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1. Biology Advanced info > 5.2.1 Photosynthesis > Flashcards

5.2.1 Photosynthesis Flashcards

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1
Q

what is photosynthesis?

A

the synthesis of complex organic molecules using light

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2
Q

what is the overall equation for photosynthesis?

A

6H2O + 6CO2 -> 6O2 + C6H12O6

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3
Q

what is the overall equation for respiration??

A

6O2 + C6H12O6 -> 6H2O + 6CO2

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4
Q

what is the Chloroplast envelope?

A

The of inner and outer membranes – these membranes are partially permeable and allow entry of carbon dioxide (by diffusion) and water (by osmosis) and exit of oxygen (by diffusion) and glucose (by facilitated diffusion);

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5
Q

what are the Thylakoids

A

each thylakoid is a circular, membrane bound disc; the thylakoid membranes are the site of the light‐dependent stage of photosynthesis and contain:
o Chlorophyll and other (accessory) pigments for light absorption, located within structures called Photosystems;

o components used in the electron transfer chains, some of which act as proton pumps;

o ATP synthase enzymes, which synthesise ATP via photophosphorylation

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6
Q

what is the grana/granum?

A

dark, dense structures on TEM images; each granum consists of a stack of many thylakoids;

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7
Q

what is the lamellae?

A

these are elongated thylakoids that join grana to one another;

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8
Q

what is the stroma?

A

the background fluid that fills the space within the chloroplast; it is the site of the LIGHT-INDEPENDANT stage of photosynthesis (the Calvin Cycle).
Within the stroma are the following important components:
o small circular DNA molecule, containing genes coding for some of the proteins needed by the chloroplast;

o 18nm (70S) ribosomes, carrying out protein synthesis;

o enzymes involved in the light independent stage of photosynthesis, including RuBisCO;

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9
Q

What are Starch grains and lipid droplets?

A

storage polymers are formed from excess
glucose that is not immediately needed for respiration etc.

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10
Q

what is this? label it.

A
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11
Q

what gives the drive for the synthesis of ATP and reduced NADP (NADPH)

A

light energy

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12
Q

how many forms of chlorophyll are there, and what are they? plus what do they do (alongside what)

A

2.
chlorophyll a
chlorophyll b

absorb light energy (w/ accessory pigments)

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13
Q

what is the structural difference between chlorophyll a and chlorophyll b?

A

small structural difference and subtle differences in the wavelengths of light they most strongly absorb.

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14
Q

what colour light does chlorophyll asorb and reflect?

A

Chlorophyll (of types a and b) only absorbs red and blue light strongly, reflecting green light

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15
Q

where is chlorophyll and accessory pigments located?

A

Thylakoid membranes

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16
Q

when does chlorophyll look green?

A

it is reflecting green light

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17
Q

how does chlorophyll extend the range of wavelengths it can absorb?

A

accessory pigments (usually present in smaller quantities than chlorophyll)

yellow, orange and red colours (indicating that they do absorb green light).

These accessory pigments include carotenoids (such as β‐ carotene) and xanthophylls.

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18
Q

what is unique about each pigment?

A

Each pigment has its own distinctive absorption spectrum

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19
Q

what is only chlorophyll a capable of?

A

Only chlorophyll a is capable of losing an electron and passing it on into an electron transfer chain.

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20
Q

what is chlorophyll a often referred as?

A

The primary photosynthetic pigment

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21
Q

what is a chlorophyll a molecule that can lose an electron is known as?

A

Reaction Centre chlorophyll

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22
Q

where is the reaction centre chlorophyll known as? what does it do?

A

it is located in the middle of a complex made of other accessory pigments and proteins known as a Photosystem.

it helps maximise light absorbtion

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23
Q

what are the two different types of photosystems?

A 
Photosystem I (PSI) 
Photosystem II (PSII)
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24
Q

What is Photosystem I (PSI)?

A

this contains a Reaction Centre chlorophyll which is a molecule of chlorophyll a known as P700 (because it most strongly absorbs light of wavelength 700nm); this Photosystem can take part in cyclic photophosphorylation or in non‐cyclic photophosphorylation;

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25
Q

What is Photosystem II (PSII)?

A

Photosystem II (PSII) – this contains a Reaction Centre chlorophyll which is a molecule of chlorophyll a known as P680 (because it most strongly absorbs light of wavelength 680nm); this Photosystem can only take part in non‐cyclic photophosphorylation

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26
Q

what surrounds the Reaction Centre of each Photosystem?

