17 Photosynthesis

Question 1

Chloroplast

Structure of membranes/ thylakoids/ granum → Function

Answer 1

1. Double membrane surrounding chloroplast ie. chloroplast envelope → compartmentalises to provide optimum temperature for enzymatic reactions to occur
2. Impermeable thylakoid membrane to H⁺ → accumulation of H⁺ without leakage → proton gradient can be generated for chemiosmosis

Question 2

Chloroplast

Structure of enzymes → Function

Answer 2

1. Stacking of thylakoid discs to form a granum + connected grana through sheet-like thylakoids ie. intergranal lamella → ↑ SA for attachment of ↑ chlorophyll, photosystems, ATP synthase, e^- & H⁺ pumps → maximises rate of light dependent reactions
2. Photosystems & e^- are close to e/o & in sequence → e^- can be efficiently passed down ETCs with progressively lower energy levels

Question 3

Chloroplast

Structure of stroma → Function

Answer 3

1. Semi-fluid space enclosed by the chloroplast envelope & abundance of enzymes → Enables it to serve as site for the heavily enzyme-dependent Calvin Cycle
2. Contains circular chloroplast DNA & 70s ribosomes → Essential proteins can be synthesised via gene expression

Question 4

Adsorption Spectrum

Definition

Answer 4

The amount of light absorbed at varying wavelengths by the various photosynthetic pigments eg. chlorophyll a, b etc.

Question 5

Photosynthetic Pigments

Wavelengths

Answer 5

Chlorophyll a: 400-450 nm
Chlorophyll b: 626-642 nm

Question 6

Action Spectrum

Definition

Answer 6

Shows how the rate of photosynthesis is affected by the spectrum of light (of varying wavelengths).

Question 7

Accessory Pigments

Function

ie. Carotenoids

Answer 7

Broadens the spectrum of light wavelength that can be absorbed.

Question 8

Resonance Energy Transfer (RET)

Definition

ie. Light harvesting

Answer 8

Light energy is absorbed by a chlorophyll pigment molecule & becomes excitied. Excitation energy is passed from one pigment to another untill all pigments are excited by light.

Question 9

Light-dependent Stage

Non-cyclic Photophosphorylation → Photoactivation in PS II

Answer 9

1. Upon being hit by a photon of light, RET occurs until it reaches photosystem II’s (PS II) reaction centre
2. An e^- in the specialised chlorophyll a molecule, P680, of each PSII becomes photoexcited
3. Primary e^- acceptor captures the e^- & emits it to the e^- carriers of the ETC, leaving all P680s e^--deficient

Question 10

Light-dependent Stage

Non-cyclic photophosphorylation → Photolysis of H2O

Answer 10

1. An enzyme catalyses photolysis of H₂O to produce H⁺, e^- & O_2(g)
2. e^- from the photolysis of H₂O replenishes the e^--deficient P680s within PS II while O atom combines with another to form O_2(g) as a by-product. H⁺ remains in the thylakoid space.

Question 11

Light-dependent Stage

Non-cyclic photophosphorylation → Proton Gradient Generation

Answer 11

1. e^- flow from PS II to PS I via the 1st ETC of increasingly electronegative & progressively lower energy levels e^- carriers, through a series of redox reactions.
2. Energy lost is used to pump H+ against the concentration gradient from the stroma to thylakoid space.
3. H+ accumulates in the thylakoid space, generating a proton gradient which is used later on for chemiosmosis

Question 12

Light-dependent Stage

Non-cyclic photophosphorylation → Light harvesting at PS I

Answer 12

1. RET occurs until one of the 2 P700 chlorophyll a molecules have been excited.
2. The e^- is captured by the primary e^- acceptor in PS I, making it e^--deficient
3. e^- from PS II are used to replenish the e^--deficient P700.

Question 13

Light-dependent Stage

Non-cyclic photophosphorylation → NADP [R]

Answer 13

1. Excited e^- from P700 passed onton the 2nd ETC
2. e^- & H+ are used to [R] NADP, the final e^- acceptor, to NADPH by the enzyme, NADP reductase in the stroma

NADPH provides the [R] power for the Calvin cycle

Question 14

Light-dependent Stage

Cyclic photophosphorylation (Thylakoid membrane)

Answer 14

1. Photo-excited e^- from P700 is captured by primary e^- acceptor of PS I which is passed onto the middle part of the 1st ETC, leaving P700 e^--deficient
2. e^- travels down ETC of progressively lower energy e^- carriers via a series of redox reactions
3. Energy lost is used to pump H+ from the stroma to thylakoid space.
4. H+ accumulates in the thylakoid space, generating a proton gradient for chemiosmosis

NO NADPH since e^- did not pass down 2nd ETC; O₂ since no photolysis occurred

Question 15

Light-dependent Stage

Reasons why cyclic phosphorylation is cyclic

Answer 15

e^- used for generation of ATP replenishes the e^--deficient P700 of PS I

Question 16

Photophosphorylation

Chemiosmosis

Answer 16

1. e^- travel down increasingly EN & progressively lower energy e^- carriers within the ETC
2. Energy lost is used to pump H+ across the membrane, against the concentration gradient from the stroma to thylakoid space
3. H+ accumulates, generating a proton gradient & proton motive force, maintained by photolysis of H2O & usage of H+ in NADP [R]
4. Chemiosmosis occurs when H+ diffuses across the membrane through ATP synthase, ADP is phosphorylated to produce ATP

Question 17

Photophosphorylation

Definition

Answer 17

Process of generating ATP from ADP and Pi by means of a proton gradient/ proton-motive force generated across the thylakoid membrane of the chloroplast using light energy absorbed during light-dependent reactions of photosynthesis

Question 18

Light Independent Stage

Calvin Cycle (Stroma) → CO₂ fixation

Answer 18

1. Rubisco combines a CO₂ with RuBP to form a unstable 6C compound
2. 6C compound breaks down into 2 3C compounds, glycerate phosphate (GP)/ phosphoglyceric acid (PGA)/ 3-phosphoglycerate (PG)

where Rubisco = RUBP carboxylase oxygenase, RuBP = Ribulose biphosphate

Question 19

Light Dependent Stage

Calvin Cycle (Stroma) → [R]

Answer 19

1. GP is phosphorylated by ATP to produce ADP for use in the light-dependent stage & an intermediate 1,3-biphosphoglycerate (1,3-BPG)
2. NADP [R] 1,3-BPG to form glyceraldehyde-3-phosphate (G3P)/ phosphoglyceraldehyde (PGAL)/ triose phosphate (TP)
   ie. ATP & NADPH are used to [R] GP to form 2 G3P

G3P is the product & 1st sugar formed in photosynthesis & is a basic sugar for synthesis of other carbohydrates

Question 20

Light-dependent Stage

Calvin Cycle (Stroma) → Regeneration of RuBP

Answer 20

For 1 CO₂, 2 G3Ps are produced → for every 3 cycles, 6 G3Ps are produced
1. 6 G3Ps are split up into 5 for regeneration of RuBP & 1 for formation of glucose molecules
2. During the regeneration of RuBP, 3 ATP from the light-dependent stage is used
For formation of 1 glucose molecule, 2 G3Ps needed → 12 cycles needed
3. G3P combines to form either α/ β-glucose which is used to synthesis other carbohydrates