Table of Contents

Life Processes

Something which is `living` (not dead) is said to be `alive`. In most simple terms, `alive` means `having life`. Alive is called `jeevit`or `zinda`in hindi. We are alive and you are also alive. Those things which are alive are called living things All the plants and animals tincluding human beings) are alive or living things. Now, an important question arises: What criteria do we use to decide whether something is alive Thus is discussed below.

The most important criterion to decide whether something is alive for not is the movement. Movement is one of the most important signs of lite in an organism. All the living things (which are alive) move by themselves without any external help. In some cases the movements of living things are quite fast which can be easily observed by us but in other cases the movements are very slow and hence observed with difficulty Fur example the movements in most of the animals are fast and can be observed easily but the movements in plants are usually slow and observed with difficulty.

Animals and plants move in Altered wars. This will become clear from the following discussion Animals can move from one place to another or they can move their body parts. For example, a frog moves when it jumps into a pond, a bird moves when it flies in the sky, an athlete moves when he runs a fish moves when it swims in water tsee Figure 11 We move our hands when we clap and our c moves up and down when we breathe And a dog can wag its tail. All these movements show that a bird, fish, dog and human beings are alive for living things)

The plants are fixed in the soil at a place, so they cannot move like animals from place to place plants can only move parts of their body such as leaves, flowers, shoots and roots. The plant parts moves towards a stimulus such as sunlight, gravity or water, etc. For example, the shoot, the leaves and flower of a sunflower, plant move by bending towards the sum so as to face the sunlight see Figure 21. The leaves of a Moneta pudica plant (sensitive plant) move by folding up when touched with a finger. Plants also show movement by growing their roots and shoots bigger

Non-living things (which are not alive cannot move by themselves. For example, a stone is a non-living thing which cannot move by itself from one place to another or show any other type of movement. We will have to move it by applying force from outside.

All the living things (plants and animals) are made up of tiny living units called cells. The cells themselves are made up of still smaller particles called molecules. The movements over very small scale (as those in the molecules of living things) are invisible to the naked eye. The invisible molecular movement is, however, necessary for the existence of life In fact, viruses do not show any molecular movement in them (until they infect some cell ) and this has created controversy about whether they truly alive or not. In addition to movement, the living things also show some other characteristics. The are discussed below . 

moves when it jumps into a pond, a hird moves when it flies in the sky, an athlete moves when he runs a fish moves when it swims in water tsee Figure 11 We move our hands when we clap and our c moves up and down when we breathe And a dog can wag its tail. All these movemerits show that a bird, fish, dog and human beings are alive for living things)

The plants are tised in the soil at a place, so they cannot move like animals from place to place plants can only move parts of their body such as leaves, flowers, shoots and roots. The plant parts moves towards a stimulus such as sunlight, gravity or water, etc. For example, the shoot, the leaves and flower of a sunflower, plant move by bending towards the sum so as to face the sunlight see Figure 21. The leaves of a Moneta pudica plant (sensitive plant) move by folding up when touched with a finger. Plants also show movement by growing their roots and shoots bigger

Non-living things (which are not alive cannot move by themselves. For example, a stone is a non-living thing which cannot move by itself from one place to another or show any other type of movement. We will have to move it by applying force from outside.

All the living things (plants and animals) are made up of tiny living units called cells. The cells themselves are made up of still smaller particles called molecules. The movements over very small scale (as those in the molecules of living things) are invisible to the naked eye. The invisible molecular movement is, however, necessary for the existence of life In fact, viruses do not show any molecular movement in them (until they infect some cell ) and this has created controversy about whether they truly alive or not. In addition to movement, the living things also show some other characteristics. The are discussed below . 

All the living things which are alive, have some common characteristics (or features) which make them different from non-living things. The characteristics of living things are as follows

  • (1). Living things can move by themselves.
  • (2). Living things need food, air and water.
  • (3). Living things can grow.
  • (4). Living things can respond to changes around them. They are sensitive.
  • (5). Living things respire (release energy from food)
  • (6). Living things excrete (get rid of waste materials from their body)
  • (7). Living things can reproduce. They can have young ones.

What are Life Processes

All the organisms perform some basic functions to keep themselves alive. The basic functions performed by living organisms to maintain their life on this earth are called life processes The basic lite processes common to all the living organisms are Nutrition and Respiration, Transport and Excretion, Control and Coordination (Response to stimuli), Growth, Movement and Reproduction. The process of nutrition involves the taking of food inside the body and converting it into smaller molecules which can be absorbed by the body. Respiration is the process which releases energy from the food absorbed by the body.

Transport is the process in which a substance absorbed or made in one part of the body is moved to other parts of the body. Excretion is the process in which the waste materials produced in the cells of the body are removed from the body. Control and coordination (or response to stimuli) is a process which helps the living organisms to survive in the changing environment around them. The process of growth involves the change from a small organism to a big organism (or an adult organism). In movement, the organism either moves from one place to another or moves its body parts while remaining at the same place. The process of reproduction involves the making of more organisms from the existing ones, so that organisms could live on this earth for ever.

Energy is Needed for the Life Processes

All the living organisms need energy to perform various life processes. They get this energy from food. Food is a kind of fuel which provides energy to all the living organisms. The living organisms use the chemical energy for carrying out various life processes. They get this chemical energy from food through chemical reactions. Actually, living organisms continuously need energy for their various life processes and other activities which they perform. For example, energy is required by an organism even during sleep. This is because when we are asleep, a number of biological processes keep on occurring in the body which require energy.

Our heart beats non-stop even when we are asleep to pump blood throughout the body. And this beating of heart requires energy. Thus, the working of heart requires a continuous supply of energy. The energy required by an organism comes from the food that the organism eats. Thus, food is the basic requirement of all the living organisms for obtaining energy. In this chapter we will first study the process of intake and utilisation of the food by an organism (called nutrition) and the liberation of energy from the food (called respiration). After that we will study the process of moving the digested food and other materials to the various parts of the body (called transport) and the removal of waste materials from the body (called excretion). Let us start with nutrition

Nutrition

Food is an organic substance. The simplest food is glucose. It is also called simple sugar. A more complex food is starch. Starch is made from glucose. The general name of substances like glucose (sugar)

Life Processes

and starch is ‘carbohydrates’. Carbohydrates are the most common foods for getting energy. Fats and proteins are also foods. (A wider definition of food, however, also includes mineral salts, vitamins and water which are essential for the normal growth and development of an organism). The process of taking in food (consuming food) and utilizing it is called nutrition. It is a process in which food is obtained in order to utilize it to provide energy for performing various metabolic activities of the organism.

