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Biology OCR AS Level > Photosynthesis > Flashcards

Photosynthesis Flashcards

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1
Q

What is photosynthesis?

A

The physiological process used by plants, algae and some bacteria to convert light energy from the sun to chemical energy in order to synthesize large organic molecules from simple inorganic molecules like water and carbon dioxide. Glucose and oxygen are produced.
This is an example of autotrophic nutrition

*Most forms of life rely directly or indirectly on photosynthesis

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2
Q

What is autotrophic nutrition?

What is photo-autotrophic nutrition?

A

Autotropic nutrition is when simple inorganic molecules are used to synthesise organic molecules. Energy is needed for this.
In photo-autotrophic nutrition, light is the energy source.

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3
Q

Plants, algae and some bacteria are photoautotrophs. What are photoautotrophs?

A

Photoautotrophs carry out photo-autotrophic nutrition.

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4
Q

Why are photo-autotrophs also described as producers?

A

This is because they are at the beginning of a food chain i.e. first trophic level.
For this reasons they provide the energy and organic molecules to other non-photosynthetic organisms.

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5
Q

What is the general equation for photosysnthesis

A

6CO₂ + 6H₂O + (energy from photons) → C₆H₁₂O₆ + 6O₂

*chlorophyll on arrow

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6
Q

What is a photon?

A

Carries light energy from sun

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7
Q

What type of sugar is produced in photosynthesis?
What can it be converted to for transport?
What can it be converted to for storage?

A
  • A monosaccharide of glucose
  • Can be converted to disaccharide for transport
  • Can then be converted to starch for storage.
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8
Q

What process is photosynthesis an example of?

A

Carbon fixation.

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9
Q

What is carbon fixation?

A

The process by which carbon dioxide is converted into sugars.

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10
Q

Why is carbon fixation important? What does it produce?

A

Carbon fixation is important because it provides the carbon for synthesizing all types of organic molecules.
Carbon fixations helps regulate the concentration of carbon dioxide in the atmosphere and oceans.

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11
Q

Is carbon fixation endothermic or exothermic?

A

Endothermic so requires energy.

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12
Q

Is carbon fixation a reduction or oxidation and why?

A

Carbon fixation is reduction because it needs electrons.

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13
Q

What are heterotrophs?

A

Organisms that ingest and digest complex organic molecules (as they cannot make it themselves), releasing the chemical potential energy stored in them.
They break down the large organic molecules into smaller ones like glucose, which can then be used as respiratory substrates to release the energy.

E.g. Humans`

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14
Q

What is the general equation for aerobic respiration respiration?

A

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

*opposite equation to phtosynthesis.

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15
Q

Is aerobic respiration exothermic or endothermic?

A

Exothermic.

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16
Q

How do photosynthesise and aerobic respiration relate?

A

The products of one process are the raw materials for the other process.
AR removes oxygen from the atmosphere and carbon dioxide while photosynthesis does the opposite.

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17
Q

What times do plants respire and photosynthesis?

A

Plants respire all the time, but only photosynthesise during daylight.

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18
Q

Why do plants only photosynthesise during daylight?

A

This is because light is needed for the first stage of phtosynthesis.

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19
Q

What happens to the rate of photosynthesis as the light intensity increases?

A

As light intensity increases, the rate of photosynthesis increases. The light must be sufficient to allow photosynthesis to occur at a rate that replenishes the carbohydrate stores used up by respiration.

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20
Q

Draw the graph of oxygen production nad oxygen absorption against time.
Photosynthesis in sun plants, Photosynthesis in shaded plants. Respiration of the plants.

A

PG115

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21
Q

What is the compensation point?

A

When photosynthesis and respiration proceed at the same rate, so that there is no net gain or loss of carbohydrate.
This is different for different plant species.

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22
Q

What is the compensation period?

A

The time a plant takes to reach its compensation point.

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23
Q

How are shade plants adapted to low light intensity?

A

Shaded plants can photosynthesize using light of lower intensity, than other plants, therefore, if a sun plant and shade plant are both taken out of the shade, the shaded plant would reach its compensation point earlier than the sun plant.

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24
Q

Which organelle does photosynthesis take place in?

A

Photosynthesis takes place in chloroplasts.

  • Algae + plants have chloroplasts
  • Photosynthetic bacteria does not have chloroplasts.
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25
Q

Structure of chloroplast?`

NEED TO BE ABLE TO LABEL A CHLOROPLAST.

