Photosynthesis Flashcards
(42 cards)
1
Q
Photosynthesis
A
- Converts carbon (inorganic CO₂) into organic compounds
- Conversion of light energy to chemical energy stored in glucose and other organic compounds
- Light dependent reaction and light-independent reaction
2
Q
Chloroplast
A
- Site of photosynthesis
- Plastid containing photosynthetic pigments (chlorophyll and carotenoids)
- Has an envelope (double membrane)
- Inner membrane encloses stroma
- Thylakoids → fluid-filled membranous sacs → stacks (grana) → linked by intergranal lamellae
3
Q
Photosynthetic pigments
A
- Light receptors
- Each has a certain molecular structure that absorbs light strongly at specific wavelengths → spectrophotometer
- Chlorophyll a
- Chlorophyll b
- Carotenoids
4
Q
Chlorophyll a
A
- P680 and P700
- Different absorption peaks
- Participate directly
5
Q
Chlorophyll b
A
- Accessory pigment
- Indirect role
6
Q
Carotenoids
A
- Accessory pigments
- Indirect role
- Photoprotective role → absorb and dissipate excess light energy
- Add colour to fruits and flowers
7
Q
Absorption spectrum
A
- Plotting a pigment’s light absorption vs wavelength
- Each pigment has own specific spectrum
- Each pigment has diff no., height and breadth of peaks
- Pigments hardly absorb any green wavelengths of light
8
Q
Action spectrum
A
- Effectiveness of diff wavelengths of light in stimulating photosynthesis
- Similar to absorption spectrum, but does not exactly match that of chlorophyll a
- Chlorophyll b and carotenoids broaden spectrum of wavelength → channel energy absorbed to chlorophyll a
9
Q
Photosystem
A
- Located on thylakoid membrane
- Reaction center
- Light-harvesting complex
- Light-dependent reactions
- PS II → P680(nm)
- PS I → P700(nm)
10
Q
Reaction center
A
Protein complex that includes 2 special chlorophyll a molecules and a primary electron acceptor
11
Q
Light-harvesting complex
A
Pigment molecules bound to proteins → allow light to be harvested
12
Q
Photoactivation
A
- When chlorophyll molecule absorbs photon of light → one of the molecule’s e⁻ elevated from ground state to excited state (orbital of higher potential energy)
- Excited state is unstable → falls back to ground state → excess energy released
- Energy relayed to another pigment via resonance transfer of energy
- Electron may also be captured by primary electron acceptor
13
Q
Light-independent reaction (2)
A
- Non-cyclic → predominant route
- Cyclic
- Located on thylakoid membrane
14
Q
Non-cyclic photophosphorylation (5)
A
- Drives synthesis of NADPH and ATP
1. Photoactivation
2. Photolysis of water
3. 1st e⁻ transport from PSII to PSI + photophosphorylation
4. Light harvesting at PSI
5. Electron transport from PSI to NADP⁺
15
Q
Photoactivation (4)
A
- Photon of light strikes pigment molecule in light harvesting complex → 1e⁻ excited to higher energy level → drops to ground state → energy released passed on to next pigment molecule
- Energy relayed to other pigment molecules via resonance transfer of energy until it reaches one of the two P680 chlorophyll a molecules in the PSII reaction center
- Excites one of the P680 e⁻ to a higher energy state
- Excited e⁻ captured by primary e⁻ acceptor → e⁻ hole left in PSII
16
Q
Photolysis of water (4)
A
- Enzyme splits water molecule into 2e⁻, 2H⁺ + ½O₂
- e⁻ supplied to P680 molecules → replace e⁻ lost to primary electron acceptor/fill e⁻ hole
- O atom combines with another to form O₂ → by-product
- H⁺ remains in thylakoid space → contributes to high [H⁺] in thylakoid space
17
Q
1st electron transport from PSII to PSI (2)
A
- Each photoexcited electron passes from primary e⁻ acceptor of PSII to PSI via ETC → made up of a series of e⁻ carriers with increasing electronegativity → series of redox reactions
- As excited e⁻ travels down ETC, energy lost coupled to form ATP via photophosphorylation
18
Q
Photophosphorylation (4)
A
- As e⁻ travels down a series of increasingly electronegative e⁻ carriers, energy lost used to pump H⁺ from stroma into the thylakoid space
- This generates a proton gradient across the membrane
- Chemiosmosis occurs when H⁺ diffuse down proton gradient back into stroma via ATP synthase
- ADP phosphorylated to ATP
19
Q
Light harvesting at PSI (2)
A
- Light energy relayed via pigment molecules to PSI reaction center, exciting an e⁻ of one of the two P700 chlorophyll a molecules → captured by primary e⁻ acceptor in reaction center → loses e⁻ → creates e⁻ hole
- e⁻ hole filled by displaced e⁻ from PSII when it reaches end of the 1st ETC
20
Q
Electron transport from PSI to NADP⁺ (2)
A
- Photoexcited e⁻ passed from PSI’s primary e⁻ acceptor down 2nd ETC
- e⁻ finally transferred to NADP⁺ (final e⁻ acceptor) → reduced to form NADPH, catalysed by NADP⁺ reductase → contributes to proton gradient
