Light Dependent Stage Flashcards
1
Q
Where does it occur?
A
Thylakoids
2
Q
What is a summary of what happens in the light dependent stage?
A
- Photolysis occurs (H+, e- and oxygen produced)
- High concentration of H+ in thylakoid lumen causes proton gradient
- Electrons travel through electron transport chain in thylakoid membrane
- NADP combine with H+ from stroma and e- in electron transport chain to form NADPH
- ATP produced during photophosphorylation
3
Q
What happens during photophosphorylation?
A
- Excited electrons passed along ELTC
- Electron carriers are alternately reduced and oxidised as electrons are passed from carrier to carrier
- Excited electrons release energy as they are passed through ELTC
- Energy released used to actively transport H+ across thylakoid membrane from stroma to thylakoid lumen via a proton pump
- Proton gradient created, high conc of H+ in thylakoid lumen, low conc in stroma
- H+ diffuses by facilitated diffusion across TRANSMEMBRANE ATP SYNTHASE enzymes back into the stroma
- Releases energy for the synthesis of ATP from ADP + Pi
- Can be cyclic or non-cyclic
- Cyclic involves PSI, Non-Cyclic involves PSI and PSII
4
Q
What are photosystems?
A
- PSI and PSII
- Collection of pigments that absorb light energy and transfer energy to electrons
- Different pigment molecules are arranged in funnel like structures in thylakoid membrane
- Each pigment molecule passes energy down to the next pigment molecule in the cluster until it reaches the primary pigment reaction centre
5
Q
What is PSI?
A
- Contains primary pigment P700
- Absorbs light of wavelength 700nm
- In the middle of the electron transport chain
6
Q
What is PSII?
A
- Contains primary pigment P680
- Absorbs light at a wavelength of 680nm
- At beginning of ELTC
- Where photolysis takes place
7
Q
What is cyclic phosphorylation?
A
- Light absorbed at PSI and passed to P700 (primary pigment of PSI)
- Electron in primary pigment molecule becomes excited to a higher energy level
- Photoactivation occurs
- Excited electron released from chlorophyll molecule and accepted by an electron acceptor
- Electron acceptor transports electrons to ELTC
- As electrons pass through ELTC, they provide energy, chemiosmosis occurs, releasing energy for the synthesis of ATP
- Electrons then transported BACK TO PSI
8
Q
What is non-cyclic phosphorylation?
A
- Light absorbed in PSII, passed to P680
- 2 electrons in primary pigment molecule excited to higher energy level
- Emitted from chlorophyll in process called photoionisation
- Each excited electron passed down ELTC
- Chemiosmosis occurs
- THEN ELECTRONS PASSED TO PSI
- PSII contains water-splitting enzyme called oxygen-evolving complex
- Catalyses photolysis
- H2O → 2H+ + 2e- + ½O2
- Excited electrons leave PSII to go to PS1
- They are replaced by electrons from photolysis of water
- So as photoactivation of electrons in PSII, photoionisation of electrons in PSI occurs
- Excited electrons from PSI pass along an ELTC
- These electrons combine with H+ produced by photolysis of water, and carrier molecule NADP to form NADPH
- 2H+ + 2e- + NADP → reduced NADP
- NADPH passes to light-independent reaction to be used in synthesis of carbohydrates