Photosynthesis Reaction As Co 2 example essay topic

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 (1900s) 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 photosynthesis, 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 sho 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 C 6 H 12 O 6. Oxygen can also be used as a tracer with different outcomes depending on whether it is put into the photosynthesis reaction as CO 2 or H 2 O. If the Oxygen tracer is inserted as part of the H 2 O it will become part of the starch molecule created via photosynthesis.

If the oxygen is part of the CO 2 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 Fe 3+ but it is easier to identify reduction using DCPIP (dichlorophenolindophenol) which changes colourless from its natural blue colour when reduced.

Hills 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 Bass ham 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 CO 2 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 CO 2 molecule to the RuBP briefly makes it a six-carbon sugar but it quickly splits into two three-carbon molecules glycerite 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.