Component 1

• Chloroplast


  An organelle found in plants and green algae that is the site of photosynthesis
• Structure of a chloroplast


  1. Double membrane (chloroplast envelope)

  2. Grana - stacks of flattened disks (thylakoids) that contain photosystems I and II, electron transport chain, ATP synthase

  3. Grana connected by intergranal lamellae

  4. Stroma - fluid-filled matrix containing enzymes
• Location of chloroplasts in a leaf

  Mainly found in the palisade layer
• Adaptations of chloroplasts for photosynthesis

  • Thylakoids give a large surface area for light-independent reactions
  • Photosynthetic pigments arranged into photosystems to maximise light absorption
  • Stroma directly surrounds grana - products of photosynthesis diffuse directly into the stroma
  • Contain their own DNA (cpDNA) and ribosomes
  • Inner chloroplast membrane less permeable than outer, enabling control over the movement of substances
• Adaptations of angiosperm leaves for photosynthesis

  • Large surface area to maximise light absorption
  • Thin to reduce the diffusion distance for CO2
  • Upper epidermis transparent allowing light to strike mesophyll layers
  • Palisade cells densely packed and contain many chloroplasts
  • Air spaces reduce the diffusion distance for CO2
  • Vein network transports water and minerals to the leaf and takes sugars away
  • Stomata allow CO2 to diffuse into the leaf
• Transducer


  Something that converts one type of energy into another
• Why chloroplasts are described as transducers

  Chloroplasts transduce light energy into the chemical energy of ATP
• Photosynthetic pigment


  A molecule present in chloroplasts (or photosynthetic bacteria) that absorbs light energy for photosynthesis. It absorbs specific wavelengths of light and reflects others.
• Examples of photosynthetic pigments

  • Chlorophylls a and b
  • Beta carotene
  • Xanthophylls
• Purpose of chromatography


  To separate different products from a mixture
• Photosynthesis


  A complex metabolic pathway that synthesises organic molecules from carbon dioxide and water in the presence of light. Overall: 6CO2 + 6H2O ⟶ C6H12O6 + 6O2
• Stages of photosynthesis


  • Light-dependent stage
  • Light-independent stage
• Photosystem


  • Protein complex consisting of an antenna complex and reaction centre
  • Involved in the absorption of light and transfer of electrons in photosynthesis
  • Two types: Photosystems I (PSI) and II (PSII)
• Difference between Photosystems I and II

  They absorb different wavelengths of light
• Light harvesting in photosynthesis

  1. Antenna complex absorbs light energy of varying wavelengths and transfers it quickly and efficiently to the reaction centre

  2. Energy absorbed by two chlorophyll a molecules which emit 'excited' electrons
• Absorption spectrum


  A pattern of bands that occurs when a substance absorbs the different wavelengths of light
• Action spectrum


  A graph of the rate of photosynthesis against each wavelength of light absorbed by a pigment
• Light-dependent stage of photosynthesis

  1. First stage of photosynthesis

  2. Takes place in the thylakoids of the chloroplast

  3. Uses light energy to produce ATP, reduced NADP and oxygen
• Sources of electrons for the electron transport chain
• Cyclic photophosphorylation


  The formation of ATP involving Photosystem I only, where NADP+ is not reduced
• Outline of cyclic photophosphorylation

  1. Involves Photosystem Ⅰ only (no electrons supplied from PS ⅠⅠ)

  2. Excited electrons enter the electron transport chain to produce ATP and then return to Photosystem I

  3. No reduction of NADP and no water required to replace lost electrons
• Purpose of cyclic photophosphorylation

  Produces additional ATP to meet surplus energy demands of the cell
• Non-cyclic photophosphorylation


  The formation of ATP and reduced NADP involving both Photosystems I and II
• Outline of non-cyclic photophosphorylation

  1. Involves Photosystem Ⅰ and II

  2. Excited electrons enter the electron transport chain to produce ATP

  3. NADP acts as a final electron acceptor and is reduced

  4. Water is split to replace lost electrons
• Cyclic photophosphorylation


  Produces additional ATP to meet surplus energy demands of the cell
• Non-cyclic photophosphorylation


  1. Involves Photosystem I and II

  2. Excited electrons enter the electron transport chain to produce ATP

  3. NADP acts as a final electron acceptor and is reduced

  4. Water is photolysed to compensate for electrons lost from Photosystem II
• Purpose of non-cyclic photophosphorylation

  Produces ATP and reduced NADP for the Calvin cycle
• How chemiosmosis produces ATP in the light-dependent stage

  1. Protons flow down their concentration gradient from the thylakoid space into the stroma via ATP synthase

  2. ATP synthase phosphorylates ADP to form ATP as protons flow through it
• Photolysis


  The splitting of a molecule of water in the presence of light (energy from the sun) that occurs during the light-dependent stage of photosynthesis. This produces protons, electrons and oxygen
• What happens to the products of photolysis

  1. H+ - used in proton pumping and to reduce NADP

  2. - replaces electrons lost from chlorophyll a in PSII

  3. O2 - by-product, used for respiration or diffuses out of the leaf as waste gas
• How the electron transfer chain results in the production of reduced NADP


  NADP acts as a final electron acceptor, and is subsequently reduced
• Light-independent stage of photosynthesis

  • Second stage of photosynthesis
  • Calvin Cycle produces glucose
  • Does not require light energy and takes place in the stroma
  • Uses carbon dioxide and the products of the light-dependent stage to build organic molecules
• Light-independent stage


  The Calvin cycle
• Stages of the Calvin cycle

  • Carbon fixation
  • Reduction
  • Regeneration
• Carbon fixation of the Calvin cycle

  1. Reaction between CO2 and ribulose bisphosphate (RuBP) catalysed by enzyme RuBisCo

  2. Forms unstable 6C intermediate that breaks down into two molecules of glycerate 3-phosphate (G3P)
• Reduction of the Calvin cycle

  1. 2× G3P are reduced to 2× triose phosphate (TP)

  2. Requires 2× reduced NADP and 2× ATP formed during the light-dependent reaction

  3. Forms 2× NADP and 2× ADP that enter the light-dependent reaction
• Regeneration of the Calvin cycle

  1. After 1C leaves the cycle, the 5C compound ribulose monophosphate (RuP) forms

  2. Ribulose biphosphate (RuBP) is regenerated from RuP using 1× ATP

  3. Forms 1× ADP
• Nutrients produced as a result of photosynthesis

  • Formation of amino acids from GP (requires nitrates and sulfates)
  • TP molecules used to produce sugars e.g. glucose, fructose, sucrose
• Limiting factor


  A variable that limits the rate of a particular reaction
• Factors that limit the rate of photosynthesis

  • Light intensity - light-dependent stage
  • Light wavelength - absorption by chlorophyll
  • CO2 concentration - light-independent stage
  • Temperature - enzyme-controlled reactions (Carbon fixation)
  • pH - enzyme-controlled reactions