Investigation Into the Factors Affecting the Rate of Photosynthesis

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INVESTIGATION INTO THE FACTORS AFFECTING THE RATE OF PHOTOSYNTHESIS

AIM- To identify the factors affecting the rate of photosynthesis and, choosing one factor, to ascertain the effects it has.

Photosynthesis is an endothermic reaction that occurs in plants, by which plants use light energy to make glucose. It needs energy from the photons of light and it is their anabolic effect on the plant that gives the energy for the reaction to take place. During this process carbon dioxide combines with water to from glucose, and oxygen is released. The glucose made then has many uses in the plant: respiration, making ATP, active uptake….

 Sunlight and chlorophyll must be present for the reaction to take place, and the light is trapped in the chlorophyll:

                           

       sunlight

Carbon dioxide + Water                    Glucose + Oxygen.

                   Chlorophyll

The amount of oxygen given off is an indication of the rate of photosynthesis. The more oxygen being given off, clearly the faster the rate of the reaction, and the more photosynthesis occurring / the faster the rate of photosynthesis.

POSSIBLE VARIABLES-

from background research and previous experiments I know the following variables/ limiting factors to affect the rate of photosynthesis:

  • Light Intensity – the basic energy source
  • Temperature- increases enzyme reactions until the point of denature.
  • Water- a basic reagent- a lack of water also causes stomata to close inhibiting diffusion of CO2 in and out of the leaf.
  • Chlorophyll- this is what traps the light energy for the reaction
  • Carbon dioxide – the more CO2  in the air, the more that can diffuse into the leaf to be a basic reagent for the photosynthesis reaction.

Of these variables I have chosen to investigate light intensity because there are various reasons why other variables would not be suitable:

  • Temperature-  this variable is not specific to increasing the rate of photosynthesis, but rather to general rates of reaction, as I have seen in previous experiments into reaction rates.

  • Water- this would be too difficult to control as lowering the water levels too much would kill the plant and ruin the investigation.

  • Chlorophyll- again this variable would be too hard to control, as we could not get a whole range of results. Leaves come in variegated form, where parts either contain chlorophyll or they don’t. There is no way with our basic equipment to ascertain precise chlorophyll levels in the plant leaves.

  • Carbon Dioxide-   again with this variable there is either carbon dioxide present or not (adding soda lime). It would be very difficult to obtain or measure precise carbon dioxide levels in the air, or keep that environment from contamination of normal carbon dioxide levels.

  • I Chose light intensity-  as it is possible to vary this more (resulting in a range of results) by increasing distances between the plant and the lamp gradually to diminish light intensity. Also light is the key variable for photosynthesis- without it no photosynthesis would occur as there would be no energy source.

 

My aim therefore is to investigate the effect of light intensity on the rate of photosynthesis by varying the distance of a lamp from pondweed and measuring the volume of Oxygen given off.

OUTPUT VARIABLE-   the volume of oxygen given off.

PREDICTION-  I predict that as light intensity increases (distance from bulb decreases) so will the rate of photosynthesis increase. Light is a key factor of photosynthesis and without it plants cannot get enough energy to make glucose. Light intensity itself is directly proportional to the rate of photosynthesis as the more light energy a plant receives and traps in the cholorophyll, the more it can produce and so doubling energy in = doubling energy out.

HOWEVER………

From scientific research I know that the relationship between light intensity and distance is:

Light intensity = 1/ d2

 This shows that light intensity is inversely proportional to the distance squared because the light energy spreads out as it travels further away from its light source (ie as distance increases).

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 This is because light energy travels along the circumference of an expanding circle. As the circle expands and distance becomes greater, this causes the light intensity to decrease as the same amount of light energy must be equally dispersed over a larger area/ circumference. This is not a linear relationship because doubling the distance causes the spreading out light energy to reduce by more than a half as the circumference of a circle = 2    r and this is not a linear quality. Also the equation backs this up, as it is a quadratic quality.

Therefore by doubling the ...

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