Topic 5: photosynthesis Flashcards
Where do the light-dependent and light-independent reactions occur in plants?
light-dependent: in the thylakoids of chloroplasts
light-independent: in the stroma
Explain the role of light in photoionisation
Chlorophyll molecules absorb energy from photons of light.
This excites two electrons (raises them to a higher energy level), causing them to be released from the chlorophyll
Name the two main stages involved in the production of ATP in the light-dependent reaction.
1) electron transfer train
2) chemiosmosis
What happens in the electron transfer chain?
Electrons released from chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane and undergo a series of redox reactions, which releases energy.
How is a proton gradient established during chemiosmosis?
Some energy released from the ETC is coupled to the active transport of H+ ions from the stroma into the thylakoid space
How does chemiosmosis produce ATP in the light-dependent stage?
H+ ions move down their conc gradient from the thylakoid space into the stroma via channel protein ATP synthase.
ATP synthase catalyses ADP + Pi –> ATP
Explain the role of light in photolysis
Light energy splits molecules of water
2H2O –> 4H+ + O2
What happens to the products of photolysis of water?
- H+ ions: move out of thylakoid space via ATP synthase and are used to reduce the 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 reactions?
- NADP +2H+ +2e- –> reduced NADP
- Catalysed by dehyrdogenase enzymes
- Stroma of chloroplasts
Where do H+ ions and electrons used to reduce NADP come from?
H+ ions: photolysis of water
Electrons: NADP acts as final electron acceptor of ECT
How does the light-independent reaction result in the production of useful organic substances?
1C /6 leaves the cycle and converted into useful organic molecules
State the roles of ATP and reduced NADP in the light-independent reaction
ATP: reduction of GP to TP and provides phosphate group to convert RuP into RuBP
reduced NADP: coenzyme transports electrons needed for reduction of GP to TP
State the no. of carbon atoms in RuBP, GP and TP
RuBP: 5
GP: 3
TP: 3
Describe the structure of a chloroplast
- usually disc-shaped
- double membrane
- thylakoids
- stroma
How does the structure of the chloroplast maximise the rate of the light-dependent reaction?
- ATP synthase channels with granal membrane
- large surface area of thylakoid membrane for ETC
- photosystems position of chlorophyll to enable max absorption of light
How does the structure of the chloroplast maximise the rate of the light-independent reaction?
- own DNA and ribosomes for synthesis of enzymes e.g. rubisco
- concentration of enzymes and substrates in stroma is high
Define limiting factor
factor that determines the max rate of a reaction, even if other factors change to become more favourable
Name 4 environmental factors that can limit the rate of photosynthesis
- light intensity
- CO2 levels
- temperature
- mineral/ magnesium levels
Write the symbol equation for photolysis.
H2O–> 1⁄2O2 + 2e- + 2H+
Describe photoionisation of chlorophyll.
Light energy is absorbed by the chlorophyll and the energy results in the electrons becoming excited and raising up an energy level to leave the chlorophyll. Therefore the chlorophyll has been ionised by light. Some of the energy from the released electrons is used to make ATP and reduced NADP in chemiosmosis.
Where does the light-dependent reaction (LDR) occur?
Thylakoid membrane
What are the three key reactions in the LDR?
- Photoionisation of chlorophyll -
- Photolysis
- Chemiosmosis
Describe chemiosmosis in 4 key stages
Step 1: The electrons that gained energy and left the chlorophyll move along a series of proteins embedded within the thylakoid membrane.
Step 2: As they move along, they release energy and some of the energy from electrons is used to pump the protons across chloroplast membranes.
Step 3: An electrochemical gradient is created. The protons pass through the enzyme ATP synthase, which results in the production of ATP.
Step 4: The protons combine with the co-enzyme NADP to become reduced NADP. As the protons move from a high to low concentration gradient this is known as chemiosmosis.
Why is the LIR temperature sensitive?
It involves the enzyme Rubisco
What is the role of ATP in the Calvin cycle?
- To provide the energy to reduce GP to TP
- To provide the energy to regenerate RuBP from TP
What is the role of NADPH in the Calvin cycle?
To provide an H to reduce GP to TP
What is the triose phosphate used for?
1- To donate one carbon each turn of the cycle to go towards making a hexose sugar
2- To regenerate RuBP so the cycle can continue
Which molecules from the LDR are used in the LIR?
1 ATP
2 Reduced NADP
Where does the LIR occur?
The stroma of the chloroplast
What can the hexose sugars made be used for?
Whilst glucose is the product, this monosaccharide can join to form disaccharides such as sucrose, and
polysaccharides such as cellulose and starch. It can also be converted into glycerol and therefore combine with fatty acids to make lipids for the plant.
Draw the Calvin cycle.
check work sheets
