3.8 Photosynthesis

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3.8.1  State that photosynthesis involves the conversion of light energy into chemical energy

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


3.8.2  State that light from the Sun is composed of a range of wavelengths (colours)

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)


3.8.3  State that chlorophyll is the main photosynthetic pigment

Chlorophyll is the main site of light absorption in the light dependent stage of photosynthesis

There are a number of different chlorophyll molecules, each with their own distinct absorption spectra (the spectrum of light absorbed by a substance)

When chlorophyll absorbs light energy, they release electrons which are used to make ATP (chemical energy)

Chlorophyll and Photosystems


3.8.4  Outline the difference in absorption of red, green and blue light by chlorophyll

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

  • This explains why leaves are green - excepting when the presence of other pigmented substances (e.g. anthocyanins) produces a different colour
  • Deciduous trees stop producing high amounts of chlorophyll in the winter (due to insufficient sunlight), allowing other photosynthetic pigments (e.g. xanthophylls, carotenoids) to come to the fore, which changes the colour of the leaf


3.8.5  State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen

The first part of photosynthesis is the light dependent reaction, which uses light energy to make ATP


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


3.8.6  State that ATP and hydrogen (derived from the photolysis of water) are used to fix carbon molecules to make organic molecules

The second part of photosynthesis is the light independent reaction, which makes organic compounds from the products of the light dependent reactions


Light Independent Reaction

  • ATP and hydrogen (carried by NADPH) are products of the light dependent reactions
  • They are used to fix carbon molecules together (add CO2 to basic carbon compounds)
  • This allows for the production of more complex organic molecules (e.g. sugars)
  • These organic molecules can then be stored to use in cellular respiration as required 


3.8.7  Explain that the rate of photosynthesis can be measured directly by the production of oxygen or the uptake of carbon dioxide, or indirectly by an increase in biomass

  • The rate of photosynthesis can be measured by changes in the amounts of inputs (CO2) or outputs (O2 or glucose) of the photosynthesis equation
  • Water cannot be measured as it is involved in a number of essential processes besides photosynthesis (e.g. condensation and hydrolysis reactions)


Measuring CO2 Uptake

  • CO2 uptake can be measured by placing a plant in an enclosed space with water
  • Carbon dioxide interacts with the water molecules, producing bicarbonate and hydrogen ions, which increases the acidity of the resulting solution
  • The change in pH can therefore provide a measure of CO2 uptake by a plant (increased CO2 uptake = more alkaline pH)


Measuring O2 Production

  • O2 production can be measured by submerging a plant in an enclosed space with water attached to a sealed gas syringe
  • Any oxygen gas produced will bubble out of solution and can be measured by a change in water level (via the position of the meniscus)


Measuring Biomass (Indirect)

  • Glucose production can be indirectly measured by a change in a plant's biomass (weight)
  • This requires the plant to be completely dehydrated prior to weighing to ensure the change in biomass reflects a change in organic matter and not water content
  • An alternative method for measuring glucose production is to determine the change in starch levels in a plant (glucose is stored as starch)
  • Starch can be identified via iodine staining (resulting solution turns purple) and quantitated using a colorimeter


3.8.8  Outline the effect of temperature, light intensity and carbon dioxide concentration on the rate of 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