Energy Transfers in and Between Organisms: Photosynthesis - Light-Dependent Reactions

Question 1

Where does the light-dependent reaction of photosynthesis occur?

Answer 1

• In thylakoid membrane

Question 2

Give a brief overview of the light-dependent reactions in photosynthesis

Answer 2

• Chlorophyll and other photosynthetic pigments absorbs light energy in the photosystems
• Light energy excites the electrons, so they gain energy
• The electrons are released from the chlorophyll molecule, process called photoionisation

Question 3

How is the energy released from the electrons during photoionisation used?

Answer 3

• Photophosphorylation - add a phosphate group to ADP to form ATP using light
• Reduce NADP to form NADPH
• Photolysis of water into protons, electrons and oxygen

Question 4

What are the two types of photophosphorylation that occur in the light-dependent reaction?

Answer 4

• Non-cyclic

- Cyclic

Question 5

What are the main stages of non-cyclic photophosphorylation?

Answer 5

• Photoionisation of chlorophyll
• Photolysis of water
• Photophosphorylation
• Reduction of NADP

Question 6

What happens during photoionisation in non-cyclic photophosphorylation?

Answer 6

• PSII absorbs light energy
• Light energy excites electrons
• Electrons gain energy
• Electrons are released from chlorophyll
• Electrons move down electron transport chain, using electron carriers, to PSI

Question 7

What happens during the photolysis of water in non-cyclic photophosphorylation?

Answer 7

• Photolysis - breakdown of water molecule using light
• Electrons that left chlorophyll need to be replaced
• Light energy splits water into protons (H+ ions), electrons and oxygen
• H2O → 2H+ + 1/2O2

Question 8

What happens during photophosphorylation in non-cyclic photophosphorylation?

Answer 8

• Photophosphorylation - adding phosphate to a molecule using light

• Chemiosmosis

• Excited electrons lose energy while moving down electron transport chain
• Energy is used to actively transport protons into thylakoid
• Thylakoid has higher concentration of protons than stroma
• Protons diffuse down a concentration gradient, into stroma via ATP synthase (embedded in thylakoid membrane)
• Energy from this movement is used to react ADP and Pi to form ATP

Question 9

What happens during generation of reduced NADP in non-cyclic photophosphorylation?

Answer 9

• PSI absorbs light energy
• Electrons are excited again, but to a higher energy level
• Electrons are transferred to NADP with a proton (H+ ion) from the stroma to produce reduced NADP (NADPH)

Question 10

Describe the whole process of non-cyclic photophosphorylation

Answer 10

• Photoionisation of chlorophyll

• PSII absorbs light energy
• Light energy excites electrons
• Electrons gain energy
• Electrons are released from chlorophyll
• Electrons move down electron transport chain, using electron carriers, to PSI

• Photolysis of water

• Photolysis - breakdown of water molecule using light
• Electrons that left chlorophyll need to be replaced
• Light energy splits water into protons (H+ ions), electrons and oxygen
• H2O → 2H+ + 1/2O2

• Photophosphorylation and chemiosmosis

• Photophosphorylation - adding phosphate to a molecule using light
• Excited electrons lose energy while moving down electron transport chain
• Energy is used to actively transport protons into thylakoid
• Thylakoid has higher concentration of protons than stroma
• Protons diffuse down a concentration gradient, into stroma via ATP synthase (embedded in thylakoid membrane)
• Energy from this movement is used to react ADP and Pi to form ATP

• Reduction of NADP

• PSI absorbs light energy
• Electrons are excited again, but to a higher energy level
• Electrons are transferred to NADP with a proton (H+ ion) from the stroma to produce reduced NADP (NADPH)

Question 11

What does chemiosmosis / chemiosmotic theory mean?

Answer 11

• Movement of ions down their concentration gradient through a partially permeable membrane
• Or osmosis of ions

Question 12

How does chemiosmosis happen in chloroplasts?

Answer 12

• In chloroplasts, it involves the passive movement of protons from thylakoid to stroma via ATP synthase through the membrane
• Occurs due to movement of electrons down electron transport chain
• This releases energy which is used to actively transport protons from stroma into thylakoid
• Thylakoid has higher proton concentration than stroma

Question 13

What occurs during cyclic photophosphorylation?

Answer 13

• When light is absorbed, the electrons become excited
• Electrons are transferred from chlorophyll to electron carriers and back to PSI
• Movement releases energy for active transport of protons into thylakoid
• Protons undergo facilitated diffusion and leave thylakoid through ATP synthase
• Energy used to produce small amounts of ATP