Microorganisms

Definition: An organism that is too small to be seen by the unaided eye, especially a single celled organism, such as a bacterium is called microorganism.


Microorganisms are classified into four major groups. These groups are bacteria, fungi, protozoa and some algae.


Viruses are also microscopic. They, however, reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal.


Where do Microorganisms Live


They can survive under all types of environment, ranging from ice cold climate to hot springs and deserts to marshy lands. They are also found inside the bodies of animals including humans. Some microorganisms grow on other organisms while others exist freely. Microorganisms like amoeba can live alone, while fungi and bacteria may live in colonies.


Microorganisms and Us


Microorganisms play an important role in our lives. Some of them are beneficial in many ways whereas some others are harmful and cause diseases.


Commercial Use of Microorganisms


Microorganisms are used for the large scale production of alcohol, wine and acetic acid (vinegar). Yeast is used for commercial production of alcohol and wine. For this purpose yeast is grown on natural sugars present in grains like barley, wheat, rice and crushed fruit juices, etc.


Medicinal Use of Microorganisms


Whenever we fall ill the doctor may give us some antibiotic tablets, capsules or injections such as of penicillin. The source of these medicines is microorganisms. These medicines kill or stop the growth of the disease-causing microorganisms. Such medicines are called antibiotics. These days a number of antibiotics are being produced from bacteria and fungi. Streptomycin, tetracycline and erythromycin are some of the commonly known antibiotics which are made from fungi and bacteria. The antibiotics are manufactured by growing specific microorganisms and are used to cure a variety of diseases. Antibiotics are even mixed with the feed of livestock and poultry to check microbial infection in animals. They are also used to control many plant diseases.


Vaccine


When a disease-carrying microbe enters our body, the body produces antibodies to fight the invader. The body also remembers how to fight the microbe if it enters again. So, if dead or weakened microbes are introduced in a healthy body, the body fights and kills them by producing suitable antibodies. The antibodies remain in the body and we are protected from the disease causing microbes. This is how a vaccine works. Several diseases, including cholera, tuberculosis, smallpox and hepatitis can be prevented by vaccination.


Increasing Soil Fertility


Some bacteria and blue green algae are able to fix nitrogen from the atmosphere to enrich soil with nitrogen and increase its fertility. These microbes are commonly called biological nitrogen fixers.


Commensals:


Some microorganisms live inside our body. They help in certain functions of our bdoy systems. For example some bacteria in our gastrointestinal tract help in digestion. These type of organisms which reside in our body and are beneficial as well are called commensals.


Apart from them certain food like curd, contain beneficial bacteria.


Harmful Microorganisms


Microorganisms are harmful in many ways. Some of the microorganisms cause diseases in human beings, plants and animals. Such disease-causing microorganisms are called pathogens. Some microorganisms spoil food, clothing and leather.


Disease— causing Microorganisms in Humans


Pathogens enter our body through the air we breathe, the water we drink or the food we eat. They can also get transmitted by direct contact with an infected person or carried through an animal. Microbial diseases that can spread from an infected person to a healthy person through air, water, food or physical contact are called communicable diseases. Examples of such diseases include cholera, common cold, chicken pox and tuberculosis.


There are some insects and animals which act as carriers of disease causing microbes. Housefly is one such carrier. The flies sit on the garbage and animal excreta. Pathogens stick to their bodies. When these flies sit on uncovered food they may transfer the pathogens. Whoever eats the contaminated food is likely to get sick. Another example of a carrier is the female Anopheles mosquito, which carries the parasite of malaria. Female Aedes mosquito acts as carrier of dengue virus.


Disease— causing Microorganisms in Animals


Several microorganisms not only cause diseases in humans and plants, but also in other animals. For example, anthrax is a dangerous human and cattle disease caused by a bacterium. Foot and mouth disease of cattle is caused by a virus.


Disease— causing Microorganisms in Plants


Several microorganisms cause diseases in plants like wheat, rice, potato, sugarcane, orange, apple and others. The diseases reduce the yield of crops. They can be controlled by the use of certain chemicals which kill the microbes.


Food Poisoning


Food poisoning could be due to the consumption of food spoilt by some microorganisms. Microorganisms that grow on our food sometimes produce toxic substances. These make the food poisonous causing serious illness and even death. So, it is very important that we preserve food to prevent it from being spoilt.


Food Preservation


Microorganisms spoil our food. Spoiled food emits bad smell and has a bad taste and changed colour.


Common methods to preserve food in our homes.


Chemical Method: Salts and edible oils are the common chemicals generally used to check the growth of microorganisms. Therefore they are called preservatives. We add salt or acid preservatives to pickles to prevent the attack of microbes. Sodium benzoate and sodium metabisulphite are common preservatives. These are also used in the jams and squashes to check their spoilage.


Preservation by Common Salt: Common salt has been used to preserve meat and fish for ages. Meat and fish are covered with dry salt to check the growth of bacteria. Salting is also used to preserve amla, raw mangoes, tamarind, etc.


Preservation by Sugar: Jams, jellies and squashes are preserved by sugar. Sugar reduces the moisture content which inhibits the growth of bacteria which spoil food.


