PhotosynthesisPhotosynthesis

Photosynthesis as an Photosynthesis as an energy transfer processenergy transfer process

ALBIO9700/2006JK

• Photosynthesis transfers light energy Photosynthesis transfers light energy into chemical potential energy of into chemical potential energy of organic moleculesorganic molecules

• This energy can then be released for This energy can then be released for work in respirationwork in respiration

• PhotoautotrophsPhotoautotrophs – green plants, the – green plants, the photosynthetic prokaryotes and both photosynthetic prokaryotes and both single-celled and many-celled single-celled and many-celled protoctists (including the green, red protoctists (including the green, red and brown algae)and brown algae)

• ChemoautotrophsChemoautotrophs – nitrifying – nitrifying bacteria (obtain their energy from bacteria (obtain their energy from oxidising ammonia to nitrite, or nitrite oxidising ammonia to nitrite, or nitrite to nitrate)to nitrate)

ALBIO9700/2006JK

ALBIO9700/2006JK

• Photosynthesis is the trapping (fixation) of COPhotosynthesis is the trapping (fixation) of CO22 and its and its subsequent reduction to carbohydrate, using H from subsequent reduction to carbohydrate, using H from HH22OO

• Overall equation for photosynthesis in green plants is:Overall equation for photosynthesis in green plants is: light energylight energy

nnCOCO22 + + nnHH22O (CHO (CH22O)O)nn + + nnOO22

chlorophyllchlorophyll

• Hexose sugars and starch are commonly formed:Hexose sugars and starch are commonly formed: light energylight energy

6CO6CO22 + 6H + 6H22O CO C66HH1212OO66 + 6O + 6O22

chlorophyllchlorophyll

Outline of the processOutline of the process

ALBIO9700/2006JK

• 2 sets of reactions involved:2 sets of reactions involved:– Light-dependent reactionsLight-dependent reactions (light (light

energy necessary)energy necessary)• Only takes place in the presence of suitable Only takes place in the presence of suitable

pigments which absorb certain wavelengths of pigments which absorb certain wavelengths of lightlight

• Light energy is necessary:Light energy is necessary:– for the splitting of water into hydrogen and oxygenfor the splitting of water into hydrogen and oxygen– to provide chemical energy (ATP) for the reduction to provide chemical energy (ATP) for the reduction

of COof CO22 to carbohydrate in the light-independent to carbohydrate in the light-independent reactionsreactions

– Light-independent reactionsLight-independent reactions (light (light energy not needed)energy not needed)

ALBIO9700/2006JK

The light-dependent The light-dependent reactionsreactions

• Include the Include the synthesis of ATPsynthesis of ATP in in photophosphorylation and the splitting photophosphorylation and the splitting of water by of water by photolysisphotolysis to give H to give H++

• HH++ + NADP NADPH + NADP NADPH• ATP and NADPH - passed from the ATP and NADPH - passed from the

light-dependent to the light-light-dependent to the light-independent reactionsindependent reactions

• Photophosphorylation of ADP to ATP:Photophosphorylation of ADP to ATP:– CyclicCyclic– Non-cyclicNon-cyclic

ALBIO9700/2006JK

• Cyclic photophosphorylationCyclic photophosphorylation– Only photosystem IOnly photosystem I– Light absorbed by photosystem I and Light absorbed by photosystem I and

passed to chlorophyll passed to chlorophyll aa (P700) (P700)– An eAn e-- in the chlorophyll in the chlorophyll aa molecule is molecule is

excited and emittedexcited and emitted– Captured by an eCaptured by an e-- acceptor and acceptor and

passed back to a chlorophyll passed back to a chlorophyll aa (P700) (P700) molecule via a chain of electron molecule via a chain of electron carrierscarriers

– Synthesis of ATPSynthesis of ATP– ATP passes to light-independent ATP passes to light-independent

reactionsreactionsALBIO9700/2006JK

• Non-cyclic photophosphorylationNon-cyclic photophosphorylation– ‘‘Z scheme’Z scheme’– Light absorbed by both photosystem and Light absorbed by both photosystem and

excited eexcited e-- emitted from the primary emitted from the primary pigments of both reaction centres (P680 pigments of both reaction centres (P680 and P700)and P700)

