MP7 - Light-Dependent Reactions

Where does photosynthesis occurs in?

chloroplast

What does photoautotrophs means?

organism that use light energy to produce their own food

What will light energy be used for in photosynthesis?

it will be used in the form of photons to excite electrons

What are the two stages of photosynthesis?

light-dependent reactions and light independent reactions (Calvin cycle)

What are the 3 breakdown steps of photosynthesis?

1. Capturing light energy

2. Using captured energy to make ATP and reduced NADP⁺ (NADPH)

3. Using the free energy of ATP and the reducing power of NADPH to synthesize organic compounds (glucose) from CO₂

Light-dependent reactions occur in the

thylakoid lumen

Converts light energy to

chemical energy

What are the 3 parts light-dependent reaction, can be divided up into?

photoexcitation, electron transport chain and chemiosmosis

What happens during photoexcitiation?

absorption of a photon by an electron of chlorophyll

What happens during ETC?

transfer of the excited electron through a series of membrane-bound electron carriers, resulting in the pumping of a proton through the photosynthetic membrane, which creates an H+ reservoir and eventually reduces an electron acceptor

What happens during chemiosmosis?

the movement of protons through ATPase complexes to drive the phosphorylation of ADP to ATP

What are the end products of light-dependent reactions?

NADPH and ATP

What is needed for light-dependent reactions (PSII) ?

the water molecule to split

What happens when the water molecule splits in PSII?

transfers electrons to NADP^+ to form NADPH

What is released when H2O splits in PSII?

oxygen

What is stored in the proton gradient?

energy

How do plants capture energy?

through antenna complex, specifically chlorophyll a

What are all of these : caratenoids, chlorophyll b?

accessory pigments

Why do plants appear green?

since, the light is reflected off the different colours specially chlorophyll a making it a green colour appereance

What wavelengths of energy does each pigment absorb?

different wavelengths of energy

Where does the light-dependent reactions begin with?

PSII : P680

What is the 1st step of light dependent reaction?

a photon striking antenna complex and exciting the electrons in P680 making it P680*

Where do the excited electrons in P680* go to in PSII?

primary acceptor molecule (negatively charged)

What charge does P680* turn into when it goes to the primary acceptor molecule?

P680+

Where does the primary acceptor molecule transfer it’s electrons to?

PQ, an electron carrier

What happens to the P680+ molecule?

gets oxidized to water becoming neutral

Why does P680 need to become neutral?

for light to then be absorbed again

Why does the water molecule split in PSII?

to replace the electrons lost in PSII, pump H^+ ions into thylakoid lumen and allow oxygen to get released

H2O splits to become…

2H^+ + 1/2O2

How does water split in PSII?

through water splitting enzyme complex (z protein)

What happens when PQ gets the electrons in PSII?

then oxidized by cytochrome complex allow H^+ pump into thylakoid lumen as PQ moves along membrane

Where is the electron passed on from cytochrome complex to?

plastocyanin (mobile electron carrier)

Why does PSI need to absorb a photon of light again?

since, from PSII the energy absorbed was lost throughout when PQ was carrying the electron through the complex to plastocyanin

What process does PSI undergo?

the same process as PSII

What is the number for PSI?

P700

How does P700+ get reduced to neutral state?

through the transfer of electrons from plastocyanin

Where the electrons transferred in PSI when it given to primary electron acceptor?

towards (Fd) ferredoxin

Where does the electrons go after transferred to Fd?

through NADP^+ reductase

What happens in NADP^+ reductase?

NADP^+ converts to NADPH and 1 H^+ proton is used in the process from stroma

Where does NADPH go to?

Calvin Cycle

What different from cyclic ETC from linear ETC?

the photon of light only strikes PSI exciting P700 to P700* transferring electrons to primary electron acceptor making P700+ and then the electrons in the primary electron goes into Fd and back to PQ making a cycle through cytochrome complex. In the process of moving PQ releases H^+ to move into lumen building gradient.

What are the 3 mechanisms in chemiosmosis that create the concentration gradient between the thylakoid lumen and stroma?

• 1 – protons taken into the lumen by the REDOX reactions of PQ

• 2 – increase in proton molecules by the splitting of water (in PSII)

• 3 – the removal of H+ from the stroma to reduce NADPH (lowers H+ concentration in stroma)

What occurs during chemiosmosis?

H^+ flows through ATP synthase to make ATP phosphorlyated