Actually, the term ‘nutrition’ comes from the word ‘nutrient’. A nutrient is an organic or inorganic substance required for the maintenance of life and survival of a living organism. In most simple terms, a nutrient can be said to be a particular type of food. A nutrient can be defined as a substance which an organism obtains from its surroundings and uses it as a source of energy or for the biosynthesis of its body constituents (like tissues and organs). For example, carbohydrates and fats are the nutrients which are used by an organism mainly as a source of energy whereas proteins and mineral salts are nutrients used by an organism for the biosynthesis of its body constituents like skin, blood, etc.

The food taken in by an organism contains a large number of nutrients like carbohydrates, fats, proteins, minerals, vitamins and water, etc. We can now say that: Nutrition is a process of intake of nutrients (like carbohydrates, fats, proteins, minerals, vitamins and water) by an organism as well as the utilisation of these nutrients by the organism. We will now describe the various ways of procuring food (or obtaining food) by the different organisms. In other words, we will now describe the different modes of nutrition of the various organisms.

Mode Of Nutrition

Modes of Nutrition Modes of nutrition means methods of procuring food or obtaining food by an organism. All the organisms do not obtain their food in the same way. Different organisms have different methods of procuring food or obtaining food. In other words, organisms differ in their modes of nutrition. Depending on the mode (or method) of obtaining food, all the organisms can be classified into two groups: autotrophic and heterotrophic. Thus: There are mainly two modes of nutrition:

  1. Autotrophic, and
  2. Heterotrophic.

We will now discuss the autotrophic mode of nutrition and the heterotrophic mode of nutrition in detail, one by one.

1. Autotrophic Mode of Nutrition

The word ‘auto’ means ‘self’ and ‘trophe’ means ‘nutrition’. Thus, autotrophic means ‘self nutrition’. In autotrophic nutrition, the organism makes (or synthesizes) its own food from the inorganic raw materials like carbon dioxide and water present in the surroundings by using the sunlight energy. We can now say that: Autotrophic nutrition is that mode of nutrition in which an organism makes (or synthesizes) its own food from the simple inorganic materials like carbon dioxide and water present in the surroundings (with the help of sunlight energy).

Please note that food is an organic material (like glucose, etc.). This means that, in autotrophic nutrition, organic material (food) is made (or synthesized) from inorganic materials like carbon dioxide and water by utilizing the sunlight energy. The green plants have an autotrophic mode of nutrition. The autotrophic bacteria also obtain their food by the autotrophic mode of nutrition (though most bacteria are not autotrophic). The organisms having autotrophic mode of nutrition are called autotrophic organisms or just autotrophs.

Those organisms which can make their own food from carbon dioxide and water are called autotrophs. Carbon dioxide and water are inorganic substances. So, we can also say that: Those organisms which can make their own food from the inorganic substances present in the environment, are called autotrophs. All the green plants are autotrophs (because they can make their own food from inorganic substances like carbon dioxide and water present in the environment). Non-green plants are, however, not autotrophs. Certain bacteria called ‘autotrophic bacteria’ are also autotrophs.

The autotrophic organisms (or autotrophs) contain the green pigment called chlorophyll which is capable of trapping sunlight energy. This trapped sunlight energy is utilised by the autotrophs to make food by combining inorganic materials like carbon dioxide and water present in the environment by the process of photosynthesis. Thus, autotrophs make their own food by photosynthesis. So, autotrophs are the producers of food. The food produced by autotrophs (green plants) is also used by human beings and many, many other animals.

2. Heterotrophic Mode of Nutrition

The word ‘heteros’ means ‘others’ and ‘trophe’ refers to ‘nutrition’. Thus, ‘heterotrophic’ means ‘nutrition obtained from others’. In heterotrophic nutrition, the organism cannot make (or synthesize) its own food from the inorganic raw materials like carbon dioxide and water, and uses the food made by autotrophic organisms directly or indirectly. We can now say that: Heterotrophic nutrition is that mode of nutrition in which an organism cannot make (or synthesize) its own food from simple inorganic materials like carbon dioxide and water, and depends on other organisms for its food.

A heterotrophic organism is a consumer which derives its nutrition from other organisms. That is, a heterotrophic organism has to eat other organisms for its nutrition. All the animals have a heterotrophic mode of nutrition. Most bacteria and fungi also have heterotrophic mode of nutrition. The organisms having heterotrophic mode of nutrition are called heterotrophic organisms or just heterotrophs.

Those organisms which cannot make their own food from inorganic substances like carbon dioxide and water, and depend on other organisms for their food are called heterotrophs. All the animals are heterotrophs (because they cannot make food from inorganic substances like carbon dioxide and water and obtain their food from other plants or animals.). Thus, man, dog, cat, deer, tiger, bear, lion, cow, etc., are all heterotrophs. The non-green plants (like yeast) are also heterotrophs. Heterotrophs depend on autotrophs and other heterotrophs for their food. In other words, animals are heterotrophs which depend on plants or other animals for their food.

From the above discussion we conclude that green plants make their own food. Non-green plants and animals cannot make their own food. They obtain food from plants and other animals. We will now discuss the various types of the heterotrophic mode of nutrition.

Types of Heterotrophic Nutrition

A heterotrophic organism (or heterotroph) can obtain its food from other organisms in three ways. So, the heterotrophic mode of nutrition is of three types:

  1. Saprotrophic nutrition,
  2. Parasitic nutrition, and
  3. Holozoic nutrition.

We will now discuss the three types of heterotrophic nutrition in detail, one by one. Let us start with the saprotrophic nutrition.

1. Saprotrophic Nutrition (or Saprophytic Nutrition)

Saprotrophic nutrition is that nutrition in which an organism obtains its food from decaying organic matter of dead plants, dead animals and rotten bread, etc. ‘Sapro’ means ‘rotten’, so a saprotrophic organism draws its food from rotting wood of dead and decaying trees, rotten leaves, dead animals and household wastes like rotten bread, etc.

