Pigments

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Pigments

• A pigment is a molecule which absorbs some
  wavelengths (colours) of light but not others.
  The wavelengths it does not absorb are either
  reflected, or transmitted through the substance.
  These unabsorbed wavelengths enter our eyes, so
  we see the pigment in these colours.
  Chloroplasts of higher plants contain two
  different groups of pigments chlorophylls and
  carotenoids.

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Chlorophylls

• Chlorophylls absorb mainly in the red and blue
  violet regions of the light spectrum. They
  reflect green light, which is why plants look
  green. Carotenoids absorb mainly in the
  blue-violet region of the spectrum so look
  yellow/orange.

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Structure of chlorophylls

• Chlorophylls consist of a head and a tail. The
  head contains a porphyrin ring centered on
  magnesium (in many ways the porphyrin structure
  of chlorophyll resembles the structure of the
  haem group in animal blood). The porphyrin ring
  is attached to a long hydrocarbon tail. The
  different chlorophylls differ in the side chains
  on the head of the molecule (chlorophyll a has
  CH3 as a side chain whereas in chlorophyll b
  this side chain is CHO). The different side
  chains affect the absorption of the chlorophylls.

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The structure of chlorophyll a
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Structure of Chlorophyll b
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Primary pigments and accessory pigments

• The photosynthetic pigments fall into two groups
  primary pigments and accessory pigments. The
  primary pigments are two forms of chlorophyll a
  with slightly different absorption peaks. The
  accessory pigments include other forms of
  chlorophyll a and b and the carotenoids.

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Accessory Pigments

• Accessory pigments help in absorbing wavelengths
  of light which otherwise could not be used by the
  plant, and passing on the energy from this light
  to chlorophyll. So with different types of
  pigments in a chloroplast, a wider range of
  wavelengths of light can be used in
  photosynthesis than with any one pigment alone.

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Absorption spectrum

• A graph of the absorbance of different
  wavelengths of light by a pigment.

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Action spectrum

• If you supply a plant with light of a single
  wavelength and measure the rate of
  photosynthesis, you find that the rate is
  greatest with wavelengths around 680 to 700 nm,
  with another, lower, peak at around 460 nm. A
  graph of rate of photosynthesis against
  wavelength is called an action spectrum.

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• The shape of the action spectrum is very similar
  to the combined shapes of the absorption spectra
  of the individual pigments. This similarity can
  be taken as evidence that the light energy
  absorbed by the pigments is used in
  photosynthesis.

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Redox reactions

• Oxidation is the loss of electrons by a
  substance. When oxygen combines with hydrogen to
  form water (hydrogen oxide), oxygen takes
  electrons from hydrogen. The hydrogen is
  oxidised as it loses electrons. Oxygen is
  called an oxidising agent because it has a strong
  tendency to take electrons from other substances.
  Other oxidising agents are NAD and NADP.

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• Reduction is the gain of electrons by a
  substance. In the above example, oxygen has
  gained electrons from hydrogen and the oxygen
  becomes reduced as a result. If a substance has
  a strong tendency to lose electrons and donate
  them to other substances, it is called a reducing
  agent. Reduced NADP (NADPH) and reduced NAD
  (NADH) are reducing agents

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• Oxidation and reduction always takes place
  simultaneously, and a reaction in which they
  occur is called a redox reaction.
• In biological redox reactions, hydrogen atoms are
  often involved. A hydrogen atom is, in effect, a
  hydrogen ion and an electron. Gaining a hydrogen
  atom means gaining a hydrogen ion and an
  electron, so this is an example of reduction.
• During some of the stages of respiration, the
  coenzyme NAD accepts hydrogen ions and electrons,
  and becomes reduced