Photosynthesis Processes

Question 1

Light dependent reaction

Answer 1

The light dependent reaction takes place at the thylakoid membranes and requires water and light and produces ATP, O2 and NADPH. Photolysis is the process of light energy breaking down water and occurs within the thylakoid lumen (products are 2H+ 2e- and 1/2 O2). Photosystem ll embedded in the thylakoid membrane is where it begins : the accessory pigments in the light harvesting system absorbs photons and pass energy to the reaction centre. The electrons present at the reaction centre are exited and move to a higher energy level. These electrons are then released from PSll and move along the electron transport chain via electron carrier proteins (which are reduced and oxidised as electrons pass over them). When the electrons leave PSll they are replaced by the electrons from water. Then when the electrons arrive at PSl the same process happens and the electrons become excited and leave PSl.
Shortly afterwards the electrons are released where they combine with 2H+, and NADP+ to form NADPH.

Question 2

Chemiosmosis

Answer 2

The movement of electrons down the electron transport chain generates energy which can be used by the proton pump to actively transport H+ ions into the lumen. This creates a concentration gradient (high in lumen low in stroma) so H+ ions move by facillitated diffusion through ATP Synthase. The movement back through ATP synthase causes ADP to be phosphorylated so ATP is produced. This is called photophosphorylation (ADP phosphorylated using energy from light)

Question 3

Cyclic photophosphorylation vs non-cyclic photophosphorylation

Answer 3

At cyclic only PSl is involved. PSl absorbs photons and releases an electron which rejoins the etc from before the PSl and moves round in a loop. Therefore no NADPH produced as electrons not released, no photolysis so no O2 produced. ATP is still produced via chemiosmosis.