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Photosynthesis Revision
Photosynthesis is the process by which green plants manufacture carbohydrates using theraw
materials carbon dioxide and water using energy from sunlight
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6CO2 +6H2O >>>> C6H12O6 +6O2
Photosynthesis takes place in
the chloroplast found mainly
in the palisade mesophyll
layer of the leaves
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 Xanthophylls= yellow, carotenes= red
-Primary and Accessory Pigments
 Chlorophyll a is primary pigment, chlorophyll b and carotenoids are accessory pigments
 Primary pigment drives chemical reactions for photosynthesis
 Accessory pigments pass light energy they absorb to primary pigments- can pick up light at
various wavelengths to increase efficiency of absorption
Pigments are chemicals that absorb visible light and this causes electrons on the 3d orbital to
become excited from the ground state to an excited state as they absorb energy
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The energised electrons leave the pigment
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Flouresced
light has less energy and longer wavelength
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 It is in the reaction centre that chlorophyll a (P680 or
P700) produces an electron leaving +ve holes in the molecule
 Chlorophyll >>>> Chlorophyll+ + e Each electron lost is accepted by an electron
acceptor, so a redox reaction occurs
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Energy given out is used to form ATP
 Meanwhile, light falls on PSI, causing electrons to be lost from P700 to an acceptor Y, which
is reduced
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Electrons from PSI cycle back to it by
feeding electrons through chain of electron carriers
Light dependent reaction produces ATP, NADPH2, and oxygen
ATP provides energy to drive light independent reactions whilst the NADPH 2 provides the reducing
power needed to convert carbon dioxide into carbohydrate
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This distorts the equilibria so more electrons are produced using OH -, so more
water splits
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Photolysis increases electrochemical gradient as there are more protons
in the thylakoid compartment
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To convert the glycerate- 3- phosphate into carbohydrates a reduction reaction is required;
the reducing power is sourced from NADPH2
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The 3C molecules formed are triose phosphate
So products of light dependent reaction are used to power the light independent reaction
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 This is the calvin cycle
Calvin researched into dark step using chromatography and radioactive carbon

Factors Affecting Photosynthesis;
 Light intnesiy, Light wavelength- Increased excitation of electrons so more ATP is
synthesised and more NADP is reduced by Z-scheme in a give time
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 Temperature- Enzymes and substrates gain energy so move around more due to increased
KE
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 Carbon Dioxide Concentration- More carbon dioxide is available to react with the ribulose
biphosphate to produce glycerate- 3 phosphate and then triose phosphate
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If its quantity is changed it
will directly affect the process
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At low light intensity
the limiting factor of photosynthesis is the light dependent reaction, at high light intensity the
limiting factor is respiration

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