Photosynthesis Revision

A2 Edexcel Biology, Unit 4 photosynthesis revision

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  • Created on: 06-01-13 17:14
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Describe the overall reaction of photosynthesis as requiring energy
from light to split apart the strong bonds in water molecules, strong
hydrogen in a fuel (glucose) by combining it with carbon dioxide and
releasing oxygen into the atmosphere.
Key terms:
Photolysis ­ The splitting (lysis) of a molecule (in this case H2O) using
light (photo)
H2O O2 + 2H+ +2e-
Coenzyme ­ aids the work of an enzyme by transferring a chemical
group from one molecule to another
The importance of photosynthesis:
1. Organic molecules (containing energy ) are passed through food webs
to other organisms
2. Allows plants to release oxygen which organisms need for aerobic
Plants need energy for: Photosynthesis; active transport (take minerals up via
roots); DNA replication; cell division and protein division
Animals need energy for: muscle contraction; maintenance of body
temperature; active transport; DNA replication, cell division and protein
Photosynthesis stores energy in glucose
1) Energy from light breaks bonds in H2O and the hydrogen molecule is
combined with C02, which is reduced to form carbohydrate fuel glucose (this
can be stored or converted to other organic compounds). O2 is released into the
6CO2 + 6H2O + Energy (Photons) C6H12O + 6O2
2) The energy needed to break the bonds is greater than the energy released
when the products are formed
The carbohydrate glucose acts as a store for the remaining energy.
Some of the glucose can be converted into a variety of substances
needed by the plants to grow cells: starch, cellulose, fats, amino
acids/proteins and nucleic acids (Extra elements needed for these
compounds such as nitrogen and phosphorous are taken up by roots of
the plants by the soil).
3) Animals obtain the glucose by eating plants (or other animals)
Cells release energy from glucose by respiration
1) The energy (in plants and animals) is released by respiration when
hydrogen in glucose reacts with oxygen
2) This is aerobic respiration as it uses oxygen, anaerobic respiration doesn't
C6H12O +6O2 6CO2 + 6H2O + Energy (Photons)
Photosynthesis: General outline
Light- dependent: Uses energy from light and hydrogen from hydrolysis of
water to produce reduced NADP, ATP and the waste product oxygen

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Light- independent reaction: Uses reduced NADP and ATP from light
dependent reaction to reduce CO2 to carbohydrate
Coenzyme used: NADP transfers hydrogen from one molecule to another so it
can reduce or oxidise a molecule. It is reduced and oxidised itself when it picks
up and drops off electrons.
Extra fact: CO2 and H2O have no direct contact ­ Hydrogen, electrons and
energy needed for reduction of CO2 is transferred indirectly by reduce NADP
and ATP.…read more

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Describe the light-dependent reactions of photosynthesis including how
light energy is trapped by exciting electrons in chlorophyll and the role
of these electrons in generating ATP, and reducing NADP in
photophosphorylation and producing oxygen through photolysis of
Occurs in thylakoid membrane in a granum
Non-cyclic photophosphorylation (adding phosphate to a molecule using light)
1) Light energy (photons) is absorbed by PSII. This energy exerts
electrons in the chlorophyll to a higher energy level.…read more

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The high-energy electron leaves PSII and passes along a series of
electron carrier molecules (which are embedded in the thylakoid
membrane) towards PSI ­ the molecule and photosystem constitute the
electron transport chain.
3) The electrons pass from one carrier (electron acceptor) to another in a
series of oxidation and reduction reaction, losing energy (that needs to
be replaced), in the process.…read more

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Occurs in the stroma of the chloroplast
Also called calvin fixation as carbon from CO2 is fixed `into' organic
Includes reduction of CO2 - CO2 combined with hydrogen makes a triose
phosphate that can be used to make glucose and other organic
Energy and hydrogen ions keep it going.…read more

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Nucleic acids ­ the sugars in RNA (ribose) is made using triose
The Calvin cycle needs to turn 6 times to make one glucose molecule
3 turns of the Calvin cycle make 6 molecules of GALP (2 GALP for every
1 CO2)
5 out of 6 GALP molecules are used to regenerate RuBP
The other molecule is used to make glucose
2 GALP molecules are needed to form one glucose (as glucose has 6
Therefore 6 turns produce one glucose molecule
6 turns…read more


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