– ee-- absorbed by e absorbed by e-- acceptors and pass acceptors and pass along chains of ealong chains of e-- carriers leaving the carriers leaving the photosystems positively chargedphotosystems positively charged

– The P700 of photosystem I absorbs The P700 of photosystem I absorbs electrons from photosystem II electrons from photosystem II

– P680 receives replacement eP680 receives replacement e-- from the from the splitting (photolysis) of watersplitting (photolysis) of water

– ATP synthesisedATP synthesisedALBIO9700/2006JK

• Photolysis of water:Photolysis of water:– Photosystem II includes a water-splitting Photosystem II includes a water-splitting

enzymes which catalyses the breakdown enzymes which catalyses the breakdown of water:of water:HH22O O → 2H→ 2H++ + 2e + 2e-- + ½O + ½O22

– HH++ combine with e combine with e-- from photosystem I from photosystem I and the carrier molecule NADP to give and the carrier molecule NADP to give reduced NADPreduced NADP2H2H++ + 2e + 2e-- + NADP + NADP → reduced NADP→ reduced NADP

– This passes to the light-independent This passes to the light-independent reactions and is used in the synthesis of reactions and is used in the synthesis of carbohydratecarbohydrate

ALBIO9700/2006JK

ALBIO9700/2006JK

Light-independent reactionsLight-independent reactions

• The fixation of COThe fixation of CO22

• COCO22 combines with a 5C sugar {ribulose combines with a 5C sugar {ribulose biphosphate (RuBP)} 2 molecules of a 3C biphosphate (RuBP)} 2 molecules of a 3C compound {glycerate-3-phosphate (GP/PGA)}compound {glycerate-3-phosphate (GP/PGA)}

• GP is reduced to triose phosphate (3C sugar) GP is reduced to triose phosphate (3C sugar) in the presence of ATP and NADPHin the presence of ATP and NADPH

• Some condense to form hexose phosphates, Some condense to form hexose phosphates, sucrose, starch and cellulose or are converted sucrose, starch and cellulose or are converted to acetyl CoA to make amino acids and lipidsto acetyl CoA to make amino acids and lipids

• Others regenerate RuBPOthers regenerate RuBP• The enzyme ribulose biphosphate carboxylase The enzyme ribulose biphosphate carboxylase

(rubisco), catalyses the combination of CO(rubisco), catalyses the combination of CO22 and RuBPand RuBP

ALBIO9700/2006JK

Calvin cycleCalvin cycle

ALBIO9700/2006JK

Leaf structure and functionLeaf structure and function• Has a broad, thin lamina, a midrib and a network of Has a broad, thin lamina, a midrib and a network of

veins, leaf stalk (petiole)veins, leaf stalk (petiole)• To perform its function the leaf must:To perform its function the leaf must:

– Contain chlorophyll and other photosynthetic pigments Contain chlorophyll and other photosynthetic pigments arranged in such a way that they can absorb lightarranged in such a way that they can absorb light

– Absorb COAbsorb CO22 and dispose of the waste product O and dispose of the waste product O22

– Have a water supply and be able to export manufactured Have a water supply and be able to export manufactured carbohydrate to the rest of the plantcarbohydrate to the rest of the plant

• Large surface area of Large surface area of laminalamina makes it easier to makes it easier to absorb light and thinness minimises diffusion pathway absorb light and thinness minimises diffusion pathway for gaseous exchangefor gaseous exchange

• Upper Upper epidermisepidermis is made of thin, flat, transparent is made of thin, flat, transparent cells which allow light through to the cells of the cells which allow light through to the cells of the mesophyllmesophyll, where photosynthesis takes place, where photosynthesis takes place

• A waxy transparent A waxy transparent cuticlecuticle provides a watertight provides a watertight layerlayer