A
  • 2-10µm long
  • Double membrane organelle
  • Grana (singular = granum) are stacks of flattened discs.
  • Each disk is called a thylakoid and the thylakoid contains the photosynthetic pigment, chlorophyll.
  • Grana are connected to each other by intergranal lamellae.
  • Surrounding the thylakoid membrane (the network of grana) is a fluid called the stroma. This contains, proteins enzymes, starch grains and the chloroplast’s own DNA + ribosomes. All enclosed in the inner membrane.
  • The space inside the thylakoid is called the thylakoid lumen
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26
Q

Which stage of photosynthesis takes place in the grana?

A

The light-dependant stage.

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27
Q

How many membranes does a chloroplast have and what are they?
They form three separate internal compartments which are….

A

3 membranes:
Inner membrane
Outer membrane
Thylakoid membrane

…thylakoid lumen, stroma and intermembrane space.

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28
Q

State the permeability of the three membranes in the chloroplasts (most permeable to least permeable)

A

Outer membrane = most permeable
Inner membrane = less permeable
Thylakoid membrane = least permeable

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29
Q

What is the benefit of the large number of grana in every chloroplasts and a large number of chloroplasts in each photosynthetic cell? Orange

A

Provides a HUGE surface area for:

  • the distribution of the photosystems that contain photosynthetic pigments that trap sunlight energy = maximises rate of photosynthesis.
  • the electron carriers and ATP synthase enzymes that are needed to convert light energy into ATP = maximises rate of photosynthesis.
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30
Q

Why is the grana surrounded by the stroma?

A

This is so that the products of the light dependant stage can easily pass into the stroma to be used in the light independent stage.

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31
Q

Why does the stroma contain enzymes and why?

What else does it contain

A

It contains the enzymes needed to catalyse the reactions of the light independent stage of photosynthesis.
It also contains starch grains, small ribosomes similar to those in prokaryotes and DNA.

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32
Q

What does the DNA in the stroma code for?

A

Some of the proteins needed for photosynthesis. These proteins are then assembled at the chloroplast ribosomes.

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33
Q

What are photosystems?

A

Funnel-shaped structures found in the thylakoid membrane of each chloroplast, which contain photosynthetic pigments, thats absorb light of a particular wavelength, whilst reflecting other wavelengths.

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34
Q

There are two main photosystems. What are they?

A

Photosystem I and Photosystem II

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35
Q

How can the photosynthetic pigments found in photosystems be classfied?

A

Primary pigment

Accessory pigments

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36
Q

What is the primary pigment referred to in the photosystem?

A

The primary pigment reaction centre.

37
Q

How does light reach the primary pigment?

A

The energy associated with the wavelengths of light are captured by accessory pigments in the photosystems and funnelled down to the primary pigment reaction centre

38
Q

What are the different photosynthetic pigments?

A

Chlorophyll a
Chlorophyll b
Carotenoids
Xanthophylls

39
Q

Which of the photosynthetic pigments is the primary pigment and accessory pigments in photosystem I?

A

In Photosystem I, a type of chlorophyll a is the primary pigment. Chlorophyll b, cartenoids and xanthophylls are the accessory pigments.

40
Q

Which of the photosynthetic pigments is the primary pigment and accessory pigments in photosystem II?

A

In Photosystem II, another type of chlorophyll a is the primary pigment. Chlorophyll b, cartenoids and xanthophylls are the accessory pigments.

41
Q

There are two types of chlorophyll a but they both reflect wavelengths of the same colour which is?

A

The wavelengths corresponding to blue-green.

42
Q

Which colour does the two types of chlorophyll a absorbs? State their absorption peaks.

A

Both chlorophyll a absorb red light but have different absorption peaks.
Chlorophyll a found in photosytem I has a peak absorption of red light of wavelength 700nm.
Chlorophyll a found in photosystem II has a peak absorption of red light of wavelength 680nm.
Chlorphyll a also absorbs some blue light of wavelength 440nm.

43
Q

There is only one type of chlorophyll b. What colour does it reflect and what wavelength of light does it absorb?

A

Chlrophyll b reflects the wavelengths corresponding to yellow-green.
It absorbs wavelengths of about 400- 500nm = red-blue light.

Remember its an accessory pigment.

44
Q

What colour do cartenoids reflect and what wavelength of light does it absorb?

A

Cartenoids reflect the wavelengths corresponding to yellow and orange.
It absorbs wavelengths of about 400-500nm.

45
Q

What colour do xanthophylls reflect and what wavelength of light does it absorb?

A

Xanthophylls reflect the wavelengths corrsponding to yellow.

It absorbs wavelengths of about 374-550nm.

