5.6.3: The light-dependent stage

Question 1

Where does the light-dependent stage of photosynthesis occur?

Answer 1

In the grana (thylakoids) of chloroplasts and involves photosystems.

Question 2

What are the 4 steps of the light-dependent stage?

Answer 2

1. Light harvesting at the photosystems
2. Photolysis of water
3. Photophosphorylation-the production of ATP in the presence of light.
4. The formation of reduced NADP

Question 3

What is the byproduct of photosynthesis?

Answer 3

Oxygen, which is produced in the light-dependent stage.

Question 4

Describe the differences between photosystem I and photosystem II.

Answer 4

• In PSI, the pigment in the primary reaction centre is a type of chlorophyll a which has a peak absorption of red light of wavelength 700 nm (P700).
• In PSII, the pigment in the primary reaction centre is also a type of chlorophyll a but this has a peak absorption of red light of wavelength 680 nm (P680).

Question 5

How does photolysis occur in PSII and write the balanced symbol equation?

Answer 5

• There is an enzyme in PSII which, in the presence of light, splits water molecules into protons, electrons and oxygen.
• This is called photolysis.
• 2H2O –> 4H+ +4e- + O2

Question 6

What happens to the oxygen produced in the light-dependent stage of photosynthesis?

Answer 6

• Some is used by the plant cells for aerobic respiration.
• BUT, during periods of high light intensity, the rate of photosynthesis is greater than the rate of respiration, so much of the oxygen by-product will diffuse out of the leaves, through the stomata into the atmosphere.

Question 7

What is the role of water in the light-dependent stage of photosynthesis?

Answer 7

• Source of protons that will be used in phosphorylation
• Donates electrons to chlorophyll to replace those lost when light strikes chlorophyll.
• Source of the by-product, oxygen
• Keeps plant cells turgid, enabling them to function

Question 8

What is photophosphorylation?

Answer 8

The generation of ATP from ADP and inorganic phosphate in the presence of light.

Question 9

What are the two types of photophosphorylation?

Answer 9

• Non-cyclic phosphorylation involving PSI and PSII. It produces ATP, oxygen and reduced NADP.
• Cyclic phosphorylation involving only PSI. It produces ATP but in smaller quantities than are made by non-cyclic phosphorylation.

Question 10

What are the similarities of non-cyclic and cyclic photophosphorylation?

Answer 10

-Both involve iron-containing proteins embedded in the thylakoid membrane that accept and donate electrons and form an electron transport system.

Question 11

There are twelve steps in non-cyclic photophosphorylation:

Step 1:

Answer 11

1. Photon strikes PSII (P680), its energy is channelled to the primary pigment reaction centre.

Question 12

There are twelve steps in non-cyclic photophosphorylation:
Step 1: Photon strikes PSII (P680), its energy is channelled to the primary pigment reaction centre

Step 2:

Answer 12

2. The light energy excites a pair of electrons inside the chlorophyll molecule.

Question 13

There are twelve steps in non-cyclic photophosphorylation:
Step 2: The light energy excites a pair of electrons inside the chlorophyll molecule.

Step 3:

Answer 13

3. The energised electrons escape from the chlorophyll molecule and are captured by an electron carrier, which is a protein with iron at its centre, embedded in the thylakoid membrane.

Question 14

There are twelve steps in non-cyclic photophosphorylation:
Step 3: The energised electrons escape from the chlorophyll molecule and are captured by an electron carrier, which is a protein with iron at its centre, embedded in the thylakoid membrane

Step 4:

Answer 14

4. These electrons are replaced by the products of photolysis.

Question 15

There are twelve steps in non-cyclic phosphorylation:
Step 4: These electrons are replaced by the products of photolysis.

Step 5:

Answer 15

5. When this iron ion combines with an electron it becomes reduced (Fe2+). It can then donate the electron, becoming reoxidised (Fe3+), to the next electron carrier in the chain.

Question 16

There are twelve steps in non-cyclic photophosphorylation:
Step 5: When this iron ion combines with an electron it becomes reduced (Fe2+). It can then donate the electron, becoming reoxidised (Fe3+), to the next electron carrier in the chain.

Step 6:

Answer 16

6. As electrons are passed along a chain of electron carriers embedded in the thylakoid membrane, at each step some energy associated with the electrons is released.

Question 17

There are twelve steps in non-cyclic photophosphorylation:
Step 6: As electrons are passed along a chain of electron carriers embedded in the thylakoid membrane, at each step some energy associated with the electrons is released.

Step 7:

Answer 17

7. This energy is used to pump protons across the thylakoid membrane.

Question 18

There are twelve steps in non-cyclic photophosphorylation:
Step 7: This energy is used to pump protons across the thylakoid membrane.

Step 8:

Answer 18

8. Eventually the electrons are captured by another molecule of chlorophyll a in PSI. These electrons replace the electrons lost from PSI due to excitation by light energy.

Question 19

There are twelve steps in non-cyclic photophosphorylation:
Step 8: Eventually the electrons are captured by another molecule of chlorophyll a in PSI. These electrons replace the electrons lost from PSI due to excitation by light energy.

Step 9:

Answer 19

9. A protein-iron-sulfur complex called ferredoxin accepts the electrons from PSI and passes them to NADP in the stroma.

Question 20

There are twelve steps in non-cyclic photophosphorylation:
Step 9: A protein-iron-sulfur complex called ferredoxin accepts the electrons from PSI and passes them to NADP in the stroma.

Step 10:

Answer 20

10. As protons accumulate in the thlakoid space, a proton gradient is formed across the membrane.

Question 21

There are twelve steps in non-cyclic photophosphorylation:
Step 10: As protons accumulate in the thlakoid space, a proton gradient is formed across the membrane.

Step 11:

Answer 21

11. Protons diffuse down their concentration gradient through special channels in the membrane associated with ATP synthase enzymes and , as they do so, the flow of protons causes ADP and inorganic phosphate join, forming ATP.

Question 22

There are twelve steps in non-cyclic photophosphorylation:
Step 11: Protons diffuse down their concentration gradient through special channels in the membrane associated with ATP synthase enzymes and , as they do so, the flow of protons causes ADP and inorganic phosphate join, forming ATP.

Step 12:

Answer 22

12. As the protons pass through the channel they are accepted along with the electrons , by NADP which becomes reduced. This reducytion of NADP is catalysed by NADP reductase.

Question 23

What is the result of the light-dependent stage of photosynthesis?

Answer 23

• Light energy has been converted into chemical energy in the form of ATP by photophosphorylation.
• ATP and reduced NADP are now in the stroma ready for the light-independent stage of photosynthesis.

Question 24

Describe cyclic photophosphorylation.

Answer 24

• Uses only PSI (P700)
• As light strikes PSI, a pair of electrons in the chlorophyll molecule at the reaction centre gain energy and become excited.
• They escape from the chlorophyll and pass to an electron carrier system and then pass back to PSI.
• During the passage of electrons along the electron carriers, a small amount of ATP is generated.

Question 25

What is the main difference between cyclic and non-cyclic photophosphorylation?

Answer 25

• No photolysis of water occurs, so no protons pr oxygen atoms are produced.
• No reduced NADP is generated.

Question 26

Chloroplasts in guard cells only contain PSI explain how this is useful for their function.

Answer 26

• They produce only ATP which actively brings potassium ions into the cells, lowering the water potential so that water flows by osmosis.
• This causes the guard cells to swell and open the stomata.