21
Q
Cyclic photophosphorylation (5)
A
- Photoexcited e⁻ from P700 captured by PSI’s primary e⁻ acceptor
- e⁻ transferred to middle of 1st ETC → creates e⁻ hole
- Energy lost during e⁻ transfer coupled to formation of ATP by photophosphorylation
- e⁻ is finally recycled back to PSI, filling e⁻ hole
22
Q
Cyclic photophosphorylation overview
A
- No NADPH produced
- No O₂ produced as there is no photolysis of water
- Only PSI involved
- Only ATP produced → Calvin cycle uses more ATP than NADPH
- Occurs when there is too little NADP⁺ available to accept e⁻
23
Q
Light-independent reaction/Calvin cycle overview
A
- Occurs in stroma
- Requires NADPH, ATP and CO₂
- Reduces CO₂ to produce carbohydrates
24
Q
Calvin cycle steps (3)
A
- Carbon fixation
- Reduction of glycerate phosphate (GP) by NADPH
- RuBP regeneration
25
Carbon fixation (3)
1. Fixing of CO₂ to ribulose bisphosphate (RuBP, 5C) → CO₂ acceptor
2. Catalysed by ribulose bisphosphate carboxylase oxygenase (RuBisCO)
3. Unstable 6C intermediate formed → split into 2 GP (3C)
26
Reduction of GP by NADPH (3)
1. Reducing power of NADPH and energy of ATP reduces GP to form G3P (3C)
2. NADPH oxidised to NADP⁺, ATP converted to ADP, 1 each per GP molecule, 6 in total
3. G3P is first sugar formed → end product of Calvin cycle → used to build up more complex carbohydrates
27
Regeneration of RuBP (2)
1. 5 G3P used to regenerate 3 RuBP
| 2. 3 ATP used
28
Fate of G3P
- For net synthesis of 1 G3P, 3 CO₂ have to be fixed
- G3P needed for formation of glucose molecules
- Both an intermediate and a product
29
Calvin cycle overview
- To produce 1 G3P: 3 CO₂, 9 ATP, 6 NADPH
| - To produce 1 glucose: 6 CO₂, 18 APT, 12 NADPH
30
Other names of GP (3)
1. Glycerate phosphate (GP)
2. Phosphoglyceric acid (PGA)
3. 3-phosphoglycerate
31
Other names of G3P (3)
1. Glyceraldehyde-3-phosphate (G3P)
2. Phosphoglyceraldehyde (PGAL)
3. Triose phosphate (TP)
32
Limiting factor
When a chemical process is affected by more than one factor, its rate is limited by that factor which is nearest its minimum value → directly affects a process if its magnitude is changed → proportional change
33
Saturation point
The point beyond which something is no longer a limiting factor
34
Limiting factors of photosynthesis
1. Light intensity, quality (wavelength) and duration
2. CO₂ concentration → major limiting factor
3. Temperature → enzyme controlled → rate doubles or every 10°C rise up to about 35°C → beyond 40°C → denature
35
O₂ concentration affecting photosynthesis
- Oxygenase function of RuBisCO
- Accepts O₂ as competitive inhibitor
- Occurs on bright, hot and dry days → stomata of leaf close → reduced CO₂ entering → O₂ concentration builds up cos of photosynthesis
36
Oxygenase function of RuBisCO
- RuBP split into 3C (GP) and 2C (glycolate)
- 2C compound exported to peroxisomes and mitochondria → broken down into CO₂
- Photorespiration → generates no ATP
37
Compensation point
Point when photosynthetic rate equals respiration rate → no net change of O₂ and CO₂
38
Experiments to find out how a certain limiting factor affects rate of photosynthesis
- Explain how independent variable affects dependent variable (rate of photosynthesis) → link to volume of gas evolved
- Identify constant variables to be kept optimal
- How to measure → syringe
- Controls
- Graphs to expect
39
Experimental setup
- Water plant Elodea → acclimatisation period
- Light source → light intensity ∝ 1/d²
- Water bath → temperature can be manipulated
- Bicarbonate solution → source of CO₂ → manipulate concentration
- Syringe → collect bubbles of O₂ → V = cross-sectional area x length
40
Role of NADP (4)
1. NADP⁺ is a coenzyme → carries both protons and high energy electrons
2. Final electron acceptor in non-cyclic light dependent reaction
3. Electrons carried in NADPH used in Calvin cycle in stroma to reduce GP to G3P
4. NADP⁺ regenerated to carry out its role
41
What contributes to high H⁺ concentration in thylakoid space? (4)
1. Proton pump
2. Photolysis of water in inner thylakoid membrane
3. Lack of permeability of thylakoid membrane to H⁺ due to its hydrophobic core
4. Reduction of NADP⁺ to NADPH in the stroma → reduces [H⁺] concentration → ensure steepness of gradient
42
Function of thylakoid membrane in photophosphorylation (4)
1. Provide large surface area to embed many
photosynthetic pigments for light absorption
2. Maintains sequential arrangement of e⁻ carriers of ETC for the flow of e⁻
3. Maintains proton gradient for ATP synthesis since hydrophobic core of membrane is impermeable to protons and this is essential for chemiosmosis
4. Allows many ATP synthase to be embedded so that ATP can be produced
