Chapter 13 Flashcards
1
Q
What is the overall equation of photosynthesis
A
6CO2 + 6H2O —> C6 H12 O6 + 6O2
2
Q
What are the two reactions involved in photosynthesis
A
- Light dependent reaction
- Light independent reaction
3
Q
What is the light dependent reaction
A
- trapping of light energy by photosynthetic pigments in chloroplast
- energy produced as ATP and NADPH (reduced NADP)
- energy is transferred to light independent reaction
4
Q
What is light independent reaction
A
- aka Calvin cycle
- energy from light dependent reaction is used for
- fixation of carbon dioxide / carbon fixation to produce complex organic molecules
5
Q
What do photosynthetic pigments do
A
- they trap light energy
6
Q
Where are photosynthetic pigments found
A
- found on thylakoid membranes of chloroplasts
7
Q
How are photosynthetic pigments arranged
A
- pigments are arranged in light harvesting clusters = photosystems
8
Q
What are the two groups of photosynthetic pigments
A
- different pigments absorb different wavelengths of light
- Chlorophylls
- chlorophyll a, chlorophyll b
- Carotenoids
- beta carotenoid, xanthophyll
9
Q
Which pigment is the primary pigment
A
- chlorophyll a is a primary pigment
- the rest are accessory pigments
10
Q
What do primary pigments do
A
- primary pigments absorb light and act as reaction centres in the light dependent reactions of photosynthesis
- accessory pigments surround a primary pigment
- accessory pigments may absorb different wavelengths of light
- light energy is then passed to primary pigments
11
Q
What are the two types of photosystems
A
- photosystem II
- photosystem I
12
Q
What are the colours of chlorophylls and carotenoids
A
- chlorophyll a = yellow-green colour
- chlorophyll b = blue-green colour
- beta carotene = orange
- xanthophyll = yellow
13
Q
What light do chlorophyll and carotenoids absorb and reflect
A
- Chlorophylls
- absorb mainly red and blue-violet lights
- reflects green light
- Carotenoids
- absorb mainly blue-violet lights
- reflects red light
14
Q
What does the light absorbance spectrum show
A
- graph of light absorbance by pigments at different wavelengths of light
15
Q
What does the photosynthetic action spectrum show
A
- graph of rate of photosynthesis at different wavelengths of light
- related to absorption spectrum (but different)
- also dependent on wavelength of light (shorter wavelength, more energy)
16
Q
Where does the light dependent reaction occur
A
Occurs in the thylakoids
17
Q
What happens during the light dependent reaction
A
- occurs in the thylakoids
- trap light energy
- use light energy to excite electrons in chlorophyll (photo activation) and split water (photolysis)
- for the synthesis of ATP and NADPH
- which is used in light-independent reactions
18
Q
What are the two pathways for the light dependent reaction
A
- Non cyclic photophosphorylation
- Cyclic photophosphorylation
19
Q
What are the steps of non-cyclic photophosphorylation
A
- Photoactivation
- light energy is absorbed by both photosystems (PSII and PSI)
- and passed to primary pigment at reaction centre
- at reaction centre electrons are excited to a higher energy level
- electrons are emitted from reaction centre
- electrons are captured by electron acceptors
- Electron transport chain and ATP synthesis
- electrons passed along the electron carriers of the ETC
- electrons release energy to produce ATP using chemiosmosis
- energy is used to pump H+ across membrane into the thylakoids lumen
- proton gradient is formed across the thylakoid membrane
- H+ move down the gradient back into stroma
- via ATP synthase
- to synthesise ATP (from ADP and Pi)
- ATP made is passed to light-independent reaction
- electrons are passed to PSI
- Photolysis and reduction of NADP
- occurs at PSII only
- requires enzymes
- water splits into H+ and OH-
- electrons are removed from OH-
- H2O —> 2H+ + 0.5O2 + 2ē
- electrons released replace electrons lost from PSII
- oxygen released is a waste gas which is released
- H+ ions released is combined with de-energised electrons from PSI to reduce NADP
20
Q
How are lost electrons replaced
A
Electrons lost from PSII are replaced by electrons from photolysis of water
- H2O —> 2H+ + 0.5O2 + 2ē
Electrons lost from PSI are replaced by electrons from PSII after passing through the ETC
- electron donor = H2O
- final electron acceptor = NADP
- NADP reduced to NADPH
21
Q
What happens during cyclic photophosphorylation
A
- involves only PSI
- reaction centre of PSI is photoactivated
- electrons excited and emitted from chlorophyll
