Photosynthesis

Question 1

Chloroplast structure to function
(Chloroplast envelope)

Answer 1

S: Double membrane
- ↑ permeable outer membrane, ↓ permeable inner membrane with transport proteins embedded

F: Compartmentalisation + specialisation
- enclose reactants and enzymes involved in photosynthesis tgt

F: Tpt proteins allow regulation of substances moving in and out of chloroplast

Question 2

Chloroplast structure to function
(Lamellae system)

Answer 2

S: thylakoids & intergranal lamellae

F: LDR in lamellae

F: Lamellae system = ↑ SA for attachment of photosynthetic pigments, e- carriers, ATP synthase
- ↑ light absorption
- proximity and arrangement of ETC for e- transfer

F: Compartmentalisation of thylakoid space separated from stoma
- set up proton gradient for ATP synthesis

Question 3

Chloroplast structure to function
(Stroma)

Answer 3

S: contains circular DNA, RNA, 70D ribosomes, enzymes, starch etc in dense fluid like matrix

F: enzymes catalyse CC

F: surrounds lamellae system → products of LDR enter CC in stroma

F: large area for temporary storage of sugars and starch

F: DNA codes for some chloroplast proteins (synthesised by chloroplast ribosomes)

Question 4

Chlorophyll structure to function
(Tail)

Answer 4

S: hydrophobic long hydrocarbon tail

F: ✓ embed in phospholipid bilayer of thylakoid membrane

Question 5

Chlorophyll structure to function
(Head)

Answer 5

✓ lie at surface of thylakoid membrane next to aqueous stroma

Question 6

Chlorophyll structure to function
(Orientation of head)

Answer 6

S: flat head of chlorophyll // membrane surface

F: ↑ light absorption

Question 7

Chlorophyll structure to function
(Light)

Answer 7

S: head absorbs light

F: causes emission of e- needed for LDR initiation

Question 8

Chlorophyll structure to function
(Side groups on head)

Answer 8

S: modifications on side groups on head possible

F: can change absorption spectrum ∴ diff energies of light absorbed

Question 9

Outline 3 steps of LDR, location, and initial reactants + final products

Answer 9

1. Photoactivation of chlorophyll
2. Photolysis of water
3. Generation of ATP and NADPh

In thylakoid membrane

Reactants: ADP, Pi, H2O, NADP
Products: ATP, O2, NADPH

Question 10

What are photosystems

Answer 10

Arrangement of photosynthetic pigments at thylakoid membranes
PS I and II

Question 11

Components of photosystems

Answer 11

1. Light harvesting antenna complex (cluster of chl a, b, carotenoids)
   - absorb light + transfer energy from molecule to molecule till it reaches special chl a at reaction centre
2. Reaction centre (pair of special chl a which trigger LDR)
   - donates excited e- to primary e- acceptor ∴ traps ↑ energy e- and passes e- to ETC
   - P700 in PS I
   - P680 in PS II
3. Primary e- acceptor
   - accepts and traps energised e- from special chl a

Question 12

Non-cyclic photophosphorylation .

Answer 12

1. Photoactivation of chl: excitation of e- of P680 in PS II → donated to primary e- acceptor
2. Photolysis of water: release e- to replace donated e- + O2 prod
3. Chemiosmosis: energy harnessed from e- travelling down ETC to prod ATP
4. E- enter PS I and replace e- lost from P700 due to absorption of light (by PS I)
5. Excited e- from P700 passed along another ETC → ATP
   - passed to final e- acceptor NADP+ → NADPH

Produces ATP and NADPH
Both PS I and II involved

Question 13

Cyclic phosphorylation

Answer 13

Only PS I involved
ATP is only product (for CC)
No NADPH, no CO2 release

Displaced e- from P700 in PS I will return to P700 on ETC instead of being taken up by NADP

Energy lost by excited e- along ETC on thylakoid membrane is harnessed to drive ATP synthesis through chemiosmosis

Question 14

ATP synthesis via chemiosmosis

Answer 14

1. Photolysis of H2O in thylakoid space → generates H+ that accumulate in thylakoid space (lumen)
   - Possible due to inner membrane acting as hydrophobic barrier between thylakoid space and stroma
   2. • Energy released from e- flow down ETC in PS I and PS II pumps H+ from stroma, across thylakoid membrane, into thylakoid space (proton pump) ∴ steep proton gradient across thylakoid membrane
        -↓ pH in thylakoid space
   3. ATP produced via chemiosmosis
      
      • Energy coupling mechanism that uses energy stored in form of proton gradient across membrane to drive cellular work - ATP synthesis
      • Steep proton gradient generates proton motive force (PMF) ∴ allows ATP synthase (stalked particles on thylakoid membrane) to phosphorylate ADP → ATP as H+ diffuses though from thylakoid space into stroma

Question 15

Light independent reactions / Calvin cycle
(Location, purpose, reactants, products)

Answer 15

In stroma, for synthesis of carbs from CO2

Reactants: ATP, CO2, NADPH
Products: ADP, NADP, glucose

Question 16

Light independent reactions
(Location, purpose, reactants, products)

Answer 16

In stroma, for synthesis of carbs from CO2

Reactants: ATP, CO2, NADPH
Products: ADP, NADP, glucose

Question 17

Steps in light independent reaction / Calvin cycle

Answer 17

1. Carbon fixation
   ○ In stroma, CO2 + 5C ribulose biphosphate (RuBP) → unstable 6C → 2x 3-phosphoglycerate (PGA)
   - Carboxylation reaction catalysed by RuBP carboxylase/Rubisco
   2. PGA reduction
      ○ [R] of PGA by NADPH → triose phosphate (TP) / glyceraldehyde 3-phosphate (G3P)
      - Energy in form of ATP required
      - NADPH and ATP are photochemical products of LDR
      - TP is the first carb produced in photosynthesis
      ○ 2 TP → glucose phosphate → starch (formed in stoma by condensation of many glucose phosphates)
      - Also synthesises sucrose, fatty acids, aa
   3. RuBP regeneration
      ○ For every 6 TP, only 1 exits cycle for glucose synthesis
      - Other 5 are recycled to regenerate RUBP for continuation of cycle → ATP required