book 3 (photosynthesis)

Question 1

outline the process of light-harvesting at PSII (step 1) in non-cyclic photophosphorylation

Answer 1

1. a photon of light strikes the accessory pigment molecule in light-harvesting complex of PSII in thylakoid membrane
2. pigment molecule absorbs the light energy and it’s electron is excited to a higher energy level
3. when the excited electron returns to ‘ground state’, the energy released is passed on to neighbouring accessory pigment molecules
4. energy will then be channelled to P680 special chlorophyll a molecule at the reaction centre, located in thylakoid membrane
5. an electron from special chlorophyll a molecule will be boosted to a very high level and is captured by PSII’s primary electron acceptor

Question 2

outline the process of the photolysis of water (step 2) in non-cyclic photophosphorylation

Answer 2

1. the electron displaced from the PSII reaction centre is replaced via the Hill reaction whereby a water molecule is split in the photolysis of water, catalysed by an enzyme, using light energy from the sun to produce H+ ions, O2 molecules and electrons
2. the electron produced are then donated to the P680 special chlorophyll a molecules in PSII, replacing the displaced electron

Question 3

outline the process of the electron transport between PSII and PSI in non-cyclic photophosphorylation

Answer 3

1. the photo-excited electrons are passed from PSII primary electron acceptor to PSI via a series of electron carriers in an electron transport chain
2. the electrons are transported along the series of electron carriers of decreasing energy levels, whereby energy is being released
3. the energy is being used to pump protons from the stroma into the thylakoid membrane, across the thylakoid membrane
4. a proton gradient is generated where the proton concentration in thylakoid space is greater than in the stroma
5. protons then diffuse out from the thylakoid space into the stroma through the ATP synthase complex, down a concentration gradient
6. ATP synthase complex enzyme catalyses the synthesis of ATP from ADP and Pi – chemiosmosis

Question 4

outline the process of light-harvesting at PSI in non-cyclic photophosphorylation

Answer 4

1. concurrently, light is harvested by the accessory pigment in the light-harvesting system of PSI and energy is passed along to electrons in the neighbouring pigment molecules and are eventually channelled to P700 special chlorophyll a molecules in the reaction centre of PSI
2. an electron in the special chlorophyll a molecule is boosted to a very high energy level and captured by PSI’s primary electron acceptor

Question 5

outline the process of the electron transport between PSI and NADP+ and NADPH synthesis

Answer 5

1. the photo-excited electrons in PSI primary electron acceptor are passed down a second ETC to NADP+
2. protons from the stroma and the electrons from PSI reduces NADP+ to NADPH, catalysed by enzyme NADP+ reductase

Question 6

outline the processes of cyclic photophosphorylation

Answer 6

1. an alternative electron transport pathway involves only PSI and a few electron carriers
2. electrons displaced from the PSI are transferred to PSI primary electron acceptor and then on to the electron transport chain between PSI and PSII and then back to PSI
3. the energy released from the electron transport is used to pump H+ ions from the stroma into the thylakoid space, across the thylakoid membrane
4. this generates a proton gradient
5. the potential energy of the proton gradient is used for the synthesis of ATP from ADP and Pi as protons flow through the ATP synthase, down a concentration gradient, back into the stroma — chemiosmosis
6. the electrons are then returned to PSI, completing the cycle
7. no NADPH formed
8. no O2 produced as PSII not involved

Question 7

outline the process of carbon dioxide fixation in Calvin cycle

Answer 7

1. 1 CO2 molecule enters the calvin cycle and combines with 5C compound ribulose biphophate (RUBP), to form unstable intermediate 6C compound, catalysed by rubisco
2. the unstable 6C compound immediately splits into half to form 2 molecules of 3C compound, 3-phosphoglycerate (PGA)

Question 8

outline the process of PGA reduction

Answer 8

1. PGA is reduced to 3-phosphoglyceraldehyde (PGAL) 2.by using the electrons that comes from the NADPH, and using the ATP, both produced from light-dependent reactions
2. for every 3 molecules of CO2, there are 6 molecules of PGAL
3. but only one molecule of PGAL exits the cycle to be used by the plant cell to synthesise carbohydrates like glucose
4. the other 5 molecules of PGAL must be recycled to regenerate the 3 molecules of RUBP

Question 9

outline the process of RUBP regeneration

Answer 9

1. the carbon skeleton of 5 molecules of PGAL are rearranged by the last steps of the calvin cycle into 3 molecules of RUBP
2. the energy for this reaction comes from ATP generated during light dependent reactions
3. RUBP now prepared to receive CO2 again, allowing the cycle to continue