5.2.1 Photosynthesis

• Photosynthesis


  The process whereby light from the Sun is harvested and used to drive the production of chemicals, including ATP, and used to synthesise large organic molecules from inorganic molecules
• Autotrophs


  Synthesise complex organic compounds from inorganic sources
• Heterotrophs


  Obtain complex organic compounds by eating other organisms
• Photosynthesis


  • A highly efficient energy transduction process
  • Conversion of light energy into chemical energy
• Sodium hydrogen carbonate indicator solution

  Changes colour according to pH: Red at pH 7 (neutral), Purple/dark red when pH slightly above 7, Yellow at pH 6
• Investigating the rate of photosynthesis by measuring the uptake of carbon dioxide


  1. In light, there is a net uptake of CO2 from the solution, turning the indicator purplish (pH slightly above neutral)

  2. In the dark, there is net production of CO2, dissolving to form carbonic acid and lowering the pH, turning the indicator orange
• Tubes 2 and 4 are the controls, showing the indicator does not change colour without the plant present (i.e. does not change colour in response to light/dark alone)
• Chloroplast


  • Components: outer membrane, lamellae, grana, thylakoid, stroma, DNA
• Leaf


  • Upper epidermis, palisade layer, spongy mesophyll, lower epidermis, waxy cuticle, vacuole, nucleus, air space, stomatal pore, guard cell
• Chloroplast


  • 5-10 um long, double membrane, two distinct regions: stroma (site of light independent reaction) and grana (stacks of thylakoids where photosynthetic pigments are found)
• Photosynthetic pigments


  Coloured compounds located in the thylakoid membranes of a chloroplast that absorb light energy used in photosynthesis
• Chlorophyll a


  • Yellow-green primary pigment at the reaction centre of the photosystems, with absorption peaks at 420-430nm (blue-violet) and 670nm (red)
• Accessory pigments


  • Absorb other wavelengths of light and pass the energy to chlorophyll a, including chlorophyll b (blue-green), carotenoids (orange), and xanthophyll (yellow)
• Photosystems


  • Funnel-shaped light-harvesting complexes of photosynthetic pigments embedded in the thylakoid membrane, with a primary pigment reaction centre
• Absorption spectrum


  Graph showing the absorption of different wavelengths of light by a pigment or mixture of pigments
• Action spectrum


  Graph showing the rate of photosynthesis at different wavelengths of light
• Energy transfer in photosystems

  When a photon of light hits an accessory pigment, the energy is passed to the next pigment until it reaches the reaction centre
• The light dependent reaction of photosynthesis occurs on the thylakoid membranes in the chloroplasts
• Different chlorophyll pigments absorb slightly different wavelengths of light, allowing a greater range of wavelengths to be used for photosynthesis
• When light strikes chlorophyll molecules, suitable wavelengths are absorbed and the energy is used to excite electrons
• Chlorophyll a P700


  The primary pigment in Photosystem I with an absorption peak at 700nm
• Chlorophyll a P680


  The primary pigment in Photosystem II with an absorption peak at 680nm
• The difference between chlorophyll a P700 and P680 is the wavelength of light they absorb
• Chlorophyll b and carotenoids are the accessory pigments
• Absorption spectrum


  Shows the absorption of different wavelengths of light by pigments
• Action spectrum


  Shows the rate of photosynthesis at different wavelengths of light
• Chromatography


  • Separates mixtures based on the different speeds at which components travel through the material due to their size and charge, allowing calculation of a retardation factor (Rf)
• Paper chromatography


  • Separates pigments by passing the mixture through paper (cellulose)
• Thin-layer chromatography


  • Separates pigments by passing the mixture through a thin layer of adsorbent (e.g. silica gel)
• Retardation factor (Rf)


  Demonstrates how far a dissolved pigment travels through the stationary phase, with larger/less soluble molecules having a smaller Rf
• Carotenoids have the highest Rf values, chlorophyll b has a much lower Rf, and chlorophyll a has an Rf value in between</b>
• Chemical hazards


  • Anhydrous sodium sulphate (low hazard), ethyl ethanoate and propanone (highly flammable and irritant), cyclohexane (highly flammable, irritant and harmful to aquatic environment)
• Light dependent reaction


  Harvests energy from light to drive the production of ATP and NADPH
• Light independent reaction


  Uses the ATP and NADPH produced in the light dependent reaction to convert CO2 into organic compounds like glucose
• The light dependent reaction occurs on the thylakoid membranes in the chloroplasts
• Photosynthesis


  The light-dependent and light-independent stages of the process by which plants and some other organisms use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar
• The Light Dependent Reaction

  1. Light energy

  2. Photosystem II

  3. Photolysis of water

  4. Electron carrier chain

  5. Photosystem I

  6. NADP+ + H+ -> NADPH + H+
• ATP


  A high-energy molecule that stores and releases energy for use in cellular processes
• NADPH


  A reduced form of the coenzyme NADP+ that is produced in the light-dependent reactions of photosynthesis and used in the light-independent reactions to reduce carbon dioxide to carbohydrates
• 5.2.1d - Understand the light-dependent stage of photosynthesis