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Introduction
Photosynthesis is a process that releases energy by capturing sunlight. As a byproduct, it
releases oxygen, which most organisms use to respire.
Light energy is used to synthesise organic molecules such as glucose. The reactions are a
series of enzymecontrolled reactions and anything that affects the rate of enzyme activity
will affect the rate of photosynthesis.
The reactions take place inside the chloroplasts of cells in green parts of plants.…read more

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The LightDependent Reactions
The LDRs occur in the thylakoid membranes, which contain many tightly packed
chlorophyll molecules and in this light is captured by the chlorophyll, which is used to:
Provide energy to produce one molecule of ATP
Split water into two protons, two electrons and half an oxygen molecule
Excite electrons in chlorophyll and provide them with energy
It can be best illustrated in the diagram below:
3…read more

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Light strikes chlorophyll and a pair of electrons become excited (gain energy) and
become so energetic they leave chlorophyll and are picked up by an electron
carrier.
2. Now the electrons are passed down an electron transport chain across the thylakoid
membranes generating a molecule of ATP from the energy released in the redox
reactions. This is known as photophosphorylation.
3. At the same time, a water molecule is split by light into hydrogen ions, electrons and
oxygen in a process known as photolysis.…read more

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O2
N.B. Whilst the diagram above does not depict the different photosystems and the true
nature of photosynthesis, it is all that is required by the AQA specification.
LightIndependent Reactions
The lightindependent reactions do not require light, however they do need the products of
the lightdependent reactions, the ATP and reduced NADP. This means that the
lightindependent reactions stop in the absence of light due to no ATP or reduced NADP
becoming available.…read more

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CO2 combines with a 5carbon molecule known as ribulose bisphosphate (RUBP)
to form two molecules of gylcerate3phosphate (GP).
The ATP and reduced NADP produced in the lightdependent reaction reduce GP
to triose phosphate (TP). The ATP provides the energy and the reduced NADP
provides the hydrogen.
NADP is reformed and is thus recycled and returns to the lightdependent reaction
to accept more hydrogen and electrons.
One sixth of the TP is converted to carbohydrates such as glucose.…read more

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The lightindependent reaction occurs in the stroma, which contains the necessary enzymes
for the reactions to occur.
The chloroplast also contains DNA and ribosomes that allow it to carry out protein
synthesis.
Remember, AQA like to ask where certain reactions occur, so make sure you know
which occurs in the stroma and which in the thylakoids.…read more

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Temperature
As temperature increases, the rate of
photosynthesis increases as it is the
limiting factor and the enzymes that
control photosynthesis will have more
kinetic energy and hence more
enzymesubstrate complexes form.
At the peak this is the optimum
temperature.
Temperatures beyond the optimum
temperature cause the enzymes to
denature or another factor becomes
limiting.
Concentration of Carbon Dioxide
Carbon dioxide is required to carboxylate RuBp and thus provide carbon for building
complex organic molecules.…read more

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Commercial Glasshouses
The limiting factors of photosynthesis can be controlled to:
Achieve higher yields.
Grow crops out of season.
Grow crops that wouldn't usually be able to survive in a certain climate.
The faster the rates of photosynthesis, the more carbohydrates the plants make and hence
the more energy is available for growth and fruit formation.
How to control the limiting factors:
Use artificial lighting.
Light Intensity
Use lighting of specific wavelengths.
Pump CO2 directly in.…read more

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There are however costs involved when controlling the environment and hence it is
only worth doing if the increase in yield outweighs the costs.…read more

Comments

Iqra

Thanks, these notes are really good :)

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