Chapter 8.3 Photosynthesis

Measurements of photosynthesis

Photosynthesis can be measured in many ways as it involves the production of oxygen, the uptake of carbon dioxide and an increase in biomass. For example, aquatic plants release oxygen bubbles during photosynthesis and so these can be collected and measured. The uptake of carbon dioxide is more difficult to measure so it is usually done indirectly. When carbon dioxide is absorbed from water the pH of the water rises and so this can be measured with pH indicators or pH meters. Finally, photosynthesis can be measured through an increase in biomass. If batches of plants are harvested at a series of times and the biomass of these batches is calculated, the rate increase in biomass gives an indirect measure of the rate of photosynthesis in the plants.

What light does chlorophyll absorb?

Red and blue light most effectively; but BLUE the most. It reflects green light more than other colors. The first stage of photosynthesis is light absorption, whihc involves chemical substances called pigments (in plants, main one is chlorophyll). Chlorophyll reflects green light, which is why the plant seems green.

Photosynthesis

The production of glucose and carbohydrates in cells using light energy. It is an example of energy conversion, as light energy is converted into chemical energy in carbon compounds. The carbon compounds produced include carbohydrates, proteins, and lipids. -it is a metabolic pathway: ---C is fixed from CO2 to produce glucose ---H2O is split, and H is used for production of glucose, O2 excreted as waste gas ---glucose used in respiration, stored as starch, or used to build cell walls as cellulose

Main pigment in plants

chlorophyll

Why do chloroplasts contain several types of chlorophyll and other pigments?

these other pigments are called accessory pigments. These pigments absorb different ranges of light, to broaden the amount and type of light absorbed, so that photosynthesis can be done more often with more access to a greater range of light.

How can pigments be separated?

through chromatography.

5) chemiosmosis

-ATP synthase in thylakoids generates ATP using the proton gradient. -protons travel back across the membrane, down concentration gradient, by passing through ATP synthase -E released by passage of protons down concentration gradient is used to make ATP from ADP and inorganic phosphate. -when electrons reach the end of the chain of carriers they are passed to plastocyanin, a water soluble electron acceptor in the fluid inside the thylakoids. Reduced plastocyanin is needed in the next stage of photosynthesis.

3) ETC

-ETC is the transfer of excited electrons which occurs between carriers in the thylakoid membranes. -the production of ATP using E derived from light is called photophosphorylation. it is carried out by the thylakoids. -reduced plastoquinone carries the pair of excited electrons from the reaction center of PSII to the start of the chain of electron carriers.

Light-dependent reactions

-Light dependent reactions take place in the membrane of the thylakoids. -The products of these reactions are NADPH + H+ and ATP. They serve as energy sources for the light-independent reactions. -Essentially it is ETC. What happens: -photolysis -photoactivation -electron transport -chemiosmosis -ATP synthesis -reduction of NADP Reactants: H2O, NADP+ Products: O2, NADPH, ATP

RuBP regeneration

-RuBP is reformed using ATP -in the last phase of the Calvin cycle, a series of enzyme-catalyzed reactions convert triose phosphate molecules into RuBP. after the RuBP is regenerated, it can serve to fix CO2 and begin cycle again,

Light-Independent reactions

-Takes place in the stroma, a thick protein-rich medium containing enzymes for use in light-independent reactions, aka Calvin cycle. What happens: -carbon fixation -carboxylation of RuBP -production of triose phosphate -ATP and NADPH as energy sources -ATP used to regenerate RuBP -ATP used to produce carbohydrates Reactants: CO2, NADPH Products: Glucose phosphate, NADP+

