The Invisible Living World

 

A cell is the smallest structural and functional unit of life, as all living things are made up of cells.

 

CELL

Cell is the basic structural and functional unit of all the living organisms. That is why, cells are also called as the "building blocks of the body". Just like as the bricks are used to make a building, cells are assembled to make the body of a living thing.

 

The word 'cell' comes from the Latin word 'cellula', meaning 'a small room'. Even though Zaccharias Jenssen is credited with inventing the earliest microscope, it was Anton Von Leeuwenhoek who developed lenses powerful enough to see cells. For his contributions to science, Anton Von Leeuwenhoek is called the 'father of microbiology'.

 

Discovery of the cell

Robert Hooke (1665) used a simple microscope to observe the slices of cork, obtained from the bark of a tree. He observed some honeycomb-like structures, with compartments or boxes, separated from each other by a wall or partition. Hooke named these compartments 'cells. But the cells that he observed were actually the dead cells in the cork. The cells of the living organisms were observed only after the discovery of improved microscopes with very high magnification.

 

Cell theory

In 1838, Schleiden and Schwann, two German biologists, presented the cell theory of life. Cell theory given by them was further expanded by Virchow. The major points of the cell theory are: All living things are made up of cells. Cells are the basic structural and functional units of life, New cells arise from pre-existing cells by their division.

The organisation of the cells in the body of a living organism decides its structure and functions.

 

A microscope is an instrument used to view very tiny things by magnifying them. It enables us to see the different types of living cells and the structures they contain.

 

STRUCTURE OF A CELL

As you have observed in the activities, plant and animal cells show certain differences but their basic structure is similar. Both cells are enclosed by a cell membrane and in case of plant cells there is a cell wall also. Inside the cell lies the nucleus and a jelly-like substance called the cytoplasm. Many small structures are present in the cytoplasm, called cell organelles. We will discuss the shapes of different types of cells later in the chapter.

 

1. Cell wall

Plants cannot move from one place to another, so they are not able to protect themselves from harsh weather conditions and from their enemies. That is why an extra covering in the form of the cell wall is present in plant cells. Cell wall is the outermost layer present in plant cells. It is a thick, non-elastic, rigid, and non-living membrane made up of cellulose (complex carbohydrate). It is fully permeable, i.e, it allows entry and exit of all substances in the cell. Cell wall provides protection and shape to the plant cell.

 

2. Cell membrane

Cell membrane or plasma membrane is present in plants as well as in animal cells. It is a thin, elastic, delicate living membrane made up of lipids and proteins that separates the cell from its surroundings. It is selectively permeable, i.e., it allows entry and exit of only few substances in the cell thus maintaining the internal composition of the cell. Cell membrane provides shape and protection to the cell.

 

Some cell organelles
Mitochondria	These produce the energy required for various functions of the body. For this reason, mitochondria are called the 'powerhouse of the cell'. These may be oval or rod-shaped.
Vacuoles	Vacuoles are fluid-filled structures enclosed by the membranes. They store substances, like food, water, and wastes produced by the cell during its various processes. Plant cells have larger vacuoles as compared to the animal cells.
Chloroplasts	These are found only in plant cells. Chloroplasts contain chlorophyll, the green pigment necessary for the photosynthesis. Plant cells also have coloured organelles called chromoplasts which impart colour to the flowers and fruits.

 

3. Nucleus

The nucleus is a spherical or oval structure known as the 'brain of the cell'. It is covered by a nuclear membrane that separates it from the cytoplasm. The nucleus contains another smaller, round structure called nucleolus. A network of thread-like structures called chromatin is seen within the nucleus. The chromatin when condensed, appear like threads or fibres called chromosomes.

 

4. Cytoplasm

Cytoplasm is a transparent, jelly-like substance, which occupies the space between the cell membrane and the nuclear membrane. The organelles necessary for cellular function are embedded in the cytoplasm.

 

Differences between plant and animal cells
Plant cell	Animal cell
All plant cells have a cell wall as the outermost covering.	Cell membrane is the outermost covering in animal cells.
Plant cells contain chloroplasts which help in photosynthesis.	Chloroplasts are absent in animal cells and hence photosynthesis does not take place.
One or two big vacuoles are present in each cell.	Vacuoles may be absent; when present they are small and few in number.

 

VARIATIONS IN CELLS

There are millions of living organisms. All of them have different shapes and sizes, and consist of different types of cells. The variations in size, shape, and number of living cells lead to the vast variety in nature.

 

Number of cells

Different organisms have different number of cells. For example, the human body has about 100 trillion cells of different shapes and sizes. On the basis of the number of cells present in organisms, they are classified as unicellular and multicellular organisms.