A

Light Harvesting Complex (LHC)/Antennae complex

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27
Q

what is in the Light Harvesting Complex (LHC)/Antennae complex?

A

A large number of chlorophyll b molecules and accessory pigments (carotenoids and xanthophylls), supported by a protein scaffold.

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28
Q

what does the Light Harvesting Complex (LHC)/Antennae complex do?

A

The pigments in the LHC absorb light, but cannot lose electrons themselves. Instead, they must
‘funnel’ energy towards the Reaction Centre chlorophyll in the heart of their Photosystem. The Reaction Centre chlorophyll (chlorophyll a) will lose an electron when it receives sufficient energy from its LHC.

29
Q

what is the first part of photosynthesis?

A

light‐dependent stage

30
Q

what is required in the light-dependant stage?

A

light energy - provides the energy

water - source of electrons (for the electron transfer chains and the reduction of NADP

H+ ions - for chemiosmosis, by which ATP is synthesised

oxidised NADP

31
Q

how are water molecules split? What is the water product?

A

photolysis (associated with non‐cyclic photophosphorylation only)

oxygen is released as a waste product from this stage

32
Q

what are the products of the light-dependent stage?

A

Reduced NADP (NADPH)
Oxygen
ATP

33
Q

Which products from the light-dependant stage are required for the light-independent stage

A 
Reduced NADP (NADPH) 
ATP
34
Q

quick overview of Cyclic photophosphorylation

A 
This process: 
produces ATP (but not reduced NADP);

only involves PSI (not PSII);

does not involve photolysis of water and does not produce oxygen

35
Q

Cyclic photophosphorylation mechanism

A
  1. The Reaction Centre chlorophyll in PSI (a chlorophyll a molecule called P700) absorbs a photon of light (or receives energy from the accessory pigments in its Light Harvesting Complex)
  2. An electron from the Reaction Centre chlorophyll is promoted to a higher energy level and is passed into the Electron Transfer Chain (ETC): this is a series of electron carriers (mostly proteins, embedded in the thylakoid membrane) each of which is reduced (by accepting the electron) and then oxidised (by passing on the electron) in turn
  3. Some of the electron carriers in the ETC act as proton pumps: using the energy released as the electron passes from carrier to carrier, H+ ions are actively transported against their concentration gradient from the stroma into the thylakoid space (lumen)
  4. H+ ions move back down their concentration gradient into the stroma in a process called chemiosmosis, through an ATP synthase enzyme (embedded in the thylakoid membrane);
  5. The energy released during chemiosmosis drives ATP synthesis (via a condensation reaction between ADP and inorganic phosphate, catalysed by ATP synthase);
  6. The electron that originated from the Reaction Centre chlorophyll eventually returns to that same Reaction Centre chlorophyll (hence the term ‘cyclic,’ as the electron has returned to its starting point).
36
Q

what is this? label it.

A

cyclic photophosphylattion

37
Q

quick overview of Non‐cyclic photophosphorylation

A

This process:
produces ATP and reduced NADP

involves both PSI and PSII

involves photolysis of water and produces oxygen.

38
Q

how to remember the Non‐cyclic photophosphorylation diagram?

A

The Z‐scheme

39
Q

What is this? label it

A

Non‐cyclic photophosphorylation

40
Q

Non‐cyclic photophosphorylation mechanism (Wrong Come back to this)

A
  1. The Reaction Centre chlorophyll in PSI (a chlorophyll a molecule called P700) absorbs a photon of light (or receives energy from the accessory pigments in its Light Harvesting Complex);
  2. An electron from the Reaction Centre chlorophyll in PSI is promoted to a higher energy level and is passed into an Electron Transfer Chain (ETC): this is a series of electron carriers, each of which is reduced and then oxidised in turn;
  3. Some of the electron carriers in the ETC act as proton pumps: using the energy released as the electron passes from carrier to carrier, H+ ions are actively transported against their concentration gradient from the stroma into the thylakoid space (lumen);
  4. H+ ions move back down their concentration gradient by chemiosmosis via an ATP synthase enzyme (embedded in the thylakoid membrane);
  5. The energy released during chemiosmosis drives ATP synthesis (via a condensation reaction between ADP and inorganic phosphate, catalysed by ATP synthase);
  6. The electron that has been transferred along the ETC is eventually accepted by (the oxidised form of) NADP, forming reduced NADP;
  7. Since PSI does not receive its own electron back again (as it would in the cyclic process), it instead receives a replacement electron from PSII: the Reaction Centre chlorophyll in PSII (a chlorophyll a molecule called P680) loses an electron when it absorbs a photon of light (or receives energy from the accessory pigments in its Light Harvesting Complex);
  8. On its way to PSI, the electron lost from PSII passes down a chain of electron carriers (an ETC) that links PSII to PSI: some of these carriers are proton pumps and so this electron transfer results in chemiosmosis and ATP synthesis;
  9. PSII does not receive its own electron back (as it has been accepted by PSI) but instead receives replacement electrons from water molecules that have undergone photolysis: this splitting of water molecules is catalysed by an Oxygen Evolving Complex that is associated with PSII;
  10. Photolysis of water also releases H+ ions into the thylakoid space (contributing to the proton gradient that is used in chemiosmosis and hence ATP synthesis) and oxygen molecules (which eventually diffuse out of the leaf, unless used in aerobic respiration in mitochondria).
41
Q

