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06 FdSc Bioscience - Metabolism and Enzymology > Week 6 - photosynthesis and carbohydrates > Flashcards

Week 6 - photosynthesis and carbohydrates Flashcards

1
Q

What are photosynthetic organisms termed as and what does this mean?

A
  • Photosymthetic organisms are photoautotrophs
    • This means that they synthesize food directly from carbon dioxide and water using energy from light
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2
Q

Give examples of organisms where photosynthesis releases oxygen and state what sort of photosynthesis this is

A
  • Plants, algae and cyanobacteria photosynthesize to produce oxygen, this is called oxygenic photosynthesis
    • ​Although there are some differences between the oxygenic photosynthesis the overall process is quite similar in these organsims
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3
Q

Besides oxygenic photosynthesis, what are the other types of photosynthesis? give an example of organism which photosynthesizes in this way

A
  • Some type of bacteria carry out anoxygenic photosynthesis which consumes carbon dioxide but does not release oxygen
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4
Q

Outline photosynthesis and where it occurs

A
  • Photosynthesis is the mechanisms that plant cells use to convert light energy into chemical energy
  • It occurs in chloroplasts of plant ad algal cells and on the plasma membrane of some bacterial cells
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5
Q

What is the significance of photosynthesis for all organisms?

A
  • Photosynthesis is the ultimate energy source for all living organisms - supplying carbohydrate for both plant metabolism and animal metabolism
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6
Q

What is the overall chemical equation for photosynthesis?

A

6CO2 + 12H2O ———-> C6H12O6 + 6O2 + 6H2O

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7
Q

In what way is the overall equation of photosynthesis incorrect and what is the discrepancy?

A
  • The end product of photosynthesis is not a 6C sugar as shown in the overall equation but it is a 3C sugar which can be used to produce a variety of larger, more complex molecules
  • 6CO2 + 12H2O ———-> C6H12O6 + 6O2 + 6H2O
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8
Q

What is the name of the three carbon sugar produced by photosynthesis?

A
  • Glyceraldehyde-3-phosphate (PGAL)
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9
Q

What are the distinct stages of photosynthesis?

A
  • Photosynthetic reaction is split into two phases:
    • The light reaction
    • The dark reaction
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10
Q

Briefly describe the light reaction of photosynthesis?

A
  • This is where light energy is converted into ATP and NADPH
  • This occurs in the thylakoid membrane of the chloroplast and requires the presence of photosythetic pigments on the membrane
  • The products of the light reaction are used to power the dark reaction
  • Also results in the formation of O2
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11
Q

Brielfy describe the dark reaction of photosynthesis

A
  • ATP and NADPH are used to power the reduction of CO2 and H2O to carbohydrate
  • Occurs in the chloroplast stroma which contains the enzymes required for this reduction reaction
  • No light is directly involved in this reaction
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12
Q

What are photons and how are they involved in photosynthesis?

A
  • Photons are high energy particles which make up the various wavelengths of light
  • Photons hit photosynthetic pigments present in the thylakoid membrane and the energy excites one of the electrons in the pigment
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13
Q

What are the pigments involved in photosynthesis?

A
  • Main pigments are chlorophyll a and b
  • Accessory pigments include xanthophylls, phycobilins and carotenoids
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14
Q

What do the accessory pigments in photosynthesis serve to do?

A
  • They widen the range of photon wavelengths accepted by the antennae
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15
Q

Briefly outline how light energy is transferred in photosynthesis

A
  • Light energy absorbed by chlorophyll molecules results in the excitation of electrons boosting them to a higher energy level
  • All excited pigments pass their energy from one to another through a locallized collection of pigments eventually funneling the energy to a special chlorophyll pair (acceptor molecule) in the reaction centre
  • At the reaction centre the electron is boosted to a high energy and transferred to an electron acceptor resulting in charge separation
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16
Q

Define the term “reaction centre” regarding photosynthesis

A
  • A reaction centre is a transmembrane complex of proteins, pigments and oxidation/reduction cofactors
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17
Q

Describe in more detail how light energy is transferred in photosynthesis

A
  1. Chlorophyll a molecules, which form a special pair bound within a large transmembrane complex called Photosystem II, rapidly transfer high energy electrons through the complex to a mobile carrier
  2. The mobile carrier transfers the energy to a second complex called Photosystem I
  3. At Photosystem I NADPH is formed
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18
Q

What does the rapid transfer of electrons through Photosystem II results in?

