Importance of Light in Photosynthesis


Our objective is to show experimentally that light is necessary for photosynthesis.

The Theory

Photosynthesis is the process in which light energy is converted into chemical energy. Using the energy of light, carbohydrates such as sugars are synthesised from carbon dioxide and water.

The name photosynthesis is derived from the Greek words, photo for ‘light’ and synthesis meaning ‘putting together’. Oxygen is also released, as a waste product. Light is the major factor for photosynthesis to take place and by doing this experiment we need to prove that light is necessary for photosynthesis.

The Process of Photosynthesis

The process of photosynthesis occurs when green plants use the energy of light to convert carbon dioxide (CO2) and water (H2O) into carbohydrates. Light energy is absorbed by chlorophyll, a photosynthetic pigment of the plant, while air containing carbon dioxide and oxygen enters the plant through the leaf stomata. An extremely important by-product of photosynthesis is oxygen, on which most organisms depend.

Glucose, a carbohydrate processed during photosynthesis, is mostly used by plants as an energy source to build leaves, flowers, fruits, and seeds. Molecules of glucose later combine with each other to form more complex carbohydrates such as starch and cellulose. The cellulose is the structural material used in plant cell walls. Photosynthesis provides the basic energy source for virtually all organisms.

We can express the overall reaction of photosynthesis as:

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Where does Photosynthesis occur?

Photosynthesis takes place primarily in leaves and little to none occurs in stems. It takes place within specialised cell structures called chloroplasts. A leaf has a petiole or the stalk and a lamina, the flat portion of the leaf. As its area is broad, the lamina helps in the absorption of sunlight and carbon dioxide during photosynthesis. Photosynthesis takes place in the chloroplasts that have chlorophyll present in them. It is the chlorophyll that absorbs light energy from the sun. There are tiny pores called stomata that function as roadways for carbon dioxide to enter and oxygen to leave the plant.

Role of the colour of light during Photosynthesis

Did you know that the colour of light plays an important role during photosynthesis? Yes, it does. Plants use only certain colours from light for the process of photosynthesis. The chlorophyll absorbs blue, red and violet light rays. Photosynthesis occurs more in blue and red light rays and less, or not at all, in green light rays.

The light that is absorbed the best is blue, so this shows the highest rate of photosynthesis, after which comes red light. Green light cannot be absorbed by the plant, and thus cannot be used for photosynthesis. Chlorophyll looks green because it absorbs red and blue light, making these colours unavailable to be seen by our eyes. It is the green light which is not absorbed that finally reaches our eyes, making the chlorophyll appear green.

Factors affecting Photosynthesis

For a constant rate of photosynthesis, various factors are needed at an optimum level. Here are some of the factors affecting photosynthesis.

  • Light Intensity:An increased light intensity leads to a high rate of photosynthesis and a low light intensity would mean low rate of photosynthesis.
  • Concentration of CO2: Higher carbon dioxide concentration increases the rate of photosynthesis. Normally the carbon dioxide concentration of 0.03 to 0.04 percent is sufficient for photosynthesis.
  • Temperature:An efficient photosynthesis requires an optimum temperature range between 25 to 35oC.
  • Water: Water is an essential factor for photosynthesis. The lack of water also leads to a problem for carbon dioxide intake. If water is scarce,  the leaves refuse to open their stomata to keep water they have stored inside.
  • Polluted Atmosphere:The pollutants and gases (impure carbon) settle on leaves and block the stomata, making it difficult to take in carbon dioxide. A polluted atmosphere can lead to a 15 percent decrease in the rate of photosynthesis.

Learning Outcomes

  • Students understand the concept that light is necessary for photosynthesis.
  • Students understand the principle of photosynthesis and the factors affecting photosynthesis.
  • Students will be able to do the experiment more accurately in the real lab once they understand the steps through the animation and simulation.