Theory & Procedure, Importance of Light in Photosynthesis Class 10 Notes | EduRev

Objective

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:

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.

Materials Required

Real Lab Procedure

• Take a healthy potted plant and keep it in a dark place for 2-3 days, this de-starches the leaves.
• Cut out two uniform strips of black paper.
• Select a healthy de-starched leaf and cover a portion of the leaf on both sides using these two uniform pieces of black paper.
• Fix both the ends of the black paper to the leaf using paper clips.
• Now, place the potted plant in sunlight for a whole day.
• Pluck the covered leaf in the late afternoon and remove the black paper from the leaf.
• Take a beaker containing distilled water and place it over a hot plate and boil the water.
• Put the experimental leaf in to the boiling water and boil it for 5-10 minutes till the leaf becomes soft.
• Remove the beaker from the hot plate and allow it to cool for some time.
• Take another beaker containing distilled water and place it over the hot plate and boil it at about 60oC.
• Pour some alcohol into a clean boiling tube.
• Place the boiling tube in the beaker with water that is being boiled.
• Remove the leaf from the beaker using a forceps.
• Place the leaf in the boiling tube containing alcohol.
• Keep the boiling tube in the beaker till the leaf becomes colourless.
• Remove the leaf from the boiling tube using the forceps.
• Dip the leaf in a beaker containing distilled water and wash it.
• Now place the leaf in a Petri dish.
• Using a dropper take few drops of iodine solution and pour this drop by drop on the leaf.

Simulator Procedure (as performed through the Online Labs)

• To increase or decrease the watts of the power source use the ‘Select the power source’ drop down list.
• You can change the distance between the plant and the power source using the ‘Select the distance of the power source’ drop down list.
• The colour of the light can be changed from the ‘Select the colour of the filter’ drop down list.
• Click on the ‘Start’ button to start the experiment.
• You can determine the rate of photosynthesis by count the number of oxygen bubbles rising from the plant.
• Click on the ‘Stop’ button to stop the experiment.
• Once you stop the experiment, you will see tool tips that give you the number of bubbles that were produced by the plant and the time take.
• Below is given a worksheet were you can enter the readings and compare it with different conditions.
• To redo the experiment, click on the ‘Reset’ button.

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).

Observation

• After iodine treatment, the colour of the exposed portion of the leaf turns blue-black.
• The colour of the unexposed portion of the leaf turns pale yellowish brown.

Inference

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.

Precautions

• The experimental leaf should be healthy.
• Clip the black paper carefully to the leaf, so that the covered portion does not receive any sunlight.
• After boiling the leaf in the alcohol, it should be washed in water.