What is Photosynthesis?

What is Photosynthesis?

‘Photo’ means ‘light’ and ‘synthesis’ means ‘to build’ thus photosynthesis literally means “building up with the help of light”. During this process, the light energy is converted into chemical energy. Green plants are autotrophic in their mode of nutrition because they prepare their food materials through a process called photosynthesis.

The overall equation of photosynthesis can be given as follows:

The end product of photosynthesis is glucose which will be converted into starch and stored. Plants take in carbon dioxide for photosynthesis; but for its living, plants also need oxygen to carry on cellular respiration.

1. What else is needed for photosynthesis?

Photosynthesis is the process by which plants make their food. A small speck of seed grows and gains weight into a giant tree, due to photosynthesis. Almost all the other organisms rely on plants for their food directly or indirectly. Even a carnivore depends ultimately upon plants for its food, how? Can you highlight? Four important things needed by plants for photosynthesis:

1.     Chlorophyll - Green pigment present in leaves

2.     Water

3.     Carbon dioxide (from air)

4.     Light

Let us examine two of these factors

Chlorophyll

Aim: To show that chlorophyll is essential for photosynthesis.

We need Coleus (croton) plant with variegated leaves, boiling water, alcohol and iodine solution.
How to do?

Variegated leaf is plucked from Coleus plant kept in sunlight after de-starching by keeping it in dark room for 24 hours. The picture of the leaf is drawn and the patches of cholorphyll on the leaf are marked. After immersing the leaf in boiling water then in alcohol it is tested for starch with iodine solution.

What do you observe? The patches of the leaf with chlorophyll turn blue-black. The other portions remain colourless.

What you conclude? The chlorophyll is essential for photosynthesis.

Plants take up water through their roots and air through stomata of their leaves. Chlorophyll is present in the leaf. What else is needed for photosynthesis? The question that remains to be answered is whether the process of forming starch by combining carbon dioxide and water also requires light. Let us do an experiment.

Light

Aim: To show that sunlight is necessary for photosynthesis.

What do you need? potted plant, black paper, boiling water, alcohol and iodine solution.

How to do?

A potted plant is placed in a dark room for about 2 days to de-starch its leaves. One of its leaves is covered with the thin strip of black paper as shown in the picture. make sure that the leaf is covered on both sides.

The potted plant is kept in bright sunlight for 4 to 6 hours. The selected covered leaf is plucked and the black paper is removed. The leaf is immersed in boiling water for a few minutes and then in alcohol to remove chlorophyll. The leaf is now tested with iodine solution for the presence of starch. The covered part of the leaf does not turn blue-black whereas the uncovered part of the leaf turns blue-black colour. The covered part of the leaf which did not receive the sunlight was unable to synthesize starch. Hence it does not turn blue-black colour. But the uncovered part of the leaf which received sunlight was able to synthesise starch and so it turns blue-black in colour.

All these four things must be in the leaf, the site where plants make their food. That raises an interesting question. Of the four, chlorophyll is present in the leaf. Sunlight falls on the leaf. But how do plants take air and water to its leaf? How does the water reach the leaves from the roots? What path does it follow? How does air enter the leaf?

2. Exchange of gas

Roots of the plant take the water from the soil and transport it to leaves. How water and other nutrients reach the leaf from the roots? Is the only question that we need to find out? We will see this later. This leaves us with only one question? How does the plant get air? The leaves have tiny holes, called stomata, through which the exchange of air takes place. These holes are so minute that we need a microscope to see them. The air exchange takes place continuously through the stomata. Plants inhale and exhale continuously through these stomata.

3. Transpiration

The loss of water in the form of water vapour from the aerial parts of the plant body is called as transpiration. There are three types of transpiration:

1.     Stomatal transpiration - This is the most dominant form of transpiration being responsible for most of the water loss in plants. It accounts for 90-95% of the water transpired from leaves.

2.     Cuticular Transpiration – This type of transpiration is responsible for the loss of water in plants via the cuticle.

3.     Lenticular Transpiration – This type of transpiration is the loss of water from plants as vapor through the lenticels. The lenticels are tiny openings that protrude from the barks in woody stems and twigs as well as in other plant organs.

Experiment 1

If you tie a plastic bag over a leaf and place the plant in light, you will be able to see water condensing inside the plastic bag. The water is let out by the leaves.

The leaves have tiny, microscopic holes called stomata. Water evaporates through these stomata. Each stomata is surrounded by guard cells. These guard cells help in regulating the rate of transpiration by opening and closing of stomata.

Typically, only 0.1 percent of water taken up by the plant is used by the plant for producing carbohydrates. That is, if a plant absorbs one litre of water, only one millilitre will be used to produce carbohydrate. The remaining 999 millilitres evaporates from the leaf. You will be able to see how much water a plant releases in the air.

Suppose the weather is hot and the stomata close; what would be the effect of such a situation in the absorption of carbon dioxide? Will the rate of photosynthesis be the same? If the plant does not get water at this time, what effect would this have on its growth? Relate your answer to the problem of drought affecting agricultural growth. Global warming implies increased level of average temperature. Can you reason what all the effect it can have on photosynthesis?

4. Macronutrients and micronutrients for plants

Nutrients such as carbon, hydrogen, oxygen, nitrogen, potassium, calcium, magnesium, sulphur and phosphorus, are required in substantial quantity and are called macronutrient. Plants also require many other nutrients like iron, manganese, copper, boron, molybdenum, chlorine, silicon, cobalt, and zinc, but only in minute quantities, hence, they are called micronutrients. The water transportation systems of the plants take these nutrients from the soil and circulate it in the plant.