1) Photoactivation

-absorption of light by photosystems generates excited electrons -chlorophyll and accessory pigments are grouped together in light-harvesting arrays called photosystems, located in the thylakoids. -2 types of light-harvesting arrays called Photosystems II and I. They also each have reaction centers. -each photosystem contains many chlorophyll molecules which absorb light energy and pass it to 2 special chlorophyll molecules in the reaction center of the photosystem. -when these special chlorophyll molecules absorb the energy from a photon of light, and electron within the molecule becomes excited. The chlorophyll is then PHOTOACTIVATED. -the chlorophylls at the reaction center have the special property of being able to donate excited electrons to an electron acceptor. -light dependent reactions happen in Photosystem II. The electron acceptor is plastoquinone. -it collects 2 excited electrons from PSII and moves away to another position in the membrane. -Plastoquinone is hydrophobic, so it remains within the membrane. -absorption of the 2 photons of light causes the production of 1 reduced plastoquinone, with one of the chlorophylls at the reaction center having lost 2 electrons to a plastoquinone molecule. -PSII can repeat this process to produce a second reduced plastoquinone, so the chlorophyll at the reaction center has lost 4 electrons and 2 plastoquinone molecules have been reduced.

action spectrum and absorption spectrum

-action spectrum: a graph showing the rate of photosynthesis at each wavelength of light. -absorption spectrum: a graph showing the percentage of light absorbed at each wavelength by a pigment or a group of pigments -they are very similar because photosynthesis can only occur in wavelengths of light that photosynthetic pigments can absorb. it also shows how chlorophyll is the main photosynthetic pigment. also the absorption spectrum shows how there are some pigments that allow for weak absorption of green/yellow light, and these produce the lowest rate of photosynthesis.

Calvin's chromatography experiment

-chlorella algae placed in a thin vessel -algae is given light, CO2, HCO3 - -at the beginning of the experiment, carbon compounds replaced with compounds containing 14C (radioactive C) -samples of algae taken at different time intervals and fixed with methanol to stop the process of PS -carbon compounds separated by chromatography and compounds containing 14C identified by autoradiography -analyzed using autoradiograms

Chloroplast structure and function

-double membrane forming the outer chloroplast envelope -extensive system of internal membranes called thylakoids, which are of a green color -small fluid filled spaces inside thylakoids -colorless fluid around thylakoids called stroma that contains many different enxymes -grana: stacks of thylakoids.

6) involving PS I

-excited electrons from PSI are used to reduce NADP, which is needed in the light independent reactions of photosynthesis. -reduced NADP carries a pair of electrons that can be used to carry out reduction reactions -chlorophyll molecules within PSI absorb light E and pass it to the special 2 chlorophyll molecules in the reaction center. -this raises an e- in one of the chlorophylls to a high E level --> called photoactivation. -the excited ei passes along a chain of carriers in PSI, at the end passed to ferredoxin (Fd). -2 molecules of reduced Fd are used to reduce NADP. -the electron lost by PSI is replaced by an electron carried by plastocyanin. -so electrons excited in PSII are passed to PC and then transferred to PSI. -the electrons are re-excited with light energy and eventually used to reduce NADP/ -supply of NADP sometimes runs out. when this happens, the e- return to the electron transport chain that links the 2 photosystems, rather than being passed to NADP. as the e- flow back along the e- transport chain to PSI, they cause pumping of protons, allowing for ATP production --> cyclic photophosphorylation.

4) the proton gradient

-excited electrons from PSII are used to generate a proton gradient -once pq transfers its electrons, the electrons are then passed from carrier to carrier in the chain, releasing E, which is used to pump p+ across the thylakoid membrane into the space inside the thylakoids. -a concentration gradient of protons develops across the thylakoid membrane, which is a store of PE.

