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O & E > Photosynthesis > Flashcards

Photosynthesis Flashcards

1
Q

What is photosynthesis?

A
  • an anabolic process in which the energy of sunlight is captured and used to convert CO2 into more complex carbon compounds.

6Co2 + 6 H2O&raquo_space;> C6H12O6 + 6O2

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2
Q

What does the light reaction do?

A
  • Converts light energy into chemical energy in the form of ATP and the reduced election carrier NADPH acts as a reducing agent in photosynthesis and other anabolic reactions.
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3
Q

What does the light independent reaction do?

A
  • CO2 fixation
  • does not use light directly but instead uses the ATP and NADPH produced in the previous light dependant reaction also guide CO2 to produce carbohydrates.
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4
Q

What is photochemistry?

A
  • Light is a form of electromagnetic radiation whcih is propagated in waves. The amount of energy in the radiation is inversely proportoal to its wavelength. I.e the shorter the greater
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5
Q

In addition to travelling as waves how else does light behave/

A
  • As particles called photons which have mass. Receptive molecules in plant can absorb photons and harvest their energy.
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6
Q

What happens when a photon is absorbed?

A
  • the photon disappears and its energy is absorbed by the molecule.
  • energy cannot be destroyed so the energy acquired by from the photon causes the molecule to be raised from ground to excited state.
  • and e- is pushed into a shell further from its nucleus the e- is now held less firmly making the molecule unstable and more chemically reactive.
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7
Q

What are pigments?

A
  • molecules that absorb wavelengths in the visible spectrum.
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8
Q

How is an absorption spectrum produced?

A
  • by plotting light absorbed against wavelength.
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9
Q

How is an action spectrum produced?

A
  • the rate of photosynthesis carried out by an organism against the wavelength of light to whcih its exposed.
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10
Q

What is the structure of chlorophyll a?

A
  • major photosynthetic pigment
    > complex ring structure similar to harm group of heamoglobin with a magnesium ion at the centre.
    > a long hydrocarbon tail anchors the molecule to proteins within the photsystem whcih spans the thylakoids membrane.
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11
Q

What are photo systems?

A
  • light harvesting clusters consisting of a range of photosynthetic pigments
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12
Q

What happens when a pigment molecule absorbs light?

A
  • It results in an excited state. This is unstable and the molecules returns to ground state releasing energy as it does so.
    > within the photsystem the end grey released by the pigment molecules is absorbed by another adjecent molecule.
    > energy is passed in this way until it reaches a chrolophyll a moleule at the centre (the primary pigment reaction centre)
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13
Q

What a heppened when the eneergy reaches the primary pigment reaction centre?

A
  • the chlorophyll absorbs the energy to come excited chl*
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14
Q

What is the first consequence of light absorption by chlorophyll?

A
  • at the reaction centre chlorophyll (chl*) looses its excited elections in a redox reaction to become chl+.
    > as a result this transfer of an electron, the chlorophyll is oxidised, while the acceptor moleule is reduced.
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15
Q

what does the reduction of the acceptor moleule do?

A
  • leads to ATP and NADPH formation.
    The e- acceptor reduced by Chl* is the first in a chain of electron carriers in the thylakoids membrane.
    E- are passed from one carrier to another in an energetically ‘downhill’ series of redox reactions.
    > NADP+ is the final electron acceptor that gets reduced:

NADP + H+ + 2e-&raquo_space;> NADPH

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16
Q

how is the ATP produced in this reaction?

A
  • like in mitochondria ATP is produced chemiosmotically during the process of electron transport.
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17
Q

What is the structure of photosystem I?

A

Contains P700 chlorophylls at its reaction centre

- passes its excited electrons to NADP+ reducing it to NADPH

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18
Q

What is the structure of photsystem II?

A

Absorbs light energy best at 680nm, oxidises water molecules and passes its energised electrons through a series of carries to produce ATP.

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19
Q

What happens at photosystem II?