The organisms having saprotrophic mode of nutrition are called saprophytes. We can now say that: Saprophytes are the organisms which obtain their food from dead plants (like rotten leaves), dead and decaying animal bodies, and other decaying organic matter (like rotten bread). Fungi (like bread moulds, mushrooms, yeast), and many bacteria are saprophytes. We know that fungi and bacteria are a kind of plants

So, we can also say that saprophytes are the plants which feed on dead and decaying organic matter. The saprophytes break down the complex organic molecules present in dead and decaying matter and convert them into simpler substances outside their body. These simpler substances are then absorbed by saprophytes as their food. Please note that saprotrophic nutrition is also known as saprophytic nutrition.

2. Parasitic Nutrition

The parasitic nutrition is that nutrition in which an organism derives its food from the body of another living organism (called its host) without killing it. The organism which obtains the food is called a ‘parasite’, and the organism from whose body food is obtained is called the ‘host’. We can now say that A parasite is an organism (plant or animal) which feeds on another living organism called its host A parasite receives its food from the host but gives no benefit to the host in return. A parasite usually harms the host. The host may be a plant or an animal.

Most of the diseases which affect mankind, his domestic animals (like dogs and cattle) and his crops are caused by parasites. Parasitic mode of nutrition is observed in several fungi, bacteria, a few plants like Cuscuta (amarbel) and some animals like Plasmodium and roundworms. Thus, the micro-organism ‘Plasmodium’ (which causes malaria disease) is a parasite. Roundworm which causes diseases in man and domestic animals (like dogs and cattle) is also a parasite. Roundworms live inside the body of man and his domestic animals. Several fungi and bacteria, and plants like Cuscuta (amarbel) are also parasites. Some other examples of parasites are ticks, lice, leeches and tapeworms.

3. Holozoic Nutrition

‘Holozoic nutrition’ means ‘feeding on solid food’ (which may be a plant product or an animal product). Most of the animals (including human beings) take the solid food into their body by the process of ingestion. The ingested food is then digested (broken down) into simpler substances which are then absorbed into the cells of the body. And the undigested and unabsorbed waste materials are egested (thrown out) of the body.

We can now say that: The holozoic nutrition is that nutrition in which an organism takes the complex organic food materials into its body by the process of ingestion, the ingested food is digested and then absorbed into the body cells of the organism. The undigested and unabsorbed part of the food is thrown out of the body of the organism by the process of egestion. The human beings and most of the animals have a holozoic mode of nutrition. In other words, man, cat, dog, cattle, deer, tiger, lion, bear, giraffe, frog, fish and Amoeba, etc., have the holozoic mode of nutrition.

Nutrition in plants

Just like other organisms, plants also require food which can supply energy for their various metabolic activities. Though animals can move from one place to another in search of food, plants just stand still at one place and make their own food. Green plants are autotrophic and synthesize their own food by the

process of photosynthesis. ‘Photo’ means ‘light’ and ‘synthesis’ means ‘to build’, thus ‘photosynthesis’ means ‘building up by light’. The plants use the energy in sunlight to prepare food from carbon dioxide and water in the presence of chlorophyll. Chlorophyll is present in the green coloured bodies called ‘chloroplasts’ inside the plant cells. In fact, the leaves of a plant are green because they contain tiny green coloured organelles called chloroplasts (which contain chlorophyll). Keeping these points in mind, we can now define the process of 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. Oxygen gas is released during photosynthesis. The process of photosynthesis can be represented as:

The process of photosynthesis takes place in the green leaves of a plant. In other words, food is made in the green leaves of the plant. The green leaves of a plant make the food by combining carbon dioxide and water in the presence of sunlight and chlorophyll. This is shown clearly in Figure 15. The carbon dioxide gas required for making food is taken by the plant leaves from the air. This carbon dioxide enters the leaves through tiny pores in them called stomata. Water required for making food is taken from the soil. This water is transported to the leaves from the soil through the roots and stem. The sunlight provides energy required to carry out the chemical reactions involved in the preparation of food. The green pigment called chlorophyll present in green leaves helps in absorbing energy from sunlight. Oxygen gas is produced as a by-product during the preparation of food by photosynthesis. This oxygen gas goes into the air.

The food prepared by the green leaves of a plant is in the form of a simple sugar called glucose. This glucose food made in the leaves is then sent to the different parts of the plant . The extra glucose is changed into another food called starch. This starch is stored in the leaves of the plant. Glucose and starch belong to a category of foods called carbohydrates. The foods like carbohydrates prepared by photosynthesis contain chemical energy stored in them. Thus, the green plants convert sunlight energy into chemical energy by making carbohydrates (foods). The food prepared by photosynthesis provides all the energy to a plant which it needs to grow. And when we eat plant foods (like foodgrains, fruits and vegetables), the chemical energy stored in them is released in our body during respiration.

We will now describe what actually happens during the process of photosynthesis. The photosynthesis takes place in the following three steps:

  1. Absorption of sunlight energy by chlorophyll.
  2. Conversion of light energy into chemical energy, and splitting of water into hydrogen and oxygen by light energy.
  3. Reduction of carbon dioxide by hydrogen to form carbohydrate like glucose by utilizing the chemical energy (obtained by the transformation of light energy).

Please note that the three steps involved in photosynthesis need not take place one after the other immediately. They can take place at different times. For example, desert plants take up carbon dioxide at night and prepare an intermediate product which is acted upon by the sunlight energy absorbed by chlorophyll when the sun shines during the next day.

Conditions Necessary for Photosynthesis

It has been found by experiments that the presence of sunlight, chlorophyll, carbon dioxide and water is necessary for the process of photosynthesis. So, we can say that The conditions necessary for photosynthesis to take place are:

  1. Sunlight,
  2. Chlorophyll,
  3. Carbon dioxide, and
  4. Water

Please note that the conditions necessary for photosynthesis are also the conditions necessary for autotrophic nutrition. We will now describe some experiments to show that sunlight, chlorophyll and carbon dioxide are necessary for photosynthesis by green plants. These experiments will also show that leaves finally make ‘starch’ as food by photosynthesis.

