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

Photoactivation: Chlorophyll in PSII absorbs light, exciting electrons to a higher energy state.
The electrons are captured by an electron acceptor.
Chemiosmosis: Electrons move through the first ETC, generating energy which is used to pump H+ ions against the concentration gradient from the stroma into the thylakoid lumen/intermembrane space.
There is now a high concentration of H+ ions which diffuses through the ATP synthase, generating kinetic energy.
Photophosphorylation: The kinetic energy used to phosphorylate ADP to ATP.
The electrons from PS II are re-excited at PS I by light energy.
The excited electrons go through a second ETC.
Then they are transferred to an enzyme called ferredoxin.
The electrons are used to reduce NADP+ using H+ ions from the stroma.
Photolysis: 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.