3.5.1 Photosynthesis

Question 1

Describe the light-dependent reaction;

non-cyclic photophosphorylation

Answer 1

1) chlorophyll in photosystem 2 absorbs light, leading to photoionisation of chlorophyll (when an eletron pair is raised to a higher energy level by absorbed light energy and is so excited leaves chlorophyll molecule making it ionised)
2) The electron pair is passed down an electron transport train losing some of the energy which is conserved in the production of ATP and reduced NADP
3) the production of ATP happens when the electrons are transferred down the electron transfer chain lowering the energy state of the electrons. The released energy is used to move protons across chloroplast membranes (thylakoid) so from stroma into intermembranal space.
4) When the protons diffuse into stroma through ATP synthase (a protein embedded in membrane) it catalyses phosphorylation of ADP
(chemiosomotic theory)
5) NADPH is produced after the electrons reach photosystem 1 and excited again when light energy is absorbed by photosystem 1. Photolysis of water also occurs which produces protons, electrons and oxygen.The protons from photolysis bound to the electrons from photosystem 1 and the electrons from photolysis replace the electrons lost by photosystem 2. Oxygen is a by-product.

Question 2

What does non-cyclic photophosphorylation produce? what is it used for?

Answer 2

ATP- immediate energy source
NADPH- calvin cycle
O2- By-product

Question 3

Where does non-cyclic photophosphorylation occur?

Answer 3

thylakoid and stroma so within chloroplasts

Question 4

Decribe cyclic photophosphorylation

Answer 4

1) chlorophyll in photosystem 1 absorbs light, leading to photoionisation of chlorophyll (when an eletron pair is raised to a higher energy level by absorbed light energy and is so excited leaves chlorophyll molecule making it ionised)
2) The electron pair is passed down an electron transport train losing some of the energy which is conserved in the production of ATP
3) the production of ATP happens when the electrons are transferred down the electron transfer chain lowering the energy state of the electrons. The released energy is used to move protons across chloroplast membranes (thylakoid) so from stroma into intermembranal space.
4) When the protons diffuse into stroma through ATP synthase (a protein embedded in membrane) it catalyses phosphorylation of ADP
(chemiosomotic theory)
5) The electron passed back to photosystem 1.

Question 5

What wavelength of light does photosystem 1 best absorb?

Answer 5

700nm

Question 6

What wavelength of light does photosystem 2 absorb?

Answer 6

680nm

Question 7

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

Answer 7

cyclic uses PS1 , AND NOT PS2
non-cyclic uses BOTH
Cylic produces smaller amounts of ATP than non-cyclic
non-cylic produces water and NADPH as well, cyclic doesn’t
Both uses proton gradient to make ATP synthase function
photolysis of water occurs in non-cylic and not in cyclic
Both occur in thylakoid membrane

Question 8

What is chemiosmosis?

Answer 8

The process of electrons flowing down the electron transport chain creating a proton gradient across the membrane to drive ATP synthesis.