Contents
Bioinformatics combines Biology Topics with computational methods to analyze large datasets.
Process of Photosynthesis and Experiments on Photosynthesis
Plants are the living things (or living organisms). Each plant has many parts to perform various functions for its survival. The main parts of a plant are : Roots, Stem, Branches, Leaves, Flowers and Fruits. The main parts of a plant are shown in Figure. Each part of the plant performs a specific function. We will now describe what type of function each of these parts performs for the plant. Let us start with the part of plant called root.
ROOT
That part of a plant which is below the ground (in the soil), is called root (see Figure). The roots of a plant perform a number of functions. The main functions of the roots of a plant are as follows :
- Roots anchor the plant to the soil. It means that roots fix the plant firmly to the soil (or ground). This prevents the plant from being pulled out easily or blown away by the wind.
- Roots absorb water and minerals from the soil. These are needed for the manufacture of food by plant leaves.
- Roots help in holding the soil together. In this way, roots prevent the soil from being blown away by wind or washed away by water, and help in the conservation of soil.
Types of Root
Roots are mainly of two types :
- Tap roots, and
- Fibrous roots.
These are discussed below.
Tap Root
Tap root is a straight tapering root which grows vertically down into the soil and gives out branches on all the sides (see Figure). Tap root is the main root and the smaller, side roots are called lateral roots. Please note that tap root itself is quite thick but its branches (lateral roots) are much thinner. Some of the plants having tap roots are : Pea plant, Neem tree, Mango tree, Marigold, Tulsi, Gram, Carrot, Radish, Beet, and Turnip.
Fibrous Roots
Some plants do not have a main root. They have a bunch of similar roots called fibrous roots. The fibrous roots consist of many thin, fibre-like roots of a similar size (see Figure). The fibrous roots spread out in the soil and give a firm support to the plant. Some of the plants having fibrous roots are : Wheat, Paddy (Rice), Grass, Maize, Millet (Bajra), Bamboo, Sugarcane and Sweet potato.
The soil (mitti) contains water. The minerals present in soil get dissolved in this water. The roots absorb water and minerals from the soil and the stem carries them to the leaves (and other parts of the plant). The leaves prepare food. The food prepared in leaves is carried by the stem to all the parts of the plant, including roots. Some of the plants store food in their roots. We eat such roots as food. For example, carrot, radish, sweet potato, turnip and tapioca, which we eat as food are the roots of the respective plants. These are actually the modified roots (for the storage of food).
STEM
The part of a plant which rises vertically up from the ground is called its stem (see Figure). The stem supports the branches and leaves. The stem of a plant is the link between the roots and the rest of the plant. The stems of most of the plants are quite strong and can stand erect on their own. So, in most cases, the stem holds the plant upright. The stems of some of the plants are, however, weak and cannot stand erect. The stem of a tree is the strongest part of the tree and it is known as trunk. Most of the tree trunks are covered with a tough layer called ‘bark’. Bark protects the inner parts of a tree. The main functions of
the stem of a plant are as follows :
- The stem holds the plant upright (or erect).
- The stem of a plant carries water and minerals from the roots to the leaves and other parts of the plant.
- The stem carries the prepared food from the leaves to other parts of the plant.
- The stem holds the leaves in such a way that the leaves are able to get plenty of sunlight for preparing food by photosynthesis.
The stem of a plant has a large number of narrow tubes inside it. One type of tubes in the stem carry water and dissolved minerals from the roots to the leaves and other parts (see Figure). The leaves of the plant make food by photosynthesis. Another type of tubes present in the stem carry the food from the leaves to all other parts of the plant, including roots (see Figure).
Activity
We wilt now describe a simple activity to show that the stem of a plant carries water to the leaves and flowers. We cut a small branch of a plant having flowers with white petals (This branch of the plant is a kind of stem). Place this branch vertically in a vessel containing red ink and water solution. Allow the branch to stand in this coloured water for a few hours. We will see red coloured lines in the leaves.
We will also see that the white petals of the flower have turned red. This experiment shows that the stem (or branch) of the plant carries red-coloured water upwards to the leaves and flowers. It is this red-coloured water which produces red lines in the leaves. And when the red-coloured water reaches the white petals of the flower, they turn red. Just like red ink, the minerals dissolved in water also move up the stem alongwith water.
