Experiment to Investigate the Effect of Temperature on the Rate of Photosynthesis in Elodea.

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Phil Cooper

02.08.02

Experiment to Investigate the Effect of Temperature on the Rate of Photosynthesis in Elodea

Aim

The aim of this experiment is to investigate the effect of temperature on the rate of photosynthesis in Elodea (Canadian pondweed).

Introduction

Elodea is native to North America and is naturalized in Australia, Asia and Europe. It grows completely submerged beneath the water. It has multi-branched, slightly brittle stems that are clothed with whorls of sessile, medium green, pointed leaves.

Elodea grows better in cooler water. In temperatures above 70º F, it becomes spindly. This plant can be grown under 8 to 10 feet of water and will easily reach the surface of the average water garden. This plant derives most of its nourishment from the water through its leaves; its roots serve mainly to anchor it to the bottom, therefore, they may be planted in sand or pea gravel as well as soil. They may be anchored down and just dropped into the pond or planted in containers of sand or gravel. If they are only floated in the pond, too much sunlight and air will kill it.

Photosynthesis is a process which occurs in all green plants, synthesising glucose as a form of energy to be used by the plant.  Glucose (and oxygen – a waste product) are produced from a reaction between carbon dioxide and water.  The general equation for photosynthesis is shown below.

6CO2 + 6H2O              6O2 + C6H12O2

To get from the reactants to the products, there are complex processes.  The processes are the light dependant stage and the light independent (Calvin cycle) stage.  These two stages are on a cycle and rely on each other.  Both processes are essential for photosynthesis.  

Variables

The rate of photosynthesis can be measured as the volume of carbon dioxide taken in by a plant per unit time.  In laboratory investigations the rate is often estimated as the volume of oxygen released per unit time, which is more easily measured.  However this is not an accurate method of  measuring the rate of photosynthesis.  This is because some of the oxygen produced by the plant in photosynthesis is then used in the plants respiration.  Respiration is ongoing within plant cells, therefore oxygen liberation from a plant does not measure all the oxygen produced during photosynthesis.  The rate of photosynthesis is measured above a point called the compensation point.  This is the point at which the rate of photosynthesis in a plant is in exact balance with the rate of respiration, so there is no net exchange of carbon dioxide or oxygen.’

The rate of photosynthesis is limited by the factor in the shortest supply.  The law of limiting factors states that when a physiological process depends on more than one essential factor being favourable, its rate at any given moment is limited by the factor at its least favourable value and by that factor alone.  Increasing the factor in shortest supply will increase the rate of photosynthesis, until another factor becomes limiting.  If we supply the plant with all of the optimum conditions it needs, the rate of photosynthesis will be limited by factors within the plant, such as the number and location of chloroplasts.  Photosynthesis is affected by many factors, both internal and external.

External factors include:

  • Light intensity – The rate of photosynthesis is directly proportional to the light intensity.  The graph below illustrates the relationship between light intensity and rate of photosynthesis.

      (Cambridge Biology 1)

The graph levels off because the photosynthetic pigments have become saturated with light, and there is a limiting factor to prevent the reaction from going any faster.  

The light intensity at which the rate of photosynthesis is balanced by the rate of respiration is called the light compensation point.  This point varies for different plants, depending on whether the plant is classified as a sun plant or a shade plant.

Light intensity only affects the light dependant stage of photosynthesis, but this in turn affects the light independent stage.  Light is the driving factor in photosynthesis, as with no light photosynthesis would not take place.  The more light there is, the more light is absorbed by the two photosystems.  The means a greater amount of electron excitation and consequently more ATP and NADPH is synthesised in the light dependant reaction.  Due to the increased amounts of ATP and NADPH, GP is converted to GALP more frequently and the rate of this process is increased, in turn increasing the synthesis of glucose.

The light intensity will be controlled in this experiment by keeping the lamp the same distance from the Elodea at all temperatures, ensuring the light intensity is constant throughout the experiment.  This actual distance will be determined after the preliminary investigation.

