Mode of Nutrition in Plants
The process by which green plants make their own food from carbon dioxide and water by using sunlight energy in the presence of chlorophyll is called photosynthesis. Green plants also need foos to acquire energy. All living organisms need energy to perform various life prosesses. This article deals with the modes of nutrition in plants. How plants prepare their own food, what all factors are responsible for photosynthesis etc.
Plants make their own food in the presence of sunlight and hence they are autotrophs. Plants convert sunlight energy into chemical energy. They use carbon dioxide, water and sunlight to make their own food in the presence of chlorophyll.
There are two types of modes of nutrition in plants. They are:
Since we are learning about mode of nutrition in plants we will discuss only about autotrophic mode of nutrition.
Autotrophic mode of nutrition
In autotrophic mode of nutrition organisms make their own food with the help of simple inorganic materials like carbon dioxide and water, in the presence of sunlight. Also in this nutrition organic food is made from inorganic materials.
Green plants have autotrophic mode of nutrition. These organisms are called autotrophs. The autotrophs have green pigments called chlorophyll which help in trapping sunlight energy. They use the sunlight to make food by the process of photosynthesis. The food produced by autotrophs is used by human beings and animals as well.
Nutrition in Plants
Green plants synthesize their own food by the process of photosynthesis. Photo means light and synthesis means to build, so photosynthesis means ‘building up by light. It is in the presence of chlorophyll that plants use energy of sunlight to make food from inorganic materials like carbon dioxide and water.
Green plants make their own food by photosynthesis
Chlorophyll is present in green coloured bodies called chloroplast. The leaves of a plant are green due to the presence of chlorophyll in them.
In the process of photosynthesis a lot of oxygen gas is released. Let us understand with the help of equation:
Food is prepared in the green leaves of the plant. Plant requires carbon dioxide to make food which it takes from air. The carbon dioxide enters the leaves through tiny pores in them called stomata. Water required to make food is taken from soil. This water is transported to the leaves through roots and stem. The sunlight provides energy to carry out chemical reactions and chlorophyll present in green leaves help in absorbing this energy. Oxygen produced as a by-product in this process which goes out in the air.
The food prepared by the leaves is in the form of simple sugar called glucose. This glucose is then sent to other parts of the plant. The extra glucose is stored in the leaves of the plant in the form of starch. Glucose and starch belong to a category called carbohydrates. Thus green plants convert sunlight energy into chemical energy.
The steps for photosynthesis are:
i) Sunlight energy is absorbed by chlorophyll.
ii) Sunlight energy is converted into chemical energy and water is split into hydrogen and oxygen.
iii) Carbon dioxide is reduced to hydrogen so that carbohydrate like glucose is formed.
It is not necessary that these steps of photosynthesis take place one after the other.
Conditions Necessary for Photosynthesis
Following are the conditions necessary for photosynthesis:
3) Carbon Dioxide
Let us discuss some experiments to prove that sunlight, chlorophyll, carbon dioxide and water are necessary for photosynthesis. This can be proven by getting the result that green leaves make starch as food and starch gives blue-black colour with iodine solution.
Experiment to Show that Sunlight is Necessary for Photosynthesis
- Take a potted plant with green leaves and keep it in a completely dark place for three days. In this way all the starch stored in the leaves will be consumed by the plant and leaves will get destarch.
- Now wrap an aluminium foil tightly in the middle part of one leaf in such a way that a small part of a leaf is covered and remaining part is exposed to sunlight. The aluminium foil must fixed tightly using paper clips so that sunlight cannot enter from the sides.
- Keep the potted plant in sunlight for three to four days.
- Pluck the partially covered leaf and remove aluminium foil. Now immerse this leaf in boiling water so that cell membranes of the leaf cell are broken down. This will make the leaf more permeable in iodine solution.
- Before testing the leaf for the presence of starch chlorophyll has to be removed from the leaf. This is because chlorophyll interferes in the test for starch.
- Now put this leaf in a beaker containing alcohol and place this beaker in water bath.
- Heat this water bath and in this way alcohol in the small beaker will also be heated and start boiling. This boiling alcohol will remove chlorophyll from the green leaf.
- After the green colour is removed from the leaf it will become colourless.
