Light dependent stage

Question 1

where does the light dependent stage take place and what does it generally involve

Answer 1

-in thylakoid (grana)
-light harvesting at photosystems, photophosphorylation to produce ATP, photolysis of water and formation of reduced NADP

Question 2

what are two types of photosystems in grana

Answer 2

-Photosystem 1 (PS1)/ P700
-PSII / P680

Question 3

what is a photosystem

Answer 3

protein complex embedded in thylakoid membranes with numerous different photosynthetic pigments

-contains chlorophyll a at the base with accessory pigments surrounding e.g chlorophyll b, carotenes, xanthophylls

Question 4

explain why it is an advantage to the plant to have several different pigments in its chloroplasts

Answer 4

so it can absorb different wavelengths, maximising the amount of light it absorbs

Question 5

explain why chlorophyll appears green

Answer 5

can’t absorb green light= reflects it

Question 6

what are the two options for the light dependent reaction

Answer 6

-cyclic= involves only PS1 and produces small amounts of ATP
-non-cyclic involves PS1 and PSII produces ATP and reduced NADP

Question 7

explain the steps of cyclic photophosphorylation

Answer 7

1. light is absorbed by PSI (P700) causing the chlorophyll A molecule to emit a pair of electrons
2. the energised electrons are picked up by an electron acceptor
3. then the electrons are passed along the electron carrier chain releasing energy to make ATP from ADP and Pi by Chemiosmosis through ATPsynthase
   4.electrons are returned to PS1

Question 8

explain non-cyclic photophosphorylation

Answer 8

at photosystem 2:
-light absorbed by PSII passed into chlorophyll A (P680) which emits two electrons
-the energised electrons are raised to a higher energy level and are picked up by an electron carrier in the thylakoid membrane
-electron is then passed along the electron carrier chain to PSI = releases enough energy to make ATP from ADP and Pi by chemiosmosis

at photosystem 1 (simultaneously)
-energised electrons are raised to a higher energy level and are picked up by a second electron acceptor called ferredoxin
-since both chlorophylls (P680 and P700) have lost electrons they will be positive and unstable
-the two electrons released by chlorophyll a (P6800) of PSII replace the two lot by chlorophyll A (P700) of PS 1
-whereas two released by chlorophyll A of PSII receive replacement electrons from the splitting of water (photolysis)

Question 9

explain photolysis during non-cyclic photophosphorylation

Answer 9

-during photolysis the water molecule dissociates into electrons, hydrogen ions and oxygen
-the electrons go to PSII and the oxygen is lost as waste gas
-the hydrogen ions and the electrons held by the ferredoxin are accepted by NADP to form reduced NADP catalysed by NADP reductase

Question 10

what do the absorption and action spectrum graphs show

Answer 10

absorption-how much light a pigment absorbs at each wavelength
-action= shows the rate of photosynthesis at different wavelengths