The experiments on photosynthesis depend on the fact that green leaves make starch as food. And that starch gives a blue-black colour with iodine solution. Now, ordinarily all the plants have starch in their green leaves, so before we can use a plant in a photosynthesis experiment, the initial starch present in its leaves must be removed. In other words, we should starch the leaves of a plant before using it in a photosynthesis experiment. The green leaves of a plant are destarched by keeping this plant in a completely dark place in a room for at least three days.

When the plant is kept in a dark place, it cannot make more starch (food) by photosynthesis because there is no sunlight. So, the plant kept in dark place uses the starch already stored in its leaves during respiration. The plant will use up all the starch stored in its leaves in about three days’ time. So, after about three days, the plant leaves will not have any starch left in them. And we say that the leaves have been destarched. This plant with destarched leaves can now be used in the photosynthesis experiments. Please note that we will be using a plant growing in a pot in these experiments. The ‘plant growing in a pot’ is called ‘potted plant’. Let us describe the experiments now.

1. Experiment to Show that Sunlight is Necessary for Photosynthesis
  1. We take a potted plant having green leaves and place it in a completely dark place for about three days to destarch its leaves. So, in the beginning of the experiment, the leaves do not have any starch in them.
  2. Take a thin strip of aluminum foil (or black paper) and wrap it in the center of one leaf on both the sides (while the leaf is still attached to the plant). The aluminium foil should be fixed tightly to the leaf by using paper clips so that sunlight may not enter it from the sides. The aluminium foil should cover only a small part of the leaf so that the remaining part of the leaf remains uncovered and exposed to sunlight. We have covered the centre part of the leaf with aluminium foil so that sunlight may not fall on this covered part of the leaf.
  3. Keep this potted plant (with partially covered leaf) in bright sunshine for three to four days
  4. Pluck the partially covered leaf from the plant and remove its aluminum foil. Immerse this leaf in boiling water for a few minutes. This will break down the cell membranes of leaf cells and make the leaf more permeable to iodine solution (so that it may reach the starch present inside the leaf cells). This

leaf is now to be tested for the presence of starch. But before testing for starch, chlorophyll has to be removed from the leaf. This is because chlorophyll interferes in the test for starch due to its green colour.

5. Put the plucked leaf in a beaker containing some alcohol. Place the beaker containing alcohol and leaf in a water bath (A water bath can be a bigger beaker containing water). A water bath is being used here for heating alcohol because alcohol is a highly inflammable liquid. So, if alcohol is heated directly over a flame, then it will catch fire at once.

6. Heat the water in the bigger beaker (or water bath). Then the alcohol in the smaller beaker will also get heated and start boiling soon. This boiling alcohol will extract (or remove) chlorophyll from the green leaf.

7. Boil the green leaf in alcohol till all its green pigment ‘chlorophyll’ is removed. The leaf will now become almost colourless or pale (and the alcohol will turn green).

8. Remove the colourless leaf from alcohol and wash it thoroughly with hot water to soften it and remove any chlorophyll which may be sticking to it.

9. Place the colourless leaf in a petri-dish. Drop iodine solution over the decolourised leaf with the help of a dropper. Observe the change in colour of leaf.

10. The middle part of leaf which was covered with aluminum foil does not turn blue-black on adding iodine solution showing that no starch is present in this middle part of the leaf . This is because sunlight could not reach the covered ‘middle part of the leaf due to which the covered ‘middle part of leaf could not do photosynthesis to make starch.

11. The uncovered part of leaf (on both sides of the aluminium foil) which was exposed to sunlight turns blue-black on adding iodine solution showing that starch is present in this part of leaf . This means that the part of leaf which was exposed to sunlight could do photosynthesis to make starch.

12. Since the part of leaf which was covered and hidden from sunlight does not contain starch but the part of leaf which was exposed to sunlight contains starch, therefore, we conclude that sunlight is necessary for photosynthesis (to make food like starch).

From the above experiment, we actually get two conclusions. That:

  1. sunlight is necessary for the process of photosynthesis, and
  2. leaves make starch as food by photosynthesis.

Most of the common plants have leaves which are totally green (because all the parts of such leaves contain the green pigment called chlorophyll). But there are some plants whose leaves are partly green and partly white. The green part of such a leaf contains chlorophyll but the white part of such a leaf does not contain chlorophyll. The leaves which are partly green and partly white are called ‘variegated leaves’. The plants such as croton and Coleus have variegated leaves which are partly green and partly white. We will use a plant having variegated leaves in the next experiment to show that chlorophyll is necessary for the process of photosynthesis in plants.

2. Experiment to Show that Chlorophyll is Necessary for Photosynthesis

1. We take a potted plant like croton whose leaves are partly green and partly white. The green part of the leaf has chlorophyll but the white part of the leaf does not have chlorophyll.

2. Place this plant in a completely dark place for about three days to destarch its leaves.

3. Take out the potted plant from the dark place and keep it in bright sunshine for three to four days.

4. Pluck the variegated leaf from the plant, boil it in water for a few minutes and then remove its green colour ‘chlorophyll’ by boiling it in alcohol. The green parts of the leaf get decolourised. So, we get decolourised leaf .

5. Wash the decolourised leaf with hot water to soften it and remove any chlorophyll which may be sticking to it.

6. Pour iodine solution over the colourless leaf and observe the change in colour of the leaf.

7. We will find that the outer part of leaf that was originally white (without chlorophyll) does not turn blue-black on adding iodine solution showing that no starch is present in this outer part of the leaf .  From this observation we conclude that the photosynthesis to make starch does not take place without chlorophyll.

8. The inner part of leaf which was originally green (contained chlorophyll) turns blue-black on adding iodine solution showing that starch is present in this inner part of the leaf . From this observation we conclude that the photosynthesis to make starch takes place in the presence of chlorophyll. In other words, chlorophyll is necessary for the process of photosynthesis to take place.

3. Experiment to Show that Carbon Dioxide is Necessary for Photosynthesis

1. We take a potted plant having long and narrow leaves and place it in a completely dark place for about three days to destarch its leaves.