The Calvin cycle (carbon fixation occurs in light independent reactions)

-in light independent reactions a carboxylase catalyzes the carboxylation of ribulose diphosphate. -carbon fixation reaction occurs in the stroma. the product of this is G3P. -CO2 reacts with RuBP to produce G3P, catalyzed by enzyme rubisco. -stroma contains large amounts of rubisco to maximize carbon fixation. -g3P is reduced to triose phosphate using reduced NADP (providing H atoms) and ATP (providing E)

2) Photolysis

-photolysis of water comes after photoactivation -it generates electrons for use in the light dependent reactions -once the plastoquinone has been reduced, the chlorophyll in the reaction center is a powerful oxidizing agent and causes the water molecules near it to split and give up electrons to replace the ones it has lost. 2H2O --> 4 e- + 4H+ + O2. -this is how oxygen is generated in photosynthesis. O2 is a waste product and then diffuses away. -the reduced plastoquinone carries a pair of electrons and much of the E absorbed from light, which drives all the subsequent reactions of photosynthesis.

Limiting factors of photosynthesis

-temperature: At low temperatures these enzymes work slower. At high temperatures the enzymes no longer work effectively. This affects the rate of the reactions in the Calvin cycle and therefore the rate of photosynthesis will be affected. -light intensity: When the light intensity is poor, there is a shortage of ATP and NADPH, as these are products from the light dependent reactions. Without these products the light independent reactions can't occur as glycerate 3-phosphate cannot be reduced. Therefore a shortage of these products will limit the rate of photosynthesis. -carbon dioxide concentration: When the carbon dioxide concentration is low, the amount of glycerate 3-phosphate produced is limited as carbon dioxide is needed for its production and therefore the rate of photosynthesis is affected.

what occurs to triose phosphate

-triose phosphate is used to generate RuBP and produce carbs. -2 triose phosphate molecules can be combined to form hexose phosphate and hexose phosphate can be combined by condensation reactions to produce starch. -regeneration of RuBP occurs in a process of conversion of 3 carbon sugars into 5 carbon sugars -for the Calvin cycle to continue, as much RuBP must be produced as consumed. -if 3 RuBP molecules are used, 6 triose phosphates are produced. 5 of these are needed to regenerate the 3 RuBP molecules, so only 1 triose phosphate is taken to make glucose. -so to produce 1 molecule of glucose, 6 turns of the Calvin cycle are needed.

Light Dependent Reaction Description (cyclic phosphorylation)

-when the supply of NADP+ runs out, the electrons return to the ETC linking the 2 PS rather than being passed to NADP -as e- flow back along EtC to PSI, they cause pumping of p+, allowing indirectly for ATP production. • It goes from PSI to FD (feradoxin), instead of taking it through NADP+ reductase, goes to Pq. ◦ Recycling those electrons. Fd --> Pq --> cytochrome C ◦ Fd can leave the membrane and transport the electrons to Pq. ◦ Pq transfers electrons to cytochrome C, which pumps H+ protons, and this will at some point cause synthesis of ATP because gradient is created. However, there is an alternative pathway for ATP production in this case and it is called cyclic photophosphorylation. It begins with Photosystem I absorbing light and becoming photoactivated. The excited electrons from Photosystem I are then passed on to a chain of electron carriers between Photosystem I and II. These electrons travel along the chain of carriers back to Photosystem I and as they do so they cause the pumping of protons across the thylakoid membrane and therefore create a proton gradient. As explained previously, the protons move back across the thylakoid membrane through ATP synthase and as they do so, ATP is produced. Therefore, ATP can be produced even when there is a shortage of NADP+.

Light Independent Reaction Description (Calvin cycle)

1) CARBON FIXATION -CO2 reacts with RuBP (C from CO2 fixes onto RuBP) to form a 6-C compound, which immediately splits into 2 molecules of 3-PHOSPHOGLYCERATE (3 PGA) --> catalyzed by the enzyme rubisco (stroma contains large amount of rubisco) 2) REDUCTION -H is added to G3P by a reduction reaction, involving both ATP and NADPH, created by light dependent reactions -ATP provides the E required to perform reduction and NADPH provides the H atoms -the product is triose phosphate 3) REGENERATION -triose phosphate is used to regenerate RuBP and produce glucose -2 triose phosphate molecules can be combined to form starch -1 triose goes to make glucose every 3 cycles, so 1 molecule of glucose is produced every.6 turns of the Calvin cycle -regeneration of RuBP occurs in a process of conversion of 3-carbon sugars into 5-carbon sugars -in the last phase of Calvin cycle, a series of enzyme-catalyzed reactions convert triose phosphate into RuBP -after RuBP is generated, carbon fixation occurs and cycle begins again.