A

After chl* (excited chlorophyll) gives up its electron to reduce a chemical acceptor molecule the chlorophyll now lacks an e- making it v unstable.
> this makes it a very strong oxidising agent.
> the replacement electrons come from the photolysis of water.

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20
Q

What is the overall reaction for the photolysis of water by chlorophyll?

A
  • 2Chl + H2O&raquo_space;» 2Chl + 2H + 1/2 O2
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21
Q

What happens at photsystem I?

A
  • an excited electron from the chl* at the reaction centre reduces an acceptor. The oxidised chlorphyll gets an electron, this time the electron comes from eh last carrier in the electron transport system, linking the 2 photsystem son chemically.
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22
Q

What is the purpose of cyclic electron transport?

A
  • the following reactions require more ATP than NADPH.
  • the cyclic electron transport makes up for this imbalance.
    > the pathway uses photosystem 1 and the ETC to produce ATP but not NADP
    > cyclic because the excited e- is passed from an excited chlorophyll and is recycled back to the same one
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23
Q

Explain in 4 steps the process of cyclic phosphorylation.

A
  1. The chl* in the reaction centre of photosystem 1 passes electrons to an electron carrier, ferredoxin (Fd) leaving positively charged chlorophyll (chl+) and
  2. The carriers off the ETC are in turn reduced
  3. Energy from electron flow is captured for chemiosmotically synthesis of ATP.
  4. he last reduced e- carrier passes electrons to the electron-deficient chlorphyll allowing the reactions to begin again.
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24
Q

How does photo phosphorylation occur?

A
  • photo phosphorylation operates in the chloroplast, where electron transport is coupled why the transport of protons across the thylakoids membrane.
  • thylakoids membranes are orientated so that the protons are transferred from the stroma into the lumen.
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25
Q

What is the simplified photosynthesis reaction?

A
  • Co2 + H2o&raquo_space;> carbohydrates + O2
26
Q

What is the effect of the proton gradient?

A
  • the lumen becomes acidic compared to the stroma.

> resulting in an electrochemical gradient across the thylakoids membrane (who’s membrane is impermible to H+ ions)

27
Q

What else contributes to the H+ gradient?

A
  • Water oxidation creates more H+ in lumen

- NADP+ reduction removes H+ in the stroma

28
Q

How does H+ get back into the stroma?

A
  • The high H+ Conc in thylakoids space drives the movement of the H+ back into the stroma through ATP synthase channels (proton motive force)
    > ATP synthase works to couple the movement of H+ to the formation of ATP, as they do in the mitochondria
    > mitochondrial and chloroplast ATP synthase enzymes differ in their orientations
29
Q

Explain the role of QB in electron transport

A
  • QB must accept 2 electrons before entering the electron transport system via formation of QH2 absorbing 2 protons from the bacterial cytosol.
  • QH2 now diffuses through the membrane to an extra coal site on the cytochrome bc1 complex pumps an additional proton from inside the cell to outside.
  • the cytochrome bc1, complex transfers electrons to (reduces) a soluble cytochrome in the peri plastic space on the exterior of the cell; the reduced cytochrome is oxidised by the chlorophyll a+ in the reaction centre, returning the system to its original state.
30
Q

What are the 3 processes that make up the light in dependant Calvin cycle?

A
  • FIXATION of CO2- catalysed by rubisco
  • REDUCTION of 3PG to form G3P. - this series of reactions involves a phosphorylation (using ATP from light reaction) and reduction (coupled to the oxidation of NADPH)
  • REGENERATION - of the CO2 acceptor, RuBP. Most of the G3P ends up as ribs lose mono phosphate (RuMP) and ATP is used to convert this compound to RuBP. For every ‘turn’ of the cycle one CO2 is fixed and one CO2 acceptor is regenerated.
31
Q

How does CO2 fixation occur?