LEAF
The leaf is a thin, broad, flat and green part of a plant which is attached to the stem (or branch). The plural of ‘leaf’ is ‘leaves’. A plant has a large number of leaves. Leaves of different plants have different shapes and sizes, but all of them have the same basic structure.
A leaf consists mainly of two parts : lamina and petiole (see Figure). Lamina is commonly known as leaf blade and petiole is commonly known as leaf stalk. The broad, green part of the leaf is called lamina. The thin stalk with which leaf is attached to the stem (or branch) is called ‘petiole’. There is a mid-rib (main vein) in the centre of lamina (or leaf blade). A large number of veins spread out from the mid-rib to all the parts of the leaf. The mid-rib and veins consist of bundles of tiny tubes some of which carry water (and dissolved minerals) into the leaf, and others carry away the food from the leaf.
There are minute pores on the surface of a leaf (which are called stomata). Stomata are so small that we cannot see them with naked eyes. The stomata allow the gases to move in and out of the leaf. They also allow excess water vapour to go out of the leaf. The leaves of plants contain a green coloured pigment called ‘chlorophyll’. Chlorophyll imparts green colour to the leaves. Chlorophyll can absorb (or trap) energy from sunlight.
The leaves of plants have three main functions :
- The leaves make food for the plant (by photosynthesis).
- The leaves get rid of excess water from the plant (through transpiration).
- The leaves carry out the process of respiration in plants (which is the production of energy from food).
We will now describe all these functions of leaves in a little more detail. The leaves are food factories. The leaves make food for the plant by the process of photosynthesis. In photosynthesis, the green leaves of plant combine carbon dioxide and water in the presence of sunlight to make food and oxygen gas. This can be written as :
Carbon dioxide gas needed for making food is taken by the leaves from air and water is carried into leaves from the soil through the stem. Sunlight provides the energy for making food. Chlorophyll present in the green leaves helps in trapping energy from sunlight (so that it can be utilised in making food). Oxygen gas is produced in the leaves during photosynthesis. This oxygen goes into air. The simplest food prepared by the leaves by photosynthesis is glucose.
Some of the glucose is converted into starch. This starch is stored as food in various parts of the plant (including leaves). We can now define photosynthesis as follows : The process by which green plants use sunlight to make food from carbon dioxide and water, is called photosynthesis. The term ‘photosynthesis’ actually means ‘making things with light’.
A plant keeps on absorbing water from the soil all the time through its roots. Due to this, a lot of water collects in the body of the plant. Some of this water is used up by the plant in making food. The extra water present in the body of plant is given out constantly as water vapour through the tiny pores (stomata) of the leaves. The evaporation of water from the leaves of plants is called transpiration. During transpiration, the excess water present in the plant is lost into the air as water vapour. In fact, plants release a lot of water vapour into the air through the process of transpiration. This water vapour in air helps in bringing rain. We will discuss the process of respiration carried out by the leaves of plants in higher classes.
Activity To Show That Leaves Make Food By Photosynthesis
We will now describe an activity to show that the green leaves of a plant make food (like starch) by the process of photosynthesis. This can be done as follows :
1. Pluck a green leaf from a potted plant which has been kept in sunlight [see Figure (a)],
2. Put the green leaf in a small beaker and cover it completely by adding alcohol (or spirit).
3. Place the beaker containing leaf and alcohol in a bigger beaker containing water [see Figure (b)]. Heat the water in bigger beaker over a burner. The hot water will then heat alcohol in smaller beaker. The hot alcohol removes the green colour (chlorophyll) from the leaf. Keep heating till all the green colour of leaf is removed. The leaf becomes almost colourless and the alcohol becomes green. It is necessary to remove green chlorophyll from the leaf because it interferes with the test for starch (Note. Do not heat the alcohol containing beaker directly over burner. Alcohol will catch fire !).
4. Take out the decolourised leaf from alcohol and wash it thoroughly with water to remove any chlorophyll that may be sticking to it.