what is the light independant reaction?

A

2nd stage of photosynthesis which does notdireclty rely on light energy

42
Q

why is the name light independant stage slightly misleading?

A

The light independant stage requires ATP and NADPH, which are produced in the light dependant stage

43
Q

what is the light independant reaction also called?

A

Calvin cycle

44
Q

what is the aim of the light independant reaction?

A

‘fix’ inorganic carbon (carbon dioxide) into organic compounds

The desired product is a three‐carbon sugar called triose phosphate, from which glucose can be made

45
Q

What is this? label it.

A

calvin cycle/ light independant reaction

46
Q

what is the mechanism for the Calvin cycle?

A
  1. The enzyme RuBisCO (ribulose bisphosphate carboxylase oxygenase) catalyses the carboxylation of (i.e. the reaction of carbon dioxide with) the five carbon ‘acceptor molecule,’ ribulose bisphosphate (RuBP);
  2. The immediate product is an unstable six‐carbon compound, which immediately splits into two molecules of a three‐carbon organic compound called glycerate‐3‐phosphate (GP): this is the first organic product of the light‐independent stage and contains the fixed carbon from carbon dioxide;
  3. GP is reduced to a three‐carbon sugar, triose phosphate (TP), in a step that requires the reducing power of reduced NADP and energy from ATP (both products of the light-dependent stage;
  4. One sixth of the TP produced is taken out of the Calvin cycle for conversion to glucose or other organic compounds as required (and detailed below);
  5. Five sixths of the TP produced is used in the regeneration (requiring ATP) of ribulose bisphosphate, the five‐carbon acceptor molecule;
  6. Providing there is sufficient ribulose bisphosphate available, the cycle continues, with carbon dioxide combining with RuBP, such that more carbon is fixed.
47
Q

what are the products of photorespiration?

A

one molecule of GP
one molecule of phosphoglycolate (a two‐carbon compound).

48
Q

what can phosphoglycerate be converted into?

A

phosphoglycolate can be converted to GP. However, this involves combining two phosphoglycolate molecules and then undergoing a decarboxylation step (releasing one carbon dioxide molecule) in order to produce one molecule of GP.

49
Q

what does it mean when RuBisCO is promiscuous?

A

it has lower substrate specificity that most enzymes

50
Q

what does RuBisCO catalyse in photorespiration instead of CO2 and RuBP?

A

oxygen and RuBP.

51
Q

what are the negative consequences of photorespiration?

A

Decreased carbon fixation as some RuBisCO is catalysing the reaction between oxygen and RuBP rather than carbon dioxide and RuBP;

Decreased production of GP and therefore less TP and less glucose made

Decreased regeneration of the acceptor molecule RuBP, so a knock‐on effect that less carbon fixation is possible following photorespiration, since less RuBP is available to combine with carbon dioxide

Loss of previously fixed carbon via the decarboxylation step needed to convert two phosphoglycolate molecules into one GP molecule

52
Q

what does oxygen act as for the RuBisCo active site?

A

A competitive inhibitor, therefore photorespiration rates will increase and Calvin Cycle rates decrease if the ratio of oxygen to carbon dioxide increases.

53
Q

give four examples of what TP (Triose phosphate) can be converted into

A

Glucose – used as a respiratory substrate to provide energy for metabolism;

Cellulose (a polysaccharide made from glucose) – used to make cell walls;

sucrose - to bo loaded into the phloem and translocated to sink tissues

Amino acids – used in protein synthesis, to produce new enzymes etc, including RuBisCO;

54
Q
A
55
Q

what is the majority of the TP used for?