2. Take a glass bottle having a wide mouth and put some potassium hydroxide solution (KOH solution) in it. (This potassium hydroxide solution is to absorb the carbon dioxide gas from the air present in the glass bottle so that no carbon dioxide is left in the air inside the glass bottle).

3. Take a rubber cork which fits tightly into the mouth of the glass bottle and cut it into two halves.

4. Put a destarched leaf of the potted plant (while it is still attached to the plant), in-between the two halves of the cut cork and then fit the cork in the mouth of the glass bottle. The upper half of the leaf should remain outside the glass bottle and only the lower half of the leaf should be inside the glass bottle .

5. The potted plant (with its one destarched leaf half inside the glass bottle containing potassium hydroxide solution) is kept in sunlight for 3 to 4 days. During this period, the upper half of the leaf (which is outside the glass bottle) gets carbon dioxide from the air but the lower half of the leaf (which is inside the glass bottle) does not get any carbon dioxide. This is because all the carbon dioxide of the air present in the glass bottle has been absorbed by potassium hydroxide solution. And no fresh air can come into the closed glass bottle.

6. Pluck the leaf from the plant and take it out from the glass bottle. Remove the green coloured chlorophyll from the leaf by boiling it in alcohol. In this way, we get a decolourised leaf .

7. Wash the decolourised leaf with water to remove any chlorophyll which may be sticking to it.

8. Pour iodine solution over the colourless leaf and observe the change in colour of the leaf.

9. We will find that the lower half part of the leaf (which was inside the glass bottle having no carbon dioxide around it), does not turn blue-black on adding iodine solution showing that no starch is present in this lower half of the leaf . From this observation we conclude that the photosynthesis to make starch in the leaf does not take place without carbon dioxide.

10. The upper half part of the leaf (which was outside the glass bottle, having carbon dioxide around it) turns blue-black on adding iodine solution showing that starch is present in this upper half of the leaf . From this observation we conclude that photosynthesis (to make starch) takes place in the presence of carbon dioxide. In other words, carbon dioxide is necessary for the process of photosynthesis to take place.

Raw Materials for Photosynthesis

The preparation of carbohydrates (food) by plants by the process of photosynthesis requires two materials (or substances): carbon dioxide, and water. Thus, the raw materials for photosynthesis are:

  1. Carbon dioxide, and
  2. Water.

We will now describe how these two raw materials become available to plants for photosynthesis.

1. How the Plants Obtain Carbon Dioxide

There are a large number of tiny pores called stomata on the surface of the leaves of plants (The singular of stomata is stoma). The green plants take carbon dioxide from air for photosynthesis. The carbon dioxide gas enters the leaves of the plant through the stomata present on their surface. Each stomatal pore (or stoma) is surrounded by a pair of guard cells. The opening and closing of stomatal

pores is controlled by the guard cells. When water flows into the guard cells, they swell, become curved and cause the pore to open. On the other hand, when the guard cells lose water, they shrink, become straight and close the stomatal pore . A large amount of water is also lost from the cells of the plant leaves through open stomatal pores. So, when the plant does not need carbon dioxide and wants to conserve water, the stomatal pores are closed. The oxygen gas produced during photosynthesis also goes out through the stomatal pores of the leaves. Please note that in addition to leaves, the stomata are also present in the green stems (or shoots) of a plant. So, the green stems (or shoots) of a plant also carry out photosynthesis. It is clear from the above discussion that stomata allow the movement of gases in and out of plant cells. In other words, the gaseous exchange in plants takes place through the stomata in leaves (and other green parts).

Please note that in most broad-leaved plants, the stomata occur only in the lower surface of the leaf but in narrow-leaved plants, the stomata are equally distributed on both the sides of the leaf. Another point to be noted is that the aquatic plants (or water plants) use the carbon dioxide gas dissolved in water for carrying out photosynthesis.

2. How the Plants Obtain Water for Photosynthesis

The water required by the plants for photosynthesis is absorbed by the roots of the plants from the soil through the process of osmosis. The water absorbed by the roots of the plants is transported upward through the xylem vessels to the leaves where it reaches the photosynthetic cells and utilised in photosynthesis.

The two raw materials, carbon dioxide and water, are required by the plants to prepare energy foods called carbohydrates (such as glucose and starch). But the plants also need other raw materials such as nitrogen, phosphorus, iron and magnesium, etc., for building their body. The plants take materials like nitrogen, phosphorus, iron and magnesium, etc., from the soil. For example, nitrogen is an essential element used by the plants to make proteins and other compounds.

The plants take up nitrogen from the soil in the form of inorganic salts called nitrates (or nitrites), or in the form of organic compounds which are produced by bacteria from the atmospheric nitrogen.

Site of Photosynthesis: Chloroplasts

Chloroplasts are the organelles in the cells of green plants which contain chlorophyll and where photosynthesis takes place. Thus, photosynthesis occurs in the organelles called chloroplasts present in the photosynthetic cells (or mesophyll cells) of green plants.

In other words, the site of photosynthesis in a cell of the leaf are chloroplasts. Chloroplasts can be seen easily by using a light microscope. In a cross- section of a leaf, chloroplasts can be seen as numerous disc-like organelles in the photosynthetic cells (or mesophyll cells) of the palisade tissue just below the upper epidermis

In the structure of a leaf shown in , we can see that the middle layers in the leaf (palisade layer and spongy layer) contain photosynthetic cells called mesophyll cells. These cells contain more chlorophyll than other plant cells. A typical photosynthetic cell (or mesophyll cell) of a green leaf may contain 100 or more tiny chloroplasts in it, and a whole leaf may contain many thousands of photosynthetic cells. Carbon dioxide needed for photosynthesis enters from the air into the leaf through the stomata in its surface , and then diffuses into the mesophyll cells and reaches the chloroplasts. Water is carried to the leaf by xylem vessels and passes into the mesophyll cells by diffusion and reaches the chloroplasts. There is a thin, waxy protective layer called cuticle above and below a leaf which helps to reduce the loss of water from the leaf.

Nutrients in Animals

We have just studied the nutrition in plants. We have learnt that plants are autotrophic organisms which can manufacture their own food. So, plants don’t have to look to others for getting their food. They are food producers themselves. But this is not so in the case of animals. Animals are heterotrophs and hence they depend on other organisms for their food. Thus, animals need an external source of food. We will now discuss how animals obtain their food.