- captured by an electron acceptor
- passed along ETC
- energy released by electrons is used for ATP synthesis by chemiosmosis
- electrons are returned to original photosystem, PSI
- no photolysis of water involved
- no reduced NADP formed
- final electron acceptor = PSI
22
Q
Where does the light-independent reaction occur
A
- occurs in the stroma of chloroplasts
- does not require light
23
Q
What are the three steps of the light-independent reaction
A
- Fixation of carbon dioxide
- Reduction
- Regeneration
24
Q
What happens during step 1 of light independent reaction (fixation of carbon dioxide)
A
- step 1: fixation of carbon dioxide
- carbon dioxide (1C) is combined with ribulose bisphospahte (RuBP) (5C)
- to produce 2x glycerate-3-phosphate (GP) (3C)
- catalysed by the enzyme, ribulose bisphosphate carboxylase (rubisco)
25
What happens during step 2 of light independent reaction (reduction)
- step 2: reduction
- the reduction of 2 x GP (3C)
- uses ATP and reduced NADP
- some NADP regenerated
- produces 2 x triose phosphate (TP) (3C)
- 1/6 molecules of TP is converted to other molecules (e.g. carbs, amino acids, lipids for uses in the plant cell)
26
What happens during step 3 of light independent reaction (regeneration of RuBP)
- step 3: regeneration of RuBP
- 5/6 molecules of TP is used to regenerate RuBP
- uses ATP
27
How is the grana related to its function
- membrane has a large surface area
- holds pigments, enzymes, electron carriers needed for light-dependent reactions
- holds ATP synthase needed for ATP synthesis by chemiosmosis
- large number of pigment molecules arranged to absorb light
- they are arranged in light-harvesting clusters (photosystems) for efficient light absorption
- different pigments arranged in funnel-like structures
- each pigment passes energy to next pigment
- till it reaches reaction centre
28
How is the stroma related to its function
- site of light-independent reaction of the Calvin cycle
- contains enzymes of the Calvin cycle, sugars and organic acids
- bathes grana membranes so receives products of light dependent reactions
- also has 70S ribosomes, loop of DNA, liquid droplets and starch grains
- loops of DNA: codes from some chloroplast proteins
- ribosomes: produce chloroplast proteins (via translation)
29
How does the structure of a palisade mesophyll cell relate to its function (cell in leaf)
- cells are closely packed to absorb maximum light
- large surface area for diffusion of gases
- they are vertical to the surface of leaf to reduce number of cross walls
- cell walls are thin for maximum light penetration / diffusion of gases
- moist cell surfaces for diffusion of gases
- near air spaces to circulate gases / provide a reservoir of CO2
- large number of chloroplasts to absorb maximum light
- large vacuole pushes chloroplasts to edge of cell
- so short diffusion path for CO2
- can absorb maximum light
- chloroplasts can move
- towards light
- away from high light intensity to avoid damage
30
How does the structure of a dicotyledonous leaf relate to its function
- thin/flat to give large surface area to volume ratio
- held at right angles to sun to allow max light absorption
- has cuticle on upper surface to prevent water loss via cuticular transpiration
- has closely packed palisade mesophyll arranged vertical to surface of leaf
- has spongy mesophyll that provides large surface area for CO2 uptake / gaseous exchange
- mesophyll cells have most surfaces for diffusion of gases
- has stomata / guard cells for entry of CO2
- has xylem to supply water / mineral ions and act as support
- has phloem for translocation of products of photosynthesis
31
What are the limiting factors affecting the rate of photosynthesis
1. Light intensity
2. Carbon dioxide concentration
3. Temperature
32
How does light intensity affect rate of photosynthesis
- rate of photosynthesis increases with light intensity
- but levels off due to limiting factors: temperature and carbon dioxide concentration
33
How does carbon dioxide concentration affect rate of photosynthesis
- rate of photosynthesis increases with [CO2]
- but levels off due to limiting factors: temperature and light intensity
34
How does temperature affect rate of photosynthesis
- rate of photosynthesis increases with temperature
- but decreases after the optimum temperature because
- rubisco has higher tendency to catalyse reaction of O2 with RuBP, instead of CO2 (photorespiration) at high temp and high light intensity
- enzymes also start to denature
