5.1- Photosynthesis Flashcards
Where do light-dependent and light-independent reactions occur in plants?
light-dependent: in the thylakoids of chloroplasts
light-independent: stroma of chloroplasts
Explain the role of light in photoionisation
Chlorophyll molecules absorb energy from photons of light.
- This ‘excited’ 2 electrons (raises them to a 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
- electron transfer chain
- chemiosmosis
What happens in the electron transfer chain (ETC)?
Electrons released from chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane & 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 of H+ ions from the stroma into the thylakoid space
How does chemiosmosis produce ATP in the LDR?
H+ ions (protons) move down their concentration gradient from the thylakoid space into the stroma via the 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+ + 4e- + O2
What happens to the products of the photolysis of water?
- H+ ions: move out of thylakoid space via ATP synthase & 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 LDR?
- 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 & 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)?
- 2x GP are reduced to 2x triose phosphate (TP)
- Requires 2x reduced NADP & 2x ATP
- Forms 2x NADP & 2x ADP
How does the LIR 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 RuBP forms
- RuBP is regenerated from RuBP using 1x ATP
- Forms 1x ADP
State the roles of ATP and reduced NADP in LIR
- ATP: reduction of GP to TP & provides phosphate group to convert RuP into RuBP
- reduced NADP: coenzyme transports electrons needed for reduction of GP to TP
State number of carbons in RuBP, GP & 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 LDR?
- ATP synthase channels with granal membrane
- large surface area of thylakoid membrane for ETC
- photosystems position chlorophyll to enable maximum absorption of light
How does structure of chloroplast maximise the rate of LIR?
- Own DNA & ribosomes for synthesis of enzymes e.g. rubisco
- Concentration of enzymes & substrates in stroma is high
Define ‘limiting factor’
Factor that determines 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 (LDR)
- CO2 levels (LIR)
- 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 rate of photosynthesis
Dependent variable: rate of O2 production/ CO2 consumption
1. Use a potometer
2. Place balls of calcium alginate containing green algae in hydrogencarbonate indicator (colour change orange -> magenta as CO2 is consumed & 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) 1 cm 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 in chromatograms
- Rf value= distance between origin and centre of pigment spot / distance between origin and solvent front
