5.1 Photosynthesis ❗️ Flashcards
Where do light-dependent reactions occur in plants?
In the thylakoids of chloroplasts
Where do light-independent reactions occur in plants?
Stroma of chloroplasts
Explain the role of light in photoionisation
Chlorophyll molecules absorb energy from photons of light. This excites 2 electrons raising them to a higher energy level, causing them to be released from chlorophyll.
Name the 2 main stages involved in ATP production in the light-dependent reaction
- Electron transfer chain
- 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 concentration gradient established during chemiosmosis?
Some energy released from the electron transport chain is coupled to the active transport of H+ ions (protons) from the stroma into the thylakoid space
How does chemiosmosis produce ATP in the light-dependent stage?
H+ ions (protons) move down their concentration gradient from the thylakoid space into the stroma via channel protein ATP synthase
Explain the role of light in photolysis
Light energy splits molecules of water
2H2O —>4H+ + 4e- + O2
What happens to the products of the photolysis of water?
H+ ions move out of the thylakoid space via ATP synthase and are used to reduce the coenzyme NADP.
The electrons replace electrons lost from chlorophyll
O2 is used for respiration or diffuses out of the 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
Where do the H+ ions and electrons used to reduce NADP come from?
H+ ions: photolysis of water
Electrons: NADP acts as the final electron acceptor of the electron transfer chain
Name the 3 main stages in the Calvin cycle
- Carbon fixation
- Reduction
- Regeneration
What happens during carbon fixation?
Reaction between CO2 and ribulose bisphosphate (RuBP) catalysed by rubisco.
Forms unstable 6C intermediate that breaks down into 2x glycerate 3-phosphate (GP)
What happens during reduction in the Calvin cycle
2 x GP are reduced to 2 X triose phosphate (TP)
This requires 2 X reduced NADP and 2 X ATP
Forms 2 X NADP and 2 X ADP
How does the light-independent reaction result in the production of useful organic substances?
1C leaves the cycle (i.e. some of the TP is converted into useful organic molecules)
What happens during regeneration (in the Calvin cycle)?
After 1C leaves the cycle, the 5C compound RuP forms
RuBP is regenerated from RuP using 1x ATP
Forms 1x ADP
Outline the sequence of events in the light-independent reaction(Calvin cycle)
Slide 34 on pmt 5.1 photosynthesis
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 number 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 (envelope)
- Thylakoids: flattened discs stack to form grana
- intergranal lamellae: tubular extensions attach thylakoids in adjacent grana
-Stroma: fluid-filled matrix
How does the structure of the chloroplast maximise the rate of the light-dependent reaction?
-ATP synthase channels within granal membrane
- large surface area of thylakoid membrane for electron transport chain
- photosystems position chlorophyll to enable maximise 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 maximum 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 (light-dependent stage)
CO2 levels (light-independent stage)
Temperature(enzyme-controlled steps)
Mineral/magnesium levels(maintain normal functioning of chlorophyll)
Outline some common agricultural practices used to overcome the effect of limiting factors in photosynthesis
Artificial light, especially at night
Artificial heating
Addition of CO2 to greenhouse atmosphere
Why do farmers try to overcome the effect of limiting factors?
To increase yield
Additional cost must be balanced with yield to ensure maximum profit
Suggest how a student could investigate the effect of a named variable on the rate of photosynthesis
Dependent variable: rate of O2 production/ CO2 consumption
- Use potometer
- Place balls of calcium alginate containing green algae in hydrogencarbonate indicator (colour change orange —> magenta as CO2 is consumed and pH increases)
State the purpose and principle of paper chromatography
Molecules in a mixture are separated based on their relative attraction to the mobile phase (running solvent) vs the stationary phase (chromatography paper)
Outline a method for extracting photosynthetic pigments
Use pestle and mortar to grind a leaf with an extraction solvent e.g. propanone
Outline how paper chromatography can be used to separate photosynthetic pigments
- Use a capillary tube to spot pigment extract onto pencil ‘start line’ (origin) 1cm above bottom of paper.
- Place chromatography paper in solvent. (Origin should be above solvent level).
- Allow solvent to run until it almost touches the other end of the paper. Pigments move different distances.
What are Rf values? How can they be calculated?
Ratios that allow comparison of how far molecules have moved on chromatograms.
Rf values = distance between origin and centre of pigments spot/ distance between origin and solvent front
