The Light Reactions

Question 1

Ground state

Answer 1

Stage one of photoexcitation

Before light (photon) strikes the electrons of chlorophyll they are at their lowest possible potential energy level=ground state

Question 2

Stage two of photoexcitation

Answer 2

Photon hits chlorophyll and electrons gain energy and raise there potential energy to a higher level = photoexcitation

At some point electron reaches its maximum energy level, then falls back to its ground state
When this happens it emits light called fluorescence

Most of the time chlorophyll molecules do not fluorescence because they are encased in a membrane that captures and controls the excited electrons= called a primary electron acceptor

Question 3

Stage 3 photoexcitation

Answer 3

As the excited electrons return to ground state they go through a redox reaction (oxidation reduction) where the chlorophyll molecule is oxidized and the primary acceptor is reduced

Question 4

Three parts of light reactions

Answer 4

Photoexcitation- photons are absorbed by an electron of chlorophyll

Electron transport- membrane bound electron carries transfers the excites electron
-results in the pumping of a proton (H+) through the thylakoid membrane creating an electrochemical gradient

Chemiosmosis- the protons move through ATP complexes driving the phosphorylation of ADP to ATP

Question 5

Photosystem

Answer 5

Clusters of chlorophyll and accessory pigments
Made of
-antenna complex= web of chlorophyll molecules in thylakoid membrane
-reaction centre= transmembrane protein complex with chlorophyll a, whose electrons absorb light energy and start PS

Question 6

Thylakoid membrane have two photosystems

Answer 6

Photosystem 1- with chlorophyll (a) at centre which is called p700 (absorbs red light 700nm)

Photosystem 2- with chlorophyll (a) center which is called p680 (absorbs red light 680nm)

Question 7

Step 1 (a) in electron transport and Chemiosmosis  
(Photosynthesis)

Answer 7

• Light strikes photosystems 2
• the excited electron of p680 is captured by the primary electron acceptor is rapidly oxidized
• as a result of the oxidation, a high energy electron is transferred to the primary electron acceptor

Question 8

Step 1(b) electron transport and Chemiosmosis (photosynthesis)

Answer 8

• The electron is transferred through the electron transport chain (this happens twice with 2 e-s )
• the water splitting complex splits H2O releasing 2H+ ions and 2O2
• oxygen leaves as waste product
• the remaining H+ ions will form a concentration gradient and power Chemiosmosis (ATP synthesis)

Question 9

Step 2 electron transport and Chemiosmosis (photosynthesis)

Answer 9

• from the primary acceptor, the electrons transfer to PQ
• PQ moves through the bilayer (cell membrane) and shuttles e- between photosystems II and cytochrome complex
• PQ also gains protons H+ from the stroma and transports them to the thylakoid lumen, therefore increase concentration gradient

Question 10

Step 3 electron transport and Chemiosmosis (photosynthesis)

Answer 10

-from the cytochrome complex, electrons pass to a mobile carrier, PC, which carriers electrons from cytochrome complex to photosystems I

Question 11

Step 4 electron transport and Chemiosmosis (photosynthesis)

Answer 11

• The e-s from photosystem I pass through another electron transport chain containing iron-containing protein ferredoxin
• uses 2 e- to cause H+ ions to reduce NADP+ to NADPH
• NADPH goes to Calvin cycle
• H+ not pumped into thylakoid lumen here

Question 12

Step 5 electron transport and Chemiosmosis (photosynthesis)

Answer 12

• H+ that remains in the thylakoid contribute to the electrochemical gradient which drives the photophosporylation of ADP to ATP
• phosphorylation done by ATP synthase