Light Dependent Photosynthesis

Question 1

Describe what happens in non - cyclic light dependent photosynthesis

Answer 1

1. PS2 absorbs light energy, EXCITING electrons (more energy) causing them to move along an ETC to PS1 (moved by electron carriers along thylakoid membrane)
   Whole process happens in the THYLAKOID MEMBRANE of chloroplasts

Question 2

2.

Answer 2

PHOTOLYSIS (water splits into H ions, electrons and oxygen) happens using light energy to replace the electrons that have left PS2.
The H ions (protons) are transported to the stroma to help create a proton gradient.

Question 3

3.

Answer 3

Excited electrons moving down ETC lose energy.
This energy is used to pump H ions from the stroma into the thylakoid (which is impermeable to protons so they can’t diffuse out).
There is a higher concentration of protons in the thylakoid than the stroma, producing a PROTON GRADIENT across the membrane

Question 4

4.

Answer 4

Protons move down their concentration gradient into the stroma by ATP synthase and the energy used for this movement for chemiosmosis.
ADP + Pi —-> ATP

Question 5

5.

Answer 5

Light energy is absorbed by PS1, exciting electrons to an even higher energy level.
This causes them to move down an ETC until they are transferred to NADP.
NADP + H proton —–> reduced NADP
This lowers the concentration of protons in the stroma, helping to maintain a gradient.

Question 6

Explain what happens in cyclic light dependent photosynthesis

Answer 6

Light energy is absorbed by PS1.
This excites 2 electrons which move down the ETC to generate ATP (by making a proton gradient)
They then return back to PS1 (used for high ATP demands)

Question 7

Compare cyclic/non cyclic

Answer 7

NON CYCLIC :
- last electron acceptor is NADP
- produces atp, reduced NADP and oxygen
- involves both photosystems
- electron source is by photolysis

CYCLIC:
- last electron acceptor is PS1
- produces atp
- only uses PS1
electron source is PS1 (by absorbing light energy)