Contents
Exploring Biology Topics can reveal the incredible complexity and interconnectedness of living systems.
How do Plants Make Food ?
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 and water by using sunlight energy (in the presence of chlorophyll) is called photosynthesis. The process of photosynthesis can be represented by a word equation as follows :
Chlorophyll is present in the green leaves. So, the process of photosynthesis takes place in the leaves of a plant. Oxygen gas is produced during photosynthesis. This oxygen goes into the air. The oxygen gas released in photosynthesis is utilised by all the living organisms for their survival.
The process of photosynthesis 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 gets stored as food in the various parts of plant including leaves. In fact, the presence of starch in the leaves shows the occurrence of photosynthesis in a plant.
Some of the glucose is also converted into other types of 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 plant can synthesise food because they contain a green pigment chlorophyll (which is necessary for making food). 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 photosynthesis 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 gas needed 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 a large number of tiny pores called stomata on the the surface of leaves of plants (see Figure).
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 (a) shows open stomatal pores whereas Figure (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 pipes throughout 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 the roots 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 the leaves of plants. In fact, it is the presence of chlorophyll which makes the leaves look green. Chlorophyll can absorb the energy from sunlight. The sunlight energy absorbed by chlorophyll is used to combine carbon dioxide and water in the green leaves to produce food (like glucose).
We can now write the role of chlorophyll in photosynthesis as follows: Chlorophyll absorbs light energy from the sun and supplies this energy to the leaves to enable them to carry out photosynthesis for making food. Since the combination of carbon dioxide and water to make food (like glucose) occurs in the presence of sunlight, the process is called photosynthesis (Photo = light, and synthesis = to combine). Please note that chlorophyll is present in every leaf of a plant in the form of hundreds of tiny structures called chloroplasts (see Figure).
4. The Role of Sunlight in Photosynthesis. The sunlight supplies energy for the food making process called photosynthesis. The sun’s energy (or solar energy) is captured by plant leaves with the help of chlorophyll and converted into chemical energy of food. Thus, solar energy is converted into chemical energy 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 the form of chemical energy. Since all the food on this earth is made by utilising solar energy, therefore, sun is the ultimate source of energy for all the living organisms.
Activity 1
To Test the Presence of Starch in Leaves
Leaves make starch as food by photosynthesis. The presence of starch in leaves can be tested as follows:
- Pluck a green leaf from a plant.
- Boil the leaf in alcohol to remove the green pigment chlorophyll from it.
- Wash the decolourised leaf with water to remove any chlorophyll sticking to it.
- Pour dilute iodine solution from a dropper over the decolourised leaf.
- Appearance of blue-black colour in leaf shows the presence of starch in it.
Leaves of Various Colours
Most of the plants have green coloured leaves. Some of the plants. however, have leaves of other colours such as red, violet, brown, etc. (see Figure). The leaves having colours other than green also have chlorophyll in them. Actually, the large amount of red, violet, brown or other pigments in such leaves masks the green colour of chlorophyll. So, photosynthesis also takes place in leaves having colour other than 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 do photosynthesis. The stem and branches of a cactus plant are green which contain chlorophyll (see Figure). So, the green stem and
green branches of a cactus plant parry 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 (see Figure). 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 algae. 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.
Synthesis of Plant Foods Other Than Simple Carbohydrate (Glucose)
The simplest food synthesised by the plants by photosynthesis is a simple carbohydrate called glucose’. The glucose carbohydrate is made up of three elements : carbon, hydrogen and oxygen. The plants use the simple carbohydrate glucose to make many other foods such as starch, oils (or fats), proteins and vitamins. This is discussed below.
(i) Plants Make Starch as Food. Some of the simple carbohydrate ‘glucose’ made by the plants through photosynthesis is converted naturally into a complex carbohydrate called ‘starch’. The starch is a food which is stored in various parts of a plant such as roots, stem, leaves and seeds. For example, the seeds (or grains) of wheat and rice have a lot of starch in them. Potato and carrot plants store a lot of starch in their roots.
(ii) Plants Make Oils (or Fats) as Food. Certaih plants convert the simple carbohydrate glucose made during photosynthesis into oils and store them in their seeds. Such seeds are called oil-seeds and give us oil (or fats) for cooking food. For example, the seeds of sunflower plant contain a lot of oil stored in them. We can extract oil from sunflower seeds and use it as a food. The oils obtained from plant seeds are commonly known as vegetable oils. Please note that just like carbohydrates glucose and starch, oils (and fats) are also made up of the same three elements : carbon, hydrogen and oxygen.
(iii) Plants Make Proteins as Food. In addition to carbon, hydrogen and oxygen, proteins also contain nitrogen element. Plants combine some of the glucose carbohydrate made during photosynthesis with nitrate minerals (obtained from soil) to make amino acids which are then made into proteins. In this way, plants make proteins as food.
Proteins are nitrogenous substances which contain nitrogen element. We will now discuss from where do the plants obtain nitrogen for making proteins. This happens as follows : Nitrogen element is present in abundance in air in the form of nitrogen gas. However, the plants cannot absorb nitrogen gas for their needs (like making proteins). Now, the soil has certain bacteria which convert nitrogen gas of air into nitrogen compounds (like nitrates) and release them into soil. Nitrates are the water soluble nitrogen compounds which are absorbed by the plants from the soil alongwith water. In this way, the plants fulfil their requirement of nitrogen. The plants also obtain nitrogen from the nitrogenous fertilisers which the farmers add to the soil in the fields from time to time.
(iv) Plants Make Vitamins as Food. Vitamins are highly complex substances which are an important part of our food. Vitamins are made by plants. Vitamins are contained in vegetables, fruits and cereals made by plants. Animals usually cannot make vitamins.
Importance of Photosynthesis
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.