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5.1 Photosynthesis


Need for energy

ð  Autotrophic organisms make organisms compounds from carbon dioxide

ð  Heterotrophic organisms generally eat plants or other animals which have eaten plants, using the products of photosynthesis indirectly

ð  6CO2 + 6H2O à C6H12O6 + 6O2

o   Oxygen is released as a waste product


ð  Constantly available in an accessible form

ð  Found in all living organisms in exactly the same form    

o   Adenosine triphosphate (ATP)

o   Nucleotide with 3 phosphate groups attached

o   The 3rd phosphate bond can be broken by a hydrolysis reaction

§  Catalysed by the enzyme ATPase

§  The result is adenosine diphosphate (ADP) and a free inorganic phosphate

§  Reversible reaction

ð  Making ATP- electron transport chain

o   Synthesised by the removal of hydrogen atoms from several of the intermediates in a metabolic pathway

o   When two hydrogen atoms are removed from a compound they are picked up by a hydrogen carrier and acceptor

o   The acceptor become reduced

o   Electrons are passed along a series of carriers known as an electron transport chain

o   The components are reduced when they receive electrons and oxidised when they pass them on

o   Redox reactions release energy to drive the synthesis of an ATP molecule


ð  Structure

o   The membranes within a chloroplast are arranged in stacks (grana)

o   A granum is made up of stacks of membrane disks (thylakoids) where the green pigment chlorophyll is found

o   The grana are surrounded by a matrix called the stroma

ð  Chlorophyll

o   A group of 5 closely related pigments

§  Include chlorophyll a (blue green) and chlorophyll b (yellow green) and the carotenoids

o   Each of the pigments absorbs and captures light from particular areas of the spectrum

o   Photosystem 1 (PS1) and photosystem 2 (PS2) contain different combinations of chlorophyll pigments so absorbs light in a different area of the spectrum

Light-dependent stage

ð  Only occurs in light and produces materials to use in light-independent stage

ð  Occurs on the thylakoid membranes

ð  Produces ATP and splits water molecules in a photochemical reaction to provide hydrogen ions to reduce CO2

ð  When a photon of light hits a chlorophyll molecule, the energy is transferred to the electrons of that molecule

o   The electrons are excited and raised to higher energy levels

o   Excited electrons are picked up by and electron acceptor  resulting in the synthesis of ATP by cyclic and non-cyclic photophosphorylation

Cyclic and non-cyclic photophosphorylation

ð  Cyclic

o   Involves only PS1 and drives the production of ATP

o   When an electron returns to the chlorophyll molecule in PS1 it can then be excited again

ð  Non-cyclic

o   Involves PS1 and PS2 to


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