• Cuticle and epidermis together form a protective layerCuticle and epidermis together form a protective layer

ALBIO9700/2006JK

• Stomata are pores in the epidermis Stomata are pores in the epidermis through which diffusion of gases occursthrough which diffusion of gases occurs

• Each stoma is bounded by 2 sausage-Each stoma is bounded by 2 sausage-shaped guard cellsshaped guard cells

• Changes in turgidity cause them to Changes in turgidity cause them to change shape so that they open and change shape so that they open and close the poreclose the pore

• Guard cells gain and loss water by Guard cells gain and loss water by osmosisosmosis

ALBIO9700/2006JK

ALBIO9700/2006JK

• The The palisade mesophyllpalisade mesophyll is the main site of photosynthesis is the main site of photosynthesis (many chloroplasts per cell than in the spongy mesophyll)(many chloroplasts per cell than in the spongy mesophyll)

• Adaptations for light absorption:Adaptations for light absorption:– Long cylinders arranged at right-angles to the upper epidermis Long cylinders arranged at right-angles to the upper epidermis

(reduces number of light-absorbing cross walls in the upper part (reduces number of light-absorbing cross walls in the upper part of the leaf so that as much light as possible can reach the of the leaf so that as much light as possible can reach the chloroplasts) chloroplasts)

– Large vacuole with a thin layer of cytoplasm (restricts Large vacuole with a thin layer of cytoplasm (restricts chloroplasts to a layer near the outside of the cell where light chloroplasts to a layer near the outside of the cell where light can reach them most easily)can reach them most easily)

– Chloroplasts can be moved within cells (to absorb the most light Chloroplasts can be moved within cells (to absorb the most light or to protect it from excessive light intensities)or to protect it from excessive light intensities)

• Adaptations for gaseous exchange:Adaptations for gaseous exchange:– Cylindrical cells pack together with long, narrow air spaces Cylindrical cells pack together with long, narrow air spaces

between them (large surface area of contact between cell and between them (large surface area of contact between cell and air)air)

– Cell walls are thin (gases can diffuse through them easily)Cell walls are thin (gases can diffuse through them easily)• Spongy mesophyllSpongy mesophyll is adapted as a surface for the exchange is adapted as a surface for the exchange

of COof CO22 and O and O22– Smaller number of chloroplastsSmaller number of chloroplasts– Photosynthesis only at high light intensitiesPhotosynthesis only at high light intensities– Irregular packing and large air spaces produced provide a large Irregular packing and large air spaces produced provide a large

surface area of moist cell wall for gaseous exchangesurface area of moist cell wall for gaseous exchange• Veins in leaf help to support large surface area of leafVeins in leaf help to support large surface area of leaf

– contains xylem and phloemcontains xylem and phloemALBIO9700/2006JK

ALBIO9700/2006JK

Investigation of limiting Investigation of limiting factorsfactors• External factors:External factors:

– Light intensityLight intensity– TemperatureTemperature– COCO22 concentration concentration

• Light intensityLight intensity– Rate initially increases as the light intensity increasesRate initially increases as the light intensity increases– Rate reaches a plateau at higher light intensitiesRate reaches a plateau at higher light intensities

• TemperatureTemperature– At high light intensities the rate of photosynthesis At high light intensities the rate of photosynthesis

increases as the temperature is increased over a limited increases as the temperature is increased over a limited rangerange

– At low light intensities, increasing the temperature has At low light intensities, increasing the temperature has little effect on the rate of photosynthesislittle effect on the rate of photosynthesis

• Photochemical reactions are not generally affected by Photochemical reactions are not generally affected by temperaturetemperature

• Since temperature affects rate, there must be 2 sets of Since temperature affects rate, there must be 2 sets of reactionsreactions– Light-dependent photochemical stageLight-dependent photochemical stage– Light-independent, temperature-dependent stageLight-independent, temperature-dependent stage

• Limiting factorLimiting factorALBIO9700/2006JK

ALBIO9700/2006JK