Photosynthesis

Question 1

Describe the structure of a photosystem

Answer 1

• Antennae complexes made of accessory pigments: absorb some wavelengths, light energy excites electrons which allows them to be transferred to the reaction centre.
• Reaction centre contains chlorophyll A.

Question 2

What are the accessory pigments?

Answer 2

• Xanthophylls.
• Chlorophyll B
• Carotenoids

Question 3

Where are photosystems found?

Answer 3

• In the thylakoid membrane.

Question 4

What are grana?

Answer 4

• Stacks of thylakoids.

Question 5

What is photolysis?

Answer 5

• H2O split into 1/2 O2 + 2H+ + 2e-.
• Occurs in the grana.
• The H+ ions are pumped into the thylakoid lumen, move by chemiosmosis back into the stroma.

Question 6

What is the stroma?

Answer 6

• The chloroplast’s equivalent of a cytoplasm.

Question 7

Describe cyclic photophosphorylation.

Answer 7

• Cyclic photophosphorylation involves photosystem 1 (PSI) only.
• Light is absorbed by photosystem 1 (located in the thylakoid membrane) and passed to the photosystem I primary pigment (P700).
• An electron in the chlorophyll A molecule is excited to a higher energy level and is emitted from the chlorophyll molecule (photoactivation).
• This excited electron 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 chlorophyll molecule in photosystem 1.
• Electrons provide energy to transport protons (H+) from the stroma to the thylakoid lumen via a proton pump.
• 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 through ATP synthase.
• No NADPH produced.

Question 8

Describe non-cyclic photophosphorylation

Answer 8

• 2 electrons excited in PSII’s reaction centre (photoionisation).
• Emitted and passed along electron transport chain, energy used by proton pumps; chemiosmosis occurs.
• Replaced by electrons from photolysis.
• Excited electrons released from the reaction centre at PSI, chemiosmosis occurs again.
• Electrons eventually combined with H+ ions and NADP to form reduced NADP/NADPH.
• 2 e- + 2H+ + NADP.
• Products are oxygen, NADPH and ATP.

Question 9

Where does the light independent stage occur?

Answer 9

Stroma

Question 10

Describe the light-independent stage.

Answer 10

• CO2 diffuses into intercellular spaces via stomata.
• CO2 combines with RuBP to form an unstable 6-carbon molecule. This reaction is catalysed by RuBisCO.
• The 6C splits into two 3C GPs, ATP and NADPH react with this to form 2 TPs.
• 5/6 TPs go to reforming RuBP (1 ATP required per TP), 1/6 goes to make organic products.