 

 

Differences between unicellular and multicellular organisms
Unicellular organisms	Multicellular organisms.
Made up of one cell only.	Made up of many cells.
One cell carries out all the functions.	Different cells carry out different functions.
Death of one cell leads to the death of the organism.	Death of one cell does not lead to the death of the organism.
Do not show levels of organisation. (We will discuss this in the next section)	Show various levels of organisation. (We will discuss this in the next section)
Examples- amoeba, bacteria, paramecium, etc.	Examples: man, cow, dog, tree, etc.

 

Division of labour

There is a division of labour in the body of multicellular organisms. For example, the stomach digests food, the heart pumps blood and so on. Similarly, division of labour is also seen in unicellular organisms where different cell organelles, such as mitochondria, ribosomes, etc., perform different functions, like making new materials in the cell, clearing up waste material, etc. A cell is able to perform various functions because of its organelles.

 

Variation in cell size
Cell	Size
Egg of an ostrich	Largest cell in the world (170 mm 130 mm)
Neurons	Longest cell in the human body (upto 100 mm)
PPLO (Pleuro pneumonia-Like Organism)	Smallest cell in the world (0.1 to 0.5 µm)

 

LEVELS OF ORGANISATION

In multicellular organisms, different cells carry out different functions. However, in unicellular organisms, a single cell performs all the required functions.

The bodies of multicellular organisms consist of many types of cells. They are organised into different levels to coordinate various functions of an organism. These levels are:

 

(a)  Cells: The smallest living unit of the body is cell. Many cells of same type organise to form a tissue.

(b) Tissues: A group of cells performing a similar function forms a tissue. For example, cells which form the lining of the skin constitute the epithelial tissue.

(c)  Organs: Different tissues assemble to form an organ. The organs perform specialised functions. For example, stomach, heart, kidney, etc., are organs.

(d) Organ systems: Various organs with different structures work together to perform a specific function. These organs form an organ system. For example, digestive system, respiratory system, and nervous system are some organ systems.

(e)  Organism: Various organ systems in the body are organised to make an organism.

 

Consider the example of the human body. It is run by various organ systems, like the digestive system, reproductive system, circulatory system, etc. The digestive system consists of organs, like stomach, intestine, etc., which in turn are made up of tissues, like epithelial tissue. The epithelial tissue is made up of millions of cells, each of them performing a specific function. In this way, the bodies of all living organisms is organised.

 

 

In the Atharvaveda, the terms Ronindass "Krimi-Drashya" and "Adhrishya" are used to describe different types of organisms. Krimi-Drashya refers to visible worms or insects, while Adhrishya refers to invisible organisms that cannot be seen with the naked eye. These ideas show that ancient Indian texts had a basic understanding of both visible and invisible life forms, which is similar to our modern concept of microorganisms.

 

MICROORGANISMS

The organisms which cannot be observed with the naked eye and can be seen only through a microscope are called microorganisms or microbes. Microorganisms are tiny organisms that are either unicellular or have very few cells. This is what makes them invisible to the naked eye. They are said to be microscopic in size and are thus referred to as microorganisms. The branch of science which deals with the study of microorganisms is called microbiology.

 

WHERE ARE MICROORGANISMS FOUND?

Microorganisms are present almost in every place, such as air water, ice-cold water, marine water, deserts, polar regions, food, and even inside our bodies.

They can survive in extremely harsh conditions some have been known to survive even volcanic eruptions. Under unfavourable conditions, microorganisms develop a hard outer covering around themselves known as cyst. Microorganisms remain inactive within the cyst till conditions like temperature and humidity, become favourable again.

 

CLASSIFICATION OF MICROORGANISMS

Microbes are mainly classified into five major groups. These are bacteria, algae, fungi, protozoa, and viruses. Classifications of Microorganisms

(a)  Bacteria

(b) Algae

(c)  Fungi

(d) Protozoa

(e)  Viruses.

 

1. Bacteria

These are simple, single celled organisms. They can be found in air, water, soil, and in the bodies of living organisms.

Bacteria are found in three shapes namely, bacillus (rod-shaped), cocci (spherical), and spirilla (spiral-shaped). Some bacteria are autotrophic, i.e., they produce their own food with the help of light or chemical reactions. However, most bacteria are heterotrophic, showing saprotrophic mode of nutrition, i.e. from dead and decaying matter, or parasitic mode of nutrition, i.e. from living organism referred here as host.

 

2. Algae

Algae are autotrophic organisms with undifferentiated body parts. Algae occur abundantly like grasses in aquatic habitats, so they are also called as 'grasses of water. These can be single-celled, i.e., unicellular or multi-celled, i.e., multicelluar.

Chlamydomonas is a unicellular alga, while Spirogyra and Laminaria are multicellular algae. They can survive in extreme climatic conditions. They usually grow in water bodies such as ponds, lakes and in moist soil. Some algae also grow on rocks and barks of trees.

