorophyll molecule so main symptom of Mg deficiency= chlorosis- begins in veins of older leaves as existing Mg in plant is mobilised and transported to newly formed leaves- Mg ions are also activators such as for ATPase

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