46
Q

How to seperate the photosynthetic pigments?

A

Using thin layer chromotography.

47
Q

What are the two main stages of photosynthesis?

A

Light-dependant stage

Light-independant stage/The Calvin Cycle.

48
Q

Where does the light-dependant stage take place?

A

in the grana (on the thylakoids)

49
Q

Why is it called the light-dependant stage?

A

Because it involves the DIRECT use of light.

50
Q

What does the light-dependant stage consist of?

A
  1. Light harvesting at the photosystems
  2. Photolysis of water
  3. Photophosphorylation – the production of ATP in the presence of light
  4. The formation of reduced NADP
51
Q

What happens in the “light harvesting by photosystems” stage?

A

Photosystem I and Photosystem II are used to harvest light energy.
PSI contains a type of chlorophyll a as the pigment at the primary reaction centre which has a peak absorption at 700nm.
PSII contains another type of chlorophyll a as the pigment at the primary reaction centre which has a peak absorption at 680nm.
Accessory pigments also make up the photosystems and these include carotenoids and xanthophylls and chlorophyll b. They surround the reaction centres and funnel light energy of different wavelengths down to the reaction centre to boost the energy available for electron excitement.

52
Q

What happens in the ‘photolysis of water’ stage?

A

An enzyme in PSII splits water molecules, in the presence of light, into H+ ions, electrons and oxygen:
2H₂O –> 4H⁺ + 4e⁻ + O₂
The O₂ can then be used for aerobic respiration, however if rate of photosynthesis is high, they can diffuse out of the stomata.

53
Q

What is the water used for in photosynthesis?

A
  • Source of protons used up in photophosphorylation.
  • Provides the electrons to replace those lost from the chlorophyll when light strikes it.
  • Source of O₂
  • Also keeps plant turgid.
54
Q

What is photophosphorylation?

A

The production of ATP (chemical energy) from the phosphorylation of ADP using inorganic phosphate in the presence of light (light energy).

55
Q

There are two types of photophosphorylation what are they?

A

Non-cyclic photophosphorylation

Cyclic photophosphorylation

56
Q

What is Non-cyclic photophosphorylation?

A

Involves photosystems I and II and produces ATP, oxygen and NADPH

57
Q

What is Cyclic photophosphorylation

A

Involves PSI only and produces only ATP and in smaller quantities than non-cyclic.

58
Q

What happens in non-cyclic photophosphorylation?

PPRC = Primary pigment reaction centre
ECC = Electron carrier chain
A

1) A photon strikes PSII (P680) and the light energy is funnelled to the PPRC where it excites 2 electrons within the chlorophyll a molecule escape the chlorophyll molecule to be accepted by an electron carrier. The electron carrier/acceptor is a protein withan Fe3+ iron ions at its centre.
2) The iron ion combines with the electron becomes reduced to Fe2+ before being re-oxidised to Fe3+ and in doing so release the electron to reduce the Fe3+ ion in the next electron carrier in the ECC.
3) The lost electrons from the chlorophyll are replaced with those from photolysis.
4) As electrons move from one electron carrier to the next they release some energy which is used to actively pump protons across the thylakoid membrane and into the thylakoid lumen.
5) As protons accumulate in the thylakoid space a proton gradient forms across the membrane.
6) Protons diffuse down the concentration gradient through channel proteins which are associated with ATP synthase enzymes. This flow of protons causes conformational change in the ATP synthase that causes it to phosphorylate ADP to form ATP.
7) The electrons leaving this ECC are then captured by chlorophyll a (P700) in PSI that was previously lost by excitation. These electrons are then excited once again by light energy causing them to escape the chlorophyll a molecule in its PPRC to be accepted by another type of electron carrier called ferredoxin causing it to be reduced before being reoxidised passing it along this chain. Electrons leaving the electron carrier chain are accepted by the NADP in the stroma, alongside the protons diffusing down the channel causing it to form reduced NADP. This reaction is catalysed by NADP reductase.
8) The reduced NADP and ATP are now present in the stroma ready for the ligth independant stage.

59
Q

What happens in cyclic photophosphorylation?

A

Photon strikes PSI (P700) where light energy is funnelled to the PPRC to excite the 2 electrons in the chlorophyll a molecule located here, which escape from the chlorophyll to be accepted by ferredoxin, which is reduced then re-oxidised to release electron so that it can reduce the next EC in the ETC. As the electrons moves through the ETC, it releases energy which is used to phosphorylate ADP to generate a small amount of ATP. The de-excited electrons now return back to PSI to repeat the process.
No photolysis occurs, so no protons or oxygen molecules are made so no reduced NADP (NADPH) forms.

