topic 6 - phototrophic metabolism - photosynthesis

Question 1

how do reaction centres reduce a primary e- acceptor

Answer 1

• light is absorbed by an antenna pigment
• energy is transferred by inductive resonance to a reaction centre
• e- is excited as a reaction centre and is transferred to a primary e- acceptor

Question 2

what happens in photosystem II

Answer 2

energy from photons channels through antenna pigments to reduced P680

e- is excited in P680* (excited form)

transfer e- to primary e- acceptor

P680+ (oxidised)

Question 3

how to get P680+ (oxidised) back to P680

Answer 3

e- donated from water is used to reduce P680+

allows the cycle to continue

Question 4

what is the difference in H+ concen between stroma and thylakoid lumen

Answer 4

stroma = low concen of H+ (higher pH)

thylakoid lumen = high concen of H+ (low pH)

Question 5

what is plastoquinone (PQ)

Answer 5

hydrophobic e- taxi
- when reduced (gains e-) it also gains an a proton from the stroma

Question 6

what is plastocyanin (PC)

Answer 6

hydrophilic e- taxi

Question 7

how does photosystem II produce PMF

Answer 7

energy from light goes to P680

P680 reduces its PEA which passes the e- to PQ which gains a proton from the stroma

PQ reduced the cytochrome complex and releases a proton in the lumen (building H+ concen in lumen)

cytochrome reduces plastocyanin that passes e- to P700+ (oxidised)

Question 8

what happens in photosystem I

Answer 8

P700 is initially reduced
receives energy through inductive resonance and e- on P700 is excited to P700*

e- transferred to PEA

P700 oxidised to P700+

Question 9

how does P700+ get reduced back to P700

Answer 9

e- to reduce comes from plastocyanin in PS II

Question 10

how does PS I produce NADPH

Answer 10

P700 reduces the PEA which passes the e- to ferrodoxin
NADP+ reductase reduces NADP+ to NADPH in the stroma

Question 11

where/what is ferrodoxin

Answer 11

on stroma side of thylakoid membrane (peripheral protein)

hydrophilic e- taxi that reduces NADP+ reductase

Question 12

how is PMF generated through photosystems across the thylakoid membrane

Answer 12

protons in the stroma are used to reduce NADP+ to NADPH (lower H+ concen in the stroma)

oxidation of water in the lumen releases protons

PQ moves protons from the stroma to the lumen

Question 13

what occurs during photophosphorylation

Answer 13

using solar energy to generate PMF to power ATP synthase

ATP generated on the stroma side of the thylakoid

takes a lot of energy to oxidise water and reduce NADP+

Question 14

what is the role of PS II and PS I in oxidising water and reducing NADP+

Answer 14

PS II - excites e- enough to pull e- from water and generate PMF but not enough to reduce NADP+

PS I - role is to re energise the e- in order to reduce NADP+ (uses second photon of light to re energise the e-)

Question 15

where does the calvin cycle occur

Answer 15

stroma (space in chloroplast surrounding thylakoids)

Question 16

what is the calvin cycle

Answer 16

11 reactions - each reaction is catalysed by an enzyme

highly endergonic overall - building carbs from CO2 requires a lot of input of energy from ATP and NADPH from light reactions

each individual reaction is exergonic because they are coupled with the oxidation of NADPH and ATP hydrolysis

Question 17

what are the phases of the calvin cycle

Answer 17

phase 1 - fixation / carboxylation
phase 2 - reduction (requires energy)
phase 3 - regeneration (requires energy)

cycle goes around 3 times

Question 18

what happens during phase 1 of the calvin cycle

Answer 18

3 molecules of CO2 enter

react with 3 RuBP (5C molecule) with enzyme rubisco

combine 15 C atoms from RuBP with 3 from CO2 to make 6 3C molecules (18C total) of 3PGA

Question 19

what happens in phase 2 of the calvin cycle

Answer 19

reduce the C molecules

3PGA will become progressively more reduced

input = 6 ATP and 6 NADPH (for energy)

6 3PGA into 6 G3P
(one G3P released)

Question 20

what happens during phase 3 of the calvin cycle

Answer 20

need to make RuBP for the next cycle

input = 3 ATP (for energy)

have 5 G3P

Question 21

what is the total input into the calvin cycle

Answer 21

9 ATP
6 NADPH
(for 3 cycles)

Question 22

what is the purpose of cyclic e- transport

Answer 22

produces ATP without the synthesis of NADPH (supplies additional ATP for the calvin cycle)

calvin cycle needs more ATP than NADPH

cyclic e- flow allows it to stop making NADPH by making PMF to power ATP synthesis

Question 23

what happens during cyclic e- flow

Answer 23

e- will flow from PQ to P700 in a circle to generate PMF to make ATP

ferrodoxin can reduce PQ instead of NADP+ reductase

Question 24

how much is linear and cyclic e- flow used

Answer 24

80% of photosynthetic ETC use linear e- flow

20% use cyclic

Question 25

what is the product of photosynthesis

Answer 25

G3P

Question 26

what does G3P turn into

Answer 26

converted to glucose

3C molecule so need 2 to make 1 glucose

Question 27

what is glucose used for

Answer 27

used for glycolysis in the cytoplasm

linked into polymers of starch / cellulose (if the cell has enough energy)
- starch = energy storage
- cellulose = cell wall

used for synthesis of other biomolecules