[11.2] the light-dependent reaction

Question 1

what is the purpose of the light energy captured in the LDR?

Answer 1

• to add Pi to ADP, making ATP
• to split water into H+ ions (protons) and OH- ions

Question 2

what is photolysis?

Answer 2

splitting due to light

Question 3

where does the LDR take place?

Answer 3

membrane of thylakoid

Question 4

describe the process of photoionisation

Answer 4

1. chlorophyll molecule absorbs light energy
2. energy of pair of electrons in this chlorophyll is boosted, raising them to a higher level (excited state)
3. electrons become so energetic that they leave chlorophyll molecule altogether
4. chlorophyll molecule becomes ionised

Question 5

what is oxidised and reduced after photoionisation?

Answer 5

• chlorophyll molecule is oxidised as it lost a pair of electrons
• electron carrier is reduced as it has gained electrons

Question 6

what happens after the electrons leave the chlorophyll molecule?

Answer 6

1. electrons are taken up by an electron carrier
2. electrons are passed along a number of electron carriers in a series of redox reactions
3. electron carriers form a transfer chain

Question 7

how do electrons lose their energy at each stage?

Answer 7

each new carrier is at a slightly lower energy level than the previous one in the chain

Question 8

what is some of the energy lost from electrons used for?

Answer 8

to combine a Pi with ADP to make ATP

Question 9

describe chemiosmotic theory

Answer 9

• each thylakoid is an enclosed chamber into which protons (H+) are pumped from stroma using protein pumps in TM
• energy for this process is from electrons, released in photolysis
• photolysis of water also produces protons, further increasing their conc in the TS
• overall, this creates and maintains a conc grad of protons across the TM. high conc inside TS, low conc in stroma
• protons can only cross TM through ATP synthase channel proteins
• as the protons pass through ATP synthase channels, they cause changes to the structure of the enzyme
• this catalyses the combination of ADP with Pi to form ATP

Question 10

describe the photolysis of water

Answer 10

• loss of electrosn when light strikes a chlorophyll molecules leaves it short of electrons
• electrons must be replaced if the chlorophyll molecule is to continue absorbing light energy
• replacement electrons are provided from water molecules that are split using light energy
• this photolysis of water also yields protons

Question 11

what is the equation for the photolysis of water?

Answer 11

2H₂O ➞ 4H⁺ + 4e⁻ + O₂

Question 12

what happens to the protons released from photolysis?

Answer 12

• pass out of TS through ATP synthase channels
• taken up by an electron carrier NADP
• when NADP takes up protons it becomes reduced

Question 13

what happens to NADP?

Answer 13

• main product of LDS
• enters LIDR, taking electrons from chlorophyll molecules with it

Question 14

why is reduced NADP important?

Answer 14

it is a further potential source of chemical energy to the plant

Question 15

what happens to the oxgen produced during photolysis?

Answer 15

• used in respiration
• diffuses out of the leaf as a waste product of photosynthesis

Question 16

how are chloroplasts structurally adapted to capture sunlight and carry out LDR?

Answer 16

• TM provides large SA for the attachment of chlorophyll, electron carriers and enzymes that carry out the LDR
• network of proteins in grana hold the C in a very precise manner that allows maximum absorption of light
• granal membranes that ATP synthase channels within them, which catalyse production of ATP
• granal membranes are selectively permeable, which allows proton gradient to be established
• C contain DNA and ribosomes so can quickly and easily manufacture some of the proteins involved in LDR