B11 Photosynthesis Flashcards
Explain the role of light in photoionisation.
Chlorophyll molecules absorb energy from photons of light
This ’excites’ 2 electrons (raises them to higher energy level), causing them to be released from the chlorophyll
Name the 2 main stages involved in ATP production in the light-dependent reaction.
1 - electron transfer chain
2 - chemiosmosis
What happens in the electron transfer chain (ETC)?
Electrons released from chlorophyll move down series of carrier proteins embedded in thylakoid membrane & undergo series of redox reaction, which releases energy.
How is a proton concentration gradient established during chemiosmosis?
Some energy is released from ETC is coupled to active transport of H+ from stroma into thylakoid space.
How does chemiosmosis produce ATP in the light-dependent stage?
H+ move down their conc gradient down from thylakoid space into stroma via channel protein ATP synthase.
ATP synthases catalyses ADP + Pi -> ATP
Explain the role of light in photolysis.
Light energy splits molecules of water
H2O -> 2H+ +2e- + 1/2O2
What happens to the products of photolysis of water?
. H+ ions: move out of thylakoid space via ATP synthase & are used to reduce coenzyme NADP
.e-: replace electrons lost from chlorophyll
. O2: used for respiration or diffuses out of leaf as waste gas
How and where is reduced NADP produced in the light-dependent reaction?
. NADP + 2H+ + 2e- —> reduced NADP
. Catalysed by dehydrogenase enzymes
. Stroma of chloroplasts
What are the 2 stages of photosynthesis
Light independent reaction
And
Light dependent reaction
Where does LDR occur
Thylakoid membrane or grana
Where does LIR occur
stroma of chloroplasts
Does LDR need light
LDR requires light
Light energy + water used to create ATP and reduced NADP - needed for LIR
What are the 4 stages that occur in LDR
- Photoionisation of chlorophyll
- Production of ATP and reduced NADP
- Chemiomosis
- Photolysis
What happens in photolysis of water
Photo (light) Lysis (splitting)
Light energy absorbed by chlorophyll and splits water into O2, H+ and e-.
H20 → ½ 02 + 2e + 2H+
H+ picked up by NADP to form NADPH and used in LIR.
e- passed along ETC
O2 either used for respiration or diffuses out of leaf through stomata.
Equation for photolysis of water
H20 → ½ 02 + 2e + 2H+
What happens in chemiosmosis in photosynthesis
electrons that gained energy and left chlorophyll move along ETC embedded within thylakoid membrane.
As move along release energy and some of energy from electrons used to pump protons across chloroplast membranes.
electrochemical gradient is created.
protons pass through enzyme ATP synthase, results in production of ATP.
protons combine with co-enzyme NADP to become reduced NADP. Because protons move from high to low conc gradient this is known as chemiosmosis.
What is the LIR also known as
Calvin cycle
What enzyme catalyses the Calvin cycle
Enzyme rubisco found in fluid in stroma
Why is the Calvin cycle temp sensitive
It involves enzymes
What are ATP and reduced NADP used for in the calving cycle
Atp is hydrolysed to provide energy from this reaction
Reduced NADP donates H to reduce molecules GP in cycle
What are the limiting factors of photosynthesis
Temp
CO2 conc
Light intensity
Why is it important to remove limiting factors of photosynthesis
Maximise photosynthesis & plant growth
What are some techniques used in agricultural practices to remove limiting factors
These range from growing plants under artificial lighting to maximum the light intensity, heating a greenhouse to increase the temperature and burning fuel, such as paraffin burners, to release more carbon dioxide.
What is the extent that needs to be considered of the techniques used in removing limiting factors
consider in terms of profit.
If extra growth from photosynthesis is minimal, not cost effective to pay for heating/lighting/fuel.
Process of light-dependent reaction
Stage 1 = light energy excites electrons in chlorophyll (photoionisation)
- light energy absorbed by PSII
- excites pair of electrons in chlorophyll molecule
- electrons leave chlorophyll + move to higher energy level (chlorophyll has become ionised)
- electrons taken up by electron carrier [chlorophyll been oxidised and electron carrier been reduced]
- high energy electrons move along electron transfer chain towards PSI
Stage 2 = photolysis of H2O produces H+, electrons and O2
- as excited electrons from chlorophyll leave PSII to move along ETC, they must be replaced
- light energy splits H2O into H+, electrons and O2
H2O —> 2H+ +1/2O2 + 2e-
Stage 3 = energy from excited electrons makes ATP (chemiosmosis)
- excited e- lose energy as move along ETC
- this energy used to transport p+ into thylakoid (via p+ pump) so that thylakoid has higher conc of p+ than in stroma. This forms p+ gradient across membrane
- p+ move down conc gradient into stroma, via enzyme ATP synthase, energy from this movement used to combine ADP + Pi to form ATP
Stage 4 = generation of reduced NADP
- light energy absorbed by PSI, which excites electrons again to an even higher energy level
- electrons transferred to NADP along with H+ from stroma to form reduced NADP
2e- + 2H+ NADP ——————> reduced NADP
Process of light-independent reaction
Stage 1 = CO2 combines with RuBP
- CO2 enters leaf through stomata + diffuses into stroma of chloroplasts
- where it combines with RuBP to form a 6C unstable compound, which quickly breaks down to 2 molecules of 3C compound called ‘glycerate 3 phosphate’ (GP)
- enzyme rubsico catalyses reaction between the 2
Stage 2 = ATP & reduced NADP reduce GP to TP
- ATP from LDR provides energy to turn 3C GP into a diff 3C compound called TP (triose phosphate)
- this reaction also requires H+ which come from reduced NADP (reduced NADP recycled to NADP)
- TP then converted into many useful organic compounds e.g. glucose
Stage 3 = RuBP regenerated
- 5 out of every 6 TP is used to regenerate RuBP
- regenerating RuBP takes the rest of ATP produced by LDR
- can also be converted into glycerol and therefore combine with fatty acids to make lipids for the plant
What are the products of LIR that pass back into LDR
NADP
ADP
Pi
