Photosynthesis

Question 1

Light dependent reactions
( PS2 —> O2)

Answer 1

1. A photon of light is absorbed by an accessory pigment of the light harvesting complex of PS2. AN electron is excited to a higher energy level, and when it drops to ground state, it passes on the energy to the next pigment molecule, this resonance transfer of energy continues until it reaches a special chlorophyll a molecule in the reaction centre, P680.
2. P680 absorbs the energy from the. accessory pigments, it releases the electron, creating an electron hole in PS2. The displaced electron is accepted by the primary electron acceptor (X) in the reaction centre.
3. The electron hole is filled by the electron released by the splitting of water in an enzyme-catalysed reaction, to form O2 as a byproduct.

Question 2

Light dependent reactions (pri electron acceptor) –> NADP

Answer 2

4. The primary electron acceptor then passes the electron down the first ETC, with a series of increasingly electronegative electron carriers, which generates energy which is used to actively pump protons from the stroma to the thylakoid space to generate a proton gradient.
5. Chemiosmosis occurs when the H+ diffuse through ATP synthase back into the stroma, where ADP is phosphorylated to ATP
6. Meanwhile, PS1 loses an electron in a similar way to PS2, via a special chlorophyll a molecule, P700 which emits the electron to be accepted by primary electron acceptor, leaving an electron hole. The electron hole is filled by the displaced electron from PS2, which reaches the end of the 1st ETC.
7. The primary electron acceptor then passes the electron down the second ETC until it reaches the final electron acceptor NADP, which is reduced to NADPH by NADP reductase.

Question 3

Cyclic photophosphorylation

Answer 3

• electrons displaced from P700 of PS1 is accepted by the primary electron acceptor and transferred to the middle of the 1st ETC.
• the electron is transported down the ETC and is finally recycled back to PS1. energy lost during electron transfer is coupled to the formation of ATP

Question 4

Calvin cycle

Answer 4

1. Carbon fixation
   - CO2 combines with RuBP in the presence of RuBisco, ribulose bisphosphate carboxylase to form unstable 6C which breaks down into 2 molecules of GP
2. Reduction
   - GP is reduced to G3P using ATP as a source of energy and ADPH reducing power
3. Regeneration of RuBP
   - G3P molecules can be regenerated to form RuBP to allows carbon fixation to continue. ATP needed

Question 5

products of light independent reactions

Answer 5

• G3P
• NADP and ADP

Question 6

Absorption spectrum

Answer 6

a record of the amount of light absorbed by each pigment at each wavelength of light

Question 7

action spectrum

Answer 7

graph showing the effectiveness of different wavelengths of light in stimulating photosynthesis

Question 8

limiting factor of photosynthesis

Answer 8

limiting factor: any factor that directly affects the process, hence rate of reaction if its magintude its changed