Photosynthesis Part 2 (LDR) - (Week 3)

Question 1

Name the 5 steps of the light reaction

Answer 1

1. Resonance energy transfer
2. Charge separation
3. Electron transfer
4. Photolysis
5. Electron transport chain (ETC)

Question 2

What happens in the resonance energy transfer stage of the light reaction?

Answer 2

• When light hits the complex, electrons are excited within the pigments & energy is transferred from electron to electron (& pigment to pigment)
• Energy arrives at a specific pair of chlorophyll molecules (in the reaction centre), causing charge separation

Question 3

What happens at the charge separation stage of the light reaction?

Answer 3

Charge separation is when an excited (high energy) electron in the special pair (of chlorophyll molecules) is physically transferred to a neighbouring electron acceptor (pheophytin)

Question 4

Where does the electron travel in the electron transfer stage of the light reaction?

Answer 4

The electron is transferred from electron acceptor (pheophytin) -> plastoquinone q a -> plastoquinone q b -> the next electron acceptor - this all happens in the ETC

Question 5

What happens in the photolysis stage of the light reaction?

Answer 5

A water molecule is split & the electron produced replaces the missing one in the special pair (of chlorophyll molecules) for future reactions

Question 6

What happens as a result of photolysis in the ETC?

Answer 6

Causes a build up of H+

Question 7

How is H+ removed?

Answer 7

As the electron travels down the ETC it drives the transport of H+ through the membrane into the stroma, creating a proton gradient

Question 8

What happens as a result of a proton gradient being created?

Answer 8

It powers ATP synthase, so ATP is produced

Question 9

How is NADPH produced in the ETC stage of the light reaction?

Answer 9

• When the electron arrives at plastocyanin, it acts as an electron donor to PSI
• Energy is low at plastocyanin as it has been used ip for transport of H+
• Electrons are excited again in PSI
• This energy is used to produce NADPH (as well as H+ in stroma)

Question 10

Briefly describe where the electron travels along the ETC

Answer 10

PSI (pheophytin) -> PQ (plastoquinone q) a&b -> Cytochrome b6f -> plastocyanin -> PSI

Question 11

In photolysis how many water molecules are split? How many electrons & H+ are produced?

Answer 11

2H20 -> 4H^+ + O2 + 4e^-

Question 12

Describe cyclic phosphorylation

Answer 12

• When PSI is switched to cyclic mode
• After going to PSI electron goes back to PQ instead
• Same electron goes round & electron
• This does NOT involve PSII

Question 13

Give the benefits of cyclic phosphorylation

Answer 13

• Energy powers more H+ transport (creating proton gradient)
• So more ATP is produced but no NADPH is made
• Electron is ‘recycled’ back to ETC as it goes round so no need for photolysis

Question 14

What happens to the products of the light reaction?

Answer 14

The products go into the dark reaction as they need to be converted so they can be readily used (as they can’t diffuse through the membrane)