Lesson 3: Photosynthesis

Question 1

What are photosystems?

Answer 1

structures in the thylakoid membrane made up of chlorophyll and accessory pigments.

Question 2

What wavelength of light does PS 1 absorb?

Answer 2

700 nm

Question 3

What wavelength of light does PS 2 absorb?

Answer 3

680 nm

Question 4

where does the light dependent stage take place?

Answer 4

thylakoid membrane

Question 5

what are the two main stages of photosynthesis?

Answer 5

1. light dependent
2. light independent

Question 6

summarise the light dependent stage (non-cyclic photophosphorylation)

Answer 6

1. a photon of light is absorbed by a pigment molecule in PS II and is transferred to other pigment molecules until it reaches the reaction centre.
2. The photon energy excites an electron to a higher energy state.
3. The electron is captured by an electron acceptor
4. Excited electrons are passed along the ETC, losing energy at each exchange
5. The energy is used to pump H+ ions to from the stroma to intermembrane space, creating a high concentration
6. H+ ions diffuse through the ATP synthase to create kinetic energy used to phosphorylate ADP to ATP (photophosphorylation).
7. The de-energised electron from PS II is passed on to PS I.
8. Electrons are re-excited by light energy and passes along a 2nd ETC. Final carrier = ferredoxin.
9. Electrons and H+ ions from stroma convert NADP to NADPH
10. At the same time, photolysis occurs. Water molecules are being split by light energy. This produces electrons to replace those lost by PS II.

Question 7

What is the equation for photolysis?

Answer 7

H₂O–> 2H+ + 1/2O₂ + 2e-

Question 8

What two paths can the light dependent stage take?

Answer 8

1. cyclic photophosphorylation
2. non-cyclic photophosphorylation

Question 9

What is the key difference between non-cyclic and cyclic photophosphorylation?

Answer 9

non-cyclic: de-energised electrons move from PS II to PS I.
cyclic: de-energised electrons cycle back to PS II.

Question 10

Where does the light independent stage take place?

Answer 10

stroma

Question 11

Summarise the light independent stage

Answer 11

1. 6 CO₂ combines with 6 RuBP with the help of rubisco enzyme to form an unstable intermediate, six 6 carbon molecule.
2. The 6 carbon molecule splits into 2 molecules of GP (3 carbons)
3. GP is reduced by ATP and NADPH to form 2 TP (12 in total).
4. 2 TPs are used to form 1 molecule of glucose.
5. The remaining 10 molecules is used to regenerate 6 RuBP using 6 molecules of ATP.