A

5/6 of the TP made in the Calvin Cycle must be used to regenerate the acceptor molecule, RuBP (ribulose bisphosphate). This is vital as it means that the Calvin Cycle can continue and more carbon dioxide can be fixed.

56
Q

what would happen if more than 1/6 of TP made was taken out the calvin cycle?

A

decrease in the rate of carbon fixation

rate of TP production would fall - shortage of RuBP to combine with CO2.

57
Q

what are the three principal limiting factors in photosynthesis

A

carbon dioxide concentration (not high enough);
light intensity (not high enough);
temperature (too low or too high).

58
Q

why is water availability not usually a limiting factor?

A

Water availability is NOT usually a limiting factor, despite it being a raw material for the light-dependent stage of photosynthesis. This is because water is so plentiful in the cytoplasm and chloroplast stroma.

However, if a plant is suffering from extreme water stress (e.g. during prolonged hot, dry conditions), stomatal closure could become a limiting factor for photosynthesis.

59
Q

why is carbon dioxide usually a limiting factor?

A

The concentration of carbon dioxide in atmospheric air is low (only 0.04%). Therefore, when light intensity is high and temperature is optimal, it is not surprising that (inadequate) carbon dioxide concentration becomes the limiting factor on the photosynthesis rate. Raw material in the calvin cycle

60
Q

draw the co2 conc graph, and desrcibe it

A

increases at a proportional rate, until its no longer a limiting factor and plateaus

61
Q

why is light intensity a limiting factor?

A

Light from the sun is the energy source for photosynthesis: in the light‐dependent stage, light energy is converted to chemical energy in ATP via photophosphorylation. Light is essential for the production of both ATP and reduced NADP in chloroplasts, which are themselves essential in the light dependent stage.

In conditions of low light intensity, e.g. plants growing in partial shade below a tree canopy, or any plant at dawn or dusk, light intensity will be the factor limiting photosynthesis. However, in bright sunlight, another factor will become limiting instead (either carbon dioxide concentration or temperature).

62
Q

draw the light intensity graph

A

increases at a proportional rate, until its no longer a limiting factor and plateaus

63
Q

why is temperature a limiting factor when below the optimum?

A

When the temperature is below the optimum, increasing temperature will increase rate. This is because higher temperatures give molecules more kinetic energy, higher rates of successful collisions and so more ESC formation.

64
Q

why is temperature a limiting factor when above the optimum?

A

When the temperature is above optimum, decreasing temperature will increase rate. This is because lower temperatures do not cause denaturation of enzymes. At high temperatures, when kinetic energy is excessive, hydrogen bonds holding an enzyme’s tertiary structure break, causing a change in the 3D shape of the active site such that it is no longer complementary to the substrate. STATE RUBISCO

65
Q

draw the temperature graph and describe it

A
66
Q

what happens to the calvin cycle if we decrease light intensity? what happens if we increase it again?

A

Levels of GP increase: this is because GP is still being produced by carbon dioxide combining with RuBP, as this step doesn’t directly depend on products from the lightdependent stage; however the GP accumulates because it can no longer be converted into TP, since the relevant reactions do require ATP and reduced NADP, which are now in short supply as the light‐dependent stage cannot occur.

Levels of TP decrease: this is because TP cannot be made from GP, since there is insufficient ATP and reduced NADP due to the light‐dependent stage not occurring.

if light intensity increases again, since ATP and reduced NADP are made in the light‐dependent reactions and allow the Calvin Cycle to run normally.

67
Q

what happens to the calvin cycle if we decrease co2 concentrations? what happens if we increase it again?

A

Levels of GP decrease: this is because there is not enough carbon dioxide to combine with RuBP, so less GP is formed as a product.

Levels of TP decrease: this is because there is less GP being made and so less GP available to convert to TP; any TP that is available may be converted to RuBP.

These effects will quickly be reversed if carbon dioxide levels increase and RuBP can once again combine with carbon dioxide to produce GP.

68
Q

what happens to the calvin cycle if we change the temperatures below or above the optimum? what happens if we increase it again?

A

The effects of lower temperatures are likely to be reversible. However, the effects of very high temperature may not be reversible, if enzymes have (permanently) denatured.

69
Q

explain photolysis

A

water molecules are split into hydrogen ion, electrons and oxygen molecules

electrons replace the electrons lost from PS II

H+ ions go into thylakoids to increase the proton gradient for chemiososmosis. Once in the stroma, an h+ ions and electron from PS I bind with NADP to form NADPH (removing this electron from the stroma maintains a high proton gradient)

1. Biology Advanced info (23 decks)

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