Solutions-Class 7-Science-Chapter-11-Cell Structure and Micro-Organisms-Maha Board

Cell is the fundamental structural and functional unit of living organisms. Cellular organization is the primary characteristic of all living organisms.

Cell ->Tissue -> Organ -> Organ System -> Body – Living Organisms.

The discovery of cells was first made by Robert Hooke. While examining a section of a cork under the microscope, he observed small compartment-like structures and named them cells. However, Hooke observed dead cells under the microscope as cork is made up of dead cells.

The body of all living organisms is made up of minute component called cell. The number of cells are different and specific for every living organism.

Name of the cell organelles are : Nucleus, Cell membrane, cell wall, vacuole, plastids, endoplasmic reticulum, chloroplast, Golgi body, ribosomes, mitochondria, nucleus, lysosomes, centrosome and inclusion bodies.

The living organisms on the earth which are all around us, but we are unable to see them are called micro-organisms. They can be seen only under compound microscope.

Microorganisms were observed for the first time by Anton von Leeuwenhoek in 1674, using a microscope of his own.

There are 5 major different types of Microorganisms, like:

Bacteria- Bacteria are the most primitive and diverse unicellular organisms found in living world. Because of their unique biochemical pathways and properties, they are of high economic importance to the human beings.

Fungi- Fungi is a group of eukaryotic, unicellular as well as multicellular, non-photosynthetic organisms that includes mushrooms, moulds and yeasts.

Algae- Algae are a diverse group of aquatic organisms that have the ability to conduct photosynthesis.

Protozoa- Many protozoa cause disease in animals and humans.

​Viruses- Viruses are ultra microscopic and non cellular organisms. They have simple structure having genetic material coated by protein coat. They are obligate parasites and highly pathogenic.

(a) The organelle called the chloroplast is present in plant cells only.

(b) Garbage is converted into compost by micro-organisms.

(c) In the cell, photosynthesis is carried out with the help of chloroplast.

(d) An electron microscope is necessary for the study of cells .

Plant Cell :

• Cell membrane: A cell is composed of cell membrane, cytoplasm and nucleus. The cytoplasm and nucleus enclosed in the cell membrane together constitute the plasma membrane. It checks the transport of substances in the cell.
• Cell wall: In plants, an extra protective covering known as cellulose is present. It is called cell wall and protects the plant cell from environment variations.
• Cytoplasm: It is a jelly-like substance present between the cell membrane and nucleus. It contains various cell organelles such as mitochondria and Golgi body.

• Nucleus: It is a dense spherical body located at the centre of the cell. It is surrounded by a porous nuclear membrane. It contains a spherical body called nucleolus and thread-like structures called chromosomes. Chromosomes are the structures that carry genes and play an important role in inheritance. The entire living substance in a cell is known as protoplast.
• Vacuoles: Vacuoles are fluid-filled membrane-bound structures in the cell. In plant cells, a single large vacuole is present. In animal cells, numerous small vacuoles are present.
• Plastids: They are present only in plant cells. Chloroplast is a plastid containing the green pigment called chlorophyll, which is required for photosynthesis.
• Mitochondria: They are involved in cellular respiration, and hence, are called ‘power houses of the cell’.

Animal Cell :

(1) The outermost covering is plasma membrane.

(2) Inside this there is cytoplasm in which the cell organelles are embedded.

(3) The cell organelles are as following : Nucleus, Golgi body, endoplasmic reticulum and many smaller vacuoles.

Microorganisms have found wide scale application in various industries. Some of their applications are:

Use of Microorganisms in Food Industry

(i) Lactobacillus bacterium promotes the conversion of milk into curd.

(ii) Yeast is used in preparation of breads, pastries, and cakes.

Use of Microorganisms in Vaccine Production

(i) Protection of the body from the attack of various disease-causing microorganisms through vaccines is known as vaccination.

(ii) Vaccine includes dead or weakened microbes that trigger the production of antibodies in the body.

Use of Microorganisms in Increasing Soil Fertility

(i) Blue green algae and Rhizobium are called biological nitrogen fixers.

(ii) They fix atmospheric free nitrogen to enhance soil fertility.

Harmful Microorganisms

These are just few examples which highlight the commercial importance of these microorganisms. However, they are not always beneficial can lead to various kinds of damage.

(1) Micro-organisms spoil the food. In hot and humid air there is fugal attack on food.

(2) Microbes produce enterotoxins and cause food toxic resulting into food poisoning.

(3) Harmful microbes cause diseases in plants, animals and human. Amoebiasis, typhoid, cholera, hepatitis, gastro are caused by water and food borne microbes.

(4) Microbes cause diseases like malaria, dengue, elephantiasis, yellow fever, chikungunya, Zika fever, etc. These diseases are caused after mosquito transfers the microbes into human body. Common cold, cough, diphtheria, pneumonia, tuberculosis are diseases of respiratory tract which are caused by microbes

Diseases spread on a large scale during periods of heavy rainfall and floods because it provides the optimum conditions for the growth of vectors which transmit diseases.

After heavy rainfall and floods, the water and food gets contaminated.

The water logging causes growth of micro-organisms and house flies. They spread the pathogens rapidly.

Water gets accumulated at places and acts as breeding grounds for mosquitoes and other vectors.

The humid atmosphere induces food spoilage.

Even if flood water recedes, the poodle formed helps in the breeding of mosquitoes.

For example, malaria is caused by a virus which is transmitted by a mosquito.

All such reasons are responsible for epidemic spread of diseases.

The stale food is likely to have microbes in it. Some of the microbes produce enterotoxins. The enterotoxins cause food poisoning. Bacteria and fungi can infect the foods which are stored for longer duration of time. Therefore there is a possibility of food poisoning if we eat stale food.

Tilling is a mechanical procedure of agitating soil where the top layer is mixed with the lower layers of soil.

There are some useful bacteria in the soil. They help to decompose organic substances into inorganic nutrients. Thereby they produce fertilizers.

Some microbes also help in nitrogen fixation. They help the plants to get nitrogen.

Thus to have better yleld of crop, the soil is turned over during tilling to mix the bacteria.

Fungus does not grow in dry conditions. The fungus needs suitable conditions for the growth. The moist and humid conditions are favourable to it. 

Refrigerator is used in almost every home because it prevents the spoilage of food.

The harmful microbes do not grow in cold temperatures. they grow only in the temperature range of 15° C to 35° C.

Refrigerator has very low temperatures. Food can be stored there without spoiling.

Therefore, to store perishable food items refrigerators have become essential in every home.

Bread rises during baking because of addition of yeast to its dough.

Yeast is a type of fungus, It undergoes process of fermentation in the batter of bread and releases carbon dioxide.

In the process of fermentation the volume of material rises, Therefore, the bread rises during baking, that makes the dough very soft and fluffy, and thus improves the quality of the bread. 

By adding the fodder into water, it is fermented. By fermentation, the vitamin content rises in the fodder, it also becomes easier for digestion. The cattle is thus well nourished. Therefore, fodder is soaked in water before offering to cattle.

In this way, simple microscope is used for general observations and compound microscope is used for specialized observations that require greater magnification.