Experiments Which Explain Photosynthesis Essay Research Paper

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