60
Q

When does cyclic photophosphorylation take place?

A

It occurs, when the plant has sufficient amounts of NADPH/reduced NADP, but still needs to produce ATP. ATP is produced at a lower cost (less resources used) as only photosystem I is involved and no photolysis of water occurs.

61
Q

Differences between cyclic and non-cyclic phosphorylation?

A

Cyclic:

  • Only PS I involved
  • No water required
  • Oxygen not produced
  • reduced NADP is not produced
  • Use: used to produce products for light-independant reaction, but only occurs when additional ATP is needed to meet the energy demands when lots of NADP is present.

Non-cyclic:
Opposite to cyclic
Last bullet point: used to produce products for the light-independant reaction.

62
Q

Where does the light independant stage of photosynthesis take place?

A

The stroma

63
Q

Why is it called the light independant stage?

A

Does not use light directly, but uses products of the light-dependant stage. If enough ATP and NADP are not present, this stage cannot take place.

64
Q

Role of carbon dioxide?

A
  • Source of carbon for the production of all organic molecules e.g. glucose (Organic molecules may also be used to make structures like antigens, proteins and enzymes)
65
Q

How does carbon dioxide enter the stroma?

A
  • Diffuses into Stomata –> diffuses through spongy mesophyll –> Duffuses across chloroplast inner and otuer membrane into stroma.
  • May also come directly from aerobic respiration that has occured in the plant.
66
Q

What happens to carbon dioxide in the stroma and why is it important?

A

Carbon fixation occurs to maintan a steep concentration gradient that allows the diffusion of carbon dioxide into the stroma.

67
Q

Stages of the light-independant stage/the Calvin cycle?

A

1) CO₂ combines with ribulose bisphosphate (RuBP), a five carbon CO₂ acceptor. This is catalysed by the enzyme, RuBisCO (ribulose bisphosphate carboxylase-oxygenase).
2) RuBP, by accepting, the COO- group, becomes carboxylated forming a six-carbon unstable intermediate, PGA, which immediately breaks down into two molecules of GP (glycerate-3-phosphate), a three carbon compund.
3) The carbon has now been fixed.
4) The GP is then reduced, using the hydrogens from NADPH, made in the light-dependant stage, to form two molecules of TP (triose phosphate). Energy from ATP, also made in the light dependant stage, is also used at this stage. Reducing each molecule of GP requires 1 ATP (so 2 ATP used at this point as there are 2 GP).
5) After 6 turns of the cycle (so after 6CO₂ are fixed), 12 TP are produced. 2/12 TP, leave the cycle and are used to generate organic compounds like glucose. The remaining 10/12 TP are then rearranged to make 6 molecules of RuBP. This process requires phosphate groups, so 5 molecules of ATP are de-phosphorylated to form ADP and Pi. The cycle can then be repeated.
The first TP molecule leaves the cycle and goes twoards making glucose after 3 turns of cycle (3CO₂ have been fixed). The second after 6 turns.

68
Q

What PH does RuBisCO work best in and how is it maintained?

A

RuBisCO works best at PH8. During the light dependant stage, protons are pumped from the stroma into the thylakoid lumen, so concentration of protons in stroma decreases, raising PH to 8.

69
Q

How is RuBisCO activated?

A
  • Presence of extra ATP in stroma
  • In daylight, concentration of Magnesium ions increases in stroma - these bind to active site of RuBis CO, acting as cofactors - activates it.
  • Ferredoxin, that is reduced by electrons from PS I activates enzymes in the Calvin cycle like RuBisCO
70
Q

What is the TP that leaves the calvin cycle used to snythesize?

A
  • Glucose, which is converted to sucrose for transport and to starch for storage.
  • Amino acids
  • TP can be converted back into GP to make fatty acids, which then can combine with glycerol (made directly from TP) to produce lipids.
71
Q

Why do chloroplasts contain low levels of RuBP?

A

This is because whilst it is converted to GP, it is continuosly regenerated.

72
Q

What are the limiting factors of photosynthesis?

A

Carbon dioxide concentration (in the atmosphere/aquatic environments)
Light intensity
Water availability
Availability of chlorophyll, electron carriers and the relevant enzymes.
Temperature
Turgidity of cells

73
Q

Why does light intensity affect photosynthesis?

A

Light provides the energy needed for the light-dependant stage to make the ATP and reduced NADP needed for the light-independant stage.
Light also causes the stomata to open so that gaseous exchange can occur, so that cells recieve the CO2 needed for photsysynthesis.
As stomata are open, transpiration occurs, so that the uptake of water occurs and is delivered to the cells for photolysis in photosynthesis to occur.

