3.5 Contd: Photosynthesis (light Reaction)

Question 1

Chemical energy is in which two forms?

Answer 1

• NADPH (reduced state(NADP+ would be oxidized state if ever))
• ATP

Question 2

How is the cell able to accomplish the conversion of light energy to chemical energy? I think this is what it is asking

Answer 2

• cell accomplishes this conversion by using light energy (photons) to excite electrons

Question 3

Why is light important to chlorophyll? (7)

Answer 3

1. Chlorophyll absorbs a photon of light
2. Electron is boosted from ground state to an excited state
3. Electron is unstable
4. Falls back to ground state
5. Releases energy as heat
6. Emits photons as fluorescence

Question 4

What are photosystems?

Answer 4

• reaction center and light capturing complexes

Question 5

What is the reaction center of photosystems?

Answer 5

• a complex of proteins associated with chlorophyll a and an electron acceptor

Question 6

What are the light capturing complexes of photosystems?

Answer 6

• pigments associated with proteins

Think: antenna for the reaction centers

Question 7

In the thylakoid membrane, there are two photosystems. These photosystems are named in order of their discovery. What is photosystem 2?

Answer 7

• reaction center P680
• absorbs light at 680 nm
  Come before photosystem 1

Question 8

What is photosystem 1? (2)

Answer 8

• reaction center P700

- absorbs light at 700 nm

Question 9

What occurs in photosystem 2? This is long, see page 35 for diagrams

Answer 9

1. Light energy (photons) causes an e to go from excited back to ground. Repeats until it reaches P680 pair of chlorphyll a molecules
2. The e is transferred to a primary electron acceptor, forming P680+
3. Water is split into: 2e that reduce P680+, 2H+ that is released into thylakoid space, 1 oxygen atom which immediately binds to another oxygen to form O2 gas
   - water replenishes e which are miving down the ETC
4. Linear e flow: each excited e will pass from PSII to PSI via the ETC

Question 10

What is the supplier of O2 for aerobic organisms?

Answer 10

• water

Question 11

Where does the energy to form ATP come from?

Answer 11

The “fall” of e from PSII to PSI

Question 12

The H+ gradient is a form of what energy?

Answer 12

Potential

Question 13

ATP synthase does what in the light reaction to form ATP?

Answer 13

• couples the diffusion of H+ to the formation of ATP

Question 14

What happens inside photosystem 1? (3)

Answer 14

1. light energy excites electrons in the P700 chlorophyll molecules
   - becomes P700+
2. Electrons go down a second transport chain
3. NADP+ reductase catalyzes the transfer of electroms from Fd to NADP+

Question 15

Photosystem 1 abosrbs what from PSII?

Answer 15

Electrons

Question 16

Why is the synthesis of NADPH in PSI essential?

Answer 16

• for thr purpose of transferring H from water during the light rxn to CO2 in the Calvin Cycle to make glucose, for example

Question 17

What are the inputs of the light reaction? (3)

Answer 17

• H2O
• ADP
• NADP+

Question 18

What are the outputs of the light reaction?

Answer 18

• O2
• ATP
• NADPH

Question 19

What is the summary of occurences in the light reaction?

Answer 19

1. Converts solar energy into chemical energy (2 forms: NADPH AND ATP)
2. Water is split to rpofide a source of electrons and protons (H+) and releases O2 as a byproduct
3. Light absorbed by chlorophyll drives the transfer of electrons and H+ from H2O to e acceptor NADP+ (final e acceptor at end of ETC)
4. NADP+ is reduced to NADPH
5. Generates ATP by phosphorylating ADP