Red and brown algae are used in manufacturing culture medium called 'agar', used in laboratories and hospitals. Some substances obtained from algae are used to thicken foods, such as ice creams and jellies.

 

3. Fungi  

These are a group of plant like organisms exhibiting heterotrophic (saprotrophic or parasitic) nutrition. They can be seen growing on moist bread, leather, cotton, paper, etc. Fungi grow vigorously in damp, warm, and dark places. Fungi may be unicellular or multicellular. For example, yeast is a unicellular fungus, while Rhizopus (common bread mould), Agaricus, Penicillium, and Aspergillus are multicellular fungi.

 

4. Protozoa

Unicellular organisms that exhibit animal-like characteristics are called protozoa. For example, amoeba, paramecium, giardia, etc. The word protozoan literally means 'the first animal'.

Most protozoa are heterotrophic. Some protozoa live in fresh or salt water and some live in soil. Some live as parasites in the bodies of other organisms, including human beings.

Protozoa have locomotory structures that allow them to move from place to place. For example, amoeba has pseudopodia and paramecium has cilia for locomotion.

 

5. Viruses

These are the smallest microorganisms. They can only be seen with the help of an electron microscope which has the capacity to magnify an object upto 2,00,000 times.

 

Viruses living or non-living?

v Viruses exhibit characteristics of living as well as non-living things. They cannot reproduce by themselves. They lack the cell organelles necessary for multiplication. Hence, they behave like non-living objects when present outside the host cell.

v But once they enter a host cell, they utilise its energy and start multiplying. In the process, they often kill the host cell. Viruses are considered a biological puzzle as they fall on the line between the living and the non-living.

 

USEFUL MICROORGANISMS

Microorganisms play an important role in our lives. They are useful in a number of fields:

 

Uses of microorganisms in the field of agriculture

v Certain bacteria (like rhizobium) and blue-green algae are able to fix nitrogen from the atmosphere to enrich soil with nitrogen and increase its fertility. These are commonly called nitrogen fixers. Rhizobium is found in soil and also in the root nodules of leguminous plants like peas, beans, etc.

v Various species of bacteria help in maintaining the nitrogen cycle in nature.

v Certain microorganisms help in the formation of humus from dead organic matter. Humus is used as manure for the plants.

 

Uses of microorganisms in food

v Lactobacillus bacteria help in making curd. When curd is added to warm milk, the lactobacillus bacteria present in curd

v multiply and convert the milk into curd. Bacteria are also useful in the preparation of cheese, pickles, and many other food items.

v Yeast (fungus) is used in baking and brewery industries for making bread, alcohol, and wine. Yeast breaks down glucose present in bread, fruit juices, etc., into ethyl alcohol and carbon dioxide in the absence of oxygen. This process is called fermentation and is used to make some dishes like dhoklas, idlis and dosas. The carbon dioxide released during fermentation makes dhoklas and idlis soft and fluffy.

v Louis Pasteur discovered fermentation in 1857. Yeast used in baking industry is called baker's yeast.

 

Medicinal use of microbes

v Antibiotics: These are medicines capable of killing harmful microbes. Bacteria and fungi are used to make them. For example, penicillin is obtained from a fungus called Penicillium notatum. This fungus is capable of destroying Staphylococcus bacteria that causes cold.

v Other examples of antibiotics obtained from fungus and bacteria are streptomycin, tetracycline, and erythromycin.

v Antibiotics are even mixed with the feed of livestock and poultry to protect them from diseases and to check any microbial infection. They are also used to control many plant diseases caused by microorganisms.

v While taking antibiotics, it is important to complete the course prescribed by the doctor. This is because bacteria can develop resistance against antibiotics, especially if they have been taken unnecessarily or in wrong doses. This may make the drug less effective when taken in future. Also, antibiotics taken when not required may kill beneficial bacteria in the body.

 

*        Antibiotics are not effective against diseases caused by viruses. *        Edward Jenner discovered the vaccine for small pox in 1798. A worldwide campaign against small pox has finally led to its eradication from most parts of the world. *        Vaccines are given to children to prevent diseases. *        Immunity is the ability of our body to fight against diseases.

 

Vaccines

v Microorganisms are also used to make vaccines which protect human beings and animals from diseases, like typhoid, tuberculosis, hepatitis, measles, polio, small pox, etc.

v Vaccine is a suspension of dead or weak microbes which, when introduced into the body, stimulates our immune system to produce special proteins called antibodies to fight diseases. These antibodies remain in the body and protect it from any microbial attack in the future.

v Vaccines, therefore, provide immunity to our body against diseases.

v The process of introducing a vaccine in an organism's body for protection against disease is called vaccination.

v Insulin, a hormone that controls blood sugar in the human body, can also be obtained from bacteria.

v Some types of bacteria and yeast are used in the synthesis of vitamin B complex tablets.

