Photo

• Describe the light dependent reaction (LDR) in photosynthesis.

  •Occurs in the thylakoid membrane or grana•First stage of photosynthesis and requires light•Light energy and water are used to create ATP and reduced NADP which are needed for the light independent stage
• Explain the 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.

  Some of the energy from the released electrons is used to make ATP and reduced NADP in chemiosmosis
• Name the 2 main stages involved in ATP production in the light-dependent reaction.

  1. Electron Transfer Chain

  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 & undergo a series of redox reactions, which releases energy.
• How is a proton concentration gradient established during chemiosis?

  Some energy released from the ETC 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 the channel protein ATP synthase.

  
  

  ATP synthase catalyses ADP + Pi ---> ATP
• Explain the role of light in photolysis?

  Light energy is absorbed and splits molecules of water into oxygen, H+ and water

  H2O ---> 2H+ + 2e- + 0.5O2

  
  

  The H+ is picked up by NADP to form NADPH and is used in the LIR.

  The e- are passed along a chain of electron carrier proteins.

  The oxygen is either used for respiration or diffuses out of the leaf through the stomata
• 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 electron 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 three main stages in the Calvin cycle.

  1.Carbon fixation

  2.Reduction

  3.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?

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

  ● Requires 2 x reduced NADP & 2 x ATP

  ● Forms 2 x NADP & 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 regenration (in the Calvin cycle)?

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

  RUBP is regenerated from RuP using 1 ATP

  Forms 1 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: coenzyme 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 (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-independent 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?

  •Concentration of enzymes and substrates in stroma in high•Own DNA and ribosomes for synthesis of enzymes e.g. rubsico
• Define limiting factor.


  Factor that determines maximum rate of a reaction, even if other factors change to become more favourable.
• Name the 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

  1.Use a potometer2.Place balls of calcium alginate containing green algae in hydrogencarbonate indicator (colour change orange --> magneta 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.

  1. Use a capillary tube to spot pigment extract onto pencil 'start line' (origin' 1cm above bottom of paper.

  2. Place chromatography paper in solvent. (origin should be above solvent level)

  3. 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.
• What are the four key stages in the light dependent reaction (LDR) in photosynthesis.

  1.Photolysis

  2.Photoionisation of chlorophyll

  3.Chemiosmosis

  4.Production of ATP and reduced NADP
• Explain the process of chemiosmosis.

  •The electrons that gained energy and left the chlorophyll move along a series of proteins embedded within the thylakoid membrane•As they move along, they release energy and some of the energy from electrons is used to pump the protons across chloroplast membranes•An electrochemical gradient is created- the protons pass through ATP synthase which results in the production of ATP•The protons combine with the co-enzyme NADP to become reduced NADP- this is known as chemiosmosis as the protons move from a high to low concentration gradient
• Describe the light-independent reaction of photosynthesis (LIR).

  •Calvin Cycle

  •Occurs in the stroma which contains enzyme RuBisCo, which catalyses the reaction

  •This stage is temperature-sensitive as it involves enzymes

  •Uses carbon dioxide, reduced NADP and ATP to form a hexose sugar

  •ATP is hydrolysed to provide energy for this reaction and the reduced NADP donates the electron to reduce molecules of GP in the cycle
• Describe the overall process of the Calvin Cycle.

  •Carbon dioxide reacts with ribulose bisphosphate (RuBP) to form two molecules of glycerate-3-phosphate (GP), a 3 carbon compound- this reaction is catalysed by RuBisCo

  •GP is reduced to triose phosphate (TP) using energy from ATP and by accepting a H from reduced NADP

  •One carbon from TP leaves the cycle each turn to be converted into useful organic substances

  •The rest of the molecule is used to regenerate RuBP, with the energy from ATP
• What is role of DCPIP solution in photosynthesis experiment?

  DCPIP is a redox indicator which is blue when oxidised and turns colourless when reduced.

  It can pick up the electrons from LDR instead of NADP
• What is the role of ammonium hydroxide in the photosynthesis experiment?

  Ammonium hydroxide is alkaline so could denature the dehydrogenase enzyme.

  Ammonium hydroxide can also accept electrons, so could pick up electrons instead of DCPIP and NADP
• What is the aim of the photosynthesis practical?

  To investigate the effect of ammonium hydroxide has on the rate of of dehydrogenase enzyme in chloroplasts
• Why must all solutions in the photosynthesis practical be cold?

  To slow the activity of the enzymes which could damage the chloroplasts that were released
• Why were the spinach leaves blended in the photosynthesis practical?

  To release the chloroplasts by breaking open the cells
• Why must the isolation medium be an isotonic solution in the photosynthesis practical?

  Isotonic means the solution has the same water potential as the chloroplasts and therefore will prevent the chloroplast bursting and shrivelling
• Why was the blended spinach filtered in the photosynthesis practical?

  To remove large pieces of cell debris and other organelles