Light Dependent Reaction Description (non-cyclic phosphorylation)

1) Light enters PSII. Chlorophyll pigments absorb light, transferring E from pigment to pigment until it reaches the reaction center 2) the E is passed to 2 special chlorophyll molecules called chlorophyll P680 3) When chlorophyll P680 absorbs the E from a photon of light, an electron within the molecule becomes excited. Chlorophyll P680 is thus PHOTOACTIVATED 4) Plastoquinone (pq) is the electron acceptor that collects the 2 excited electrons (once the P680 molecule is photo activated again) and moves away to another position in the membrane. pq is hydrophobic, so it stays within the membrane 5) 1 of the chlorophyll P680 molecules has lost 2 electrons to pq, and the pq is reduced. 6) after photoactivation, PHOTOLYSIS occurs. -H2O is split, and e- are released. These e- replace the ones that have been lost in PSII -once pq has been reduced, the P680 is a powerful oxidizing agent which causes H2O molecules to be split and give up e- to replace the ones it has lost. 2H2O --> 4e- + 4H+ + O2 -O2 is a waste product and diffuses away 7) the excited e- from PSII create a proton gradient 8) when the electrons are passed from carrier to carrier, E is released, pumping H+ ions across the membrane into the lumen --> bc the lumen has a small volume, the gradient can be built up quickly --> protons are pumped through Cyt-C. 9) Chemiosmosis occurs: -protons travel back down the membrane, down the concentration gradient, through ATP synthase -E released by the passage of protons down the gradient is used to make ATP from ADP and inorganic phosphate -when electrons reach the end of the chain of carriers, they are passed to plastocyanin (water soluble electrons acceptor in the membrane) -reduced pc is needed for PSI stage 10) light enters into PSI, and chlorophyll pigments pass the E to the 2 chlorophyll P700 molecules in the reaction center. 11) photoactivation occurs, since the electrons in the chlorophyll molecules are excited 12) the excited electrons to ferredoxin (Fd) 13) the electrons reduce NADP+ to create NADPH + H+ 14) the NADPH is carried to light independent reactions --> 2 molecules of reduced Fd are used to reduce NADP to NADPH + H+ 15) the e- lost by PSI are replaced by e- carried by the plastocyanin (pc) --e- excited by PSII are passed to pc and transferred to PSI --these e- are re-excited with light E and eventually used to reduce NADP+ --these electrons fill the void in P700 --photolysis does NOT occur

Calvin's chromatography experiment improvements in technology

1) radioactive labelling --using radioisotopes to label organic compounds 2) discovery of 14C in 1945 3) use of autoradiography to produce patterns of radioactive decay emissions (autoradiograms) 4) double-way paper chromatograph --use to separate small organic compounds --run 2 solvents perpendicular to each other up paper --used to separate/identify products of carbon fixation

Production of carbohydrates

Energy is needed to produce carbs and other carbon compounds from carbon dioxide. Photosynthesis is an endothermic reaction since energy is required to create the product.

2 parts of photosynthesis

Light dependent (consists of ETC) (thylakoid membrane) and light independent (Calvin cycle) (stroma)

The Calvin Cycle

Light independent reaction that is an anabolic pathway requiring endergonic reactions to be coupled to the hydrolysis of ATP and the oxidation of reduced NADP (NADPH + H+)

Oxygen production in photosynthesis

Oxygen is produced in photosynthesis from photolysis of water. This is the splitting of molecules of water to release electrons needed in other stages. H2O --> 4 e- + 4H+ + O2. O2 is s waste product and diffuses away.