3.5.1 Photosynthesis

• Photosynthesis- a 2 step process which takes place in the chloroplasts, consisting of light dependent reactions and light independent reactions
• ATP- cyclic molecule which is key in energy transfer as a high energy form
• Chlorophyll- light absorbing pigment which is embedded in the thylacoid membrane to maximise light energy, absorbing two specific wavelengths of light
• Photolysis- the splitting of water molecules into electrons, hydrogen ions and oxygen
• Photoionisation- when light of the correct wavelength strikes chlorophyll molecules, exciting electrons, physically elevating them to higher energy levels
• Light Dependent reactions:

  1. Light of the correct wavelength strikes PS2, causing photoionisation.
  2. Elevated electron is accepted by electron carrier, which is reduced. Electrons are passed down a series of electron carriers in a series of redox reactions.
  3. Energy from the excitation of electrons between electron carriers moves hydrogen ions against their concentration gradient.
  4. Electrons are accepted by PS1.
  5. Light of the correct wavelength strikes PS1, causing photoionisation. Phase 2 repeats.
  6. NADP accepts electrons and hydrogen ions, and is reduced to NADPH.
• Significance of hydrogen ion gradient- allows energy to be generated to create ATP from ADP and Pi:

  1. Hydrogen ions attempt to diffuse across membrane from high concentration in thylacoid space to low concentration in stroma.
  2. Hydrogen ions cascade and flood the channel protein (ATP synthase).
  3. Kinetic energy from hydrogen ion movement through the channel protein spins sub-units.
  4. Energy generated from hydrogen ion movement binds ADP and Pi in a phosphorylation reaction, forming ATP.
• Light-independent reactions- series of enzyme catalysed reactions which occur within the stroma, called the Calvin Cycle:

  1. 1 RuBP molecule with 5 carbon molecules combines with carbon dioxide, catalysed by Rubisco.
  2. 2 GP molecules are produced, each with 3 carbon molecules.
  3. 2 GP molecules are reduced to Triose phosphate, each with 3 carbon molecules, using ATP energy and NADPH.
  4. 2 Triose Phosphate molecules are converted back into 1 RuBP molecule, using ATP energy, also producing 1/6 of a glucose molecule.
• If light is turned off:

  • RuBP levels decrease- no more light energy to produce ATP energy which is used to produce RuBP, so any remaining RuBP is converted into GP.
  • GP levels would increase- all remaining RuBP molecules would react with carbon dioxide and convert into GP.
  • Triose phosphate levels would stay the same- no more molecules can be made or converted due to lack of ATP energy or NADPH.
• Limiting factors- when a process is influenced by multiple factors, the factor in shortest supply becomes the limiting factor
• In photosynthesis, there are multiple factors which could become limiting factors (light intensity, water availability, carbon dioxide concentration, temperature) and for most plants, carbon dioxide concentration is the limiting factor.
• A - light intensity is the limiting factor,as the rate of photosynthesis increases as light intensity increases.

  B - another factor could now be the limiting factor, not light intensity.

  
• Factors which could influence photosynthesis:

  • Length of plant
  • Age of plant
  • Plant species
  • Surface area
  • Carbon dioxide concentration
  • pH
  • Temperature
  • Hydrogen ion concentration
  • Light intensity
  • Light wavelength
  • Number of leaves
  • Density of chloroplasts
• Measuring the rate of photosynthesis per unit of time using:

  • Oxygen production- volume of bubbles
  • Carbon dioxide uptake- concentration
  • Glucose formation- organic molecules
  • Water uptake- almost impossible as it is used in many other reactions
• Chromatography- separation of pigments, based on the solubility of the pigments in the solvent
• The same pigments separated in the same solvent have the same RF values. In a different solvent, there is different solubility of the pigments, which means different RF values.