Preservation by Oil and Vinegar: Use of oil and vinegar prevents spoilage of pickles because bacteria cannot live in such an environment. Vegetables, fruits, fish and meat are often preserved by this method.


Heat and Cold Treatments: You must have observed your mother boiling milk before it is stored or used. Boiling kills many microorganisms.


Similarly, we keep our food in the refrigerator. Low temperature inhibits the growth of microbes. Pasteurized milk can be consumed without boiling as it is free from harmful microbes. The milk is heated to about 700C for 15 to 30 seconds and then suddenly chilled and stored. By doing so, it prevents the growth of microbes. This process was discovered by Louis Pasteur. It is called pasteurization.


Storage and Packing: These days dry fruits and even vegetables are sold in sealed air tight packets to prevent the attack of microbes.


Nitrogen Fixation: Rhizobium is involved in the fixation of nitrogen in leguminous plants (pulses). Sometimes nitrogen gets fixed through the action of lightning. But the amount of nitrogen in the atmosphere remains constant.
Nitrogen cycle


Our atmosphere has 78% nitrogen gas. Nitrogen is one of the essential constituents of all living organisms as part of proteins, chlorophyll, nucleic acids and vitamins. Certain bacteria and blue green algae present in the soil fix nitrogen from the atmosphere and convert into compounds of nitrogen. Once nitrogen is converted into these usable compounds, it can be utilised by plants from the soil through their root system. Nitrogen is then used for the synthesis of plant proteins and other compounds. Animals feeding on plants get these proteins and other nitrogen compounds.


When plants and animals die, bacteria and fungi present in the soil convert the nitrogenous wastes into nitrogenous compounds to be used by plants again. Certain other bacteria convert some part of them to nitrogen gas which goes back into the atmosphere. As a result, the percentage of nitrogen in the atmosphere remains more or less constant.

Cell A Fundamental Unit of Life Notes for classs 8

Fundamental Unit of Life
Cells are the basic unit of life, all living things, are made up of one or more cells. Organisms that exist as single cells are called unicellular and organisms that are made up of groups of cells working together are called multicellular. There are two kingdoms of unicellular organisms (Archaea and Bacteria), and three kingdoms of multicellular organisms (Animals, Fungi and Plants), and one kingdom which contains a mixture of both unicellular and multicellular organisms (the Protista).
Each living cell has the capacity to perform certain basic functions that are characteristic of all living forms. A cell is able to live and perform all its functions because of these organelles. These organelles together constitute the basic unit called the cell.
1)Discovery of the cell
In 1965 Robert Hookeobserved slices of cork from the bark of a tree under a simple magnifying glass. He noticed small boxes or compartments in the cork slice. The boxes were separated from each other by a wall or partition. He named each box as a Cell.The cells which Hooke observed were actually dead cells of plants.
2) The Cell
Cellsare the basic structural units of living organisms.
Some organisms are made up of a single cell and some are made up of many cells.
The number of cells in a tall tree or an animal like an elephant or human being are billions and trillions of cells.
Organisms made up of many cells are calledmulticellularorganisms.Eg: a tree, an elephant.
Organisms made up of only one cell are calledunicellularorganisms.Eg: amoeba, paramaecium.
3) Shape of cells
Cells are of different shapes.
Some cells keep changing their shape. Eg : amoeba, white blood cells in human beings.
Some cells are spherical, round, elongated, spindle shaped, or branched.
4) Size of cells
Cells in living organisms are of different sizes. They may be as small as millionth of a meter (micrometre or micron) or large as a few centimetres.
The smallest cell is the cell of bacteria (0.1 to 0.5 micrometre).
The largest cell is the egg of an ostrich (17cm x 13cm).
Structural Organization of a Cell











