The aim of our investigation is to find out how the amount of light, that is light intensity, affects the rate of photosynthesis.

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Halina Patel

Investigating Photosynthesis

Aims: The aim of our investigation is to find out how the amount of light, that is light intensity, affects the rate of photosynthesis.

Background information: What variables are involved.

Dependant variable: Light intensity, distance, lamp and plant.

Key factors to control:  

  • temperature
  • number of leaves
  • presence of chlorophyll
  • time

Preliminary practical work:

Carbon dioxide + water         glucose + oxygen

           Light and chlorophyll  

      6CO  + 6H O                      C H O + 60

  • Photosynthesis is when green plants produce glucose from sunlight.
  • Light energy, water, chlorophyll, glucose and carbon dioxide are needed for photosynthesis to take place.
  • Sunlight, chlorophyll, water and carbon dioxide are needed during photosynthesis.
  • Sodium hydrogencarbonate increases the levels of carbon dioxide in a plant.
  • It does matter if all the leaves are of different sizes because then the experiment will not be fair and therefore results will not be reliable.
  • We can assume that each leaf is approximately the same size because there is no need to measure each leaf exactly, an estimate would be more preferable within the width of the leaf. Each leaf should have similar sizes to be approximate.
  • The amount of light on a plant may be a limiting factor that means that it is something which is less in the plant and reducing. Not enough light slows down the rate of photosynthesis.
  • The amount of carbon dioxide in the water may be a limiting factor because there is too little of it, it could be decreasing. Too little carbon dioxide also slows the rate of photosynthesis. If carbon dioxide is a limiting factor then either temperature or light intensity may also be a limiting factor.
  • If a lamp is moved to be twice as far away from a plant, then the amount of light reaching the plant would be limiting and the rate of photosynthesis will be limiting and slowing down and there would be a decrease in the rate.
  • A lamp produces heat as well as light.
  • Heat will affect the rate of photosynthesis because it will affect the results/experiment by interfering. The water may warm up and the Elodea will warm up causing anomalous results. The Elodea will warm up; the enzymes in the Elodea will warm up. If enzymes become too hot, they become denatured, they cannot work and the rate of photosynthesis will then stop.

Apparatus:

        I need pond water because Elodea grows in pond water. If Elodea is put in tap water, then the chlorine in tap water might kill the enzymes. I need a beaker because I can put the Elodea and the water into the beaker and I can do my experiment by counting the number of bubbles and the beaker helps me do this and help keep the contents in the beaker. It can help keep the water to act as a water bath for a heat shield for my experiment, which can help reduce anomalous results. I will need a stopclock because I can use this to count the bubbles each minute whilst doing my experiment accurately. I will obviously need Elodea crispa to carry out the experiment successfully using a pondweed to find how light intensity affects photosynthesis. I will use sodium hydrogencarbonate to increase the rate of photosynthesis and the carbon dioxide level the plant is receiving. I will use a spatula to use the sodium hydrogencarbonate on the spatula using an equal amount for a fair test. I will need the boiling tube to hold the Elodea crispa in that so I can carry out the experiment.

Thermometer will be needed to take readings of the water and recording the temperature, but always making sure that the temperature does not exceed above 40    and making sure it is even for a fair test, such as up to 40      . I will use a 200W lamp to use as the amount of light the plant is receiving and which is fair for the experiment to be successfully be carried out. A light shield will be used to protect other sources of light entering the Elodea within which my experiment can be altered. I will use a water bath- to act as a heat shield to ensure that the temperature is kept constant.

Prediction:

I predict that as the light level is raised the rate of photosynthesis increases steadily but only up to a certain point, but beyond that, it won’t make any difference because then it’ll be either the temperature or the carbon dioxide level that is the limiting factor. Also with carbon dioxide it will only increase the rate of photosynthesis up to a point. If light and carbon dioxide were in plentiful supply then the factor limiting photosynthesis would be temperature. Finally if temperature gets too high it will destroy the chlorophyll enzymes.

        The rate of photosynthesis is affected by the amount of light energy to perform photosynthesis. It can only do it as fast as the light energy is arriving. Chlorophyll actually absorbs the red and blue ends of the visible light spectrum, but not the green light in the middle, which is reflected back. This is why the plant looks green.

        It’s affected by the amount of carbon dioxide. Carbon dioxide and water are raw materials. Water is not in short supply in a plant but only 0.03% of the air around is carbon dioxide, so it’s quite scarce.

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        The temperature also affects it. Chlorophyll is like an enzyme so it works best when it’s warm but not too hot. Chlorophyll is NOT like an enzyme, but the mechanism of photosynthesis is controlled by enzymes.

        Four factors affect photosynthesis they are light- the more light there is, the faster the rate of photosynthesis. Water- too little water makes the photosynthesis slow down. Temperature- the optimum temperature is around 30    . Photosynthesis still works fine up to about 40    . But once you get much above 40     it slows down dramatically. Carbon dioxide- if paraffin is ...

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