3.2 Photosynthesis Flashcards
Adaptation of the cuticle for photosynthesis
- stops the leaf from losing water
- transparent so light can penetrate to mesophyll
Adaptation of the epidermis for photosynthesis
- protective layer
Adaptations of palisade cells for photosynthesis
- cylindrical and elongated at right angles to the surface of the leaf
- enable light to reach the chloroplasts to accommodate large numbers
Adaptations of air spaces for photosynthesis
- create diffusion gradient for gas exchange
- increase SA to exchange gas
Adaptations of stoma for photosynthesis
- able to let CO2 in and O2 out for gas exchange and photosynthesis
Adaptations of guard cells for photosynthesis
- control the opening and closing of stomata based on external conditions
Adaptations of the vein for photosynthesis
- steady supply of water to leaf
Define an absorption spectrum and an action spectrum
Absorption: a graph showing how much light is absorbed at different wavelengths
Action: a graph showing the rate of photosynthesis at different wavelengths
—> rate of photosynthesis at different wavelengths of light as measured by the mass of carbohydrate synthesised by plants exposed to different wavelengths
Rf equation
Rf = distance travelled by pigment
——————————————
distance traveled by solvent front
What is meant by saying chloroplasts are transducers?
- they convert light energy to chemical energy in ATP
Name the photosynthetic pigments and the value of having multiple
- chlorophyll a and b, beta carotene, and xanthophylls
- different pigments absorb light at different wavelengths, utilising most of the visible light spectrum
What is the most significant pigment of the reaction centre?
- chlorophyll a and b
- they excite electrons
Where are photosystems found?
Thylakoid membrane
Describe a photosystem
- antenna complex containing photosynthetic pigments
—> array of protein and pigment molecules that transfer energy from light to chlorophyll a
—> harvest energy to pass onto reaction centre for excitation - reaction centre that contains 2 molecules of chlorophyll a that get excited, emitting an electron
—> electrons are passed down electron carriers and gradually lose energy to make ATP
—> electrons fall back to photosystem or go to diff one to end up making water
Describe the two types of reaction centre
- PSI - two chlorophyll a molecules with an absorption peak of 700nm
- PSII - two chlorophyll a molecules with an absorption peak of 680nm
Define cyclic photophosphorylation
ATP can be produced by electrons that take a cyclical pathway and are recycled back into the chlorophyll a in PSI
Define non-cyclic photophosphorylation
ATP can be produced by electrons that take a linear pathway from water, through PSII and PSI to NADP, which they reduce
Explain the light dependant stage and what it produces
- light as energy source and using water
- produces ATP to synthesise energy rich molecules
- produces NADP which provides reducing power to produce molecules like CO2
- oxygen is a byproduct derived from water
Explain the passage of electrons in cyclic photophosphorylation
- PSI absorbs photons which excites electrons in the chlorophyll a molecules in its reaction centre
- these are emitted and picked up by an electron acceptor which passes them down a chain of electron carriers back to PSI
- energy released phosphorylates ADP to ATP
- electrons have flowed from PSI to the electron acceptor back to PSI, so this is described as cyclic
Explain the passage of electrons in non-cyclic photophosphorylation
- electrons are transferred from the electron acceptor to oxidised NAP in the stroma which is reduced
- electrons have not been returned to PSI so its chlorophyll is left with a positive charge
- the positive charge is neutralised by electrons from PSII that have been excited to a higher energy level by light absorption, picked up by an electron acceptor and passed down the ETC to PSI
- transport down the ETC makes energy available for the phosphorylation of ADP
- chlorophyll in PSII is left with a positive charge which is neutralised by electrons from the photolysis of water
Equation for the photolysis of water
2H2O —> 4H+ + 4e- + O2
Explain the photolysis of water
- water molecules absorb light which indirectly causes them to dissociate into hydrogen, oxygen and electrons
—> enhanced by a protein complex in PSII which is the only known enzyme to cause water to be oxidised - electrons replace those lost from PSII
- protons from water and electrons from PSI reduce NADP
- oxygen diffuses out of stomata
Explain the photolysis of water
- water molecules absorb light which indirectly causes them to dissociate into hydrogen, oxygen and electrons
—> enhanced by a protein complex in PSII which is the only known enzyme to cause water to be oxidised - electrons replace those lost from PSII
- protons from water and electrons from PSI reduce NADP
- oxygen diffuses out of stomata
Explain the passage of protons and phosphorylation
- electrons pass through a proton pump in the thylakoid membrane providing energy to pump protons from stroma into thylakoid space
- protons join H+ ions from the photolysis of water and accumulate
- generate an electrochemical gradient which is a source of potential energy
- chemiosmosis occurs - H+ ions diffuse down electrochemical gradient through ATP synthetase into the stroma which makes energy available
- ADP is phosphorylated to ATP
- H+ ions are passed to oxidised NADP
What are the 3 factors maintaining the proton gradient?