How is this neutralised?

A
  • Rapid transfer of electrons through Photosystem II results in charge separation leaving the pair of chlorophyll a molecules with a positive charge
  • This is neutralised by the extraction of electrons from water bound at a manganese centre resulting in the production of O2
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19
Q

What happens as electrons are transferred through Photosystem II and Photosystem I?

A
  • As electrons pass through Photosystem II & I protons are transferred across the thylakoid membrane forming a proton gradient which is responsible for the formation of ATP
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20
Q

Where are the photosystems located in the chloroplast?

A
  • Photosystems are anchord to the hydrophobic side chains within a thylakoid or photosynthetic bacterial membrane
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21
Q

What, structurally speaking, are photosystems?

Where are they located?

A
  • Structurally they are chlorophyll binding proteins which stabilises the photosystem and helps to modify the absorption spectrum
  • They are anchored to the hydrophobic side chains within a thylakoid of bacterial photosynthetic membrane
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22
Q

What are the photosystem complexes surrounded by?

What is their purpose?

A
  • Photosystem complexes in the thylakoid or photosynthetic bacterial membrane are surrounded by Light Harvesting Complexes (LHCs) which have no reaction centre
  • They are used to absorb photon energy and pass it on to the photosystems
  • They are often rich in chlorophylls and accessory pigments
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23
Q

How many variations of the light reaction are there?

What are the different variations?

A
  • There are two variations of the light reaction:
    • Noncyclic photophosphorylation which occurs in eukaryotic cells
    • Cyclic photophosphorylation which occurs in bacterial cells
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24
Q

Where does noncyclic photophosphorylation occur?

What does it result in the formation of?

What does it involve the use of?

A
  • Noncyclic photophosphorylation (oxygenic) occurs in eukaryotic plant and algae cells
  • Results in the formation of ATP, NADPH, O2
  • It uses H2O and requires the use of photosystems II and I in parallel
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25
Q

Describe, at a high level, what happens with photosystem II in noncyclic photophosphorylation (oxygenic)

A
  1. Photons release energy to photosynthetic pigments and high energy electrons are passes to the reaction centre
  2. Chlorophyll a passes high energy e-s to the electron transport chain (ETC) in the thylakoid membrane
  3. High energy e-s are passed down the ETC releasing energy which is converted to ATP
  4. Electron gap in PSII is filled by breaking H2O down to release 2H+ + 2 e- + O
  5. Oxygen is released as a biproduct, e-s fill the gap after chlorophyll A has given high energy e-s to ETC, H+s are used in the formation of NADPH
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26
Q

Describe what happens, at a high level, with photosystem I during noncyclic photophosphorylation (oxygenic) and when it happens in relation to the reactions at PSII

A
  • Happens simultaneously as reactions in PSII
  1. Photons release their energy to photosynthetic pigments and the high energy e-s are passed to the reaction centre
  2. Chlorophyll A passes the high energy e-s to NADP+ which picks up the e-s and a H+ to form NADPH
  3. Chlorophyll A is missing an electron and the gap is filled by the electron which came from PSII after it had gone down the ETC
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27
Q

Summarise the actions of PSII and PSI in noncyclic photophosphorylation

A
  • PSII and PSI work together to produce ATP and NADPH which will be used in the dark reaction
  • O2 is produced from the breakdown of H2O to release electrons to fill the gap at PSII reaction centre
28
Q

Where does cyclic photophosphorylation occur?

What does it result in the formation of?

What does it involve the use of?

