Photosynthesis

• 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 in light

  ~ This excites two electrons and raises them to a higher energy level

  ~ This causes them to be released from the chlorophyll
• Name the two 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 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 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 concentration gradient from the thylakoid space into the stroma via the channel protein ATP synthase

  ~ ADP + Pi = ATP
• Explain the role of light in photolysis

  ~ Light energy splits molecules of water

  ~ 2H2O →→ 4e- + O2 + 4H+ 
• What happens to the products of the 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 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: used for photolysis of water

  ~ Electrons: NADP acts as the final electron acceptor of the electron transfer chain
• Name the three 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

  ~ 2x GP are reduced to 2x triose phosphate (TP)

  ~ Requires 2x reduced NADP and 2x ATP

  ~ Forms 2x NADP and 2x ADP
• How does the light independent reaction result in the production of useful organic substances

  ~1C leaves the cycle

  ~ Some of the TP is converted into useful organic molecules
• What happens during regeneration

  ~ After 1C leaves the cycle, the 5C compound RuP forms

  ~ RuBP is regenerated from RuBP 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 phosphate group to convert RuP into RuBP

  ~ Reduced NADP: coenzymes transports electrons needed for reduction of GP to TP
• State the 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

  ~ 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 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

  ~ 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 more favourable
• Name four environmental factors that can limit the rate of photosynthesis

  ~ Light intensity

  ~ CO2 levels

  ~ Temperature

  ~ Mineral levels
• 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
• State the purpose and principle of paper chromatography

  Molecules in a mixture are separated based on their relative attraction to the mobile phase vs the stationary phase
• Outline a method for extracting photosynthetic pigments

  Use a pestle and mortar to grind a leaf with an extraction solvent
• Outline how paper chromatography can be used to separate photosynthetic pigments

  ~ Use a capillary tube to spot pigment extract onto pencil start line 1cm above bottom of paper

  ~ Place chromatography paper in solvent

  ~ Allow solvent to run until it almost touches the other end of the paper

  ~ Pigments move different distances
• What are Rf values and 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