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created 13 days ago by Renata_SidorukSołoducha
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Photosynthesis is the process by which

cells synthesise organic compounds (e.g. glucose) from inorganic molecules (CO2 and H2O) in the presence of sunlight


This process requires

a photosynthetic pigment (chlorophyll) and can only occur in certain organisms (plants, certain bacteria)


Photosynthesis Equation

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Photosynthetic organisms use the light energy from the sun to create chemical energy (ATP)

This chemical energy can either be used directly by the organism or used to synthesise organic compounds (e.g. glucose)


Animals then consume these organic compounds as food and release the stored energy via cell respiration

  • Photosynthesis (anabolic synthesis of organic compounds) is essentially the reverse of cell respiration (catabolic breakdown)

Relationship between Photosynthesis and Cell Respiration

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The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation

The Sun emits its peak power in the visible region of this spectrum (white light ~ 400 – 700 nm)

Colours are different wavelengths of white light and range from red (~700 nm) to violet (~400 nm)

The colours of the visible spectrum are (from longest to shortest wavelength):

Red Orange Yellow Green Blue Indigo V iolet (Mnemonic: Roy G. Biv)


The Electromagnetic Spectrum

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Chlorophyll is a green pigment found in photosynthetic organisms that is responsible for light absorption

  • When chlorophyll absorbs light, it releases electrons which are used to synthesise ATP (chemical energy)

There are a number of different chlorophyll molecules, each with their own absorption spectra, however collectively:

  • Chlorophyll absorbs light most strongly in the blue portion of the visible spectrum, followed by the red portion
  • Chlorophyll reflects light most strongly in the green portion of the visible spectrum (hence the green colour of leaves)

Diagram of a Typical Chlorophyll Molecule

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Pigments absorb light as a source of energy for photosynthesis

  • The absorption spectrum indicates the wavelengths of light absorbed by each pigment (e.g. chlorophyll)
  • The action spectrum indicates the overall rate of photosynthesis at each wavelength of light

There is a strong correlation between the cumulative absorption spectra of all pigments and the action spectrum

  • Both display two main peaks – a larger peak at the blue region (~450 nm) and a smaller peak at the red region (~670 nm)
  • Both display a trough in the green / yellow portion of the visible spectra (~550 nm)

Absorption and Action Spectra

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Photosynthesis is a two step process:

  • The light dependent reactions convert light energy from the Sun into chemical energy (ATP)
  • The light independent reactions use the chemical energy to synthesise organic compounds (e.g. carbohydrates)

Step 1: Light Dependent Reactions

  • Light is absorbed by chlorophyll, which results in the production of ATP (chemical energy)
  • Light is also absorbed by water, which is split (photolysis) to produce oxygen and hydrogen
  • The hydrogen and ATP are used in the light independent reactions, the oxygen is released from stomata as a waste product

Step 2: Light Independent Reactions

  • ATP and hydrogen (carried by NADPH) are transferred to the site of the light independent reactions
  • The hydrogen is combined with carbon dioxide to form complex organic compounds (e.g. carbohydrates, amino acids, etc.)
  • The ATP provides the required energy to power these anabolic reactions and fix the carbon molecules together

Summary of the Overall Process of Photosynthesis

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Photosynthetic organisms do not rely on a single pigment to absorb light, but instead benefit from the combined action of many

  • These pigments include chlorophylls, xanthophyll and carotenes

Chromatography is an experimental technique by which mixtures can be separated

  • A mixture is dissolved in a fluid (called the mobile phase) and passed through a static material (called the stationary phase)
  • The different components of the mixture travel at different speeds, causing them to separate
  • A retardation factor can then be calculated (Rf value = distance component travels ÷ distance solvent travels)

Two of the most common techniques for separating photosynthetic pigments are:

  • Paper chromatography – uses paper (cellulose) as the stationary bed
  • Thin layer chromatography – uses a thin layer of adsorbent (e.g. silica gel) which runs faster and has better separation

Overview of the Chomatographic Separation of Photosynthetic Pigments

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The law of limiting factors states that when

a chemical process depends on more than one essential condition being favourable, the rate of reaction will be limited by the factor that is nearest its minimum value


Photosynthesis is dependent on a number of favourable conditions, including:

  • Temperature
  • Light intensity
  • Carbon dioxide concentration


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

The Effect of Temperature on Photosynthetic Rate

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Light Intensity

  • Light is absorbed by chlorophyll, which convert the radiant energy into chemical energy (ATP)
  • As light intensity increases reaction rate will increase, as more chlorophyll are being photo-activated
  • At a certain light intensity photosynthetic rate will plateau, as all available chlorophyll are saturated with light
  • Different wavelengths of light will have different effects on the rate of photosynthesis (e.g. green light is reflected)

The Effect of Light Intensity on Photosynthetic Rate

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Carbon Dioxide Concentration