- Remove the colourless leaf from alcohol and wash it with hot water.
- Place this leaf in a petri-dish and drop iodine solution over it. You will notice a change in colour.
- The middle part of the leaf which was covered with iodine solution will not turn blue-black on adding iodine solution. This shows that no starch is present in this part of the leaf. This is because sunlight could not reach over the covered part of the leaf and it could not do photosynthesis to make starch.
- The uncovered part of the leaf which was exposed to sunlight turns blue-black on adding iodine solution. This is because it could do photosynthesis in the presence of sunlight and make starch.
- Therefore, we conclude that sunlight is necessary for photosynthesis and leaves make starch as food by photosynthesis
There are certain plants that have partially white and partially green leaves. Such leaves are called variegated leaves. Croton and coleus have variegated leaves.
Experiment to show that sunlight is necessary for photosynthesis.
Experiment to Show that Chlorophyll is necessary for Photosynthesis
- Take a potted plant croton, as its leaves are partially white and partially green.
- Place this plant in a completely dark place for three days to destarch its leaves.
- Now take out the potted plant and place in sunshine for three to four days.
- Pluck the variegated leaf and boil it in water for a few minutes. Remove this leaf and boil it in alcohol so as to remove its green colour.
- Wash the decolourised leaf with hot water.
- Pour iodine solution over decolourised leaf and notice the change in colour.
- We will notice that the part of leaf which was originally white will not turn blue-black. This shows that no starch is present in this part of the leaf. Also it shows that photosynthesis to make starch does not take place without chlorophyll.
- The inner part of leaf which was originally green turns blue-black on adding iodine solution. This shows that this part of leaf contains starch. Thus, photosynthesis to make starch takes place in the presence of chlorophyll. From this we conclude that chlorophyll is necessary for the process of photosynthesis to take place.
Experiment to show that chlorophyll is necessary for photosynthesis
Experiment to Show that Carbon Dioxide is Necessary for Photosynthesis
- Take a potted plant with long and narrow leaves and place it in a dark place for three days to destarch its leaves.
- Take a glass bottle having wide mouth and put some potassium hydroxide solution in it. This solution will absorb all the carbon dioxide gas from the air present in glass bottle.
- Take a rubber cork which fits nicely into the mouth of the glass bottle and make a slit or cut into it.
- Put the destarched leaf while it still attached to the plant in-between the slit of the cork. The leaf should be put in the cork in such a way that upper half of the leaf remains outside the bottle. Fix this cork to the bottle. Now half of the leaf is inside the bottle and half outside it.
- Now keep this potted plant in sunlight for three to four days. Now the upper part of the leaf gets carbon dioxide from air and the lower part which is inside glass bottle does not get any carbon dioxide.
- Pluck the leaf from the plant and take it out from the glass bottle. Remove green colour of the leaf by boiling it in alcohol.
- Wash the decolourised leaf with water.
- Pour iodine solution over the colourless leaf. There is a change in the colour of the leaf.
- We will notice that lower part of the leaf which was inside glass bottle does not turn blue-black. This shows that no starch is present in this part of the leaf. Thus, we conclude that carbon dioxide is necessary for the plant to make starch in the process of photosynthesis.
- The upper part of the leaf which was outside bottle turns blue-black. This shows that this part of leaf has starch in it. Thus, we conclude that carbon dioxide is necessary for the process of photosynthesis to take place.
Experiment to show that carbon dioxide is necessary for photosynthesis.
How Plants Obtain Water for Photosynthesis?
Plants obtain water for the process of photosynthesis from soil. Roots of a plant absorb water from the soil which transported upward to the leaves through xylem vessel and utilised for photosynthesis.
Plants use carbon dioxide and water as raw materials to prepare energy food called carbohydrates. Other material like nitrogen, phosphorus, iron and magnesium which are required by the plant are taken from soil.
Site of Photosynthesis: Chloroplasts
The cell organelles of green plants which contain chlorophyll are called chloroplasts. It is here that the photosynthesis takes place. Chloroplasts are just below the upper epidermis of the leaf in palisade tissue.
The structure of a leaf to show chloroplasts in it (the small circles in above diagram are all chloroplasts).
From the article we have seen that how plants prepare theor own food.