- proton pump associated with ETC pushes protons into thylakoid space
- photolysis of water in thylakoid space
- removal of protons from stroma, reducing NADP
Explain the light independent stage / Calvin cycle
- a 5C acceptor molecule ribulose bisphosphate combines with carbon dioxide catalysed by the enzyme ribulose bisphosphate carboxylase
- produces an unstable 6C compound which immediately splits into two molecules of a 3C compound glycerate-3-phosphate
- GP is reduced to triose phosphate by reduced NADP
—> energy provided by ATP made in light dependant stage - NADP is reformed
- some of the triose phosphate is converted to glucose phosphate, then into starch by condensation
- most of triose phosphate goes through reactions that regenerate RuBP for the cycle to continue
3 phases of the Calvin cycle
- carbon fixing
- reduction
- regeneration
Production of carbohydrates from the Calvin cycle
- fructose bisphosphate
- converted to glucose and combined with fructose to make sucrose
- alpha glucose converted to starch
- beta glucose polymerised to cellulose
Production of fats from the Calvin cycle
- AcCoA can be synthesised from glycerate-3-phosphate and converted to fatty acids
- triose phosphate can be directly converted to glycerol
- fatty acids and glycerol undergo condensation to make triglycerides
Production of proteins from the Calvin cycle
- glycerate-3-phosphate can be converted to amino acids for protein synthesis
Name the requirements of plants
- CO2 and H2O
- light at high intensity and suitable wavelength
- suitable temperature
Define a limiting factor
- a factor that limits the rate of a physical process by being in short supply
- an increase in a limiting factor increases the rate of the process
Explain CO2 as a limiting factor
- limiting until hits >0.5% conc
Explain light intensity as a limiting factor
- light dependant stage not possible in dark so no O2 evolved
- as intensity increases efficiency increases
- when exceeds max it damages pigments so decreases efficiency
Define light compensation point
- light intensity at which a plant has no net gas exchange as the volume of gases used and produced in respiration and photosynthesis are equal to the
Draw the Calvin cycle
Use of nitrogen in plants
- humus, organic molecules, decaying organisms
- amino acids, protein, chlorophylls and nucleotides
Use of magnesium in plants
- major component of chlorophyll
- enzyme activators for ATPase
Magnesium and nitrogen deficiency symptoms
Nitrogen: stunted growth and chlorosis
Magnesium: chlorosis
Describe how glycerate-3-phosphate is converted to triose phosphate
- reduced using reduced NADP from light dependant reactions
- NADP reformed
- formation of TP
Explain the effects of Diuron on non-cyclic photophosphorylation and why cyclic is not affected
- blocked reduction of NADP
- electron emitted by PSII cannot travel down ETC
- cyclic only involves PSI and the carrier involved is not impacted
Why would a plant die when Diuron is sprayed onto it?
- plant cannot generate NADPH
- Calvin cycle cannot work
- no glucose so no respiration
How does blocking electron transport systems from photosystems lead to the death of a plant?
- destruction of chlorophyll makes unable to absorb light energy
- no reduced NADP for Calvin cycle
- no glucose for respiration
- no ATP for active transport of minerals and nutrients
- destruction of cell membranes
Explain shape of the rate graph for CO2 and RuBP
- initially CO2 and RuBP reaction continues
-GP cannot be converted to TP as ATP and reduced NADH needed - only produced in light dependant reactions which can’t happen if dark
- less TP to regenerate RuBP
- rate of reaction decreases
Explain why tomato plants grown at 27°C produce sweeter fruit than those at 40°C
- at 27°C rate of respiration is lower than the rate of photosynthesis
- more sugar produced than used
- more sugar stored in tomato
Describe differences in LDR with bacteria and green plants
- 1 photo system vs 2
- H2S not H2O
- sulfur released no oxygen
- reduced NAD vs reduced NADP
- protons pumped out of cell vs into thylakoid
Describe LDR
- requires light to create ATP and reduced NADP
- light energy excites electrons which then leave the chlorphyll
- electrons move along ETC releasing energy from redox reactions
- energy used to pump protons across thylakoid membrane
- protons move in down ATP synthetase, producing ATP
- protons combine with co-enzyme NADP to become reduced in chemiosmosis
