Green plants are autotrophic and synthesise (or make) their own food by the process of photosynthesis.
The green plants make their food from simple inorganic substances like carbon dioxide and water in the
presence of sunlight. The plants use the energy in sunlight to prepare food in the presence of a green
colouring matter called ‘chlorophyll’ present in the leaves of a green plant. We can now define.
photosynthesis as follows :
The process by which green plants make their own food (like glucose) from carbon dioxide andw ater by using sunlight energy (in the presence of chlorophyll) is called photosynthesis.
The process ofphotosynthesis can be represented by a word equation as follows :Carbon dioxide + Water → Glucose + Oxygen
Chlorophyll is present in the green leaves. So, the process of photosynthesis takes place in the leavesof a plant. Oxygen gas is produced during photosynthesis. This oxygen goes into the air. The oxygen gasreleased in photosynthesis is utilised by all the living organisms for their survival. The process ofphotosynthesis can be shown with the help of a diagram given in Figure The process of photosynthesis first produces a simple carbohydrate called ‘glucose’ as food.
The glucose carbohydrate then gets converted into a complex carbohydrate called ‘starch’. This starch getsstored as food in the various parts of plant including leaves. In fact, the presence of starch in the leavess hows the occurrence of photosynthesis in a plant.
Some of the glucose is also converted into other typeso f plant foods such as fats and oils, proteins as well as vitamins. The synthesis of food (or making of food) occurs in the leaves of a plant (or tree). So, leaves are the food factories of a plant.
The leaves of a plantc an synthesise food because they contain a green pigment chlorophyll (which is necessary for makingf ood). Other parts of a plant usually cannot synthesise food because they do not contain chlorophyll.
Conditions Necessary for Photosynthesis:The presence of carbon dioxide, water, chlorophyll and sunlight is necessary for the process of potosynthesis to take place. Photosynthesis cannot occur in the absence of any one of these conditions.
We will now describe how the leaves of a plant get carbon dioxide and water required for making food by
photosynthesis and what are the roles of chlorophyll and sunlight in photosynthesis.
1. How the Plants Obtain Carbon Dioxide for Photosynthesis. The plants take carbon dioxide gasn eeded for photosynthesis from the air. The plants take carbon dioxide gas from air through the tiny pores( called stomata) present on the surface of leaves (The singular of stomata is stoma). Actually, there are al arge number of tiny pores called stomata on the the surface of leaves of plants (see Figure 4). Each pore or stoma) is surrounded by a pair of guard cells. The opening and closing of stomatal pores in the leaves is controlled by the guard cells. Figure 4(a) shows open stomatal pores whereas Figure 4(b) shows the stomatal pores in closed position. The carbon dioxide gas present in air enters the leaves of a plant through the stomatal pores present on their surface and utilised in photosynthesis. The oxygen gas produced in the leaves during photosynthesis goes out into air through the same stomatal pores.The stomatal pores of leaves open only when carbon dioxide is to be taken in or oxygen is to be released otherwise they remain closed.
2. How the Plants Obtain Water for Photosynthesis. The plants take water needed for Photosynthesis from the soil. Soil always contains some water in it. Water present in the soil is absorbed by the roots of a plant and then transported to the leaves through the vessels which run like inter-connected pipesh Troughout the roots, stem, branches and leaves. The tiny, pipe-like vessels which transport water from the roots of a plant to its leaves are called xylem.
The plants also need minerals to make foods other than carbohydrates. For example, plants need. Nitrogen mineral to make proteins. The minerals are present in the soil (and have to be transported to the leaves). The minerals dissolve in water present in the soil and get transported with it. So, we can now say that : Water and minerals present in the soil are absorbed by theroots of a plant and transported to its leaves through the inter-connected pipe-like xylem vessels present throughout the roots,stem, branches and leaves of the plant.
3. The Role of Chlorophyll in Photosynthesis.Chlorophyll is a green substance which is present in theleaves of plants. In fact, it is the presence of chlorophyllwhich makes the leaves look green. Chlorophyll canabsorb the energy from sunlight. The sunlight energyabsorbed by chlorophyll is used to combine carbondioxide and water in the green leaves to produce food(like glucose). We can now write the role of chlorophyll inphotosynthesis as follows : Chlorophyll absorbs lightenergy from the sun and supplies this energy to theleaves to enable them to carry out photosynthesis formaking food. Since the combination of carbon dioxideand water to make food (like glucose) occurs in thepresence of sunlight, the process is called photosynthesis(Photo = light, and synthesis = to combine). Please note that chlorophyll is present in every leaf of a plantin the form of hundreds of tiny structures called chloroplasts (see Figure 5).
4. The Role of Sunlight in Photosynthesis. The sunlight supplies energy for the food making processcalled photosynthesis. The sun’s energy (or solar energy) is captured by plant leaves with the help ofchlorophyll and converted into chemical energy of food. Thus, solar energy is converted into chemicalenergy during photosynthesis. This chemical energy gets stored in the form of plant food. So, when plants(or animals) utilise the food made by photosynthesis, they actually use the solar energy stored in it in theform of chemical energy. Since all the food on this earth is made by utilising solar energy, therefore, sunis the ultimate source of energy for all the living organisms.
Leaves of Various Colours:Most of the plants have green coloured leaves. Some of the plants, however, have leaves of othercolours such as red, violet, brown, etc. (see Figure 6). The leaves having colours other than green also havechlorophyll in them. Actually, the large amount of red, violet, brown or other pigments in such leavesmasks the green colour of chlorophyll. So, photosynthesis also takes place in leaves having colour otherthan green
Photosynthesis by Plant Parts Other Than Leaves:Normally photosynthesis takes place only in the leaves of plants.
In some plants, however, photosynthesis also takes place in other parts of plants such as “green stems” and “green branches”. The green stems and green branches can do photosynthesis because they contain chlorophyll. For example, the desert plants such as cactus have tiny, spine-like leaves to reduce the loss of water by transpiration. These tiny, spine-like leaves of a cactus plant cannot dop hotosynthesis. The stem and branches of a cactus plant are green which contain chlorophyll (see Figure 7). So, the green stem and green branches of a cactus plant carry out the process of photosynthesis to make food for the plant. Please note that only those stems and branches which are green in colour can do photosynthesis. Now, the stems and branches of all the plants and trees are not green. So, the stems and branches of all the plants and trees cannot do photosynthesis.Photosynthesis by Algae:Many times we have seen patches of slimy, green layer floating on the surface of a pond or lake, or even in the stagnant parts of a river. It also develops in the swimming pools which have not been cleaned for a long time. This green layer is formed by the growth of tiny green plant-like organisms called algae (Algae is pronounced as algee. The singular of algae is alga). It is called shaiwal in Hindi. Algae are a large group of simple, plant-like organisms. Algae contain chlorophyll and produce food by photosynthesis just like plants. Algae, however, differ from plants because they do not have proper roots, stems and leaves. The green colour of algae is due to the presence of chlorophyll in them.Importance of Photosynthesi:Photosynthesis is important for the existence of life on this earth. In the absence of photosynthesis, life would be impossible on this earth. This is due to the following reasons :
(i) Photosynthesis by plants provides food to animals (including human beings). So, the survival of animals (including human beings) depends on the food made by plants by photosynthesis. In the absence of photosynthesis, there would be no plants on this earth and hence no animals will survive.
(ii) The process of photosynthesis by plants puts oxygen gas into the air. It is this oxygen gas which the animals (including human beings) use for breathing and respiration. In the absence of photosynthesis, there would be no oxygen in air and hence no animals could exist on this earth.
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