5.6.5 The Light-Dependent stage

Question 1

What is another name for the light dependent stage of photosynthesis

Answer 1

Photophosphorylation

Question 2

Describe the overall process of using light energy and the electron transport chain to phosphorylate ADP to ATP (photophosphorylation)

Answer 2

1. Excited electrons are passed along a chain of electron carriers (electron transport chain)
2. The electron carriers are alternately reduced (as they gain an electron) and then oxidised (as they lose the electron by passing it to the next carrier)
3. The excited elecrtons gradually release their energy as they pass through the electron transport chain
4. The released energy is used to pump protons (H+) across the thylakoid membrane, from the stroma to the thylakoid lumen - proton pump.
5. This creates a proton gradient - high conc in the thylakoid lumen and low conc in the stroma
6. Protons return to the stroma, by faciltated diffusion (down the proton gradient), through transmembrane ATP synthase enzymes in a process known as chemiosmosis
7. This process provides the energy needed to synthesise ATP by adding an inorganic phosphate group (Pi) to ADP
8. The energy carried by the ATP is then used during the light-independent reactions of photosynthesis
   This whole process is photophosphorylation as light provides the initial energy source for ATP synthesis.
   This can be cyclic or non cyclic, depending on the pattern of electron flow in PSI or PSII or both
   Cyclic = PSI
   Non cyclic = PSI and PSII

Question 3

How are photosystems used during the light dependent stage

Answer 3

PSII (P680) is located at the beginning of the electron transport chain and is where the photolysis of water takes place.
PSI (P700) is in the middle of the electron transport chain.

Question 4

What photosystem is involved in cyclic photophosphorylation?

Answer 4

Photosystem I only (P700)

Question 5

Describe the process of cyclic photophosphorylation

Answer 5

1. Light is absorbed by photosystem I (thylakoid membrane) and passed to the photosystem I primary pigment (P700)
2. An electron in the primary pigment molecule (i.e. the chlorophyll molecule) is excited to a higher energy level as is emitted from the chlorophyll molecule in a process known as photoactivation
3. This excited elecrton is captured by an electron acceptor, transported via a chain of electron carriers known as an electron transport chain before being passed back to the chlorphyll molecule in photosystem I (hence cyclic)
4. As electrons pass through the electron transport chain they provide energy to transport protons (H+) from the stroma to the thylakoid lumen via a proton pump
5. A build up of protons in the thylakoid lumen can then be used to drive the synthesis of ATP from ADP and an inorganic phosphate group (Pi) by the process of chemiosmosis

Question 6

What photosystem is used in non-cyclic phosphorylation?

Answer 6

PSII (P680) and PSI (P700)

Question 7

Describe the process of non-cyclic photophosphorylation

Answer 7

1. Light is absorbed from PSII and passed to the PSII primary pigment (P680)
2. Two electrons in the primary pigment molecule (i.e. the chlorophyll molecule) are excited to a higher energy level and are emitted from the chlorophyll molecule in a process known as photoionisation
3. Each excited electron is passed down a chain of electro carriers known as an electron transport chain, before being passed onto photosystem I
4. During this process chemiosmosis occurs:
   - The energy given by the electrons moving through the electron transport chain enables H+ ions to pass from low conc (stroma) to high conc (lumen). The creation of this proton gradient later drives the synthesis of ATP in photophosphorylation
5. PSII contains a water-splitting enzyme called the oxygen-evolving complex which catalyses the breakdown (photolysis) of water by light
6. As the excited electrons leave the primary pigment of PSII and are passed onto PSI, they are replaced by electrons from the photolysis of water
7. At the same time as photoactivation of electrons in PSII, electrons in PSI also undergo photoionisation
8. The excited elecrtons from PSI also pass along an electron transport chain, alternatively reducing and oxidising proteins as they are aceppted then passed on
9. These electrons combine with H ions (produced in photolysis and transported out by ATP synthase) and the carrier molecule NADP to give reduced NADP
10. The reduced NADP then passes to the light-independent reactions to be used in the synthesis of carbohydrates
    - The electrons lost by PSI are replaced by the de-energised electrons from PSII

Question 8

How does cyclic and non cyclic photophosphorylation differ in five key ways?

Answer 8

• Cyclic = only PSI whereas non cyclic = PSI and PSII
• Cyclic = no NADP produced whereas non cyclic = NADP produced
• Cyclic = no photolysis whereas non cyclic = photolysis
• Cyclic = electron donor is P700 in PSI whereas non cyclic = electron donor is water
• Cyclic = final electron acceptor is P700 in PSI whereas non cyclic = final electron acceptor is NADP