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level: 11.2 The light-dependent reaction

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level questions: 11.2 The light-dependent reaction

QuestionAnswer
What is the light-dependent reaction- Needs light energy - Takes place in the thylakoid membranes of the chloroplasts - Includes two types of photophosphorylation
hOverview of the light-dependent reaction- Light energy is absorbed by chlorophyll in the photosystems - It excites electrons in the chlorophyll, releasing them from mol. - Chlorophyll has been photoionised - Some energy released by electrons used to add a phosphate to ADP to form ATP - Some used to form reduced NADP - ATP transfers energy, reduced NADP transfers H to the light-independent reaction - During process H2O is oxidised to O2
Structure of a chloroplast (photosytems)- Have photosynthetic pigments - Coloured substances, absorb light energy for photosyn. - Pigments found in thylakoid membranes, attached to proteins - Protein + pigment = photosystem - There are 2 photosystems
The 2 photosystems- PSI absorbs light best at longer wavelengths - PSII absorbs light best at shorter wavelengths
How is the energy captured from the light in the light-dependent reaction used- Making ATP from ADP + inorganic phosphate (photophosphorylation) - Making reduced NADP from NADP - Splitting water into protons (H+ ions), electrons + O2 (photolysis)
Oxidation- Loss of electrons or loss of hydrogen or gain of oxygen
Reduction- Gain of electrons or gain of hydrogen or loss of oxygen
What are the two types of photophosphorylation in the light-dependent reaction- Non-cyclic (produces ATP, reduced NADP, O2) - Cyclic (only produces ATP)
Non-cyclic photophosphorylation key steps (all happening together btw)- Light energy excites electrons in chlorophyll - Photolysis of water produces protons (H+ ions), electrons, O2 - Energy from the excited electrons makes ATP (chemiosmosis) - Energy from the excited electrons generates reduced NADP
Light energy excites electrons in chlorophyll- Light energy is absorbed by PSII - It excites electrons in chlorophyll - Electrons move to a higher energy level (they have more energy) - These high energy electrons are released, move down ETC to PSI
Photolysis of water produces protons (H+ ions), electrons, O2- As electrons leave PSII to move down ETC, they must be replaced - Light energy splits water into protons (H+ ions), electrons, O2 by photolysis
Equation for the photolysis of water2H20 ---------> 4H+ + 4e- + O2 water ----------> protons + electrons + oxygen
Chemiosmosis meaning- Electrons flowing down ETC, creating proton gradient across membrane - To drive ATP synthesis
Energy from the excited electrons makes ATP (chemiosmosis)- Excited electrons lose energy as they move down ETC - Energy used to transport protons into thylakoid via protein pumps - Thylakoid has a higher conc. of protons than the stroma - Protons move down their conc. gradient, into stroma, via ATP synthase channel proteins - As the protons move, they change structure of the enzyme
Energy from the excited electrons generates reduced NADP- Light energy is absorbed by PSI, which excites the electrons again to an even higher energy level - Electrons are transferred to NADP, along with a proton (H+ ion) from the stroma, to form reduced NADP
Cyclic photophosphorylation- Only uses PSI - Electrons from chlorophyll aren't transferred to NADP, but are transferred to PSI via electron carriers - So electrons are recycled + can repeatedly flow through PSI
How are chloroplasts adapted to carry out light-dependent reactions- LSA of thylakoid membranes for attachment of chlorophyll - Granal membranes have ATP synthase, catalyse production of ATP - GM selectively permeable, allows formation of proton gradient - Contain DNA + ribosomes, quickly + easily make proteins involved in reaction