Chapter 13 Flashcards
What is the overall equation of photosynthesis
6CO2 + 6H2O —> C6 H12 O6 + 6O2
What are the two reactions involved in photosynthesis
- Light dependent reaction
- Light independent reaction
What is the light dependent reaction
- trapping of light energy by photosynthetic pigments in chloroplast
- energy produced as ATP and NADPH (reduced NADP)
- energy is transferred to light independent reaction
What is light independent reaction
- aka Calvin cycle
- energy from light dependent reaction is used for
- fixation of carbon dioxide / carbon fixation to produce complex organic molecules
What do photosynthetic pigments do
- they trap light energy
Where are photosynthetic pigments found
- found on thylakoid membranes of chloroplasts
How are photosynthetic pigments arranged
- pigments are arranged in light harvesting clusters = photosystems
What are the two groups of photosynthetic pigments
- different pigments absorb different wavelengths of light
- Chlorophylls
- chlorophyll a, chlorophyll b
- Carotenoids
- beta carotenoid, xanthophyll
Which pigment is the primary pigment
- chlorophyll a is a primary pigment
- the rest are accessory pigments
What do primary pigments do
- 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
What are the two types of photosystems
- photosystem II
- photosystem I
What are the colours of chlorophylls and carotenoids
- chlorophyll a = yellow-green colour
- chlorophyll b = blue-green colour
- beta carotene = orange
- xanthophyll = yellow
What light do chlorophyll and carotenoids absorb and reflect
- Chlorophylls
- absorb mainly red and blue-violet lights
- reflects green light
- Carotenoids
- absorb mainly blue-violet lights
- reflects red light
What does the light absorbance spectrum show
- graph of light absorbance by pigments at different wavelengths of light
What does the photosynthetic action spectrum show
- 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)
Where does the light dependent reaction occur
Occurs in the thylakoids
What happens during the light dependent reaction
- 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
What are the two pathways for the light dependent reaction
- Non cyclic photophosphorylation
- Cyclic photophosphorylation
What are the steps of non-cyclic photophosphorylation
- 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
How are lost electrons replaced
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
What happens during cyclic photophosphorylation
- 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
Where does the light-independent reaction occur
- occurs in the stroma of chloroplasts
- does not require light
What are the three steps of the light-independent reaction
- Fixation of carbon dioxide
- Reduction
- Regeneration
What happens during step 1 of light independent reaction (fixation of carbon dioxide)
- 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)
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)
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
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
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)
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
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
What are the limiting factors affecting the rate of photosynthesis
- Light intensity
- Carbon dioxide concentration
- Temperature
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
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
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
