Our objective is to show experimentally that light is necessary for photosynthesis.
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:
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.
Real Lab Procedure
Simulator Procedure (as performed through the Online Labs)
Note: Oxygen is the by-product of photosynthesis. A higher number of oxygen bubbles indicate a higher rate of photosynthesis, whereas lesser number of oxygen bubbles indicates a lower rate of photosynthesis. You can determine the rate of photosynthesis by altering the power sources (40W, 100W), increasing or decreasing the distance between the power source and the plant (50cm, 100 cm) and varying the colours of the light source (blue, green and red).
We know that starch is one of the end products of photosynthesis and our observation shows that only those areas of the leaf exposed to sunlight turned blue-black on contact with iodine. Since starch turns blue-black on contact with iodine, the portions of the leaf that turned blue-black indicates photosynthetic activity, while the unexposed portion shows the reverse. This clearly indicates that light is essential for photosynthesis.