Investigation to ascertain the extent to which light intensity is a limiting factor to photosynthesis.

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Investigation to ascertain the extent to which light intensity is a limiting factor to photosynthesis.                                                                        

Gina Collier

Abstract

The aim of this investigation is to ascertain the extent to which light intensity is a limiting factor on the rate of photosynthesis of elodea canadensis. The hypothesis was that light intensity will have a significant effect on the rate of photosynthesis. As light intensity increases so too will the rate of photosynthesis. The length of gas bubble produced in five minutes was taken at differing light intensities and temperatures. The results showed that light intensity does have a significant effect on the rate of photosynthesis. As light intensity increased so too did rate of photosynthesis until it was limited by another factor such as temperature. When the temperature was raised, the rate of photosynthesis continued to rise.

Introduction

Null hypothesis:         Light intensity will not have any effect on the rate of photosynthesis in elodea canadensis.

Experimental hypothesis:        Light intensity will have a significant effect on the rate of photosynthesis in elodea canadensis. As light intensity increases so too will the rate of photosynthesis

Biological knowledge to support hypotheses:

In a freshwater environment PFD (wavelengths of sunlight used for photosynthesis) is low for submerged leaves, because light penetration is reduced when passed through the water. At the surface there is unobstructed full sun for a photosynthetic organ floating. An emergent canopy may intercept high PFD, which may be harmful to the plant. The concentration of carbon dioxide and oxygen dissolved in water is low. Minerals and nutrients are scarce or dilute within the water medium, when compared with drier soil.

Elodea canadensis is a leafy submerged aquatic hydrophyte originating from North America. It is commonly found in still or slow flowing waters in various locations around the state. Elodea thrives in temperate climatic zones and grows prolifically during summer once water temperatures exceed 15°C

Elodea canadensis is also known by several other common names such as Canadian waterweed, common elodea, or anacharis. It has been introduced to several countries where it is not native, and is now considered a noxious weed in those regions (parts of Europe, Australia, Africa, Asia, and New Zealand).  

Elodea canadensis lives entirely underwater with the exception of small white flowers which bloom at the surface and are attached to the plant by delicate stalks. It produces winter buds from the stem tips which over winter on the lake bottom. In the autumn leafy stalks will detach from the parent plant, float away, root, and start new plants.1 

A hydropyhte is a plant that has adapted to live partially or totally submerged in water. They have no osmotic problems in freshwater because of the pressure potential caused by the cell walls. Elodea canadensis has only submerged leaves arranged three per whorl, which have many specialised features. There are a large number of small leaves which are long and cylindrical. The leaves are quite delicate so that there is less resistance to movement of water to reduce damage.

Other specialised features of the plant are that there are large intercellular spaces which form air filled cavities extending throughout the plant to provide internal air passages for gas exchange and a carbon dioxide store for photosynthesis. Bubbles are produced from these when the plant is photosynthesising rapidly. Hence when the stem is cut bubbles will come from that point. The cavities provide buoyancy to keep the plant upright in the water so that the leaves can obtain maximum light for photosynthesis.

There is an absence of mechanical and vascular tissue and root systems, this lack of rigidity enables bending with water currents.2 

Literally, photosynthesis means 'synthesis with the help of light.' This covers a variety of processes. However, the term is usually applied to one reaction only -the synthesis of organic matter by plants in light- a process also called 'carbon assimilation.' This is a fundamental process of life. It creates living material out of inert inorganic materials, replenishes our supply of oxygen in the atmosphere and stores energy provided by the sun to support life activities of organisms.3 

        The equation for this process is:

                        6CO2  +  6H2O  => C6H12O6  +  6O2

                     Carbon dioxide   + water -------- glucose   + oxygen 

The extent of photosynthesis performed by a plant depends on a number of factors both internal and external. The external ones are: light intensity; ambient temperature; concentration of carbon dioxide. These are the ones this experiment is concerned with.

Photosynthesis put simply is this; the capture of the energy of a photon of light by a pigment molecule, in this case chlorophyll, the formation of an electronic 'excited' state, the use of the 'excited' electron to reduce a chemical substance and to form energy rich molecules. These energy rich molecules are used to form other more complex organic molecules4.

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At low light intensities the rate of photosynthesis is low. This is because the electrons are receiving a smaller amount of energy and so fewer of them reach the 'excited' state. When the intensity is increased, the rate of photosynthesis will increase until the saturation point is reached. This is the point when the rate cannot go any higher. A factor other than light intensity is limiting it. A limiting factor is any variable that prevents the reaction from exceeding a particular level.5

Temperature affects the rate of photosynthesis in a different way. As temperature increases, more energy is provided. ...

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