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Biology active recall > 3.2 Photosynthesis > Flashcards

3.2 Photosynthesis Flashcards

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1
Q

Definition of photosynthesis

A

The conversion of light energy into chemical energy that is stored in glucose and other organic compounds

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2
Q

What are the two stages in photosynthesis?

A

Light dependent stage

Light independent stage

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3
Q

Briefly describe the light dependent reaction

A

A series of photochemical events that converts energy from visible light into high energy intermediates and ATP for the light independent reaction

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4
Q

Briefly describe the light independent reaction

A

The fixing of atmospheric carbon in the form of carbon dioxide into sugar molecules using the high energy intermediates and ATP generated from the light dependent reaction

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5
Q

What are the functions of photosynthesis?

A 
Muscular contraction
Reproduction
Synthesise raw materials
Active transport systems
Drive chemical reactions
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6
Q

What makes ATP the universal energy currency?

A

Highly mobile energy carrier due to its small size
Easily hydrolysed (in a single step) to release energy
Involved in diverse reactions and can easily be reformed from ADP and Pi according to demand​

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7
Q

What is the organ and site of photosythesis?

A

Organ: Leaf
Site: Chloroplast

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8
Q

Describe the organ for photosynthesis and explain how it is adapted for photosynthesis

A
  1. Palisade mesophyll cells contain 3-5 times as many chloroplasts as spongy mesophyll cells. and they are located near the upper epidermis of the leaf where more sunlight is received
  2. Palisade mesophyll cells have a large surface area to allow for efficient gaseous exchange and thus efficient carbon fixation
  3. Palisade mesophyll cells arranged with the long axis perpendicular to the surface of the leaf to minimise amount of light scattered and absorbed by cell wall before reaching the chloroplasts
  4. Leaf is thin which allows CO2 to diffuse through the intermembrane space to the palisade mesophyll cells quickly
  5. Chloroplasts move via cytoplasmic streaming for optimal capturing of light
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9
Q

Describe the site of photosynthesis

A

The chloroplast is a plasmid containing photosynthetic pigments
The chloroplast envelope separates the inside of the chloroplast from the rest of the cell
The inner membrane encloses the stroma which is a concentrated solution of enzymes including the enzymes required for carbohydrate synthesis
The stroma surrounds the thylakoids (fluid filled membranous sacs) that contain photosynthetic pigments such as chlorophyll and carotenoids
Stacks of thylakoids form the grana
Intergranal lamallae link the grana

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10
Q

Describe the three groups of photosynthetic pigments incuding their role in photosynthesis

A

Chlorophyll a:
Most abundant
P680 and P700
Direct role in photosynthesis

Chlorophyll b:
Accessory pigment
Channels light energy absorbed to chlorophyll a
Indirect role in photosynthesis

Carotenoids:
Accessory pigments (e.g. Carotene, Xanthophylls etc.)
Photoprotective role as they absorb and dissipate excess light energy that could damage chlorophyll/ interact with oxygen to form oxidative molecules dangerous to the cell
Add colour to fruits and flowers which helps in dispersal and pollination

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11
Q

Where are the photosystems located?

A

Thylakoid membrane

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12
Q

Describe the process of photoactivation

A

A chlorophyll molecule absorbs a photon of light
One of the pigment molecule’s electrons is elevated from its ground state to its excited state
The excited state is unstable; when the electron quickly falls back from its excited state to its ground state orbital, excess energy is released
Energy is relayed to another pigment molecule via resonance transfer of energy
OR electron is captured by primary electron acceptor in the reaction centre of a photosystem

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13
Q

Where does the non-cyclic light dependent reaction occur?

A

Thylakoid membrane

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14
Q

What does the thylakoid membrane contain that makes it suitable as the site for the non-cyclic light dependent reaction?

A

Photosystems II and I
Enzymes (e.g. ATP synthase)
ETC

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15
Q

What is synthesised in the non-cyclic light dependent reaction?

