Photosynthesis Topic Revision

Revision notes I made for my 2015 topic exam on Photosynthesis in Biology.

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  • Created by: hgrattan
  • Created on: 07-12-15 21:38
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The carbon dioxide enters the plant by stomata (in the leaves) or lenticels (in the stem). The water enters through
the roots and moves up through the plant in xylem vessels. Light is required as an energy source for the reaction
and chlorophyll is necessary to absorb the light.
The glucose made may be used to make other substances or it may be used in respiration. The oxygen may diffuse
out through the stomata or it may be used in respiration. Light energy is necessary but it must be harvested and
trapped by the photosynthetic pigments to be of any use.
The action spectrum shows the rate of photosynthesis at different wavelengths.
The absorption spectrum shows how strongly the pigments absorb at different wavelengths.
The absorption spectrum and action spectrum show that the wavelengths that are most strongly absorbed (red
and blue) are the ones that cause photosynthesis to proceed at the fastest rate. Green is not strongly absorbed;
rather it is reflected, causing leaves to look green.
The shorter the wavelength, the more energy it contains. During photosynthesis the light energy is converted into
chemical energy. The absorbed light excites electrons in the pigment molecules and the energy can be passed on
to be used by the plant.
The chloroplasts are the organelles where the two stages of photosynthesis take place.
The granum (plural grana) is a stack of membranes which contains the photosynthetic pigments; this is
where the light dependent stage takes place
The stroma is the fluid part of the chloroplast which contains the enzymes controllingthe carbon fixation

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Light dependant reactions:
1. Energy from light raises 2 electrons in each chlorophyll molecule to a higher energy level. The electrons
then leave the excited chlorophyll molecules.
2. The electrons pass along the embedded electron transport chain in the thylakoid membranes.
3. The electrons pass from one carrier protein to the next in a series of oxidation and reduction reactions,
losing energy in the process.
4.…read more

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Note:No ATP has been made during this process.
Cylic Photophosphorylation
However, if there is plenty of NADPH, something different happens.
The electron from PS I is passed to the electron carriers used in PS II. ATP is formed and the electrons return to PS
I to fill the space. This makes PS II redundant as no electrons are needed from there to fill the space in PS I. Only
PS I is active. This is called cyclic photophosphorylation.
To summarise, in cyclic photophosphorylation...…read more

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The rest of the
GALP's are used to regenerate the RuBP using the phosphate from ATP.…read more


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