Animals Obtain their Food from Plants or Other Animals

Since animals cannot make their own food, they depend on readymade food. This readymade food comes either from ‘plants’ or from ‘other animals’. Thus, animals obtain their food from plants or other animals (which they eat). We (human beings) are also animals. We obtain the foods like wheat, rice, pulses (dal), fruits and vegetables from plants. And the foods like milk, curd, cheese and eggs are obtained from animals.

Some people also eat meat, chicken and fish as food. These foods are also obtained from animals. Many other animals obtain their food by eating the flesh of other animals. For example, the fish, birds, snakes and insects, all obtain their food from other animals. The big fish eats small fish; the birds eat worms and insects; the snake eats frogs and the insects eat dead bodies of animals. The non-green plants

(which cannot make their own food by photosynthesis) also obtain their food from other plants and animals. Yeast plant is one such example. Even the plants can eat insects. For example, the pitcher plant and the venus fly-trap are the two plants which eat insects. All the animals can be divided into three groups on the basis of their food habits (or eating habits). These are:

  • (i) Herbivores,
  • (ii) Carnivores, and
  • (iii) Omnivores.

We will now discuss herbivores, carnivores and omnivores in somewhat detail. Let us start with the herbivores.

1. Herbivores

Some animals eat only plants (or their products). Those animals which eat only plants are called herbivores. The herbivores may eat grasses, leaves, grains, fruits or the bark of trees. Some of the examples of herbivores are: Goat, Cow, Buffalo, Sheep, Horse, Deer, Camel, Ass, Ox, Elephant, Monkey, Squirrel, Rabbit, Grasshopper and Hippopotamus. Cow is called a herbivore because it eats only plants as food. Thus, herbivores are plant eaters. Herbivores are also called herbivorous animals.

2. Carnivores

Some animals eat only other animals. They do not eat plant food at all. Those animals which eat only other animals as food are called carnivores. Carnivores eat only the meat (or flesh) of other animals. So, we can also say that: Those animals which eat only the meat (or flesh) of other animals are called carnivores.

Some of the examples of carnivores are: Lion, Tiger, Frog, Vulture, Kingfisher, Lizard, Wolf, Snake and Hawk. Lion is called a carnivore because it eats only the meat (or flesh) of other animals like deer, rabbit, goat, etc. Thus, carnivores are meat eaters. Carnivores are also called carnivorous animals.

3. Omnivores

Some animals eat both, plants as well as other animals as food. Those animals which eat both, plants and animals, are called omnivores. In other words, the omnivores eat plant food as well as the meat (or flesh) of other animals. Some of the examples of omnivores are: Man (Human beings), Dog, Crow, Sparrow, Bear, Mynah, and Ant.

Man is called an omnivore because he eats the plant food (such as grains, pulses, fruits and vegetables) as well as the meat of animals (such as goat, chicken and fish). Thus, omnivores are plant eaters as well as meat eaters. Omnivores are also called omnivorous animals.

All the living things on earth actually depend on the sun for their food. This has been shown clearly in given below:

Plants use the energy of sun and prepare food by photosynthesis. The plants utilise this food for maintaining their life. These plants (and their products) are also eaten up by herbivores and omnivores as food. And the carnivores eat herbivores as food. In this way, it is the energy of the sun which provides food for plants, and animals (herbivores, carnivores and omnivores).

In , the goat is a herbivore which eats plants; man is an omnivore who eats both, plants and meat of goat; and lion is a carnivore which eats the flesh of goat (The man usually does not get eaten up by lion because he is a very clever fellow !).

An organism either makes its own food from raw materials as green plants do or takes in readymade food as animals do. The process of obtaining food and then using it for obtaining energy, growth and repair of the body, is called nutrition. We will now discuss the animal nutrition in detail.

Different Steps in the Process of Nutrition in Animals

There are five main processes concerned with the use of food by animals. In other words, there are five steps in the process of nutrition in animals. These are: Ingestion, Digestion, Absorption, Assimilation and Egestion. All these steps are discussed below:

1. Ingestion

In order to provide the energy necessary for growth and carry on life’s activities, we must ‘eat food’ or ‘take food into the body’. The process of taking food into the body is called ingestion. In most simple terms, ingestion means ‘eating of food’ by the animal. When we put food into our mouth with hands, we are ingesting (the food).

2. Digestion

The food of most animals consists of large insoluble molecules which cannot be absorbed by the animal’s body in this form. So, before the food can be used by the animal for various functions like getting energy or for growth, it must be broken down into small, water soluble molecules which can be absorbed by the body.

The process in which the food containing large, insoluble molecules is broken down into small, water soluble molecules (which can be absorbed by the body) is called digestion. In most simple terms, digestion is the dissolving of the solid food.

Digestion makes the food soluble so that it can be utilised by the animal’s body. Most animals use both, physical and chemical methods for digesting (breaking up) the large food molecules. Physical methods include chewing and grinding the food in mouth and chemical methods include the addition of digestive juices (enzymes) to food by the body itself.

3. Absorption

After digestion, the food molecules become small and soluble. The soluble food molecules can pass through the walls of our intestine and go into blood. The process in which the digested food passes through the intestinal wall into blood stream is called absorption.

4. Assimilation

Blood carries the absorbed food to all the parts of the body. The food then enters each and every cell of the body where it is used for producing energy and for making materials for the growth and repair of the body. The process in which the absorbed food is taken in by body cells and used for energy, growth and repair, is called assimilation.

5. Egestion

The whole food which we eat is not digested by our body. A part of the food which we eat remains undigested (or insoluble) which cannot be used by the body. This undigested part of the food is then removed from the body in the form of faeces when we go to toilet. The process in which the undigested food is removed from the body is called egestion

Nutrition in Simple Animals

Amoeba and Paramecium are two very simple animals. The body of each one of them consists of a single cell only. They are called unicellular animals. In unicellular animals, all the processes of nutrition are performed by the single cell. This point will become more clear from the following example of the nutrition in Amoeba.

