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OCR Biology Paper 1 > 5.6 - Photosynthesis > Flashcards

5.6 - Photosynthesis Flashcards

1
Q

Define photosynthesis

A

The production of carbon compounds in cells using light energy

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2
Q

What is the word and symbol equation for photosynthesis?

A
  • Carbon dioxide + Water (+ Light energy) → Oxygen + Glucose
  • 6CO2 + 6H2O → 6O2 + C6H12O6
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3
Q

Where does photosynthesis take place?

A

Chloroplasts

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4
Q

Describe the structure of chloroplasts

A
  • Thylakoids - large surface area for light absorbing photosystems - Provides site for electron transport chain and chemiosmosis
  • Thylakoid space - very small volume so a steep proton gradient builds up quickly
  • Granum - stack of thylakoid membranes to maximise light absorption
  • Stroma - contains enzymes needed for Calvin cycle (e.g. rubisco)
  • Starch grain - storage of carbohydrate
  • Outer and inner membranes
  • 70S ribosomes & DNA
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5
Q

Describe how the structure of a chloroplast is related to its function

A
  • Large surface area of thylakoids/grana for light absorption
  • Space inside thylakoids for accumulation of protons
  • Stroma contains enzymes used in Calvin cycle/light independent reactions
  • Arrangement of photosystems to allow electron transport to take place
  • Double membrane on the outside allows separation from rest of cell
  • Presence of DNA & ribosomes for protein synthesis
  • Starch grains store carbohydrates (from photosynthesis)
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6
Q

What is the electromagnetic (EM) spectrum?

A
  • Range of all possible frequencies of electromagnetic radiation
  • Visible spectrum is one region of the EM spectrum
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7
Q

What is the wavelength of visible light?

A

Between 400 and 700 nm

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8
Q

Compare wavelengths of light in the visible part of the EM spectrum

A
  • Red - longest wavelength
  • Violet - shortest wavelength
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9
Q

What is a pigment?

A
  • A substance that absorbs light
  • Colour of pigment comes from the wavelengths of light reflected (not absorbed)
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10
Q

What is chlorophyll?

A
  • Pigment responsible for absorbing light
  • Releases electrons used to produce ATP
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11
Q

Which photosynthetic pigments are found in the chloroplast?

A
  • Chlorophyll a (main pigment)
  • Chlorophyll b (accessory pigment)
  • Carotenoids (accessory pigment)
  • Xanthophylls (accessory pigment)
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12
Q

Where are the photosynthetic pigments found in the chloroplasts?

A

In photosystems

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13
Q

Where in a photosystem are accessory pigments found?

A

Light harvesting system (antenna complex)

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14
Q

What is the role of the pigments in the light harvesting system?

A
  • Absorb light energy of different wavelengths
  • Transfer energy to reaction centre
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15
Q

Where in a photosystem is chlorophyll a found?

A

Reaction centre

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16
Q

What is the role of the reaction centre?

A
  • Releases high energy electrons
  • Used in the light-dependent stage of photosynthesis
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17
Q

Outline the role of plant pigments in the process of photosynthesis

A
  • Pigments in photosystems absorb photons of light
  • Light energy excites electrons
  • Energy passed from pigment to pigment
  • Energy reaches reaction centre where chlorophyll a is found
  • Accessory pigments allow for wider range of wavelengths to be absorbed
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18
Q

Compare chlorophyll’s absorption in the different parts of the visible spectrum

A
  • Absorbs blue light most strongly, followed by red
  • Reflects green light most strongly
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19
Q

Describe what an absorption spectrum shows

A

The wavelengths of light absorbed by each pigment (e.g. chlorophyll)

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20
Q

Define action spectrum

A

Graph showing overall rate of photosynthesis at each wavelength of light

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21
Q

What is an action spectrum of photosynthesis?

A
  • Graph showing overall rate of photosynthesis at each wavelength of light
  • Maximum rates occur in blue light, followed by red light
  • Lowest rates occur in green light
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22
Q

Distinguish between absorption spectrum and action spectrum

A
  • Absorption spectrum shows wavelengths of light which are absorbed by each pigment
  • Action spectrum shows overall rate of photosynthesis at each wavelength of light
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23
Q

Chlorophyll reflects most green light yet there is still use of green light on the action spectrum.
Explain how this is possible

A

Accessory pigments which absorb green light

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24
Q

How may mixtures of photosynthetic pigments be separated?

A

Using thin layer chromatography

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25
Q

Describe the process of thin layer chromatography (TLC)

A
  • Stationary phase - small strip of TLC plate used
  • Grind leaves with organic solvent (mobile phase)
  • e.g. propanone
  • Apply drop of extract to TLC strip near to one end
  • Repeat until concentrated spot produced
  • Place TLC strip in test tube
  • Put solvent into test tube so level of solvent below spot
  • Ensure strip not touching sides of tube
  • Leave solvent to run up strip
  • Remove TLC strip before solvent reaches top of strip
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26
Q

How are different pigments identified after carrying out TLC?