74
Q

When is light intensity (LI) a limiting factor?

A

At a favourable, constant temp and CO2 conc, LI is the limiting factor (relatively low). ∴ as LI ↑, rate of photosysnthesis ↑.
However at a certain point, even when LI, is increased, the rate of photosynthesis does not increase. At this point, LI is no longer the limiting factor.

LOOK AT GRAPH ON PG 124 `

75
Q

Effect of low light intensity on the Calvin cycle?

A

1) GP cannot be reduced to TP
2) TP levels fall and GP accumulates
3) If TP falls, RuBP cannot be regenerated.

LOOK AT THE THIRD GRAPH ON PG124.

76
Q

When is CO2 the limiting factor?

A

If rate of photosynthesis increases with increasing CO2 cncentration, while other variables remain constant it is the limiting factor.

77
Q

Effect of low CO2 conc on the calvin cycle? (low conc = below 0.01%)

A

1) RuBP cannot accept it, and accumulates.
2) GP cannot be made
3) Therefore TP cannot be made

78
Q

Effect of changing temp on the Calvin cycle?

A
  • From low temperatures to temperatures of about 25-30°C, at sufficient levels of water, light intensity and CO2 conc, rate of photosynthesis increases with temp.
  • At temperatures above 30°C, rate of photosynthesis may decrease due to photorespiration: oxygen successfully competes with carbon dioxide for the RuBisCO’s active site. This means less GP is made and ∴ less TP is made. Conequently, not enough TP to regenerate the initial RuBP.
  • At temperatures above 45°C, enzymes in photosynthesis like RuBisCO will become denatured. ∴ Reduced production of GP and TP and eventually no regeneration of RuBP.
79
Q

At low temperatures, why is rate of photosnthesis slow?

A

Talk about enzymes, active sites and susbtrates have less KE = move slower = less frequent collisions between active site and substrate = less ESCs formed = less EPCs formed = less product molecules formed.

80
Q

What is water stress?

A

The condition a plant will expereince when the water supply becomes limiting.

81
Q

Why is water needed in a plant?

A
  • As it travels up the transpiration stream, has a cooling effect on the plant.
  • The water travelling up also keeps the plant turgid so that it can function. Turgid gurad are needed to keep the stomata open for gaseous exchange.
82
Q

Effect of low water availability (water stress)?

A

1) Roots unable to take up enough water that was lost via transpiration
2) Cells lose water and become plasmolysed
3) Plant roots produce abscisic acid, that when translocated to leaves, causes stomata to close (reducing gaseous exchange)
4) Tissue become flaccid and leaves wilt
5) Rate of photosynthesis decreases greatly.

83
Q

It is important to remember that in water stress:

“Cells become __________ but tissues become ________”

A

1) Plasmolysed

2) Flaccid

84
Q

How is the rate of photosynthesis measured?

A
  • Rate of uptake of raw materials like CO2
  • Rate of O2 production

To see the effect of changing limiting factors, you can change CO2 conc, light intensity and temperature and measure the volume of oxygen produced.

85
Q

What are the limitations of measuring the volume of oxygen produced?

A
  • Some of the oxygen produced by the plant will be used for respiration.
  • There may be some dissolved nitrogen in the gas collected.
86
Q

How can the volume of oxygen collected be measured?

A

Using a photosynthometer - contains a capillary tube that should be air tight and contains no air bubbles at the start of the experiment. As oxygen is released (as bubbles), it is collected in the capillary tube. If we measure the length of bubble/gas collected and the radius of the capillary tube, we can use the equation, πr²h = volume of gas collected.

87
Q

What is the relationship between light intensity and distance (from plant)?

A

Light intensity is inversely proportional to the distance squared.
L = 1/d²

88
Q

When investigating light intensity, how do we ensure that carbon dioxide is not the limiting factor?

A

Add sufficient amounts of sodium hydrogencarbonate to the water.

89
Q

LOOK AT PG127 AND PG128 TO FIND OUT THE METHOD TO INVESTIGATE THE RATE OF PHOTOSYNTHESIS.

A

ASK MISS HOW THE SECOND METHOD CAN BE ALTERED TO MEASURE EFFECT OF CO2.

*FOR LIGHT INTENSITY CHANGE DISTANCE FROM LIGHT LAMP AND FOR TEMPERATURE, JUST ALTER IT.

Biology OCR AS Level (10 decks)

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