A
  • Enzymes in the stroma use the energy in ATP and NADP to reduce CO2.
  • Production of ATP and NADPH is light-dependant therefore CO@ fixation must also take place in the light
    > the Calvin cycle is stimulated by light 2 processes occur to connect the light reactions with the CO2 fixation pathway. Both indirect but significant:
  • protons pumped from stroma into thylakoids increase the pH which favours the activation of rubisco.
  • Electron flow from photosystem I reduces disulphide bonds to activate 4 of the Calvin cycle enzymes
32
Q

How does the light breaking disulphide bonds activate the enzymes?

A
  • when ferredoxin is reduced in photosystem I it passes some electrons to a small soluble protein called thioredoxin this protein passes e- to 4 enzymes in the CO2 fixation pathway. Reduction of the sulphurs in the DS bridges causes SH groups to form and breaks the bridges.
  • the resulting changes in the 3D shape activate the enzymes and increases the rate at which the Calvin cycle operates.
33
Q

Explain in 4 steps the Calvin cycle.

A
  1. CO2 combines with its acceptor RuBP for in 3PG
  2. 3PG is reduced to G3P in a two-step reaction reaction requiring ATP and NADPH.
  3. ABout 1/6 of the G3P molecules are used to make sugars - the output of the cycle
  4. The remaining 5/6 of the G3P molecules are processed in the series of the reactions that produce RuMP
  5. RuMP is converted to RuBP in a reaction requiring ATP. RuBP is ready to accept another electron.
34
Q

What goes in and comes out of carbon fixation?

A
  • 6 RuBP to start
  • 6CO2 is added
  • 12 3PG is produced
35
Q

What goes in and comes out in the reduction and sugar pathway?

A 
12 3PG goes in
12 ATP dephospohrylated to form ADP. 
12 NADPH >> 12NADP+ + 12H+ >>>> 12 Pi
Producing 12 G3P
2 of which are used to make sugar
36
Q

What goes in and comes out in the process of regeneration of RuBP?

A

10 G3P go in
6 RuMP are added
6 ATP converted to ADP
And 6 RuBP produced to begin again

37
Q

What does it mean when we say ‘rubisco is an oxygenate as well as a carboxylase.

A
  • it can add O2 to RuBP instead to CO2, reducing the amount of CO2 converted to carbohydrates may limit plant growth.
38
Q

What are the products of RuBP + O2?

A

Produces phosphoglycolate and 3-phosphoglycerate (3PG)

39
Q

What is photorespiration?

A
  • Consumes O2 releases CO2 and takes place in light.
40
Q

What happens to the phosphoglycolate? As it doesn’t enter the CO2 cycle

A
  • the phosphoglycolate moves into peroxisomes and is converted to glycine.
  • glycine diffuses into mitochondria, 2 glycines are converted into glycerine +CO2.
41
Q

When is photorespiration more likely to occur?

A
  • In the leaf if CO2 concentration is high, photorespiration occurs. If CO2 Conc is high it is fixed.
  • Photorespiration is more likely at high tempratures, such as hot days when stomata (leaf pores) are closed.
42
Q

Following n from earlier what are the 5 steps involved in the photorespiration pathway?

A
  1. In the chloroplast stroma, RuBP reacts with O2. Glycol ate is formed.
  2. Glycol ate diffuses into a peroxisomes, where it is converted to glycine
  3. Glycine moves to the mitochondrion and is converted to serine, releasing CO2.
  4. Serine moves back to the peroxisomes and is converted to glycerate.
  5. Glycerate moves to the chloroplast, where it is converted to 3PG and enters the Calvin cycle.

> > > > > 3PG can then enter the Calvin cycle!!!

43
Q

Why is there a C4 cycle?

A

Ce plants undergo photorespiration, C4 plants do not.

44
Q

What happens in the C4 cycle?

A
  • fixes atmospheric CO2 into carbon skeletons in one compartment and releases CO2 in another compartment to increase CO2 Conc from rubisco for refixing via the Calvin-benson cycle.
  • C4 cycle: involves five stages in two different compartments, with phosphoenolpyruvate carboxylase not rubisco catalysing the primary carboxylation.
45
Q

Why are some plants C4 plants?

