Photosynthesis

• Photosystem II is the site where water molecules are split into oxygen, electrons, and protons.
• The light-dependent reactions occur in the thylakoid membranes of chloroplasts.
• Chlorophyll absorbs most strongly at wavelengths around 430 nm (blue) and 662 nm (red).
• Photosynthesis takes place in chloroplasts. Chloroplasts contain photosynthetic pigments which absorb light energy at particular wavelengths of light. Examples of pigments include – chlorophyll a and b, carotene and xanthophylls. Chloroplasts are found in mesophyll tissues, predominantly in the palisade mesophyll cells. Chloroplasts orientate themselves to receive the maximum exposure to light. 
• In low light conditions


  Chloroplasts distribute evenly throughout the cytoplasm to maximise absorption of the available light
• In high light intensity

  Chloroplasts line up in vertical columns against the cell wall, side on to the light to prevent damage by over-exposure
• Photosynthetic pigments can be separated


  Using chromatography
• Pigments


  • Chlorophyll a
  • Chlorophyll b
  • Carotenoids
• Chromatogram


  A record of the separation of a mixture by chromatography
• Rf value


  Relative position of a pigment on a chromatogram
• Rf values can be compared to the Rf values of known pigments to identify pigments
• Photosynthesis


  1. Capture light energy

  2. Transduce it into chemical energy stored in molecules of carbohydrates
• During the day


  Respiration releases carbon dioxide which, in the leaves, is used in photosynthesis
• Respiration


  Takes place during the night (dark)
• As light intensity increases

  The rate of photosynthesis also increases
• Up to a certain point, respiration can provide the CO2 needed by photosynthesis until the rates of these reactions are the same. This is called the compensation point
• Plants adapted to live in the shade

  • Have a higher rate of photosynthesis at lower light intensities
  • Have a lower compensation point than plants adapted to living in full sunlight
• Limiting factors:

  The law of limiting factors states that:

  When a chemical process is affected by more than one factor its rate is limited by that factor which is nearest its minimum value. If its quantity is changed it will directly affect a process.

  This means that a limiting factor is something which directly affects the rate of a process if its quantity is changed.

  
• Photosynthesis

  • Requires:
  • - Energy in the form of light (usually from the sun)
  • - Chlorophyll and other pigments to absorb light energy
  • - Water to combine with carbon dioxide to form organic compounds
  • - Suitable temperature to provide optimum conditions for the enzymes that control photosynthetic reactions
  • - Carbon dioxide which will be fixed in the Calvin cycle
  • - Enzymes such as Rubisco to catalyse reactions
• A shortage of any of these factors will limit the maximum rate at which photosynthesis can take place
• An increase in a factor, such as light intensity, causes no further increase in the rate of photosynthesis
• Palisade tissue


  • Site of photosynthesis in leaves
  • Location in dicot leaves
• Adaptations of dicot leaves

  Change position during the day so that the upper surface is always at right angles to the direction of light
• Palisade mesophyll cells


  Cells in the palisade tissue of a leaf
• Chloroplasts


  Organelles in plant cells that contain chlorophyll and are the site of photosynthesis
• Intracellular movement of chloroplasts

  1. Movement of chloroplasts within a cell in response to light intensity

  2. Orientation of chloroplasts to absorb more light

  3. Movement of chloroplasts to areas of a cell that receive more light
• The different pigments can absorb light of different wavelengths.

  If you shine the light of different wavelengths through a solution of chlorophyll molecules, short/blue wavelengths and long/red wavelengths are absorbed but the green light is reflected/transmitted.

  The % absorption of light absorbed at different wavelengths is called an absorption spectrum
• Each pigment absorbs light of slightly different wavelengths – overall the plant can carry out photosynthesis at a wider range of wavelengths and is more effective.

  The rate of photosynthesis at different wavelengths of light corresponds closely to the wavelengths absorbed – this shows that it is the wavelengths of light that are absorbed that are used in photosynthesis.
• The rate of photosynthesis is usually measured as the volume of O2 produced per minute.

  If the rate of photosynthesis is plotted against wavelength of light this is called an action spectrum – it shows the wavelengths of light actually used in photosynthesis.
• Plants which are adapted to living in habitats with different light availabilities may produce different proportions of photosynthetic pigments and even contain different pigments. This way, plants can maximise the amount of light energy they can absorb and ensure that photosynthesis can continue.
• adaptations of sun and shade-adapted plants

  
  

  Sun plant

  Large palisade mesophyll with small chloroplasts

  Few grana

  Low chlorophyll content

  High compensation point

  More chlorophyll A than B

  The rate of photosynthesis is higher in short wavelengths of light

  Small thick leaves held vertically

  
  

  Shade plants

  Short palisade cells with larger chloroplasts

  Many grana

  High chlorophyll content

  Low compensation point

  Less chlorophyll A then B

  Rate of photosynteiss hight in longer wavelenghts of light

  Large thin leaves held horizontally.
• An absorption spectrum is a graph which shows how much light is absorbed by a pigment at different wavelengths of light.

  An action spectrum is a graph which shows the rate of photosynthesis at different wavelengths of light. If you overlay an action spectrum onto an absorption spectrum the peaks show a very close correlation. This suggests that the wavelengths of light absorbed, by the photosynthetic pigments, are actually used for photosynthesis.
• Thomas Englemann


  Devised an experiment to determine which wavelengths of light were used most for photosynthesis
• Englemann's experiment

  1. Placed algae spirogyra in a suspension of motile aerobic bacteria

  2. Used a prism to refract white light into its constituent rainbow colours

  3. Observed the oxygen production

  4. Concluded that the blue and red regions of the spectrum caused the most photosynthetic activity
• Algae spirogyra


  Used in Englemann's experiment
• Motile aerobic bacteria


  Used in Englemann's experiment
• White light


  Refracted in Englemann's experiment
• Prism


  Used in Englemann's experiment to refract white light into its constituent rainbow colours
• Oxygen


  Produced during photosynthesis
• Aerobic bacteria


  Need oxygen for respiration and migrate towards regions with the highest concentration of oxygen