A
  • Cyclic photophosphorylation (Anoxygenic) is a variation of the light reaction which occurs in some bacterial cells
  • It only uses PSI and results only in the production of ATP energy
  • Bacterial cells attain the reducing power from another source such as H2S or H2(S2O3)
29
Q

Describe what happens, at a high level, in the reaction of cyclic photophosphorylation (anoxygenic)

A
  1. In PSI photons release their energy to photosynthetic pigments and the high energy electrons are passed to the reaction centre
  2. Chlorophyll A passes the high energy electrons to the electron transport chain where it is used to generate ATP for the dark reaction
  3. Chlorophyll A is missing an electron and this gap must be filled before it can give away any more electrons
  4. The gap is filled by the electron from PSI after it has been down the electron transport chain
30
Q

Summarise cyclic photophosphorylation (anoxygenic)

A
  • Only product is ATP
    • No NADPH or O2 is generated and no H2O is used
  • The ATP that is generated is used for the dark reaction
31
Q

What is photophosphorylation?

When does it occur?

A
  • Photophosphorylation is the production of ATP during photosynthesis
  • During the cyclic and noncyclic options of the light reaction ATP is produced by photophosphorylation
32
Q

What is photoreduction?

When does it occur?

A
  • Photoreduction is the production of NADPH during photosynthesis
  • It occurs during the noncyclic option of the light reaction where NADPH is produced by photoreduction
33
Q

In noncyclic photophosphorylation, describe in detail the first stage of the reactions of PSII

A
  1. Photon energy is channels to a modified chlorophyll A molecule called Pheophytin
  2. Pheophytin passes the excited e-​ through two compounds which are also part of the reaction centre
    1. Firstly, e- is passed to Plastoquinone then to Plastoquinol
  3. Plastoquinol transferrs the excited electrons to the cytochrome complex in the thylakoid membrane
34
Q

In noncyclic photophosphorylation, describe in detail the second stage of the reaction of PSII

A
  1. Electrons are passed from Plastoquinol to a Cytochrome B6/FeS/F (cyt b6/f) complex
  2. Cyt b6/f complex strips the electron of its energy and uses it to move H+ across the thylakoid membrane from the chloroplast stroma to the thylakoid space / lumen
  3. Electron which has been stripped of its energy is passed to Plastocyanin which donates the electron to fill the gap in PSI
35
Q

In noncyclic photophosphorylation, describe in detail the formation of ATP

A
  • In the thylakoid membrane there are ATP synthase complexes similar to those found in the mitochondria, referred to as CF0CF1 complexes
  • The CF1 is a stalk composed of 5 polypeptides protruding into the stroma and the CF0 forms a hydophillic channel between the stroma and the thylakoid lumen to allow H+ translocation
  • H+ in thylakoid space result in a H+ concentration gradient, H+ move through CF0CF1 complexes into the stroma which is coupled with ATP production
  • 3 ATP are generated for every 8 H+ which passes through the ATP synthase complex
36
Q

Summarise, in detail, the reactions occuring in PSII in noncyclic photophosphorylation

A
  • 6CO2 + 12H2O ———-> C6H12O6 + 6O2 + 6H2O
  • 12 H2O are used to generate 1 x 6C compound which releases 24 e-s which fill the gap in PSII and allows the release of 24 excited e-s from PSII
  • 24 e-s pass to cyt b6/f complex and the energy from each electron moves 2 H+ across the thylakoid membrane
  • A total of 48 H+ can be moved across the membrane which are then translocated through the thylakoid membrane and product 18 ATP molecules (3ATP for 8 H+)
37
Q

In noncyclic photophosphorylation, describe in detail the reactions occuring at PSI

A
  1. Plastocyanin donates electrons stripped of their energy to fill the e- gaps in PSI
    1. This allows the release of electrons which are used in the generation of reducing power NADPH
  2. Excited e-s are channeled to a modified chlorophyll A molecule (Ao) in the reaction centre
  3. Ao passes the e- through a molecule of Phylloquinone (A1) and then through three FeS centres (FX, FA/B, Fd)
  4. Fd passes the e- onto the NADPH complex
  5. 12NADP + + 24H+ + 24 e- —–> 12NADPH + 12H+
38
Q

How many ATP and NADPD are produced in the nonycyclic light reaction?

