Leaves & photosynthetic pathways Flashcards
What are the main parts of a leaf?
Blade = >apex >margin >vein >base
Petiole
Axial bud
Stipule
Which fibres strengthen the leaf?
Sclerenchyma
What does the spongy mesophyll contain?
Xylem + phloem surrounded by bundle sheath cells
What is phyllotaxy?
Leaf arrangement
- efficiently distributing leaves to reduce overlap and maximise light capture
What type of phyllotaxy do most angiosperms have?
Alternate phyllotaxy
= leaves arrange din ascending spiral around stem - each leaf emerging 137.5 degrees from the previous one
What are the 3 general types of dicotyledonous leaves?
Simple
Compound
Doubly compound
How do veins differ in monocots & dicots?
Monocots: parallel veins
Dicots: branched network of major veins
During the redox reaction of photosynthesis, which compound is reduced and which is oxidised?
CO2 reduced into glucose
H2O oxidised into O2
Chlorophyll absorbing light results in what?
Water splitting into protons + electrons
How many protons + electrons are transferred to NADP+?
What is produced?
1 proton + 2 electrons
NADPH
reduced NADP
Light energy is initially converted into chemical in the form of which 2 compounds?
NADPH
= source of electrons as ‘reducing power’ that can be passed on to another electron acceptor
ATP
by photophosphorylation of ADP
Photosynthesis is made of which 2 reactions?
Light reaction
Calvin cycle
What is the main objective of the Calvin cycle?
Forms sugar (G3P) from CO2 using ATP + NADPH
What do photosynthetic pigments do?
Absorb light
- diff pigments absorb diff wavelengths
What happens to wavelengths that aren’t absorbed?
Give an example.
Reflected
Green (500-600nm) = why plants are green
What is the main photosynthetic pigment?
What are the 2 accessory pigments?
Chlorophyll a
> Chlorophyll b - broadens spectrum for photosynthesis
> Carotenoids - absorb excessive light that would damage chlorophyll
Describe the excitation of chlorophyll by light
- chlorophyll absorbs light
- energy from absorbed photons excites electrons
- electrons move from ground state = unstable
- electrons drop back to ground state releasing heat/light
What are light-harvesting systems made up of?
What function do they carry out?
chlorophyll a, chlorophyll b + carotenoids bound to proteins
transfer photon energy to reaction centre
What is the reaction centre?
An organised association of proteins holding a special pair of chlorophyll a molecules
Describe how a photosystem harvests light
- photon excites electron in pigment
- energy transferred through other pigments
- excited electron transferred to special pair of chlorophyll a molecules in reaction centre
- pass excited electron to primary electron acceptor - becomes reduced
What are the 2 photosystems and the wavelengths they absorb?
PS II = 680nm
PS I = 700nm
What happens in PS II once an electron is passed from the P680 chlorophyll a to the primary electron acceptor?
P680 with a missing electron
= P680+
(has an electron hole)
An enzyme then catalyses the photolysis of what…?
H2O
–>
2 e + 2 H+ + 1/2 O2
What are the products of the photolysis of water used for?
Electrons supplied to the P680+ –> replacing those passed to primary electron acceptor
H+ released into thylakoid lumen
O atom immediately combines w/ another O atom
What is the 1st ETC made up of?
> Plastoquinone
a cytochrome complex
Plastocyanin
What happens as electrons pass through the cytochrome complex?
H+ (protons) are pumped into thylakoid lumen
–> contributes to proton gradient used in chemiosmosis
What happens in PS I when P700 transfers an excited electron to the primary electron acceptor?
P700 converted to electron deficient form = P700+
electrons replaced via ETC
What is the 2nd ETC made of?
Ferredoxin
What does NADP+ reductase do?
Catalyses the transfer of e from ferredoxin to NADP+
What happens in the conversion of NADP+ to NADPH?
2 electrons required for the reduction
Removes 1 H+ from stroma
What are the 3 main phases of the Calvin cycle?
- Carbon fixation (using Rubisco)
- Reduction
- Regeneration of RuBP
What is the sugar produced in the Calvin cycle?
G3P = glyceraldehyde 3-phosphate
What happens in hot, arid environments?
These conditions favour what process?
Stomata close
–> O2 builds up
photorespiration
What happens in photorespiration?
Rubisco adds O2 instead of CO2 in the Calvin cycle
–> produces 2-C compound
Why isn’t photorespiration good?
Consumes O2 + organic fuel
Releases CO2
Doesn’t produce ATP or sugar
Why might photorespiration be an evolutionary relic?
Rubsico 1st evolved at a time when atmosphere had much less O2 + more CO2
It limits damaging products of light reactions that build up in absence of Calvin cycle
Why are C4 plants special?
Minimise cost of photorespiration by incorporating CO2 into 4C compounds in mesophyll cells.
- -> 4C compounds exported to bundle-sheath cells
- -> release CO2 for Calvin cycle
Which enzyme do C4 plants need?
PEP carboxylase
- higher affinity for CO2 than Rubisco
What do CAM plants do?
Use crassulacean acid metabolism to fix carbon
- Open stomata at night
- -> incorporate CO2 into organic acids - Close stomata in day
- -> CO2 released & used in Calvin cycle
What are the pros of C4 plants?
Beneficial in high light, low nutrient envrios as require less Rubisco
What are the cons of C4 plants?
Regeneration of PEP in C4 pathway requires more ATP than in C3 photosynthesis
–> bad for low light enviros
What are CAM plants usually associated with and why?
Succulence & low photosynthetic capacity
As photosynthesis is proportional to vacuolar storage
