5.6.3: The light-dependent stage Flashcards
Where does the light-dependent stage of photosynthesis occur?
In the grana (thylakoids) of chloroplasts and involves photosystems.
What are the 4 steps of the light-dependent stage?
- Light harvesting at the photosystems
- Photolysis of water
- Photophosphorylation-the production of ATP in the presence of light.
- The formation of reduced NADP
What is the byproduct of photosynthesis?
Oxygen, which is produced in the light-dependent stage.
Describe the differences between photosystem I and photosystem II.
- 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).
How does photolysis occur in PSII and write the balanced symbol equation?
- 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
What happens to the oxygen produced in the light-dependent stage of photosynthesis?
- 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.
What is the role of water in the light-dependent stage of photosynthesis?
- 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
What is photophosphorylation?
The generation of ATP from ADP and inorganic phosphate in the presence of light.
What are the two types of photophosphorylation?
- 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.
What are the similarities of non-cyclic and cyclic photophosphorylation?
-Both involve iron-containing proteins embedded in the thylakoid membrane that accept and donate electrons and form an electron transport system.
There are twelve steps in non-cyclic photophosphorylation:
Step 1:
- Photon strikes PSII (P680), its energy is channelled to the primary pigment reaction centre.
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:
- The light energy excites a pair of electrons inside the chlorophyll molecule.
There are twelve steps in non-cyclic photophosphorylation:
Step 2: The light energy excites a pair of electrons inside the chlorophyll molecule.
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.
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:
- These electrons are replaced by the products of photolysis.
There are twelve steps in non-cyclic phosphorylation:
Step 4: These electrons are replaced by the products of photolysis.
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.
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:
- 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.
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:
- This energy is used to pump protons across the thylakoid membrane.
There are twelve steps in non-cyclic photophosphorylation:
Step 7: This energy is used to pump protons across the thylakoid membrane.
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.
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:
- A protein-iron-sulfur complex called ferredoxin accepts the electrons from PSI and passes them to NADP in the stroma.
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:
- As protons accumulate in the thlakoid space, a proton gradient is formed across the membrane.
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:
- 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.
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:
- 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.
What is the result of the light-dependent stage of photosynthesis?
- 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.
Describe cyclic photophosphorylation.
- 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.
What is the main difference between cyclic and non-cyclic photophosphorylation?
- No photolysis of water occurs, so no protons pr oxygen atoms are produced.
- No reduced NADP is generated.
Chloroplasts in guard cells only contain PSI explain how this is useful for their function.
- 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.