A
  • 12 NADPH are produced by PSI using photoreduction
  • 18 ATP are produced by PSII using photophosphorylation
39
Q

Describe, in detail, the cyclic option of the light reaction

A
  1. Excited e-s in PSI are channels to the modified chlorophyll A molecule Ao in the reaction centre
  2. Ao passes the excited e- through a molecuel of Phylloquinone (A1) and then through three FeS centres (FX, FA/B, Fd)
  3. Fd passes the electron onto the cyt b6/f complex in the thylakoid membrane
  4. Cyt b6/f strips the energy from the e- and uses the energy to move H+ across the thylakoid membrane from the chloroplast stroma to the thylakoid lumen
  5. e- stripped of its enegy is passes to Plastocyanin which donates it to fill the e- gap in PSI
40
Q

Describe, in detail, the formation of ATP in the cyclic option of the light reaction

A
  1. H+ molecules accumulated in the thylakoid space resulting in a concentration gradient
    1. It’s though that for 3H+ are moved into the thylakoid space for every 2e- that move through PSI
  2. H+ move down the gradient through ATP synthase comples into the stroma
  3. Flow of H+ is coupled with the production of ATP
41
Q

When is the cyclic option of the light reaction used?

A
  • It is used when the cell doesn’t require ATP and NADPH in the quantities produced by the noncyclic system (ration 3ATP:2NADPH)
  • If there is ample NADPH or when NADPH requirements are low the cell often switches to the cyclic pathway which generates only ATP
42
Q

What is the dark reaction (what is an alternative name)?

Where does it occur?

What are the stages(briefly)?

A
  • Dark reaction - aka the Calvin cycle - is the stage of photosynthesis where CO2 and H2O are converted into a carbohydrate
  • Calvin cycle occurs in the chloroplasts and each stage is mediated by an enzyme
  • Cycle occurs in three stages:
    • CO2 fixation
    • CO2 reduction
    • Ribulose bisphosphate regeneration
43
Q

What is important to remember about the carbohydrate produced by the Calvin cycle?

A
  • Carbohydrate produced and released is phosphoglyceraldehyde (PGAL) and not glucose
  • PGAL is a 3C compound hence this is called C3 cycle
  • PGAL units may join together to form glucose and other polymers such as sucrose and starch
44
Q

What supplies the reducing power for the Calvin cycle?

A
  • ATP and NADPH from the light reaction provide the reducing power for this reduction reaction
45
Q

What are plants called that use the Calvin cycle?

A
  • Plants that use the Calvin cycle for their carbon assimilation are called C3 plants
46
Q

Outline the carbon fixation stage of the Calvin cycle

A
  • CO2 binds to a 5C compound called ribulose bisphosphate (RuBP) to become a 6C compound which immediately breaks down to 2 x 3C compounds called phosphoglyceric acid (PGA)
  • Catalysed by the enzyme ribulose bisphosphate carboxylase (RuBisCO) in C3 plants
47
Q

Describe the carbon dioxide reduction stage of the Calvin cycle

A
  • Phosphoglyceric acid (PGA) is reduced using ATP and NADPH from the light reaction into another 3C compound phosphoglyceraldehyde (PGAL)
  • This is the end product of the Calvin cycle abd PGAL can be removed and used to form larger carbohydrates such as glucose, sucrose and starch
  • 1 PGAL molecule requires 3 CO2 molecules to enter the Calvin cycle
48
Q

Describe the ribulose bisphosphate regeneration stage of the Calvin cycle

A
  • RuBP (5C) compound initiated the cycle cannot be lost as carbons from the system, therefore it must be regenerated
  • Some PGAL molecules undergo reorganisation to regenerate RuBP but this process must be balances for carbon input & outuput
  • 6CO2 + 6RuBP ——> 6 x 6C compounds —–> 12 x 3C (PGAL)

12 x 3C PGAL ——> a) 2 x 3C PGAL removed to form 1 x glucose

b) 10 x 3C PGAL recycled to for 6 x 5C RuBP.

49
Q

How many molecules of CO2 need to enter the Calvin cycle to form one glucose molecule?

A
  • An initial input of 6 CO2 into the Calvin cycle is required to produce 2 x 3C PGAL which ultimately forms 1 x glucose (6C)
50
Q

What are the main differences between cyclic and non-cyclic photophosphorylation?