A
  • Different components of the mixture travel at different speeds, causing them to separate
  • An Rf value can then be calculated and compared to data tables
  • Rf value = distance component travels
    distance solvent travels
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27
Q

Give a suitable solvent for extracting photosynthetic pigments from plant tissue

A

Propanone, alcohol, ether

28
Q

What are the two steps involved in photosynthesis?

A
  • Light-dependent stage
  • Light-independent stage
29
Q

Where do light-dependent reactions take place?

A

Thylakoids

30
Q

What are the products of the light-dependent reactions?

A
  • Reduced NADP (NADPH)
  • ATP
  • Oxygen
31
Q

Define photolysis

A
  • Splitting of water molecules
  • Using energy from light
  • Electrons and protons formed used in light-dependent stage
  • Oxygen is a waste product
32
Q

How is oxygen produced in photosynthesis?

A

Photolysis of water

33
Q

Where are photosystems located?

A

Thylakoid membrane

34
Q

Explain the processes involved in light absorption by photosystems

A
  • Pigments in light harvesting system absorb certain wavelengths of light
  • Light energy transferred to chlorophyll a in reaction centre
  • Causes an electron in the pigment to be raised to a higher energy level
  • Chlorophyll a passes excited electron pair to electron acceptors in thylakoid membrane
  • Two types of photosystem - I and II
35
Q

Define photophosphorylation

A

Production of ATP in chloroplasts

36
Q

How is a proton gradient built up in the chloroplast?

A
  • Pair of excited electrons from reaction centre of photosystem II passed to a chain of
    electron carriers
  • Electrons give up energy as they pass from one carrier to next
  • Energy released used to pump protons across thylakoid membrane from stroma into
    thylakoid lumen
  • Establishes proton gradient
37
Q

Describe the process of chemiosmosis

A
  • ATP synthase located in thylakoid membranes
  • Allows protons to diffuse back across the membrane to the stroma
  • Uses energy that protons release as they diffuse down concentration gradient to produce ATP
38
Q

What is NADP+?

A
  • Co-enzyme
  • Reduced by electrons and H+ ions to form reduced NADP (NADPH)
39
Q

What is used to reduce NADP+ in the light-dependent reactions of photosynthesis?

A
  • Protons from the thylakoid space
  • Electrons from electron transport chain
40
Q

What is the role of NADPH?

A

Used as reducing agent in light-independent stage

41
Q

Outline the light-dependent reactions of photosynthesis

A
  • Light produces an excited electron from photolysis of water
  • Photosynthetic pigments in photosystem II absorb light
  • Excited electron pair pass along electron transport chain
  • Protons from photolysis of water pumped into thylakoid space
  • ATP produced by the light dependent reactions
  • ATP production by chemiosmosis by ATP synthase
  • Electrons from photosystem II passed to photosystem I
  • Light excites electrons in photosystem I to higher energy level
  • Leads to production of NADPH
  • In non-cyclic photophosphorylation electrons from photolysis needed for photosystem II
  • In cyclic photophosphorylation electrons from photosystem I return to it
  • Oxygen from photolysis is a waste product
42
Q

What is the difference between cyclic and non-cyclic photophosphorylation?

A

Non-cyclic photophosphorylation
- Photolysis of water required to provide electrons
- Absorption of light in photosystem II provides electrons for photosystem I
- Light excites electrons in photosystem I to higher energy level
- Leads to production of NADPH and ATP

Cyclic photophosphorylation
- Electron returns to photosystem I (does not enter photosystem II)
- Generates ATP from H+ by chemiosmosis
- Does not produce NADPH
- Occurs when additional ATP required for cellular processes

43
Q

Why is cyclic photophosphorylation sometimes necessary?

A
  • ATP produced, reduced NADPH not produced
  • Electrons not required from photolysis
  • Less ATP used in Calvin cycle
  • More ATP available for other metabolic processes
44
Q

Which products of the light-dependent reactions are used in the light-independent reactions?

A

ATP and NADPH

45
Q

What other natural product is required for the light-independent reactions?

A

Carbon dioxide from atmosphere

46
Q

Where do light-independent reactions take place?

A

Stroma

47
Q

Outline the light-independent reactions of photosynthesis

A
  • Calvin cycle
  • Takes place in the stroma of the chloroplast
  • Produces carbohydrates
  • Ribulose bisphosphate (RuBP) is a five carbon compound
  • Carbon dioxide added to RuBP by RuBP carboxylase enzyme (Rubisco)
  • Example of carboxylation
  • Forms unstable six carbon compound
  • This splits into two molecules of glycerate-3-phosphate (GP)
  • ATP produced in light-dependent reaction provides the energy
  • NADPH produced in light-dependent reaction provides hydrogen
  • GP reduced to triose phosphate (TP)
  • Some TP sugars go to form hexose sugars (e.g. glucose)
  • Some go towards regenerating RuBP
48
Q

State the role of ribulose bisphosphate carboxylase (Rubisco) in the Calvin cycle

A

Fixes carbon dioxide to RuBP

49
Q

How is triose phosphate used by plants?

