Photosynthesis


Photosynthesis is the process by which plants synthesise organic compounds (e.g. glucose) from inorganic compounds (CO2 and H2O) in the presence of sunlight


Photosynthesis is a two step process: 

1.  The light dependent reactions convert the light energy into chemical energy (ATP)

2.  The light independent reactions use the chemical energy to synthesise organic compounds (e.g. glucose)

The organic molecules produced in photosynthesis can be used in cellular respiration to provide the energy needed by the organism


Overview of Photosynthesis


Light

Sunlight is white light, made up of all the colours of the visible spectrum

Colours are different wavelengths of light and range from ~ 400 nm - 700 nm

The colours of the visible spectrum are (from longer to shorter wavelength):

Red    Orange    Yellow    Green    Blue    Indigo    Violet    (R.O.Y.G.B.I.V)


Chlorophyll

  • Chlorophyll is the main site of light absorption in the light dependent stage of photosynthesis
  • The main colours of light absorbed by chlorophyll are red and blue light
  • The main colour of light not absorbed (it is reflected) by chlorophyll is green light
  • There are a number of different chlorophyll molecules, each with their own distinct absorption spectra (the spectrum of light absorbed by a substance)
  • These chlorophyll molecules cooperate to generate an action spectrum (the spectrum of light used in photosynthesis) 


Absorption Spectrum versus Action Spectrum


Chloroplast

  • The chloroplast is the organelle in plants where photosynthesis occurs (it will not be present in non-photosynthesising tissues though)
  • It contains a series of flattened membrane sacs (called thylakoids) arranged into stacks called grana
  • Chlorophyll is grouped into photosystems on the thylakoid membrane and this is where the light dependent reactions occur
  • The internal fluid of the chloroplast is called the stroma and is where the light independent reactions occur


Structure of Chloroplasts

                  2D Representation                                                           3D Representation                                                               Electron Micrograph


Light Dependent Reaction

  • The light dependent reactions occur on the thylakoid membrane and involves chlorophyll
  • Light energy and water are used to produce ATP and NADPH (and O2 as a byproduct)


Overview of the Light Dependent Reaction

  • Light stimulates chlorophyll to release electrons, which results in the production of ATP
  • Light energy also splits water molecules (photolysis), producing oxygen and hydrogen
  • The hydrogen is taken up by a hydrogen carrier (NADP+) to form NADPH
  • The splitting of water also releases electrons, which replace those lost by the chlorophyll
  • The ATP and hydrogen (NADPH) are taken to the site of the light independent reactions


Light Dependent Reaction in Detail


Light Independent Reaction

  • The light independent reaction occurs in the stroma and uses the ATP and NADPH produced by the light dependent reaction
  • The light independent reaction is also known as the Calvin cycle and occurs via three main steps:


1.  Carbon Fixation

  • The enzyme rubisco (RuBP carboxylase) catalyses the attachment of CO2 to the 5C compound ribulose bisphosphate (RuBP)
  • The unstable 6C compound that is formed immediately breaks down into two 3C molecules called glycerate-3-phosphate (GP)


2.  Reduction

  • Each GP molecule is then phosphorylated by ATP and reduced by NADPH
  • This converts each GP molecule into a triose phosphate (TP) called glyceraldehyde phosphate


3.  Regeneration of RuBP

  • For every six molecules of TP produced, only one may be used to form half a sugar molecule (need two cycles to form a complete glucose)
  • The remaining TP molecules are reorganised to regenerate stocks of RuBP in a reaction that involves ATP
  • With RuBP regenerated, this cycle will repeat many times and be used to construct chains of sugars (e.g. sucrose) for use by the plant


The Light Independent Reaction (Calvin Cycle)


Factors Affecting Photosynthesis

Temperature

  • Photosynthesis is controlled by enzymes, which are sensitive to temperature
  • As temperature increases, the rate of photosynthesis will increase as reagents have greater kinetic energy and are more likely to react
  • Above a certain temperature, the rate of photosynthesis will decrease as essential enzymes begin to denature


Light Intensity

  • As light intensity increases, the rate of photosynthesis will increase up until a certain point, when photosynthesis is proceeding at its maximum rate
  • Further increases to light intensity will have no effect on photosynthesis (the rate will plateau), as chlorophyll are saturated by light
  • Different wavelengths of light will have different effects on the rate of photosynthesis (e.g. green light will not be used)


CO2 Concentration

  • As the concentration of carbon dioxide increases, the rate of photosynthesis will increase up until a certain point, when photosynthesis is proceeding at its maximum rate
  • Further increases to carbon dioxide concentration will have no effect on photosynthesis (the rate will plateau), as the enzymes responsible for carbon fixation become saturated


Factors Affecting the Rate of Photosynthesis