A
  • Only PSI is used in cyclic photophosphorylation where the e-s are passed back to the same photosystem
  • Noncyclic photophosphorylation uses both PSII and PSI where the electrons are passed from PSII to PSI via electron carriers
  • Only ATP is produced in cyclic phosphorylation whereas both ATP and NAPHD are produced in non-cyclic photophosphorylation
51
Q

Write the overall balanced equation for the Calvin cycle

A
  • 6CO2 + 6RuBP ——> 6 x 6C compounds —–> 12 x 3C (PGAL)

12 x 3C PGAL ——> a) 2 x 3C PGAL removed to form 1 x glucose

b) 10 x 3C PGAL recycled to for 6 x 5C RuBP.
* 6 CO2 molecules are required to produce 1 glucose molecule because 10 of 12 PGAL molecules are required to regenerate RuBP

52
Q

What happens to the products of photosynthesis (PGAL)?

A
  • In the stroma of the chloroplast the 3C PGAL is converted to glucose-1-phosphate which is coverted into starch for storage
  • 6C sugars cannot pass from the stroma to the cytosol therefore PGAL is exported to the cytosol of the cell where it is converted into fructose-6-phosphate, glucose-6-phosphate then glucose-1-phosphate which can be used to assimilate sucrose
    • Sucrose is then transported through the vascular bundle to the plant tissues and used for energy and carbon
53
Q

Describe what happens to PGAL from the Calvin cycle when it is in excess

A
  • PGAL is converted into glycose-1-phosphate which can be assimilated into starch with the input of ATP energy
  • Process is enzyme regulated and starch is only produced when there is an excess of PGAL
  • Starch serves as a carbohydrate reserve and it is stored as large granules in the stroma in period of excess photosynthetic capacity
  • When needed, starch can be hydrolyzed to release 6C compounds which are fed direclty into cellular respiration or can be used to assimilate sucrose to be transported through the plant and used as required
54
Q

Outline photorespiration

A
  1. RuBisCO in C3 plants can also fix O2 to 5C RuBP, instead of CO2, to produce 1 x 3C phosphoglycerate and 1 x 2C phosphoglycolate
  2. Phosphoglycolate can be broken down to release CO2 from the system
  • If CO2 is fixed the Calvin cycle occurs as normal but if [CO2] is low O2 is fixed instead since CO2 and O2 compete with other for RuBisCO therefore the [] of both is crucial
55
Q

Describe the plants in which photorespiration occurs, when and why it occurs

A
  • C3 plants that use RuBisCO general live in temperate climates <25oC
  • Fixation of CO2 occurs in mesophyll cells and the products are transferred to the bundle sheath cells and into the vascular system for transport round the plant
  • Photorespiration tends to occur on hot days when the plants close their stomata to converve H2O, without the flow of CO2 into the cells the [CO2] decrease and the [O2] increase
  • Rise in [O2] results in RuBisCO fixing O2
56
Q

Why have plants evolved to undergo photorespiration?

A
  • Historically when C3 plants developed this fixation system (Calvin cycle) the atmosphere had high [O2] therefore photorespiration was not an issue
  • When the oxidising atmosphere developed the problem of photorespiration arose
  • Photorespiration doesn’t benefit the plant since no energy is generated and little C is fixed meaning it is considered to be wasteful
  • It takes C out of the Calvin cycle as opposed to putting it in which can results in a 50% reduction in C fixation in some plants
  • C3 plants have maintained this throughout evolution
57
Q

Give a brief overview of how C4 plants have avoided photorespiration

A
  • C4 plants have an additional pathway before the Calvin cycle which is responsible for the initial CO2 fixation
  • The initial fixation occurs in the mesophyll cells and its purpose is to concentrate and fix the CO2 when [CO2] are low and the concentrated CO2 is passed to the Calvin cycle which occurs in the bundle sheath cells
58
Q