  • Carbon dioxide is involved in the fixation of carbon atoms to form organic molecules
  • As carbon dioxide concentration increases reaction rate will increase, as more organic molecules are being produced
  • At a certain concentration of CO2 photosynthetic rate will plateau, as the enzymes responsible for carbon fixation are saturated

Effect of Carbon Dioxide Concentration on Photosynthetic Rate

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Photosynthesis can be measured directly via the uptake of CO2 or production of O2, or indirectly via a change in biomass

  • It is important to recognise that these levels may be influenced by the relative amount of cell respiration occurring in the tissue

Measuring CO2 Uptake

  • Carbon dioxide uptake can be measured by placing leaf tissue in an enclosed space with water
  • Water free of dissolved carbon dioxide can initially be produced by boiling and cooling water
  • Carbon dioxide interacts with the water molecules, producing bicarbonate and hydrogen ions, which changes the pH (↑ acidity)
  • Increased uptake of CO2 by the plant will lower the concentration in solution and increase the alkalinity (measure with probe)
  • Alternatively, carbon dioxide levels may be monitored via a data logger

Measuring O2 Production

  • Oxygen production can be measured by submerging a plant in an enclosed water-filled space attached to a sealed gas syringe
  • Any oxygen gas produced will bubble out of solution and can be measured by a change in meniscus level on the syringe
  • Alternatively, oxygen production could be measured by the time taken for submerged leaf discs to surface
  • Oxygen levels can also be measured with a data logger if the appropriate probe is available

Measuring Biomass (Indirect)

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

Only one significant source of oxygen gas exists in the known universe – biological photosynthesis

  • Before the evolution of photosynthetic organisms, any free oxygen produced was chemically captured and stored

Approximately 2.3 billion years ago, photosynthetic organisms began to saturate the environment with oxygen

  • This led to changes in the Earth’s atmosphere, oceans, rock deposition and biological life


  • Earth’s oceans initially had high levels of dissolved iron (released from the crust by underwater volcanic vents)
  • When iron reacts with oxygen gas it undergoes a chemical reaction to form an insoluble precipitate (iron oxide)
  • When the iron in the ocean was completely consumed, oxygen gas started accumulating in the atmosphere


  • For the first 2 billion years after the Earth was formed, its atmosphere was anoxic (oxygen-free)
  • The current concentration of oxygen gas within the atmosphere is approximately 20%

Rock Deposition

  • The reaction between dissolved iron and oxygen gas created oceanic deposits called banded iron formations (BIFs)
  • These deposits are not commonly found in oceanic sedimentary rock younger than 1.8 billion years old
    • This likely reflects the time when oxygen levels caused the near complete consumption of dissolved iron levels
  • As BIF deposition slowed in oceans, iron rich layers started to form on land due to the rise in atmospheric O2 levels

Biological Life

  • Free oxygen is toxic to obligate anaerobes and an increase in O2 levels may have wiped out many of these species
  • Conversely, rising O2 levels was a critical determinant to the evolution of aerobically respiring organisms

Changes to Oxygen Levels on Earth

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Plants have evolved a specialised organelle responsible for photosynthesis

the chloroplast


The chloroplast contains membrane sacs (called thylakoids) arranged into stacks (called grana)

  • These membrane sacs contain chlorophyll and are the site of the light dependent reactions

The surrounding fluid matrix is called the stroma and contains carbon fixating enzymes


  • This is the site of the light independent reactions

The Chloroplast

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Representations of the Transfer of Energy by Antenna Pigments

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Photosynthetic organisms do not rely on a single pigment to absorb light, but instead benefit from the combined action of many

  • These photosynthetic pigments are grouped into photosystems that absorb and funnel light energy
  • By grouping pigments that have individualised absorption spectra together, the cell maximises its light absorption

When a pigment is energised by light, it releases high energy electrons (ionisation)

  • Antenna pigments transfer their energised electrons to a central reaction centre
  • From the reaction centre, electrons are passed on to an acceptor molecule in an electron transport chain to synthesise ATP

The presence of accessory pigments explains why not all leaves are green

  • While chlorophyll possesses a green colouration, other pigments (e.g. anthocyanins) may produce different colours
  • Deciduous trees change colour when leaves stop producing chlorophyll in winter when levels of available light are low

visible light spectrum

portion of the light spectrum
which is visible to the human eye,
includes wavelengths of 400 nm
to 700 nm



main pigment
involved in the process of
photosynthesis, absorbs light



a substance with colour,
able to absorb light energy in the
process of photosynthesis



instrument used
to carry out chromatography



process used
to separate the comp



pattern, usually
of colours, formed as a result of



distance moved of separate
pigments compared to distance
moved of the solvent, expressed
as a decimal



process in
photosynthesis where water
molecules are split using the
energy from light


action spectrum

a graph
showing photosynthetic rate in
relation to light wavelength


absorption spectrum

a graph
showing absorption of light at
various wavelengths in the process
of photosynthesis