5. Place the decolourised leaf in a petri-dish [see Figure (c)]. Pour dilute iodine solution over the decolourised leaf with the help of a dropper. The leaf turns blue-black showing the presence of starch in it [see Figure (d)]. This starch is the food which has been prepared by the leaf by photosynthesis.
From the above activity we conclude that leaves of a plant make starch as food by photosynthesis.
An Important Discussion. In the next activity we will show that sunlight is necessary for photosynthesis by plants. That is, sunlight is necessary for making starch as food by the plants. Ordinarily, all the plants have some starch stored in their green leaves, so before we can use a leaf in the next activity, we have to remove all the starch which is already present in it. In other words, we have to destarch the leaves of a plant before using it in the next activity.
The green leaves of a plant can be destarched by keeping the plant in a completely dark place in a room for 2 to 3 days. When the plant is kept in a dark place for 2 to 3 days, it cannot make any fresh starch by photosynthesis because there is no sunlight. And the starch which is already present in the leaves of the plant kept in the dark, is all used up by the plant itself for meeting its own energy needs during these 2 or 3 days. From this discussion we conclude that when a potted plant is kept in a completely dark place for 2 to 3 days, then its leaves will not have any starch left in them. We say that the leaves have been destarched. Keeping this in mind, we will now describe the next activity.
Activity To Show That Sunlight Is Necessary For Photosynthesis
We will now describe an activity to show that sunlight is necessary for making food like starch by the leaves by the process of photosynthesis. This can be done as follows :
1. Take a potted plant having green leaves and place it in a completely dark place for 2 to 3 days to destarch its leaves. So, in the beginning of the activity, the leaves do not have any starch in them.
2. Take a thin strip of aluminium foil and wrap it in the centre of one leaf on both the sides, while the
leaf is still attached to the plant [see Figure (a)]. The aluminium foil should be fixed tightly to the leaf by using paper clips so that sunlight may not enter it from the sides. Only the middle part of the leaf is covered with aluminium foil. The remaining part of leaf remains uncovered so that it is exposed to sunlight. (We can also use a black paper strip to cover a part of the green leaf in place of aluminium foil strip),
3. Keep the potted plant (with partially covered leaf) in bright sunshine for 3 to 4 days [see Figure (a)].
4. Pluck the partially covered leaf from the plant and remove the aluminium foil.
5. Remove chlorophyll from this leaf by heating in alcohol (as described in the previous activity) [see Figure (b)]. The leaf becomes colourless. Wash it with water.
6. Place the decolourised leaf in a petri-dish [see Figure (c)]. Pour dilute iodine solution over the decolourised leaf with the help of a dropper [see Figure (d)].
(a) The middle part of leaf, which was covered with aluminium foil, does not turn blue-black on adding iodine solution [see Figure (d)] showing that no starch is present in the middle part of the leaf. This means that the middle part of leaf which did not get sunlight, could not make starch food by photosynthesis.
(b) The uncovered parts of leaf (on both sides of aluminium foil) which were exposed to sunlight, turn blue-black on adding iodine solution [see Figure (d), showing that starch is present in the outer parts of leaf. This means that the outer parts of leaf which got sunlight, could make starch food by photosynthesis.
From the above activity we conclude that sunlight is necessary to make food like starch by photosynthesis.
Activity To Show Transpiration In Plants
The leaves of plants give out water constantly in the form of water vapour (which goes into air). This process is called transpiration. We will now describe an activity to show the process of transpiration in plants.
Take a well-watered potted plant having big leaves. Enclose a leafy stem (or branch) of the plant in a clean polythene bag and seal the mouth of polythene bag properly with cellotape (see Figure). Keep this potted plant in the sunshine for 2 to 3 hours. We will see a number of water droplets on the inner side of the polythene bag (see Figure). This can be explained as follows : The leaves of plant give out water vapour.
Since the leaves are enclosed in polythene bag, the water vapour cannot escape into air. The water vapour given out by the covered leaves keeps on collecting inside the polythene bag. Some of this water vapour condenses (liquefies) to form tiny drops of water. It is these droplets of water which we see sticking to the inner sides of the polythene bag. This activity shows that the leaves of a plant lose water through transpiration.