 

Role of microbes in cleaning the environment

v Microorganisms such as bacteria and fungi help in cleaning the environment by decomposing dead and decaying organic matter. They release the nutrients trapped in the bodies of dead plants and animals back into the soil, thus maintaining the nutrient cycle in nature.

v Some bacteria like pseudomonas decompose sewage and other wastes in water. This helps in cleaning the environment and water.

v Algae, being autotrophic, release oxygen in water during photosynthesis.

 

Other uses of microbes

v Certain species of bacteria are used in the leather industry for tanning. Some bacteria are used for rotting of jute fibres, curing of tea leaves, etc.

v Some bacteria and protozoa reside in the digestive systems of ruminant animals, like cows, sheep, etc., and help in the digestion of cellulose.

v Anaerobic breakdown of animal wastes, leafy wastes, etc., by bacteria produces methane gas which is used as a fuel

v Microalgae are microscopic, plant-like organisms found in freshwater and marine environments. They are autotrophic, meaning they produce their own food using sunlight through photosynthesis, and in doing so, they release a large amount of oxygen into the atmosphere-making them important for the health of our planet.

v These tiny organisms are rich in nutrients like proteins,

v vitamins, and omega-3 fatty acids, so they are often used in health supplements. Microalgae are also being studied as a source of biofuel, which can be a cleaner alternative to fossil fuels. In addition, they help clean polluted water by absorbing harmful substances and excess nutrients. Some common examples of microalgae include Chlorella, Spirulina, and Dunaliella. However, pollution and climate change are major threats to their survival. Since they play a key role in producing oxygen, cleaning water, and offering health and energy benefits, it is important to protect and conserve microalgae by keeping our water bodies clean and supporting eco-friendly technologies.

 

*        Spirulina is a type of blue-green microalgae that grows in freshwater and alkaline lakes. It is packed with proteins, vitamins, minerals, and antioxidants, making it a popular health supplement. *        Spirulina is used to boost immunity, improve energy, and fight malnutrition. It is also used in space missions by astronauts as a nutritious food source. Due to its high nutritional value, spirulina is often called a superfood.

 

CELLS OF MICROORGANISMS

Just like plants and animals, microorganisms are also made up of cells. Some microorganisms, like fungi and algae can be multicellular or unicellular organisms. While bacteria and protozoa are unicellular organisms. Cells of all microorganisms have a cell membrane.

 

Fungi also have a cell-wall like plant-cells. Bacteria do not have a well-defined nucleus and nuclear membrane. Instead, they have nuclear material lying in the cytoplasm. This is called nucleoid. Such cells are called prokaryotic cells. Thus, bacteria are prokaryotes. Those cells that have a well-defined nucleus with a membrane and cell organelles are called eukaryotic cells. Most microorganisms, plants and animals are eukaryotes.

 

Antibiotic	A chemical substance that is produced by microorganisms and has the ability to kill or inhibit the growth of other microorganisms.
Microbiology	Science dealing with the study of microorganisms.
Yoghurt	A common dish made of milk curdled and fermented with a culture of Lactobacillus.
Anaerobic	A biological process that occurs in the absence of oxygen.
Nucleus	It is a spherical or oval structure also called the 'brain of the cell'.
Cytoplasm	It is a transparent, jelly-like substance, which occupies the space between the cell membrane and the nuclear membrane.
Coverslip	A small, thin, transparent piece of glass or plastic placed over a specimen on a microscope slide to protect it.

 

SUMMARY

v Cells are the structural and functional units of the body of living organisms.

v Organisms may be unicellular (single-celled) or multicellular (many-celled).

v Living cells show variations in size, shape, and number.

v Plant cells and animal cells have common features like cell membrane, nucleus and cytoplasm.

v Plant cells are surrounded by cell walls and are different from the animal cells in structure and function.

v Cell membrane is a thin, elastic, delicate, living membrane made up of lipids and proteins that separates the cell from its surroundings.

v The nucleus is a spherical or oval structure also called the 'brain of the cell'.

v Cytoplasm is a transparent, jelly-like substance, which occupies the space between the cell membrane and the nuclear membrane.

v Organisms which cannot be observed with the naked eye and need a microscope to be seen are called microorganisms or microbes.

v Microorganisms are present everywhere in air, water, food and inside the body.

v Microbes are classified into bacteria, algae, fungi. protozoa and viruses.

v Microorganisms can be very useful in contributing to health, providing vitamins, helping in nitrogen fixation in plants, production of antibiotics, etc.

v Harmful microorganisms are called pathogens and can cause many diseases in plants, animals and humans.