Investigation into how light intensity affects the rate of photosynthesis.

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Investigation into how light intensity affects the rate of photosynthesis

Aim:

The aim of this investigation is to determine whether light intensity affects the rate at which plants photosynthesise.

Introduction/Prediction:

All living things need food to survive. Photosynthesis is the chemical process, which takes place in every green plant to produce glucose, the plants food supply. Glucose is used for many different things in the plant; it can be combined with nitrates from the soil to make amino acids, which then in turn can be used to make proteins. Glucose can be turned into cellulose which is used to make strong cells that can support the plant keeping it up right. But most importantly glucose is needed in the growing parts of the plant i.e. shoots, roots, developing buds, flowers and fruits. These areas of the plant need the sugars as a source of energy and as materials in the growth.

The chemical equation for photosynthesis is:

                                                      Sunlight

Carbon dioxide     +     Water                                        Glucose     and      Oxygen

                                                     Chlorophyll

                                                       Sunlight

6CO2                   +       6H2O                                       C6 H12 O6   and      6O2                

                                                     Chlorophyll

Sunlight is trapped by a green pigment called chlorophyll in the chloroplast cells of the leaf. The chlorophyll then turns the light energy it trapped from the sun into chemical energy which is used to join together water and carbon molecules to make glucose (a sugar); oxygen is also produced as a by-product. This glucose can be transported up and down the plant by the phloem (one of the plants veins that transports sugars up and down the stem) to where it is needed.

      If all the glucose is not needed all at once then some of it can be stored until it is needed. Glucose is not an easy substance to store as it is soluble and would cause water to get into the storage cells in the roots, causing them to swell up by osmosis (the movement of water through a semi-permeable membrane) and burst. So instead the plant turns the glucose it does not need into starch, which is insoluble and can be easily stored with out osmosis occurring. This starch can then be used when there is little sunlight around, especially in the winter months.

      Oxygen is also made as a by-product of photosynthesis; some of this oxygen is diffused out of the plant through the stomata a small amount of oxygen is kept and used by the plant in respiration (used to make energy for other functions in the plant).

     The plant is well adapted for making glucose through photosynthesis. Its thin, broad structure gives a large surface area for absorbing as much sunlight as possible. The think waxy cuticle on the surface of the leaf gives a waterproof layer preventing water loss from the leaf. The palisade cells which are packed full of chloroplasts are found near the top of the leaf where they can absorb all the suns light. On the underside of the leaf there are tiny holes called stomata which let carbon dioxide and oxygen diffuse in and out of them which is essential in photosynthesis. In addition they also control how much water is let in and out of the plant, also essential in photosynthesis.  

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Diagram of a cross-section of a leaf.

For photosynthesis to be successful, there are 4 main things the plant needs. They are: Carbon dioxide, Water, light and chlorophyll.

Carbon dioxide

Carbon dioxide enters the leaf through the stomata cells on the underside of the leaf. The layers of palisade and spongy mesophyll cells are full of air pockets; these spaces allow the carbon dioxide to diffuse into the palisade cells and on into the chloroplasts where it can be joined with oxygen to produce glucose.  

Water

Water is obtained through the root ...

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