A

NADH
[Nicotinamide Adenine Dinucleotide Phosphate]
ATP

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16
Q

Non-cyclic light dependent reaction: Photoactivation at PSII

A

A photon of light strikes a pigment molecule in the light harvesting complex and energy is relayed to another pigment molecule via resonance transfer of energy until it reaches one of the two P680 chlorophyll a in the reaction centre exciting one of the P680 electrons to a higher energy state
The excited electron is captured by the primary electron acceptor and needs to be replaced

17
Q

Non-cyclic light dependent reaction: Photolysis of water

A

An enzyme splits water into 2e, 2H+ and O
O reacts with another O to form O2 as by-product
H+ remains in the thylakoid space
electrons are supplied to P680, replacing electron lost

18
Q

Non-cyclic light dependent reaction: 1st electron transport from PSII to PSI

A

Photoexcited electrons passes from the primary electron acceptor of PSII to PSI via ETC (electron carriers of increasing electronegativity)
A series of redox reactions take place as the electrons are transferred down the ETC from one electron carrier to the next
As excited electron travels down ETC consisting of electron carriers of progressively lower energy levels, energy lost is coupled to the formation of ATP

19
Q

Non-cyclic light dependent reaction: light harvesting at PSI

A

Light energy relayed via pigment molecules via resonance transfer of energy to the PSI reaction centre, exciting an electron from one of the two P700 chlorophyll a molecules
Electron is captured by the primary electron acceptor of PSI; electron “hole” is created that is filled by electron from PSII

20
Q

Non-cyclic light dependent reaction: Electron transport from PSI to NADP+

A

Photoexcited electron is passed from the primary electron centre in PSI down the 2nd ETC
No ATP
electrons react with NADP+ to form NADPH
Reduction in stroma
NADP+ final electron acceptor
Since high energy electrons are stored in NADPH, it provides reducing power for synthesis of sugar in the light-independent reaction

21
Q

Describe the cyclic light-dependent reaction

A

Photoexcited electron from P700 is captured by the primary electron acceptor of PSI which passes it on to the middle part of the first ETC (electron “hole” created in P700)
Loss of excitation energy coupled to formation of ATP
electron “hole” in P700 is filled
No NADPH
No O2
electrons are recycled

22
Q

Why doesn’t the electron from PSI get passed to the 2nd ETC?

A

Ratio of NADPH:NADP+ too high
Insufficient NADP+ to act as final electron acceptor
High ATP requirements may drive plant to carry out cyclic phosphorylation

23
Q

Define Photophosphorylation

A

The process of generating ATP from ADP and Pi using proton-motive force generated across thylakoid membrane of chloroplast using light energy absorbed during the light-dependent reaction of photosynthesis

24
Q

Describe the process of photophosphorylation

A

As electrons travel down electron carriers that are progressively more electronegative, certain ETC proteins in the thylakoid membrane pump H+ across the thylakoid membrane into the thylakoid space
H+ accummulate in the thylakoid space acting as H+ reservoir [ETC transforms redox energy to proton-motive force]
Chemiosmosis occurs when H+ diffuses down conc gradient via ATP synthase
ADP phophorylated to form ATP
Photolysis of water further increases conc of H+ which further maintains electrochemical proton gradient
Uptake of H+ from stroma when NADP+ is reduced to NADPH by NADP+ reductase further contributes to proton gradient

25
Q

Where does the light-independent reaction: The Calvin cycle occur?

A

Stroma

26
Q

What makes the stroma suitable for the calvin cycle?

A 
Contains RuBisCO
Ribulose
Bisphosphate
Carboxylase
Oxygenase

enzymes that catalyse reactions in the Calvin cycle

27
Q

What are the prerequisites for the Calvin cycle?

A

ATP
NADPH
CO2
from the light-dependent reaction

28
Q

Describe the Calvin cycle

A 
1. Carbon dioxide fixation
Fix CO2 to RuBP (5C)
Catalysed by RuBisCO
RuBP is the CO2 acceptor
Forms unstable 6C intermediate that splits to form 2 glycerate phosphate (GP) molecules (3C)
  1. Reduction of GP by NADPH
    Reducing power of NADPH and energy from ATP generated in light-dependent reaction
    GP is reduced to G3P (3C)- first sugar
  2. Regeneration of RuBP
    G3P used to regenerate RuBP CO2 acceptor
    5 G3P [53=15C] regenerates 3 RuBP [35=15C]
    3 ATP from light-dependent reaction used
  3. Fate of G3P
    For net synthesis of 1 G3P for carbohydrate synthesis, 3 CO2 molecules must be fixed
    G3P needed for formation of glucose
    G3P both a product and an intermediate
    Depending on type of glucose isomers formed, alpha-glucose polymerises to form starch, beta-glucose polymerises to form cellulose
29
Q

What are the limiting factors of photosynthesis?

A

Light
Temperature
[CO2]
[O2] oxygenase function of RuBisCO

30
Q

Define “compensation point”

A

The point where photosynthetic rate = rate of respiration

no net change of O2 and CO2

Biology active recall (7 decks)

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