Nutrition in Amoeba

Amoeba is a unicellular animal. Amoeba eats tiny (microscopic) plants and animals as food which float in water in which it lives. The mode of nutrition in Amoeba is holozoic. The process of obtaining food by Amoeba is called phagocytosis (‘Phagocytosis’ means ‘cell feeding’). The various steps involved in the nutrition of Amoeba are ingestion, digestion, absorption, assimilation, and egestion. All the processes of nutrition are performed by the single cell of Amoeba. This is described below.

1. Ingestion

Amoeba has no mouth or a fixed place for the ingestion of food (intake of food). Amoeba ingests food by using its pseudopodia. When a food particle comes near Amoeba, then Amoeba ingests this food particle by forming temporary finger-like projections called pseudopodia around it . The food is engulfed with a little surrounding water to form a food vacuole inside the Amoeba. This food vacuole can be considered to be a ‘temporary stomach’ of Amoeba.

2. Digestion

In Amoeba, food is digested in the food vacuole by digestive enzymes. The enzymes from surrounding cytoplasm enter into the food vacuole and break down the food into small and soluble molecules by chemical reactions. Thus, digestion in Amoeba takes place inside the food vacuole due to which the food dissolves (or food becomes soluble).

3. Absorption

The digested food present in the food vacuole of Amoeba is absorbed directly into the cytoplasm of Amoeba cell by diffusion. Since Amoeba consists of only one small cell, it does not require blood system to carry the digested food. The digested food just spreads out from the food vacuole into the whole of Amoeba cell. After absorption of food, the food vacuole disappears.

4. Assimilation

A part of the food absorbed in Amoeba cell is used to obtain energy through respiration. The remaining part of absorbed food is used to make the parts of Amoeba cell which lead to the growth of Amoeba. Thus, on assimilating food Amoeba grows in size . And then Amoeba can reproduce by dividing into two daughter cells.

5. Egestion

Amoeba has no fixed place (like anus) for removing the undigested part of food. When a considerable amount of undigested food collects inside Amoeba, then its cell membrane suddenly ruptures at any place and the undigested food is thrown out of the body of Amoeba Paramecium is also a tiny unicellular animal which lives in water. Paramecium uses its hair like structures called cilia to sweep the food particles from water and put them into its mouth . The Paramecium has thin, hair-like cilia all over its body. The cilia move back and forth rapidly in water.

When the cilia present around the mouth region of Paramecium move back and forth, they sweep the food particles present in water into the mouth of Paramecium . This is the first step in the nutrition of Paramecium which is called ingestion. Ingestion is followed by other steps such as digestion, absorption, assimilation and egestion (as explained in the case of Amoeba).

Nutrition in Complex Multicellular Animals

In the complex multicellular animals like man (humans), grasshopper, fish and frog, etc., all the processes involved in nutrition are performed by a combination of digestive organs. This combination of digestive organs is called digestive system. We will now describe all the processes in the nutrition of complex multicellular animals by taking the example of nutrition in human beings. Please note that a long tube running from mouth to anus of a human being (or other animals) in which digestion and absorption of food takes place is called alimentary canal. Alimentary canal is also called ‘gut’. Let us now study the nutrition in human beings.

Nutrition in human beings (Human Digestive System)

The nutrition in human beings (or man) takes place through human digestive system. The human digestive system consists of the alimentary canal and its associated glands. The various organs of the human digestive system in sequence are: Mouth, Oesophagus (or Food pipe), Stomach, Small intestine and Large intestine. The glands which are associated with the human digestive system and form a part of the human digestive system are: Salivary glands, Liver and Pancreas.

The human alimentary canal which runs from mouth to anus is about 9 metres long tube. The ducts of various glands open into the alimentary canal and pour the secretions of the digestive juices into the alimentary canal. The human digestive system . We will now describe the various steps of nutrition in human beings (or man)

1. Ingestion

The human beings have a special organ for the ingestion of food. It is called mouth. So, in human beings, food is ingested through the mouth. The food is put into the mouth with the help of hands.

2. Digestion

In human beings, the digestion of food begins in the mouth itself. In fact, the digestion of food starts as soon as we put food in our mouth. This happens as follows: The mouth cavity (or buccal cavity) contains teeth, tongue, and salivary glands. The teeth cut the food into small pieces, chew and grind it. So, the teeth help in physical digestion. The salivary glands in our mouth produce saliva. Our tongue helps in mixing this saliva with food. Saliva is a watery liquid so it wets the food in our mouth. The wetted food can be swallowed more easily.

Many times we have observed that when we see or eat a food which we really like, our mouth ‘waters’. This watering of mouth is due to the production of saliva by the salivary glands in the mouth. The salivary glands help in chemical digestion by secreting enzymes. The human saliva contains an enzyme called salivary amylase which digests the starch present in food into sugar. Thus, the digestion of starch (carbohydrate) begins in the mouth itself. Since the food remains in the mouth only for a short time, so the digestion of food remains incomplete in mouth.

The slightly digested food in the mouth is swallowed by the tongue and goes down the food pipe called esophagus . The esophagus carries food to the stomach. This happens as follows: The walls of food pipe have muscles which can contract and expand alternately. When the slightly digested food enters the food pipe, the walls of food pipe start contraction and expansion movements. The contraction and expansion movement of the walls of food pipe is called peristaltic movement. This peristaltic movement of food pipe (or esophagus) pushes the slightly digested food into the stomach (In fact, the peristaltic movement moves the food in all the digestive organs throughout the alimentary canal).

The stomach is a J-shaped organ present on the left side of the abdomen. The food is further digested in the stomach. as viewed by an endoscope. The food is churned in the stomach for about three hours. During this time, the food breaks down into still smaller pieces and forms a semi-solid paste. The stomach wall contains three tubular glands in its walls. The glands present in the walls of the stomach secrete gastric juice. The gastric juice contains three

substances: hydrochloric acid, the enzyme pepsin and mucus. Due to the presence of hydrochloric acid, the gastric juice is acidic in nature. In the acidic medium, the enzyme pepsin begins the digestion of proteins present in food to form smaller molecules. Thus, the protein digestion begins in the stomach. Please note that the protein digesting enzyme pepsin is active only in the presence of an acid. So, the function of hydrochloric acid in the stomach is to make the medium of gastric juice acidic so that the enzyme pepsin can digest the proteins properly.