Outline the first stage of the carbon fixation in C4 plants

A
  1. CO2 is fixed to phosphoenol pyruvate (PEP) to form a 4C compound oxaloacetate catalysed by phosphoenol pyruvate carboxylase (PEPCO)
    1. ​PEPCO will no fix O2 meaning even at low [CO2] PEPCO scavenges CO2 and concentrates it by fixing it to oxaloacetate in mesophyll cells
  2. Oxaloacetate is reduced to malic acid (4C malate) and malate transfers the CO2 to the bundle sheath cells into the Calvin cycle
    1. No O2 can enter the Calvin cycle
  3. Once CO2 released pyruvic acid is left which is converted back to PEP requiring 2 molecules of ATP
59
Q

Outline the second stage of carbon fixation in C4 plants

A
  1. When CO2 enters the bundle sheath cells it is in a concentrated form so RuBisCO will only fix CO2 in the Calvin cycle and no photorespiration will occur
  2. Products of the Calvin cycle are fed into the vascular system
60
Q

How is the Calvin cycle different in C4 plant to the Calvin cycle in C3 plants ?

A
  • Plant uses 18 ATP and 12 NADPH through the Calvin cycle for both C3 and C4 plants
  • However C4 plants expend two ATP recycling phosphoenol pyruvate from pyruvic acid
    • This energy is well spend is the C4 plant is not losing carbons through photorespiration
  • C4 plants use different spatial arrangement of cells, there is an additional section of pathway to counteract photorespiration but the reactions occur at the same time as in C3 plants
61
Q

Where do C4 plants grow?

Give examples of C4 plants

A
  • C4 plants live in hot climates between 30-40oC where there is a high risk of photorespiration occuring
  • C4 plants include sugar cane, corn, grasses, and several thousand species from 17 families that live in hot climates
62
Q

What are CAM plants?

How have they developed to counteract photorespiration?

A
  • CAM plants are members of the Crassulaceae family
  • They have an additional pathway before the Calvin cycle which is responsible for the initial CO2 fixation
  • Initial fixation in CAM plants occurs at night in the mesophyll cells and its purpose is to fix and concentrate CO2 when [CO2] are high
  • The concentrated CO2 is passed to the Calvin cycle in the mesophyll cells during the day when [CO2] is low due to closed stomata and when NADPH and ATP are available from the light reaction
63
Q

Outline the first stage of the CAM pathway

A
  1. At night CO2 is fixed to a 3C phosphoenol pyruvate (PEP) compound to form a 4C compound oxaloacetate
    1. Catalysed by phosphoenol pyruvate carboxylase (PEPCO) which will not fix O2
  2. Oxaloacetate is reduced to malic acid (4C malate) and CO2 remains fixed in the mesophyll cells overnight
  3. During the day when the light reaction occurs the stomata are closed and the CO2 in malate is passed to the Calvin cycle in the mesophyll cells
    1. Only CO2 is being fed to the Calvin cycle when energy is available for assimilation and not O2 is present
  4. CO2 released from malate generates 3C pyruvic acid which is converted back to PEP requiring 2 ATP
64
Q

Outline the second step of the CAM pathway

A
  1. CO2 entering the Calvin cycle in mesophyll cells is in a concentrated form meaning RuBisCO only fixes CO2 and no photorespiration occurs
  2. ATP and NADPH are available since the light reaction can occur during the day and therefore assimilation occurs
  3. The products from the Calvin cycle feed into the bundle sheath cells then into the plants vascular system
65
Q

Where do CAM plants live?

Give some example of CAM plants

A
  • CAM plants live in climates >35oC when there is a high risk of photorespiration occuring
  • Examples of CAM plants include succulent plants such as jade plants, stone crops and cacti
66
Q

How is the Calvin cycle different in CAM plants to C3 plants ?

A
  • In terms of energy CAM plants gain 18 ATP and 12 NADPH through the Calvin cycle as with C3 plants
  • However, CAM plants expend ATP recycling phosphoenol pyruvate from pyruvic acid in the additional pathway
    • This expenditure requires 2 ATP but is well spent since the plant doesn’t lose carbons through photorespiration
  • CAM plants use different times, same location and have an additional section of pathway to counteract photorespiration
67
Q

What are the different types of autotrophy?

A
  1. C3 plants
  2. C4 plants
  3. CAM plants

06 FdSc Bioscience - Metabolism and Enzymology (8 decks)

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