Colours Chloroplasts contain several different pigments the majority
of which are chlorophyll a and
chlorophyll b. Both of these types of
chlorophyll absorb similar wavelengths of light, but chlorophyll a absorbs a slightly higher wavelength
than chlorophyll b. Neither
chlorophyll absorbs much light in the green region of the spectrum thus making
it appear green. This can be plotted onto a graph (fig 1.1). This graph is
called the Absorption Spectrum. The Absorption Spectrum is very similar in
shape to the Action Spectrum (fig 1.2). The Action Spectrum is a graph showing
rate of photosynthesis with different wavelengths of light. This is evidence
that light energy absorbed by the pigments in chlorophyll a and chlorophyll b
is used in photosynthesis. Blackman (1900?s) In 1905 F.F. Blackman measured the rate of photosynthesis
under a variety of different conditions of light and carbon dioxide supply. His
work lead him to formulate the principal of limiting factors which states ?At
any given moment, the rate of a physiological process is limited by one factor
which is in shortest supply, and by that factor alone.? This means that the
factor which is nearest its minimum value determines the rate of reaction. It
is therefore known as the limiting factor. It is the only thing, which affects
the reaction. An example of this is a plant with a good supply of carbon
dioxide and at a high temperature but in the dark will not photosynthesise, as
the light intensity is the limiting factor. If the light intensity is increased
then the rate of photosynthesis will increase to a level until one of the other
factors is the limiting factor. These changes are illustrated in the graph
Blackman created. (fig 2.1) Radio Tracers Radioactive tracers can be used to show the path of certain
chemical elements within a reaction. For example a radioactive carbon can be
given to a plant as carbon dioxide and will then be changed via photosynthesis
into radioactive starch thus proving that the carbon in the carbon dioxide has
been used in the C6H12O6. Oxygen can also be
used as a tracer with different outcomes depending on whether it is put into
the photosynthesis reaction as CO2 or H2O. If the Oxygen
tracer is inserted as part of the H2O it will become part of the
starch molecule created via photosynthesis. If the oxygen is part of the CO2
it will be given off as a waste product after the reaction with water to create
starch. Robert Hill (1939), The Hill Reaction In 1939 Robert Hill showed that isolated chloroplasts had
?reducing power? this, meant that that they could remove oxygen from water in
the presence of an oxidising agent. The reducing power was illustrated by using
a redox agent which is an agent used in a redox reaction. A redox reaction is a
reaction that involves the transfer of electrons from a reducing agent to an
oxidising agent. Hill substituted the plants NADP with Fe3+ but it
is easier to identify reduction using DCPIP (dichlorophenolindophenol) which
changes colourless from its natural blue colour when reduced. Hill?s summarised
his experiments into an equation. (fig 3.1 using DCPIP as the acceptor) Melvin Calvin (1946-53), The Calvin Cycle Between 1946 and 1953 three scientists Calvin, Benson and
Bassham examined the chemical changes in Chlorella a single celled algae as
photosynthesis began and stopped. The result they collected formed the basis of
the Calvin Cycle. In the Calvin Cycle hydrogen is added to carbon dioxide to
make carbohydrates. The hydrogen comes from reduced NADP and the energy needed
to drive these reactions comes from ATP and reduced NADP. These two products
were made in the light dependant reactions. The main stages of the Calvin Cycle
are shown in fig 4.1. The CO2 from the air diffuses through the
stomata into the leaf, into the air spaces in the mesophyll, into a palisade
cell and into the chloroplast. Here in the stroma it comes into contact with
the most abundant enzyme on the planet called ribulose bisphosphate carboxylase
or RuBP carboxylase or Rubisco. Rubisco catalyses a reaction between the carbon
dioxide and a five-carbon sugar called ribulose bisphosphate, or RuBP. The
addition of this CO2 molecule to the RuBP briefly makes it a
six-carbon sugar but it quickly splits into two three-carbon molecules
glycerate 3-phosphate, or GP. This is the point when the products of the light
dependant reactions the reduced NADP and the ATP are needed. These provide the
energy to reduce the GP into triose phosphate. Triose phosphate is a three-carbon
phosphorylated sugar. About on sixth of this triose phospahte is then used to
make other carbohydrates. The rest is converted back to RuBP to prevent the
plant running out of RuBP. It is this regeneration which makes the process a
cycle. Bibliography Central Concepts in Biology?Cambridge
A-Level Biology Philips & Chilton
Advanced Biology Jones & Jones
Understanding Biology for Advanced Level – Toole & Toole
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