A
  • Starting material for synthesising glucose, lipids and amino acids
  • Recycled to regenerate RuBP
50
Q

Why is it necessary for RuBP to be regenerated?

A

Allows Calvin cycle to continue

51
Q

How can the rate of photosynthesis be measured?

A
  • By measuring the volume of oxygen released using a gas syringe
  • Measure increase in pH
  • Measure decrease in carbon dioxide
  • Indirectly - measure increase in biomass
52
Q

Define limiting factor

A
  • The factor that will determine the rate of photosynthesis
  • When at sub-optimal levels
53
Q

What limiting factors affect the rate of photosynthesis?

A
  • Temperature
  • Light intensity
  • Carbon dioxide concentration
54
Q

Why is water not considered a limiting factor of photosynthesis?

A
  • Plants close stomata in response to water shortage
  • This stops photosynthesis as no gas exchange can occur
  • Stomatal closing occurs before water potential is low enough to limit rate of photosynthesis
55
Q

Describe how you would investigate the effect of temperature on the rate of photosynthesis

A
  • Independent variable: temperature (e.g. 0°C, 10°C, 20°C, 30°C, 40°C)
  • Dependent variable: volume of oxygen produced in 1 minute
  • Control variables: carbon dioxide concentration, light intensity, species of pond weed
    (Elodea), length of pond weed
56
Q

Describe how you would investigate the effect of carbon dioxide concentration on the rate of photosynthesis

A
  • Independent variable: carbon dioxide concentration (e.g. 0, 10, 20, 30, 40, 50 mmol dm-3)
  • Dependent variable: volume of oxygen produced in 1 minute
  • Control variables: temperature, light intensity, species of pond weed (Elodea), length of pond weed
57
Q

Describe how you would investigate the effect of light intensity on the rate of photosynthesis

A
  • Independent variable: light intensity (e.g. 40W bulb held 10cm, 20cm, 30cm, 40cm, 50cm
    away)
  • Dependent variable: volume of oxygen produced in 1 minute
  • Control variables: carbon dioxide concentration, temperature, species of pond weed
    (Elodea), length of pond weed
58
Q

Outline factors that affect the rate of photosynthesis

A

Light intensity:
- Increase in light increases rate
- Until a plateau is reached at higher light intensities when another factor is limiting
- Light needed for light dependent reactions
- At low intensities insufficient ATP, NADPH and H+ produced
- Prevents Calvin cycle operating at maximum rate

Temperature:
- Increase in temperature increases rate of photosynthesis
- Above optimum temperature the rate drops as enzymes denature
- Temperature affects rate of rubisco activity in Calvin cycle

CO2 concentration:
- Increase in CO2 increases rate of photosynthesis until a plateau is reached
- At higher CO2 levels another factor is limiting
- CO2 needed for light independent reactions (Calvin cycle)

59
Q

Explain why rates of photosynthesis are low in the early morning

A
  • Low light intensity
  • Rate of photolysis is low and production of high energy electrons and H+ is limited
  • Light needed to produce ATP and NADPH in the light dependent reaction
60
Q

Explain what factor is most likely to be limiting in the middle of the day

A
  • Carbon dioxide concentration
  • Enzymes (Rubisco) cannot fix carbon dioxide effectively at low carbon dioxide levels
  • ATP and NADPH are not used as quickly as they are made
61
Q

Explain why low temperature limits the rate of photosynthesis

A
  • Rubisco cannot fix carbon dioxide effectively at low temperatures (low kinetic energy)
  • ATP and NADPH are not used as quickly as they are made
  • Results in lower concentrations of RuBP, GP and TP
62
Q

Explain why excessively high temperatures limit the rate of photosynthesis

A
  • Enzymes (e.g. Rubisco) begin to denature
  • Active site deformed
  • Substrate no longer complementary
  • Results in lower concentrations of RuBP, GP and TP
63
Q

Explain why decreasing CO2 concentration leads to an increase in RuBP

A
  • CO2 required to generate GP and TP in the Calvin cycle
  • Low CO2 concentration means less GP and TP synthesised
  • RuBP still regenerated from existing TP
  • RuBP no longer being used to fix CO2 so RuBP levels increase
64
Q

Explain why reducing light intensity leads to an increase in GP

A
  • Light required to produce ATP and NADPH in light-depended reaction
  • ATP and NADPH required to convert GP to TP in Calvin cycle
  • Low light intensity means less GP converted to TP
  • GP levels increase
  • TP and RuBP levels decrease
65
Q

What other feature of light influences the rate of oxygen production in photosynthesis?

A
  • Wavelength
  • Not all wavelengths are absorbed - some wavelengths reflected
66
Q

Give a method to measure rate of photosynthesis that is more accurate than counting bubbles or collecting gas

A
  • Use an oxygen sensor
  • Connected to a data logger
  • Independent variable (e.g. light intensity, temperature, carbon dioxide concentration) also
    measured using sensors

OCR Biology Paper 1 (20 decks)

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