Photosynthesis

Question 1

Describe the physiology of a chloroplast

Answer 1

1. Organelles in green algae and plants
2. Have a double membrane and contain unique DNA
3. The interior of the chloroplast is the stroma
4. The stroma contains flattened membrane sacks called thylakoids stacked on top of each other, forming grana
5. The light dependent reactions occur in the thylakoid membranes, catalysed by enzymes
6. The light independent reactions take place in the stroma

Question 2

Describe the light dependent reaction

Answer 2

1. Photosystem II contains Chlorophyll a, which contains an Mg2+ centre. Chlorophyll absorbs light energy and excites electrons which are passed to the ETC in the thylakoid membrane
2. Electron movement along the ETC is coupled to H+ pumping into the thylakoid lumen, generating a proton gradient. H+ diffuses through ATP synthase back into the stroma producing ATP.
3. Oxidised PSII uses Mn4+ to oxidise surrounding water molecules in the oxygen evolving complex into O2 and H+ ions, replacing the lost electrons from excitation. Thus water is the source of electrons
4. PS1 accepts lower energy electrons at the end of the ETC, where they are re-energised by absorbing another photon of light
5. This provides the energy for electrons to be transferred to NADP+ electron carrier alongside a proton produced by PSII to produce NADPH

Question 3

Describe the light independent reaction

Answer 3

1. The Calvin cycle occurs in the stroma
2. Carbon fixation: the addition of CO2 to ribulose-1,5-bisphosphate is catalysed by rubisco enzyme
3. The six carbon molecule is cleaved into 2 molecules of 3-phosphoglycerate
4. Reduction: ATP and NADPH convert 3-phosphoglycerate into glyceraldehyde-3-phosphate
5. Regeneration of RuBP and Gluconeogenesis: G3P is converted back into RuBP to continue the Calvin cycle, and some is used to synthesise glucose. (3 for 1 RuBP and 2 for 1 glucose)

Question 4

PS1 structure

Answer 4

1. PS1 is a large protein complex embedded in the thylakoid membranes
2. PS1 contains Light harvesting complexes, which have proteins that organise chlorophyll and other pigments
3. Chlorophyll a and b in LHC have different absorption profiles
4. LHC also has accessory pigments such as lutein and carotenes that extend the wavelength of light absorption

Question 5

Describe some redox reactions between photosystems

Answer 5

1. Cytochrome b/f complex links the excited electrons from PSII to PSI
2. Alternative cyclic electron flow takes electrons back from ferrodoxin (last carrier before NADPH formation) to Cytochrome b/f generating only proton gradient and ATP, but no NADPH or O2 (likely to be when ATP levels low)
3. Plants adjust NADPH/ATP ratios to satisfy what is needed for bio synthetic pathways

Question 6

How is rubisco avoids wasting energy on oxygen metabolism

Answer 6

1. The carboxysome is a large multi-subunit protein shell that keeps oxygen away from rubisco