Another function of hydrochloric acid is that it kills any bacteria which may enter the stomach with food. The mucus helps to protect the stomach wall from its own secretions of hydrochloric acid. If mucus is not secreted, hydrochloric acid will cause the erosion of inner lining of stomach leading to the formation of ulcers in the stomach. The partially digested food then goes from the stomach into the small intestine. The exit of food from stomach is regulated by a ‘sphincter muscle’ which releases it in small amounts into the small intestine.

From the stomach, the partially digested food enters the small intestine. The small intestine is the largest part of the alimentary canal. It is about 6.5 meters long in an adult man. Though the small intestine is very long, it is called small intestine because it is very narrow. The small intestine is arranged in the form of a coil in our belly . Please note that the length of the small intestine differs in various animals depending on the type of food they eat. For example, cellulose is a carbohydrate food which is digested with difficulty.

So, the herbivorous animals like cow which eat grass need a longer ‘small intestine’ to allow the cellulose present in grass to be digested completely. On the other hand, meat is a food which is easier to digest. So, the carnivorous animals like tigers which eat meat have a shorter ‘small intestine’.

The small intestine in human beings is the site of complete digestion of food (like carbohydrates, proteins and fats). This happens as follows:

  • (a) The small intestine receives the secretions of two glands: liver and pancreas. Liver secretes bile. Bile is a greenish yellow liquid made in the liver which is normally stored in the gall bladder. Bile is alkaline, and contains salts which help to emulsify or break the fats (or lipids) present in the food. Thus, bile performs two functions:
    • (i) makes the acidic food coming from the stomach alkaline so that pancreatic enzymes can act on it, and
    • bile salts break the fats present in the food into small globules making it easy for the enzymes to act and digest them.

Pancreas is a large gland which lies parallel to and beneath the stomach . Pancreas secretes pancreatic juice which contains digestive enzymes like pancreatic amylase, trypsin and lipase. The enzyme amylase breaks down the starch, the enzyme trypsin digests the proteins and the enzyme lipase breaks down the emulsified fats.

  • (b) The walls of small intestine contain glands which secrete intestinal juice. The intestinal juice contains a number of enzymes which complete the digestion of complex carbohydrates into glucose, proteins into amino acids and fats into fatty acids and glycerol. Glucose, amino acids, fatty acids and glycerol are small, water soluble molecules. In this way, the process of digestion converts the large and insoluble food molecules into small, water soluble molecules. The chemical digestion of food is brought about by biological catalysts called enzymes.
3. Absorption

After digestion, the molecules of food become so small that they can pass through the walls of the small intestine (which contain blood capillaries) and go into our blood. This is called absorption. The small intestine is the main region for the absorption of digested food. In fact, the small intestine is especially adapted for absorbing the digested food. The inner surface of small intestine has millions of tiny, finger- like projections called villi. The presence of villi gives the inner walls of the small intestine a very large surface area. And the large surface area of small intestine helps in the rapid absorption of digested food. The digested food which is absorbed through the walls of the small intestine, goes into our blood.

4. Assimilation

The blood carries digested and dissolved food to all the parts of the body where it becomes assimilated as part of the cells. This assimilated food is used by the body cells for obtaining energy as well as for growth and repair of the body. The energy is released by the oxidation of assimilated food in the cells during respiration. The digested food which is not used by our body immediately is stored in the liver in the form of a carbohydrate called ‘glycogen’. This stored glycogen can be used as a source of energy by the body as and when required.

5. Egestion

A part of the food which we eat cannot be digested by our body. This undigested food cannot be absorbed in the small intestine. So, the undigested food passes from the small intestine into a wider tube called large intestine (It is called large intestine because it is a quite wide tube). The walls of large intestine absorb most of the water from the undigested food (with the help of villi).

Due to this, the undigested part of food becomes almost solid. The last part of the large intestine called ‘rectum’ stores this undigested food for some time. And when we go to the toilet, then this undigested food is passed out (or egested) from our body through anus as faeces or ‘stool’ . The act of expelling the faeces is called egestion or defecation. The exit of faeces is controlled by the anal sphincter.

Let us solve one problem now.

Sample Problem. 1 mL of very dilute starch solution (1% starch solution) is taken in a test-tube and 1 mL of saliva is added to it. After keeping this mixture for half an hour, a few drops of dilute iodine solution are added to the test-tube. There is no change in colour on adding iodine solution. What does this tell you about the action of saliva on starch?

Answer. When a mixture of dilute starch solution and saliva is kept in a test-tube for half an hour, it does not produce a blue-black colour with iodine solution showing that no starch is left in the test-tube. This tells us that the action of saliva has broken down starch into some other substance which does not give any colour with iodine solution. Actually, saliva contains an enzyme ‘amylase’ which converts starch into a sugar.

Dental Caries

The hard, outer covering of a tooth is called enamel. Tooth enamel is the hardest material in our body. It is harder than even bones. The part of tooth below enamel is called dentine. Dentine is similar to bone. Inside the dentine is pulp cavity. The pulp cavity contains nerves and blood vessels. The formation of small cavities (or holes) in the teeth due to the action of acid-forming bacteria and improper dental care is called dental caries. This happens as follows.

When we eat sugary food, the bacteria in our mouth act on sugar to produce acids. These acids first dissolve the calcium salts from the tooth enamel and then from dentine forming small cavities (or holes) in the tooth over a period of time. The formation of cavities reduces the distance between the outside of the tooth and the pulp cavity which contains nerves and blood vessels. The acids produced by bacteria irritate the nerve endings inside the tooth and cause toothache. If the cavities caused by dental decay are not cleaned and filled by a dentist, the bacteria will get into the pulp cavity of tooth causing inflammation and infection leading to severe pain.

If the teeth are not cleaned regularly, they become covered with a sticky, yellowish layer of food particles and bacteria cells called ‘dental plaque’. Since plaque covers the teeth forming a layer over them, the alkaline saliva cannot reach the tooth surface to neutralise the acid formed by bacteria and hence tooth decay sets in. Brushing the teeth regularly, after eating food, removes the plaque before bacteria produces acids. This will prevent dental caries or tooth decay. Before we go further and discuss respiration,

class – 10 more chapter

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