5.6 - Photosynthesis Flashcards
1
Q
Define photosynthesis
A
The production of carbon compounds in cells using light energy
2
Q
What is the word and symbol equation for photosynthesis?
A
- Carbon dioxide + Water (+ Light energy) → Oxygen + Glucose
- 6CO2 + 6H2O → 6O2 + C6H12O6
3
Q
Where does photosynthesis take place?
A
Chloroplasts
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
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)
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
7
Q
What is the wavelength of visible light?
A
Between 400 and 700 nm
8
Q
Compare wavelengths of light in the visible part of the EM spectrum
A
- Red - longest wavelength
- Violet - shortest wavelength
9
Q
What is a pigment?
A
- A substance that absorbs light
- Colour of pigment comes from the wavelengths of light reflected (not absorbed)
10
Q
What is chlorophyll?
A
- Pigment responsible for absorbing light
- Releases electrons used to produce ATP
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)
12
Q
Where are the photosynthetic pigments found in the chloroplasts?
A
In photosystems
13
Q
Where in a photosystem are accessory pigments found?
A
Light harvesting system (antenna complex)
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
15
Q
Where in a photosystem is chlorophyll a found?
A
Reaction centre
16
Q
What is the role of the reaction centre?
A
- Releases high energy electrons
- Used in the light-dependent stage of photosynthesis
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
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
19
Q
Describe what an absorption spectrum shows
A
The wavelengths of light absorbed by each pigment (e.g. chlorophyll)
20
Q
Define action spectrum
A
Graph showing overall rate of photosynthesis at each wavelength of light
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
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
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
24
Q
How may mixtures of photosynthetic pigments be separated?
A
Using thin layer chromatography
25
Describe the process of thin layer chromatography (TLC)
- 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
26
How are different pigments identified after carrying out TLC?
- 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
27
Give a suitable solvent for extracting photosynthetic pigments from plant tissue
Propanone, alcohol, ether
28
What are the two steps involved in photosynthesis?
- Light-dependent stage
- Light-independent stage
29
Where do light-dependent reactions take place?
Thylakoids
30
What are the products of the light-dependent reactions?
- Reduced NADP (NADPH)
- ATP
- Oxygen
31
Define photolysis
- Splitting of water molecules
- Using energy from light
- Electrons and protons formed used in light-dependent stage
- Oxygen is a waste product
32
How is oxygen produced in photosynthesis?
Photolysis of water
33
Where are photosystems located?
Thylakoid membrane
34
Explain the processes involved in light absorption by photosystems
- 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
Define photophosphorylation
Production of ATP in chloroplasts
36
How is a proton gradient built up in the chloroplast?
- 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
Describe the process of chemiosmosis
- 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
What is NADP+?
- Co-enzyme
- Reduced by electrons and H+ ions to form reduced NADP (NADPH)
39
What is used to reduce NADP+ in the light-dependent reactions of photosynthesis?
- Protons from the thylakoid space
- Electrons from electron transport chain
40
What is the role of NADPH?
Used as reducing agent in light-independent stage
41
Outline the light-dependent reactions of photosynthesis
- 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
What is the difference between cyclic and non-cyclic photophosphorylation?
**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
Why is cyclic photophosphorylation sometimes necessary?
- 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
Which products of the light-dependent reactions are used in the light-independent reactions?
ATP and NADPH
45
What other natural product is required for the light-independent reactions?
Carbon dioxide from atmosphere
46
Where do light-independent reactions take place?
Stroma
47
Outline the light-independent reactions of photosynthesis
- 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
State the role of ribulose bisphosphate carboxylase (Rubisco) in the Calvin cycle
Fixes carbon dioxide to RuBP
49
How is triose phosphate used by plants?
- Starting material for synthesising glucose, lipids and amino acids
- Recycled to regenerate RuBP
50
Why is it necessary for RuBP to be regenerated?
Allows Calvin cycle to continue
51
How can the rate of photosynthesis be measured?
- 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
Define limiting factor
- The factor that will determine the rate of photosynthesis
- When at sub-optimal levels
53
What limiting factors affect the rate of photosynthesis?
- Temperature
- Light intensity
- Carbon dioxide concentration
54
Why is water not considered a limiting factor of photosynthesis?
- 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
Describe how you would investigate the effect of temperature on the rate of photosynthesis
- 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
Describe how you would investigate the effect of carbon dioxide concentration on the rate of photosynthesis
- 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
Describe how you would investigate the effect of light intensity on the rate of photosynthesis
- 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
Outline factors that affect the rate of photosynthesis
**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
Explain why rates of photosynthesis are low in the early morning
- 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
Explain what factor is most likely to be limiting in the middle of the day
- 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
Explain why low temperature limits the rate of photosynthesis
- 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
Explain why excessively high temperatures limit the rate of photosynthesis
- Enzymes (e.g. Rubisco) begin to denature
- Active site deformed
- Substrate no longer complementary
- Results in lower concentrations of RuBP, GP and TP
63
Explain why decreasing CO2 concentration leads to an increase in RuBP
- 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
Explain why reducing light intensity leads to an increase in GP
- 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
What other feature of light influences the rate of oxygen production in photosynthesis?
- Wavelength
- Not all wavelengths are absorbed - some wavelengths reflected
66
Give a method to measure rate of photosynthesis that is more accurate than counting bubbles or collecting gas
- Use an oxygen sensor
- Connected to a data logger
- Independent variable (e.g. light intensity, temperature, carbon dioxide concentration) also
measured using sensors