  • Carbon dioxide levels – the concentration of carbon dioxide in the air is 0.04%.  An increase in carbon dioxide concentration up to 0.5% usually results in an increase in the rate of photosynthesis.  However the leaves of the plant can be damaged in air containing more than 0.1% carbon dioxide, so the optimum rate of photosynthesis occurs when the concentration of carbon dioxide in the air is just below 0.1%.  The graph below illustrates the effect if carbon dioxide levels on the rate of photosynthesis.  Carbon dioxide is involved in fixing wit RuBP, therefore the greater the concentration of carbon dioxide in the air the greater the rate of fixation.  The demand for ATP and NADPH will be higher because the rate oh production of GP is higher.  Because of this the light dependant reaction will take place at a greater to rate to cope with the demand.  The concentration of carbon dioxide will be kept constant by keeping constant the concentration of the hydrogen carbonate solution constant throughout the experiment.  The actual concentration to be used in the experiment will be determined after the preliminary investigation.  Below is a graph showing the effect of carbon dioxide concentration on the rate of photosynthesis:

 

   (Cambridge Biology 1)

  • Temperature – The light-dependant stage of photosynthesis is hardly affected by changes in temperature because this stage is driven by light as opposed to temperature.  However temperature affects the Calvin cycle (light independent).  Reactions in the Calvin cycle are catalysed by enzymes, which are affected by temperature. If the light independent reaction slows down, the demand for ATP and NADPH decreases, slowing down the light dependant stage.  Temperature has a big effect on enzyme-controlled reactions.  As with all enzymes, there is an optimum temperature at which they work.  As plants are not usually exposed to very high temperatures, enzymes which work at an optimum of around 400C, such as those in mammals, would be inefficient.  The optimum temperature for enzymes within plants is around 300C.  Up to this temperature, the rate of reaction increases, and then starts to decrease fairly rapidly from this temperature to the point of the enzyme being denatured.  For every 100C rise in temperature up to the optimum, the rate of reaction approximately doubles.  This is known as the Q10 value.  The reason behind this is that for every 100C rise in temperature, the molecules have twice as much kinetic energy.  This causes the molecules to move around twice as fast, increasing the chances of a collision between enzyme and substrate molecules by two.  Also, when they do collide, the chance of fixation is double because the force of the collision is twice as great.  In this case, the substrate molecules (RuBP and carbon dioxide) and the enzyme molecules (rubsico) gain more kinetic energy with the increase in temperature.  As the temperature goes beyond the optimum temperature of the enzyme, the rate of reaction decreases.  This is because the kinetic energy is too great and some of the bonds within the enzyme start to break.  As the temperature is further increased more of the internal bonds are broken.  This causes the shape of the active site to deform so the substrate molecules can no longer fit it.  This is the variable that is going to be investigated in the experiment.  The way in which the temperature will be varied will be discussed in the method later on in the investigation.

  • Water availability – Water is essential in photosynthesis to keep the stomata open.  If there is not enough water, the stomata will close up, depriving the plant of the carbon dioxide it needs for photosynthesis.  Water is involved in photolysis, therefore the more water there is, the greater the amount of hydrogen ions produced from the photolysis of water.  Hydrogen ions reduce NADP to NADPH, so the greater the amount of water, the more NADPH is produced.  If the amount of water available doubles, twice as much NADPH and ATP is produced, therefore the reaction takes place at double the speed.

  • Light wavelength – An action spectrum is the rate of a physiological activity plotted against wavelength of light.  In 1881, the German plant physiologist T. W. Engelmann placed a filamentous green alga under the microscope and illuminated it with a tiny spectrum of visible light.  In the medium surrounding the strands were motile, aerobic bacteria. After a few minutes, the bacteria had congregated around the portions of the filament illuminated by red and blue light. Assuming that the bacteria were congregating in regions where oxygen was being evolved in photosynthesis, Engelmann concluded that red and blue light are the most effective colours for photosynthesis.  Below are diagrams of an action and absorption spectrum.  The action spectrum shows which wavelengths of light are most used in the process of photosynthesis.  The absorption spectrum shows the wavelengths of light which are most absorbed by the plant.
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Action spectrum                                        Absorption spectrum

The light wavelength will be controlled during the experiment by using the same             lamp at the same distance from the Elodea each time.  The lamp will be plain ‘white’  light with no coloured filter.

  • Enzyme concentration – Reactions in the light independent reaction are catalysed by enzymes.  In this case, rubisco is the enzyme and RuBP and carbon dioxide are the substrates.  At low concentrations of the enzyme, there are more substrate molecules than enzyme molecules.  This means that all the active sites are filled with substrate molecules, ...

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