Animal cells













There are three features in almost every cell; plasma membrane, nucleus and cytoplasm. All activities inside the cell and interactions of the cell with its environment are possible due to these features.
PLASMA MEMBRANE OR CELL MEMBRANE
This is the outermost covering of the cell that separates the contents of the cell from its external environment. The plasma membrane allows or permits the entry and exit of some materials in and out of the cell. It also prevents movement of some other materials. The cell membrane, therefore, is called a selectively permeable membrane.
Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion.
CELL WALL
A cell wall is a tough, flexible and sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to act as a pressure vessel, preventing over-expansion when water enters the cell. They are found in plants, bacteria, fungi, algae, and some archaea. Animals and protozoa do not have cell walls.
When a living plant cell loses water through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis.
NUCLEUS
The nucleus is a membrane-enclosed organelle found in all eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these chromosomes are the cell's nuclear genome. The function of the nucleus is to maintain the integrity of these genes and to control the activities of the cell by regulating gene expression.
The nucleus has a double layered covering called nuclear membrane. The nuclear membrane has pores which allow the transfer of material from inside the nucleus to its outside, that is, to the cytoplasm
The nucleus plays a central role in cellular reproduction, the process by which a single cell divides and forms two new cells. It also plays a crucial part, along with the environment, in determining the way the cell will develop and what form it will exhibit at maturity, by directing the chemical activities of the cell.
CYTOPLASM
The cytoplasm is the part of a cell that is enclosed within the plasma membrane. In eukaryotic cells the cytoplasm contains organelles, such as mitochondria, that are filled with liquid kept separate from the rest of the cytoplasm by biological membranes. The cytoplasm is the site where most cellular activities occur, such as many metabolic pathways, and processes such as cell division.
Cytoplasm is basically the substance that fills the cell. It is a jelly-like material that is eighty percent water and usually clears in color. It is more like a viscous (thick) gel than a watery substance, but it liquefies when shaken or stirred.
CELL ORGANELLES
An organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid membrane.
Every cell has a membrane around it to keep its own contents separate from the external environment. Large and complex cells, including cells from multicellular organisms, need a lot of chemical activities to support their complicated structure and function. To keep these activities of different kinds separate from each other, these cells use membrane-bound little structures (or ‘organelles’) within themselves. This is one of the features of the eukaryotic cells that distinguish them from prokaryotic cells. Some of these organelles are visible only with an electron microscope.
Some important examples of cell organelles are: Endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, plastids and vacuoles. They are important because they carry out some very crucial functions in cells.
ENDOPLASMIC RETICULUM (ER)
The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets. It looks like long tubules or round or oblong bags (vesicles). The ER membrane is similar in structure to the plasma membrane. There are two types of ER– rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). RER looks rough under a microscope because it has particles called ribosome attached to its surface. The ribosomes, which are present in all active cells, are the sites of protein manufacture. The manufactured proteins are then sent to various places in the cell depending on need, using the ER. The SER helps in the manufacture of fat molecules, or lipids, important for cell function. Some of these proteins and lipids help in building the cell membrane. This process is known as membrane biogenesis. Some other proteins and lipids function as enzymes and hormones. Although the ER varies greatly in appearance in different cells, it always forms a network system.
Thus, one function of the ER is to serve as channels for the transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus. The ER also functions as a cytoplasmic framework providing a surface for some of the biochemical activities of the cell.
GOLGI APPARATUS
The Golgi apparatus, first described by Camillo Golgi, consists of a system of membrane-bound vesicles arranged approximately parallel to each other in stacks called cisterns. These membranes often have connections with the membranes of ER and therefore constitute another portion of a complex cellular membrane system.
The material synthesised near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus. Its functions include the storage, modification and packaging of products in vesicles. In some cases, complex sugars may be made from simple sugars in the Golgi apparatus. The Golgi apparatus is also involved in the formation of lysosomes.
LYSOSOMES
Lysosomes are a kind of waste disposal system of the cell. Lysosomes help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles. Foreign materials entering the cell, such as bacteria or food, as well as old organelles end up in the lysosomes, which break them up into small pieces. Lysosomes are able to do this because they contain powerful digestive enzymes capable of breaking down all organic material. During the disturbance in cellular metabolism, for example, when the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are also known as the ‘suicide bags’ of a cell. Structurally, lysosomes are membrane bound sacs filled with digestive enzymes. These enzymes are made by RER.
MITOCHONDRIA
Mitochondria are known as the powerhouses of the cell. The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine triphopshate) molecules. ATP is known as the energy currency of the cell. The body uses energy stored in ATP for making new chemical compounds and for mechanical work. Mitochondria have two membrane coverings instead of just one. The outer membrane is very porous while the inner membrane is deeply folded. These folds create a large surface area for ATP generating chemical reactions. Mitochondria are strange organelles in the sense that they have their own DNA and ribosomes. Therefore, mitochondria are able to make some of their own proteins.
PLASTIDS
Plastids are present only in plant cells. There are two types of plastids – chromoplasts (coloured plastids) and leucoplasts (white or colourless plastids). Plastids containing the pigment chlorophyll are known as chloroplasts. Chloroplasts are important for photosynthesis in plants. Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll. Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules are stored.
The internal organisation of the plastids consists of numerous membrane layers embedded in a material called the stroma. Plastids are similar to mitochondria in external structure. Like the mitochondria, plastids also have their own DNA and ribosomes.
VACUOLES
Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small sized in animal cells while plant cells have very large vacuoles. The central vacuole of some plant cells may occupy 50-90% of the cell volume.
In plant cells vacuoles are full of cell sap and provide turgidity and rigidity to the cell. Many substances of importance in the life of the plant cell are stored in vacuoles. These include amino acids, sugars, various organic acids and some proteins. In single-celled organisms like Amoeba, the food vacuole contains the food items that the Amoeba has consumed. In some unicellular organisms, specialised vacuoles also play important roles in expelling excess water and some wastes from the cell.
Concept of Unit of Life
Each cell thus acquires its structure and ability to function because of the organization of its membrane and organelles in specific ways. The cell thus has a basic structural organisation. This helps the cells to perform functions like respiration, obtaining nutrition, and clearing of waste material, or forming new proteins. Thus, the cell is the fundamental structural unit of living organisms. It is also the basic functional unit of life.Comparison of Plant cell and Animal cell