A
  • in hot climates they can close their stomata on a hot day but not reduce the rate of photosynthesis.
  • they have evolved a mechanism that increase the Conc of CO2 around the rubisco enzyme whilst simultaneously isolating the rubisco from atmospheric O2. thus in these plants the carboxylase reaction is favoured over the oxygenates, the Calvin cycle operates but photorespiration does not occur.
46
Q

What is the operation of the C4 cycle driven by?

A
  • Diffusion gradients within a single cell as well as gradients between mesothelioma and bundle sheath cells.
  • light regulates the activity of key C4 cycle enzymes: NADP-maleate dehydrogenase, PEPCase and pyruvate-phosphate dikinase. The C4 cycle reduces photorespiration and water loss in hot, dry climates.
47
Q

What is the function of CAM photosynthesis?

A
  • CAM photosynthesis functions to capture atmospheric CO2 and scavenging respiratory CO2 in arid enviroments.
  • CAM is generally associated with anatomical features that minimise water loss.
  • the initial capture of CO2 and its final incorporation into the carbon skeletons are temporally separated.
    > genetics and environmental factors determine CAM expression.
48
Q

What does the the 99% of assimilatoion of CO2 at night suggest?

A
  • supports the hypothesis that CAM is adaptive because it allows CO2 fixation durin the part of the day with lower evaporative demand, making life in water limited enviroments possible.
49
Q

How does strong light damage leaves?

A
  • high PPFD from strong light causes leaf damage due to the production of free radicals.
50
Q

How can plants protect themselves from ROS?

A
  • leaf movements and a range of molecular mechanism, including the xanthophylls cycle (preventing formation of ROS), enzymatic destruction of ROS, and rapid destruction and repair of D1 protein in PSII.
51
Q

What is damage due to strong light called? (Reversible by repair mechanism)

A
  • Photoinhibiition.
    Irreversible is called = photo-oxidation
  • shade leaves have very limited capacity for damage repair and are easily photo-oxidised whereas sun leaves have a repair capacity.
52
Q

What does the xanthophylls cycle do?

A
  • dissipates excess light energy to avoid damaging the photosynthetic apparatus; chloroplast movements also limit excess light absorption.
53
Q

What is the effect of moderate levels of excess light?

A
  • decreases quantum efficiency (reduces the slope of curve)

WITHOUT reducing maximum photosynthetic rate. = DYNAMIC PHOTOINHIBITION

54
Q

Whst is the effect of excess light?

A
  • decreases quantom efficiency and maximum photosynthetic rate (chloroplast damage).
55
Q

what is dynamic PHOTOINHIBITION?

A
  • temporarily diverts excess light absorption to heat but Maintins maximal photosynthetic rate.
56
Q

What is the effect of elevated CO2 Conc on leaves?

A
  • stomata like aperture is smaller and hence leaf temperature is higher due to low transpirational cooling.
57
Q

What are the 3 levels of protection?

A 
- Heat dissipation
> using the xanthophyll cycle
- Detoxification 
> scavenging of ROS
- Repair mechanism 
> synthesis of the D1 protein
58
Q

what is the effect of UV radiation?

A
  • UV radiation penetrating cells causes acute injuries due to the high quantom energy. Longer-wave UV-A is chefly photo-oxidative; UV-B as well as being photo-oxidative also causes photoresists, particularly in bio membranes.
59
Q

What is the molecular mechanism of UV damage?

A
  • Thoguh the breakdown of the disulphide bridges in protein molecules and in dimerising the thymine groups of DNA.
60
Q

what are the counteracting processes of UV damage mechanisms?

A
  • enzymatic repair like reverse reactions though DNA photolyase during blue light UV-A and the elimination of damaged parts via endonucelases.
61
Q

what happens if there is high UV and light intensity is strong what happens?

A
  • inhibits the violate thin-deepoxidase

So when both light intensity and UV are high xanthophylls cycle cannot forfeit its protective role.

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