5.1 Photosynthesis Flashcards
Where do light-dependent and light-independent reactions occur in plants?
- Light-dependent: thylakoids of chloroplasts
- Light-independent: stroma of chloroplasts
Role of light in photoionisation
- Chlorophyll molecules absorb energy from photons of light
- This ‘excites’ 2 electrons (raises them to a higher energy level), causing them to be released from the chlorophyll
Name 2 main stages involved in ATP production in the light-dependent reaction
- Electron transfer chain
- Chemiosmosis
What happens in the electron transfer chain (ETC)?
Electron 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 ETC is coupled to the active transport H+ ion (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 in the stroma via ATP synthase
- ATP synthase catalyses ADP+Pi—> ATP
Explain role of light in photolysis
- Light energy splits molecules of water
- 2H2O —> 4H+ + 4e- + 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 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
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)
- 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)
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
State the roles of ATP and reduced NADP in the light-independent reaction
- ATP: reduction of GP to TP and provides the 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
Structure of a chloroplast
- Usually disc shaped
- Double membrane (envelope)
- Thylakoids: flattened discs stack to form grana
- Intergranel 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 SA of thylakoid membrane for ETC
- Photosystems position chlorophyll to enable maximum 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 maximum rate of a reaction, even if other factors change to become 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 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 a 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 a 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 the 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 calculated?
- Ratios that allow comparisons of how far molecules have moved in chromatograms
- Rf value = distance between origin and centre of pigment spot/distance between origin and solvent front
During the light-independent reaction of photosynthesis, carbon dioxide is converted into organic substances. Describe how.
- CO2 combines with ribulose bisphosphate/ RuBP
- Produces two glycerate (3-) phosphate/GP
- GP reduced to triose phosphate/TP
- Using reduced NADP
- Using energy from ATP
- Triose phosphate converted to glucose/named organic substance
