The Light Dependent Stage

Question 1

Where does the light dependent stage occur?

Answer 1

In the grana (thylakoids) of chloroplasts and involves the photosystem

Question 2

What is photosystem 1 ?

Answer 2

Has the primary pigment of chlorophyll a which has a peak absorption of red light of wavelength

Question 3

what is the role of water?

Answer 3

• source of protons/hydrogen ions that will be used in photophosphorylation
• donates electrons to chlorophyll to replace when chlorophyll is struck by light
• keeps plant cells turgid

Question 4

Define photophosphorylation

Answer 4

the making of ATP from ADP and Pi in the presence of light

Question 5

what is non-cyclic photophosphorylation?

Answer 5

Producing: ATP from ADP, oxygen and reduced NADP. Involving PSI and PSII.

Question 6

what is cyclic photophosphorylation?

Answer 6

Only involves PSI and produces ATP just in smaller amounts.

Question 7

explain the first stage of non-cyclic photophosphorylation

Answer 7

• photosystem II is hit by a photon of light, chlorophyll a in the primary pigment centre is hit exciting the electrons in the Mg atom.
• the electrons are excited so are released and are transported by electron carriers (proteins which contain iron which are embedded in the thylakoid membranes
• energy is released as electrons pass along the chain of electron carriers
• the photolysis of water replaces the lost electrons: 2 H+ ions are produced and 2 electrons are produced

Question 8

explain the second stage of non-cyclic photophosphorylation

Answer 8

• electrons are carried from PSII to PSI
• the energy from this is used to pump protons across the thylakoid membrane into the thylakoid membrane where they accumulate
• a proton gradient is formed so that protons will diffuse into the stroma
• the protons flow through the channels with ATP synthase (chemiosmosis)
• the protons catalyze the conversion of ADP+Pi to ATP as the proton flow creates a force

Question 9

explain the third stage of non-cyclic photophosphorylation

Answer 9

• PSI also absorbs a photon of light which hits the chlorophyll a pigment releasing two electrons from the Mg atom
• electrons from PS II replace these electrons
• these electrons join with the protons to reduce NADP to NADPH.

Question 10

explain the process of cyclic photophosphorylation

Answer 10

• only uses PSI
• light hits PSI exciting the electrons in the Mg atom in the chlorophyll a in the primary pigment centre
• they are released and pass to an electron carrier system and then back to PSI
• during the passage of electrons along the electron carriers a small amount of ATP is generated
• but there are no protons for the electrons to join with to create NADPH so electrons are recycled